C Colman
email: charlotte.colman@uliege.be

Internal sulfate attack in mortars containing contaminated fine recycled concrete aggregates

Keywords: Recycled aggregates, sulfate attack, microstructure, waste management, secondary ettringite formation

Internal sulfate attack can be caused by the gypsum residues present in fine recycled aggregates (FRA). As opposed to the better known external sulfate attack or Delayed Ettringite Formation (DEF), the sulfates in this context are provided by a gypsum contamination of the aggregates. Mortars made with contaminated FRA were subjected to different conditions, to assess which parameters had an influence on the sulfate attack reaction. Their mechanical properties and microstructure are investigated. Results showed that gypsum content, porosity, temperature and alkalinity influenced the consequences of sulfate attack. However, the gypsum size distribution and cement type did not.

Introduction

Gypsum in fine recycled concrete aggregates

One of the key points within the framework of a sustainable construction sector is the recycling of its waste products, completing the life cycle of these materials. Recycled concrete aggregates (RCA), obtained by the demolition or deconstruction of older concrete structures, can be used in a new concrete as a replacement for natural aggregates [START_REF] Oikonomou | Recycled concrete aggregates[END_REF]. 1.7 tonnes of these RCA are produced per person per year in Europe, waiting to be valorized [START_REF] Delvoie | Market analysis of recycled sands and aggregates in northwest europe: drivers and barriers[END_REF]. Gypsum (CaSO 4 .2H 2 O) is used in the construction sector firstly as an addition to Portland cement, to regulate the setting time of concrete and prevent a flash set [START_REF] Collepardi | A state-of-the-art review on delayed ettringite attack on concrete[END_REF]. Besides that, gypsum is the major constituent of plaster walls in buildings. RCA, as a consequence, will contain a certain amount of gypsum. Larger concrete and gypsum particles can be separated from each other based on a difference in color [START_REF] Asakura | Characteristics of fine processed construction and demolition waste in japan and method to obtain fines having low gypsum component and wood contents[END_REF] or density [START_REF] Ambrós | Separation in air jigs of mixed construction and demolition waste[END_REF], but these techniques are not applicable on the smallest size fractions of RCA. In fine recycled aggregates (FRA), gypsum is an important contaminant to be considered: the water soluble sulfates coming from the gypsum particles strongly limit their valorization potential [START_REF] Agrela | Limiting properties in the characterisation of mixed recycled aggregates for use in the manufacture of concrete[END_REF].

Sulfate attack: sources and mechanisms

Sulfate attack is a deteriorating process where sulfates react with water and aluminate hydrates in a hardened cement paste to form secondary ettringite. It is assumed that this mineral exerts a pressure on its surrounding cement paste and causes a volumetric deformation [START_REF] Diamond | Delayed ettringite formation -processes and problems[END_REF]. Macroscopically, the concrete structure will show swelling behavior and the formation of cracks. Ettringite is a normal hydration product in the cement paste: its formation only becomes dangerous when it occurs after setting, in a rigid cement matrix. Depending on the source of the sulfates responsible for the reaction, a distinction can be made between an external and an internal reaction. To experience external sulfate attack, the structure is submerged in a sulfate rich environment such as soil or seawater.

Diffusion mechanics and microcrack propagation from the surface inwards are determining factors here [START_REF] Yu | Mechanism of expansion of mortars immersed in sodium sulfate solutions[END_REF]. Internal sulfate attack happens when there is a delayed release of sulfates from the hardened cement matrix. In this sense, an internal source of sulfates eliminates the diffusion and microcrack necessity of external sulfate attack, possibly accelerating the reaction. Delayed Ettringite Formation (DEF), which occurs when high curing temperatures have destroyed the sulfate hydrates that were initially formed [START_REF] Diamond | Delayed ettringite formation -processes and problems[END_REF], has been known for some time. The current situation, where FRA is contaminated with gypsum residues, is a relatively new problem that has not been studied extensively.

The following aspects of the internal sulfate attack reaction were selected to be elaborated in this study:

Alkalinity. Many authors show the important role of the alkalinity of the interstitial solution, as it interferes with the equilibrium between the different sulfate phases. A higher alkalinity favors the existence of monosulfate and the absorption of sulfur on the C-S-H gel instead of the formation of ettringite [START_REF] Escadeillas | Some factors affecting delayed ettringite formation in heat-cured mortars[END_REF], so ettringite formation triggers as pH lowers. Nevertheless, a higher swelling due to ettringite formation is often found in mixes with a higher alkalinity [START_REF] Escadeillas | Some factors affecting delayed ettringite formation in heat-cured mortars[END_REF][START_REF] Pavoine | The impact of cement parameters on delayed ettringite formation[END_REF][START_REF] Halaweh | Effect of alkalis and sulfates on portland cement systems[END_REF][START_REF] Leklou | Contribution à la connaissance de la réaction sulfatique interne[END_REF][START_REF] Nguyen | The effect of natural pozzolan on delayed ettringite formation of the heat-cured mortars[END_REF]. While alkalinity speeds up hydration and increases early compressive strength [START_REF] Juenger | Effects of high alkalinity on cement pastes[END_REF], it leads to lower mechanical performances in a sulfate presence [START_REF] Escadeillas | Some factors affecting delayed ettringite formation in heat-cured mortars[END_REF][START_REF] Pavoine | The impact of cement parameters on delayed ettringite formation[END_REF]. Besides a possible interaction with sulfate attack, alkalinity is also a risk factor for the alkali-silica reaction and other durability issues.

Cement type. Using a sulfate-resisting cement allows the use of FRA with a high sulfate content [START_REF] Agrela | Influence of the sulphate content of recycled aggregates on the properties of cement-treated granular materials using sulphate-resistant portland cement[END_REF]. These types of cement contain less C 3 A, one of the reactants needed to form ettringite. Moreover, fewer gypsum is added to this cement type as a setting retarder, compensating for the additional sulfate source to which the mixture will be exposed. The SO 3 /Al 2 O 3 ratio of a cement is an important factor regarding its potential to form ettringite [START_REF] Batic | Secondary ettringite formation in concrete subjected to different curing conditions[END_REF].

Porosity. The most commonly accepted theory about the cause of expansion is the heterogeneous crystal pressure exerted by the growing ettringite crystals [START_REF] Diamond | Delayed ettringite formation -processes and problems[END_REF][START_REF] Collepardi | A state-of-the-art review on delayed ettringite attack on concrete[END_REF][START_REF] Breysse | Deterioration processes in reinforced concrete: an overview[END_REF]. In this sense, a lower porosity means more confinement and a higher internal pressure. On the other hand, in the case of external sulfate attack, a lower porosity would prevent the inwards diffusion of sulfates and thus limit the swelling potential [START_REF] Zhang | Study on the expansion of concrete under attack of sulfate and sulfate-chloride ions[END_REF].

Sulfates. Before dissolution, sulfates can be associated with different cations. It has been found that the sulfates originating from Na 2 SO 4 lead to more swelling than those from CaSO 4 [START_REF] Escadeillas | Some factors affecting delayed ettringite formation in heat-cured mortars[END_REF], and that MgSO 4 is even more damaging [START_REF] Souza | Evaluation of external sulfate attack (na2so4 and mgso4): Portland cement mortars containing fillers[END_REF]. This would suggest that a gypsum contamination is less damaging than other types of sulfate attack that have been researched. However, these are external sources.

The rapid availability of an internal CaSO 4 contamination could make this difference smaller.

Coarser gypsum particles are hypothesized to not feed early ettringite formation, but react later in an already rigid cement matrix. To keep the risk on sulfate attack at a reasonable level, the current water soluble sulfate limit in coarse recycled aggregates is established at 0.2% by EN 206 [START_REF] Standardization | Concrete: Specification, performance, production and conformity: Annex E.3 on Recycled Aggregates[END_REF], with no mention of FRA. At these quantities, sulfates are considered the limiting reagent in the ettringite formation reaction so any augmentation would hypothetically lead to more swelling. The conclusions of recent durability studies indicate a level of 0.3% should be made possible [START_REF] Rougeau | Propriétés liées à la durabilité[END_REF].

Thaumasite formation. Next to ettringite, sulfates can also contribute to the formation of the expansive mineral thaumasite. While damage caused by thaumasite is more severe than that caused by ettringite, thaumasite formation does not occur as often [START_REF] Irassar | Sulfate attack on cementitious materials containing limestone filler -a review[END_REF]. Only at temperatures lower than 10 • C and in the presence of a carbonate source, is thaumasite favored over ettringite [START_REF] Rahman | Thaumasite sulfate attack on concrete: Mechanisms, influential factors and mitigation[END_REF].

Objectives

In this study, contaminated FRA were used in mortars to research the damaging effects of sulfates. Each parameter of interest was varied while others were kept constant, to identify the factors that can worsen or mitigate the sulfate attack results. Knowing which parameters to manipulate in a mix design with highly contaminated FRA will ultimately promote the use of these recycled aggregates. The results of this study could also contribute to the ongoing discussion about the sulfate attack reaction mechanism [START_REF] Neville | The confused world of sulfate attack on concrete[END_REF]. CEM I 52. [START_REF] Ambrós | Separation in air jigs of mixed construction and demolition waste[END_REF] gave exact control of the chemical composition of the materials and removed any possible variability or contamination at the level of the aggregates by chlorides, organics, etc. This FRA was then manually contaminated with gypsum to obtain a 'clean' material where only sulfates could contribute to a deteriorating reaction.

Figure 1 and Table 2 summarize the properties of the resulting FRA. Water absorption and particle density of the FRA were determined via the method described by Zhao et al. [START_REF] Zhao | Influence of hardened cement paste content on the water absorption of fine recycled concrete aggregates[END_REF]. Characterization techniques for natural aggregates, described in EN 1097-6 [START_REF] Standardization | EN 1097-6 'Tests for mechanical and physical properties of aggregates[END_REF], consistently underestimate the water absorption of FRA because of the fineness and agglomeration issues between the particles.

The method -designed in response to this difficulty -by IFSTTAR [START_REF] Ifsttar | Test method no. 78[END_REF] seems to overestimate the water absorption of FRA but works well for particles in the 0.5/4 mm range. Thanks to an very good correlation between the hardened cement paste content or mass loss at 475 • C and the water absorption, the water absorption of the fines can then be extrapolated. Using the water absorption of each size fraction (either measured for the coarser particles or calculated for the fines) is more accurate than using either of the two mentioned experimental methods for the whole 0/4 mm bulk [START_REF] Le | Fresh behavior of mortar based on recycled sand: Influence of moisture condition[END_REF]. Even though no gypsum was

Mortar fabrication

All mortars were prepared with the FRA described in section 2.1, and were contaminated with 5% (by weight of the granular fraction) of gypsum. This gyspum, together with the residual sulfates in the model FRA, brings the total sulfate content of this mix to 3.08%. 7 days before mixing, the FRA was presaturated with its absorbed water and 10% of the mixing water. The standard procedure described in EN 196-1 [START_REF]for Standardization, EN 196-1[END_REF] for mortar fabrication was followed, where normalized sand was replaced volumetrically by FRA: using a particle density of 2.6 g/cm 3 for normalized sand and 1.95 g/cm 3 for the FRA, the aggregate envelope volume was kept constant. After a cure of 24 hours, the mortars were kept in water at 21 • C. These described compositions or conditions were then varied accordingly, depending on the parameter that was tested.

Tested parameters

The following parameters were hypothesized to have an effect on the internal sulfate attack reaction. The influence of each one was tested with an exaggerated high (+1) and a low (-1) level, and in one case also an intermediate (0) level.

Table 4 summarizes these levels and how each of them was obtained by adapting the standard mortar composition. Table 5 shows the compositions in more detail. To isolate the response of only one parameter, each series of replicates is kept in its own container, so as not to be influenced by the leaching water of another [START_REF] Taylor | Delayed ettringite formation[END_REF]. While one factor is being researched, all other parameters are kept as described in section 2.2.

Alkalinity. The lower level of this parameter is the normal alkalinity present in the used cement. The higher level is double this amount, achieved by adding NaOH to the mixing water.

Cement type. To test the influence of the available C 3 A, a HSR cement was used. A small difference in alkalinity between the HSR cement and the CEM I was mitigated by adding NaOH to the mixing water, bringing the Na 2 O eq. of both cements on the same level.

Gypsum grain size. The same fine gypsum powder used in the other samples, was hardened and subsequently crushed to obtain particles in the 2/4 mm range.

These two size distributions -the powder or the coarser particles -are used to contaminate the FRA.

Porosity. To research the influence of the available porosity, the water to cement ratio (W/C) was varied.

Sulfate content.

A sulfate amount, one order of magnitude smaller than in the other mortars, was used to demonstrate the importance of this parameter.

Thaumasite formation. These mortars were kept in water at 5

g NaOH

Table 5: The compositions, in kg, of the different mortar samples. The changes between a mix and its reference composition is placed in bold. For the C-1 samples, the difference is in the type of cement, for the G+1 samples the size of the gypsum particles. Since the gypsum contamination is expressed as a mass% of the aggregate part, a lower gypsum content in S-1 means more FRA and thus also more absorbed water.

Monitoring of the reaction

To follow the development of the internal sulfate attack reaction, the mortar specimens were subjected to different tests. On a macroscopic level, the mass, length and ultrasonic wavespeed were recorded weekly to observe features of sulfate attack such as swelling and possible internal cracking. At 7, 28, 90 and 180 days the mortars were characterized mechanically for their compressive strength [START_REF]for Standardization, EN 196-1[END_REF] and porosity by Mercury Intrusion Porosimetry. Every described test was done for 3 replicate mortars. A microstructural analysis for the samples at 90 days was carried out to examine the pore structure and ettringite deposits, to complement the swelling results and provide grounds for their interpretation.

The samples were prepared according to routine procedures of embedding and polishing [START_REF] Kjellsen | Preparation of flatpolished specimens for sem-backscattered electron imaging and x-ray microanalysis -importance of epoxy impregnation[END_REF] using a 2020 resin from Huntsman and MD System from Struers with water-free diamond pastes and lubricants. Images in scanning electron microscopy were obtained on a Hitachi S-4300/SE-N and coupled with EDS analyses.

Results and discussion

In Figure 2, the length change of the mortars is shown. Standard deviations are not shown on these figures to improve their readability, but are taken into account when performing an unpaired t-test to check whether the swelling results differ from each other statistically.

According to Table 6, C-1, G+1 and P+1 are similar to the standard mix where all parameters have their normal value. Four other samples deviate from this trend: P-1 and T-1 had a larger expansion, A+1 and S-1 had a lower expansion. Next to their length, the mechanical properties that were monitored are presented in Figure 3. The compressive strength of all samples kept steadily increasing over time but did not seem to have any correlation to the corresponding swelling amounts. The samples that showed a high or low swelling did not have a low or high compressive strength, respectively. There was a large variation of the measured strengths between 25 and 40 MPa, the parameters that did not and their reference mix, shows that 3 parameters did not change the swelling results (C-1, P+1 and G+1), and 4 did (T-1, P-1, S-1, A+1).

influence the swelling amount did influence the compressive strength. There Length change (%)

Age (days) A-1, C+1, G-1, P0, S+1, T+1 A+1 C-1 G+1 P-1 P+1 S-1 T-1

Alkalinity

Sample A+1 with an increased alkalinity did not show any evidence of cracks.

Air bubbles were often filled with ettringite, but not in a way that caused damage. Instead, a high percentage of them showed the presence of a piece of calcite or portlandite in their center, with ettringite crystals growing outwards of this center and not inwards from the cement paste. This is demonstrated in Figure 6. Calcite has been described as a nucleation center for very fast ettringite growth at high alkalinity [START_REF] Komatsu | In-situ observation of ettringite crystals[END_REF][START_REF] Hargis | Calcium sulfoaluminate (ye'elimite) hydration in the presence of gypsum, calcite, and vaterite[END_REF]. The kinetics of this reaction explain the lack of available sulfates to cause damage in a cured cement matrix. A higher alkalinity has also been observed by Juenger et al. [START_REF] Juenger | Effects of high alkalinity on cement pastes[END_REF] to increase the initial rate of hydration and cause a higher early compressive strength, which has been confirmed in Figure 3. However, the swelling and compressive strength results obtained in these experiments contradicted the findings of numerous authors described in section 1.2. This could be due to the chosen experimental setup:

the high alkali values in this study were obtained by adding NaOH, which is immediately available. Alkali's coming from the adherent cement paste of FRA would take more time to leach into the interstitial solution. The rapid initial hydration in this sample prevented the higher swelling that is normally provoked by the alkalinity of FRA. While increasing the W/C ratio did not have an effect on the reaction, lowering it raised the swelling amount. Figure 3 shows a distinctly lower porosity for sample P-1, but not an equally large difference between P0 and P+1. In Figure 7, these findings are confirmed by showing a similar morphology for P0 and P+1, but a significantly denser matrix for P-1. No superplasticizer was added to counteract a loss in workability due to the lower W/C. The microstructure of P-1 also showed the presence of more unreacted cement particles. The P-1 samples displayed many cracks throughout which were filled with ettringite crystals, as illustrated in Figure 8. A high swelling level would seemingly be a good indicator for internal damage.

Sulfates

The S-1 samples contained only 0.28% of water soluble sulfates, which is still well above the maximum allowable limit in recycled aggregates [START_REF] Standardization | Concrete: Specification, performance, production and conformity: Annex E.3 on Recycled Aggregates[END_REF]. Still, no significant damage occured, as shown by its swelling behavior in Figure 2 and microstructure in Figure 9. The long term compressive strength of these 250 samples, shown in Figure 3, was lower than those with higher sulfate contaminations, but only because it did not increase after the first month of aging. The samples kept at lower temperatures showed an important amount of swelling compared to the other mixes, but the reaction seems slower and did 255 not reach a stabilization point yet after 6 months. Compressive strength, shown in Figure 3, was much lower compared to the other samples, which is due to the limited amount of cement in this mix. Figure 10 shows the internal damage in these samples, while EDS confirmed that pores, air bubbles or cracks are filled with an ettringite/thaumasite mixture. Limiting the C 3 A content, using coarse gypsum particles, or augmenting the W/C ratio did not significantly change the swelling results. The formation of thaumasite or limiting the available porosity lead to a larger expansion. On the other hand, increasing the alkalinity of the mix or using a lower sulfate content seemed to limit the swelling results. This indicates that the maximum sulfate content established in EN206 may be too strict, which is in line with the findings of the PN RecyBéton [START_REF] Rougeau | Propriétés liées à la durabilité[END_REF] and their subsequent proposal to set this limit at 0.3%. Unrelated to their influence on the swelling amount, the variation of these parameters also changed the compressive strength of the mortars.

In general, the use of FRA in mortars led to a high air content, which was responsible for a lower compressive strength when compared to the compressive strength of a standard mortar with natural aggregates. This air content was explained by the surface roughness of recycled aggregates.

These results provide industrials with helpful information for mix designs with high sulfate contents. This could in turn help promote the use of contaminated FRA, which are up to now not valorized.

Future work should focus on upscaling these tests to concrete, and/or research a possible interaction between the parameters.

Figure 1 :

 1 Figure 1: Size distribution 0-4 mm of the used FRA

195

  was less variation between the samples in terms of porosity, only the sample with a limited W/C ratio had a distinct lower porosity. The macroscopic differences between the parameters are interpreted together with microstructural observations.

Figure 2 :

 2 Figure 2: 6 month swelling behavior of the mortar samples in function of the tested parameters

Figure 3 :

 3 Figure 3: Compressive strength and porosity of the samples at 7, 28, 90 and 180 days

Figure 4 :Figure 5 :

 45 Figure 4: Contrast between a recycled aggregate and the new cement paste with a high air content

Figure 6 :

 6 Figure 6: A piece of calcite promoted the formation of ettringite at high alkalinity at an early age

Figure 7 :Figure 8 :

 78 Figure 7: Comparison of the cement paste density between P-1, P0, and P+1

Figure 9 :

 9 Figure 9: Lowering the sulfate content resulted in an undamaged sample

  260

Figure 10 :

 10 Figure 10: The T-1 samples were damaged by a mixture of ettringite and thaumasite

Table 1 :

 1 N Water Limestone aggregates (mm) Superplasticizer Composition, in kg, of the original concrete

			0/4 2/7 7/14	14/20
	350	175	216 658 436	612	0.4%

2. Materials and methods

2.1. Used materials

FRA were made in the laboratory by fabricating a concrete and subsequently crushing it. The composition of this original concrete is given in Table

1

, and was designed to obtain a consistency class S3 and strength class C30/37. After 90 days of curing, this concrete was crushed by a jaw crusher and the resulting 0/4 mm fraction was used as FRA in all described tests. The use of this 'model' FRA

Table 2 :

 2 Characterization of the used FRA

	Chemical CEM I 52.5 N CEM I HSR
	CaO	64.3	64.6
	SiO 2	18.3	21.4
	Al 2 O 3	5.2	3.7
	Fe 2 O 3	4.0	4.6
	MgO	1.4	0.8
	Na 2 O	0.32	0.27
	K 2 O	0.43	0.40
	SO 3	3.5	2.5
	Cl -	0.06	0.06
	LOI	2.3	1.3
	C 3 A	6.6	2.4
	C 4 AF	12	14
	C 3 S	61.9	68.8
	C 2 S	11.2	9.4

addedto the model concrete, 0.18% of water soluble sulfates were measured via 115 ion chromatography, originating from the used cement. The gypsum used to contaminate this FRA was a CaSO 4 .2H 2 O powder (D50 13 µm) obtained from VWR Chemicals. The sulfates from this gypsum contamination are added to the 0.18% of water soluble sulfates already in this FRA, originating from cement particles. A CEM I 52.5 N cement from HOLCIM 120 was used as the default cement, in one test replaced by a High Sulfate Resisting (HSR) CEM I from the same manufacturer. The chemical composition of these cements is shown in Table

3

.

Table 3 :

 3 Chemical composition (mass%) of the used cement types

Table 4 :

 4 Summary of how the mortar composition was changed to obtain the levels of the

	Factor	Level
	Alkalinity (A)	-1	0.61% Na 2 O eq.
		+1	1.2% Na 2 O eq.
	C 3 A content (C)	-1	2.4% (CEM I HSR)
		+1	6.6% (CEM I)
	Gypsum grain size (G)	-1	Powder (D50 13 µm)
		+1	2/4 mm distribution
	Porosity (P)	-1	W/C 0.35
		0	W/C 0.5
		+1	W/C 0.65
	Sulfate content (S)	-1	0.47% of water soluble sulfates
		+1	3.08% of water soluble sulfates
	Temperature (T)	-1	5 • C and a carbonate addition
		+1	21 • C
	different parameters
		10

• C to promote thaumasite formation over ettringite. For thaumasite formation, a source of carbonates is necessary besides C 3 A, sulfates and water. 20% by mass of CEM I was therefore replaced with a limestone filler. Here again, NaOH was added so the alkalinity of the mix resembled the others.

Table 6 :

 6 Unpaired t-test to evaluate if there is a significant difference between the samples

	Name	Length	Stdev	Sample Difference
		(180 days) (180 days)	size	for p>0.05
	A-1, C+1, G-1, P0,	0.0675	0.007	3
	S+1, T+1
	A+1	0.017	0.002	3	yes
	C-1	0.081	0.005	3	no
	G+1	0.060	0.002	3	no
	P-1	0.113	0.004	3	yes
	P+1	0.063	0.004	3	no
	S-1	0.037	0.010	3	yes
	T-1	0.137	0.004	3	yes

Acknowledgements

This research was conducted in the framework of the Interreg project VAL-DEM 'Solutions intégrées de valorisation des flux "matériaux" issus de la démolition des bâtiments: approche transfrontalière vers une économie circulaire' (Convention n • 1.1.57 of Interreg France-Wallonie-Vlaanderen 2014-2020).

General case

The four curves on Figure 2 that stay together between the 0.06% and 0.08% marks, show that increasing the porosity, using larger gypsum grains, or limiting the available C 3 A did not influence the amount of swelling. SEM results confirmed that the morphology of these samples was very similar. Figure 4 is a typical image found with recycled materials: the recycled aggregate is a cluster of natural aggregates in the original cement paste. The new cement paste, surrounding the recycled aggregates, had a notably higher air content, which has been observed before by Bouarroudj et al. [START_REF] Bouarroudj | Use of a reference limestone fine aggregate to study the fresh and hard behavior of mortar made with recycled fine aggregate[END_REF] for the same material. This air content can be explained by the surface roughness of the recycled aggregates [START_REF] Mehdipour | Understanding the role of particle packing characteristics in rheophysical properties of cementitious suspensions: A literature review[END_REF], which captures more air into the mixture than a round (natural) aggregate.

Next to that, the difference in surface free energy between the new cement paste and the recycled aggregates could also have played a role in this elevated air content [START_REF] Courard | The evaluation of the surface free energy of liquids and solids in concrete technology[END_REF]. Figure 5 illustrates how ettringite deposits in these samples were mainly found in pores or air bubbles, which sometimes also resulted in cracks in the surrounding paste.


Calypso Chadfeau

Safiullah Omary
email: safiullah.omary@insa-strasbourg.fr

Essia Belhaj

Christophe Fond

Françoise Feugeas

CHARACTERIZATION OF THE SURFACE OF FORMWORKS -INFLUENCE OF THE SURFACE ENERGY AND SURFACE TEXTURE PARAMETERS ON THE DEMOLDING FORCES

Keywords: adhesion, formwork, cement, physical -chemical characterization, surface texture parameters, surface energy

Highlights

• Surface formwork is characterized by combining interferometry and contact angle • An original experimental device is developed to assess the demolding force • Correlations between demolding force, surface energy and morphology are found • Performances of metallic and polymer formwork surfaces are compared

Essential in the field of construction and in constant improvement, the formwork process acts on the quality of the concrete structures. The aesthetic and technical possibilities offered by the use of concrete bring a total freedom in the creation of forms, textures and colors to the structures. In addition to the concrete formulations, the concrete placing conditions, the choice of the formwork system and the treatments applied determine the quality of the final concrete finishing. Two factors are of prime importance in obtaining a quality concrete finishing: the concrete formulation and its placing. The formwork is therefore a determining factor in achieving the required concrete finishing quality and in conditioning its evolution in the environment of the structure. It will particularly influence different periods in the life of a concrete structure: during its casting, setting and hardening, and unmolding.

The metallic formwork is most commonly used for the walls of the structures. They are covered with release agents which aim to facilitate the unmolding by limiting adhesion between formwork and concrete. However, the careful application of release agents is a delicate operation. They have to be evenly sprayed onto the formwork wall. This work is tedious and can induce a risk both for the technician (risk of falling on slippery ground, chemical risk by contact for example) and for the environment (spillage on the ground, infiltration of groundwater for example). That is why this study aims to offer a solution for the treatment of steel formwork that offers an alternative to the application of release agents.

In order to improve separation between the concrete and the formwork, the parameters influencing formwork/concrete adhesion have to be analyzed. Numerous studies have been carried out on the interface between concrete, release agent and formwork [START_REF] Bouharoun | Effect of a Superplasticizer on the Properties of the Concrete/Oil/Formwork Interface[END_REF][START_REF] Courard | Influence of the Reuse of OSB and Marine Plywood Formworks on Concrete Surface Aesthetics[END_REF][START_REF] De Brito | Evaluation of the Technical Performance of Concrete Vegetable Oil Based Release Agents[END_REF][START_REF] Libessart | Influence of the Type of Release Oil on Steel Formwork Corrosion and Facing Aesthetics[END_REF]. Some focused on the impact of the formwork on the formation of the concrete skin and its properties [START_REF] Libessart | Influence of the Type of Release Oil on Steel Formwork Corrosion and Facing Aesthetics[END_REF][START_REF] Figueiras | Combined Effect of Two Sustainable Technologies: Self-Compacting Concrete (SCC) and Controlled Permeability Formwork (CPF)[END_REF][START_REF] Gueit | Surfactant-Induced Growth of a Calcium Hydroxide Coating at the Concrete Surface[END_REF][START_REF] Lemaire | Evaluating Concrete Surfaces Using an Image Analysis Process[END_REF][START_REF] Martin | Béton Apparent: Influence de l'état Du Moule Sur l'aspect Des Surfaces[END_REF][START_REF] Vanhove | Prediction of the Lateral Pressure Exerted by Self-Compacting Concrete on Formwork[END_REF]. The concrete skin is mainly composed of cement. Its thickness can vary from the micrometer to centimeter depending on the concrete mix design, the type of formwork and the curing conditions [START_REF] Bur | Porosité Des Mortiers et Bioréceptivité[END_REF]. The adhesion between concrete and formwork can be due to mechanical anchoring, electro-capillary forces, and chemical bonds [START_REF] Libessart | Correlation between Adhesion Energy of Release Agents on the Formwork and Demoulding Performances[END_REF][START_REF] Mazkewitsch | Adhesion between polymers and concrete / Adhésion entre polymères et béton: Bonding • Protection • Repair / Revêtement • Protection[END_REF][START_REF]Décollement de la pellicule de ciment[END_REF][START_REF]L'adhérence du béton au coffrage[END_REF]. A formwork surface with a rugged morphology or cavities promotes mechanical anchoring and increases mechanical adhesion. In addition, the workability of the concrete, the fineness of the cement grain, the concrete placing are parameters which also influence the concrete/formwork adhesion. The electro-capillary forces are related to the wettability and energy of the formwork surface [START_REF] Libessart | Correlation between Adhesion Energy of Release Agents on the Formwork and Demoulding Performances[END_REF][START_REF]Décollement de la pellicule de ciment[END_REF][START_REF] Młodecki | Adhesion Forces of Polymer Modified Concrete and Plain Concrete to Steel in Moulds and in Reinforced Concretes[END_REF], influencing the formation of a thin layer of water at the interface during curing. During concrete setting, chemical reactions generate a solution of ions at the formwork interface. Chemical bonds can thus appear between the ions present at the metal surface (ions OH -,Fe 2+ …) and those from the fresh paste (OH --, Ca 2+ …). Concrete/formwork adhesion is thus promoted.

In this present work, the aim is to analyze the influence of the morphological and thermodynamic characteristics of different types of formwork on the demolding strength. Correlations between surface texture parameters, the surface energy of the formwork and demolding efforts are studied. The surface characteristics of the formworks are determined beforehand, by interferometry and measurement of contact angle. An original experimental device for evaluating demolding efforts at 24 hours is designed. Indeed, few studies [START_REF] Gilabert | Determination of Strength and Debonding Energy of a Glass-Concrete Interface for Encapsulation-Based Self-Healing Concrete[END_REF][START_REF] Spitz | Quantitative and Representative Adherence Assessment of Coated and Uncoated Concrete-Formwork[END_REF] have investigated the effect of formwork surface characteristics on the ability of concrete to adhere to the formwork.

1

Materials and Methods

Cement paste

The cement paste samples are composed of cement CEM II B-S 42.5 N CE CP1 NF, provided by EQIOM company. The water-to-cement ratio is defined as W/C = 0.4 and tap water is used for mixing. The preparation of the paste complies with the steps described in standard NF EN 196-3 [START_REF] Afnor | 3 : Détermination Du Temps de Prise et de La Stabilité[END_REF]: two 90-second slow-speed mixing times separated by a 30-second paste homogenization time. The cement powder is stored in hermetically sealed drums under laboratory conditions of humidity and temperature. Cementitious paste samples are made with cement taken from a single drum.

A particle size analysis of the cement powder contained in the drum is carried out. The particle size distribution of the cement powder is determined by laser method with five samples taken from the same drum. The distribution curve describes a multimodal distribution composed of three modes: a main mode at 18.7 µm, and two secondary modes of increasing importance at 586 µm and 0.6 µm respectively. The cement powder exhibits a median diameter D50 of 15.4 µm, D10 of 2.3 µm and D90 of 70.5 µm. The mean size of the smallest particle identified is 0.166 µm and the mean size of the largest particle in the sample is 976 µm.

The cement paste is cast into cylindrical PVC molds whose dimensions are 27.5 mm for inner diameter, 32 mm for outer diameter, and 30 mm for height. The pre-crack is performed by means of an adhesive strip applied to the plate and then folded back on itself. A line is drawn 6 mm from the outer edge of the adhesive strip in order to position the mould (Figure 1). This procedure is applied for each sample allowing to create an identical pre-crack on each sample. Each mold is positioned on a formwork plate before casting. It is then filled with 8 ml of cement paste using a syringe graduated to ± 1 ml. The paste is then vibrated using a specifically designed vibrator for 30 seconds.

The samples are placed in a humidity room for a standard curing after vibration. They are maintained for 24 hours at 20.0 ± 1.0 °C and a relative humidity greater than or equal to 90% (NF EN 196-1). The formwork/cement contact area represents 541 mm 2 .

Formwork samples

Reference formwork

The reference formwork surface is made of 17% chromium ferritic stainless steel. The European designation of the steel is X6Cr17 -1.4016 according to NF EN 10088-2. This type of formwork surface is called F17 R.

The formwork samples are provided by Hussor company which uses the same steel F17 for its industrial formworks. Plates of formwork are cut into two formats: 160*94*5 mm 3 and 94*60*5 mm 3 . The second format is used in the case of polished plates.

Tests are also run with formwork surfaces covered with a release agent. For these tests, a mineral oil FDI CIMACIRE G® is used. These surfaces are called F17 O. The characteristics of the release agent are reported in table . The oil is sprayed using the same process and device for all the F17 O samples, which consists in a unique spray with a spray bottle. The thickness of the film is not measured. The automatic polishing is carried out using a Buehler Motopol 2000 automatic polishing machine set at 150 rpm, at a force of 200 N. Prior to the polishing, the plates are fixed to the polishing head with natural wax.

The first step of the mirror polishing procedure is performed by using four types of grinding paper. The paper P180 is used for 6 minutes, P320 for 1 minute 30 seconds, P600 for 1 min 30 sec and P1200 for 1 min 30 sec. Following this step, the plates are rinsed under running water, dried with cotton and then immersed in an ultrasonic bath in a mixture of distilled water and ethanol for 5 min. These steps are completed with two rinses: one with running water and the other with ethanol. The plates are then dried by hot air ventilation. The mirror surface finish is achieved by using diamond suspensions and diamond pastes with different grain sizes. Grain size of 6 µm is applied for 2 min twice and grain size of 3 µm for 5 min. The plates are successively rinsed with water and ethanol and dried with cotton. The final step consists of an ultrasonic bath and hot air drying.

The intermediate polishing procedure is carried out only with a P180 grinding paper for 6 minutes and a P320 grinding paper for 3 minutes, followed by rinsing with running water, drying with cotton and finally by an ultrasonic bath in a mixture of distilled water and ethanol for 5 minutes. The polishing procedure is completed by rinsing with running water, rinsing with ethanol and then drying by hot air ventilation.

Coated formwork

A double-layer polymer solution is developed and called C20C27. One layer has to offer adhesion properties to the F17 steel formwork and one layer has to present demolding properties without damaging the concrete finishing for its aesthetics and durability (Figure 2 a). The two layers are successively photo-polymerized. The first layer, named C20, is deposited on the F17 steel formwork and then photo-polymerized. The second layer, called C27, is then deposited on the C20 layer before being in turn photo-polymerized. The C20 polymer solution is a commercial resin chosen for its adhesion properties to metal. The C27 polymer solution is an acrylate resin selected for its moisture resistance. The thickness of each layer is about 50 µm and is measured by a mechanical micrometer (Figure 2 b). The photopolymerization were performed by the LPIM laboratory [START_REF] Rannée | Développement de Revêtements Photopolymères Pour Le Démoulage de Coffrages de Béton[END_REF].

Characterization of the formwork surfaces

Surface morphology

The surface morphology is measured by means of an interferometric microscope (Bruker Contour GT-K®).

The surface morphology of the formwork plates is evaluated according to five parameters: Sa, Sq, Sdr, Vvv and Vvc, defined in the NF EN ISO 25178-2 standard [START_REF] Afnor | Norme Européenne NF EN ISO 25178-2 -Spécification Géométrique Des Produits (GPS) -État de Surface : Surfacique -Partie 2 : Termes, Définitions et Paramètres d'états de Surface[END_REF]. Sa and Sq parameters are height parameters. They bring information about variation of the points altitude which compose the defined surface (measured surface). Sdr is a hybrid parameter. It gives information about the texture of the defined surface. Vvc and Vvv are functional parameters. They are indicative of the repartition of voids volume, relatively to the defined surface. These parameters are useful for analyzing the mechanical adhesion between formwork surface and cement paste components.

Characterization of surface morphology is related to the observation scale and the apparatus used [START_REF] Miller | Surface Roughness Criteria for Cement Paste Nanoindentation[END_REF][START_REF] Poon | Comparison of Surface Roughness Measurements by Stylus Profiler, AFM and Non-Contact Optical Profiler[END_REF][START_REF] Ahmed | Etude de l'encrassement biologique de matériaux cimentaires en eau de rivière -Analyse de l'influence des paramètres de surface des pâtes cimentaires[END_REF][START_REF] Garbacz | Characterization of Concrete Surface Roughness and Its Relation to Adhesion in Repair Systems[END_REF].

The value of the surface texture parameters depends on the dimensions of the grid chosen by the manipulator according to the measuring device. It is therefore necessary to determine these parameters as a function of the surface heterogeneities, which are of greater or lesser dimensions, in order to obtain homogeneous results for the same surface states. For that reason, a non-standardized procedure for determining a reference observation scale is used. A stitching procedure is thus performed on the F17 reference formwork surface in order to define the observation scale for which homogeneity of the surface texture parameters values is reached. The arithmetic mean of the point surface parameters (Sa1, Sa2 ...) is compared to the value of the same parameter (Sa stitch). The Sa stitch parameter is obtained from the stitching procedure. An overlap area of 10 % is considered during the stitching procedure. The interferometric measurements make it possible to select scale 0.6 x 0.5 mm² as the reference scale for the characterization of the formwork surface morphology.

Topography characterization is performed, at the reference scale, for the different formwork surfaces: F17 R, F17 M, F17 I and C20C27. A total of 100 measurements of surface topography are recorded on each formwork plate in order to assess the impact of the surface treatment. Measurements are spaced 5 mm apart.

Wettability and surface energy

Wettability of a surface, its hydrophilicity or hydrophobicity, can be assessed by the analysis of the contact angle formed by a drop of a water deposited on this surface. The physical and chemical characteristics of the surface influence its wettability. The surface is qualified as hydrophilic if the contact angle is less than 90° and hydrophobic for a contact angle greater than 90° [START_REF] Munzer | Etude de l'action d'un Bioadjuvant Aux Substances Extracellulaires Sur La Microstructure et Les Caractéristiques de Surface de Pâtes Cimentaires Pour Des Bétons plus Éco-Respectueux[END_REF].

The surface energy of the formwork surface is measured by application of the Owens-Wendt model, a method described in NF EN 828 [START_REF] Owens | Estimation of the Surface Free Energy of Polymers[END_REF][START_REF] Afnor | Norme Française NF EN 828 -Adhésifs -Mouillabilité -Détermination Par Mesurage de l'angle de Contact et de l'énergie Superficielle Libre de La Surface Solide[END_REF]. This method has been applied in different works [START_REF] Libessart | Correlation between Adhesion Energy of Release Agents on the Formwork and Demoulding Performances[END_REF][START_REF] Spitz | On Functional Signatures of Bare and Coated Formwork Skin Surfaces[END_REF] for the characterization of formwork surface properties. The model is based on the Young equation (Equation ( 1)) which can be applied for ideal, homogeneous, non-porous, non-rough surfaces [START_REF] Béranger | Approche Scientifique Des Surfaces. Caractérisation et Propriétés[END_REF]:

LV .cos =γ SV -SL (1)
In the Young equation, the contact angle relies on the equilibrium of the energies of the three interfaces solid/liquid SL , liquid/vapor LV and solid/vapor SV .

The Owens-Wendt method considers two proportions of the solid surface energy : the polar proportion and the disperse proportions . Similarly, the surface tension of a liquid comprises a polar proportion and a disperse proportion . By combining the Young equation and the Young-Dupré formula, the Owens-Wendt model describes the relation between the polar and disperse proportions of the two phases, liquid and solid, and the interfacial energy between the solid and the liquid SL as:

SL =γ +γ -2 . + . (2)
A general form of the Equation ( 2) can be written:

1 + cos . 2. = . + (3)
The previous equation ( 3) can be associated to a straight line y = ax + b, where a is equal to and b is equal to . The Owens-Wendt model uses different reference liquids. The polar and disperse proportions of these liquids are known. Therefore, the polar and disperse proportions, and , can be assessed from the measurement of the contact angle between the solid surface and each liquid. The total surface energy of the solid can be then calculated by summing the two proportions.

The surface energy is assessed for the reference formwork surface F17 R, the polished ones F17 M, F17 I and the coated ones C20C27. Three different liquids are used as reference liquids: distilled water, glycerol and ethylene glycol (Table 2). Table 2: Characteristics of the reference liquids at 20°C used for the assessment of the wettability and surface energy of the formwork surfaces with the Owens-Wendt model [START_REF] Afnor | Norme Française NF EN 828 -Adhésifs -Mouillabilité -Détermination Par Mesurage de l'angle de Contact et de l'énergie Superficielle Libre de La Surface Solide[END_REF].

An identical preparation is performed on all the formwork surfaces prior the measurement of the contact angle. Each plate of formwork is immersed in an ultrasonic bath composed of distilled water and ethanol for 5 minutes in order to be cleaned. The plate is then dried by air ventilation and stored for 24 hours in the measuring room. Before measuring the contact angle, compressed air is sprayed onto the surface of the plate to remove any particles that may have settled during storage. Static measurements are carried out with a DSA30 Kruss® drop analyzer at 22 ± 1 °C temperature and 44 ± 7 % relative humidity. For each liquid, ten drops of 5 µl are deposited on each formwork surfaces using a dosing needle of 0.5 mm outer diameter. The impact of gravity is thus negligible with respect to the capillary forces and the form of the water droplet as a cap is verified for the three liquids. Indeed, the capillarity length of each liquid is greater than the radius of the liquid droplet which is equal to 1.061 mm. The measurement starts a maximum of 10 seconds after dosing. Twenty measurements of left and right contact angle are taken for each drop and averaged. The polar and dispersed components are then calculated and summed up to obtain the surface energy of each type of formwork, according to the Owens-Wendt model.

Procedure of the demolding test with pre-crack

The adherence represents the force or the energy required in order to separate two materials linked by a shared surface. Its evaluation by a force or by an energy is related to the experimental set-up used to achieve the rupture [START_REF] Maugis | 4 -Adhesion of Solids: Mechanical Aspects[END_REF][START_REF] Qhach | Etude de la durabilité d'un primaire epoxy anticorrosion: rôle de l'interphase polymère/métal et conséquence sur l'adhérence[END_REF]. In civil engineering, usually, the adherence tests are run in order to evaluate the required high strength of bonds between concrete and rebars for example. In the case of formwork/concrete contact, adherence is aimed to be reduced or even to be eliminated. Therefore, the classical tests, such as the pull off test for the assessment of the reinforcement bar/concrete adhesion or concrete repair systems, have to be adapted to the contact between formwork and concrete.

Thus, a specific procedure of testing is developed in order to assess the force of separation required for separating cement sample from formwork. It is inspired by procedures of adherence standard tests: in particular, the pull off test, the peel test and the four points bending test, and also by the work of Gilabert et al. [START_REF] Gilabert | Determination of Strength and Debonding Energy of a Glass-Concrete Interface for Encapsulation-Based Self-Healing Concrete[END_REF]]. Moreover, during molding, the formwork surface is mainly in contact with cement paste. Therefore, samples are made with cement paste. The sample and procedure are designed to approach the in situ molding conditions without claiming to reproduce them completely.

This specific procedure consists in an original and non-standardized test. It is called demolding test with precrack, or pre-crack test. Its main mode of fracture is the mode I (opening -mode [START_REF] Maugis | 4 -Adhesion of Solids: Mechanical Aspects[END_REF][START_REF] Hasanpour | Rock Fracture Characterization Using the Modified Arcan Test Specimen[END_REF][START_REF] Cognard | Science et technologie du collage[END_REF]). The pre-crack is created at the formwork/cement interface and initiates the separation. Figure 1 illustrates the experimental design.

The force perpendicular to the surfaces in contact is applied in line with the pre-crack front by an attachment system made using a screw and two nuts connected to a hook-shaped steel wire, itself connected to a jack. Each series of pre-crack tests is carried out with a single batch and is performed 24 h ± 15 min after cement placing.

The test procedure consists in four steps:

• The formwork plate is positioned on the guide rail of the testing machine in such a way as to align the force and the pre-crack front

• The hook is anchored between the two nuts to ensure that the vertical force is applied and centered in line with the pre-crack front

• The plate is attached to the guide rail of the testing machine at both ends

•
The load is applied by the action of the jack, with a displacement speed of 0.1 mm.s -1

The test machine is a didactic test bench BED 100 from 3R company. The accuracy of the demolding forces measured by this test is 10 N. It is related to the experimental set-up and corresponds to the cable and hook tensioning and force sensor accuracy.

During the test, the force is systematically applied to the edge of the pre-crack thanks to the fixing system and the rail guidance. The application of the force is thus carried out in the same way for each specimen.

The test allows the force to be measured as a function of the displacement until the two formwork/cement surfaces are separated (Figure 1). The evaluation of the adherence between these two surfaces can thus be combined with the force or energy measured by such a separation test [START_REF] Maugis | 4 -Adhesion of Solids: Mechanical Aspects[END_REF][START_REF] Cognard | Science et technologie du collage[END_REF]. The maximum force reached, at the moment of the two elements are separated, is taken as the demolding force. The stress is then evaluated by taking the demolding force and the formwork/cement area (541 mm 2 ). The force or stress averages can be calculated.

Results and discussion

A total of 200 demolding test with pre-crack were performed to analyze the influence of the formwork surface characteristics on demolding efforts (Table 3).

Influence of the polishing and coating on the surfaces characteristics

Surface morphology

The impact of the two procedures of polishing can be noted at the macroscopic scale (naked eye) ( The mechanical action of polishing tends to concentrate the surface heights towards a common value. The Sdr parameter confirms this effect. The surface gain between the developed surface and the projected surface after polishing is less than 0.5 % in the case of the F17 M surfaces, it varies from 0.5 % to 1 % for the F17 I surfaces and it is greater than 10 % for the F17 R surfaces. According to this parameter, the surface area offered in contact with the cement is thus reduced in the case of the polished surfaces. For all the parameters, the value decreases with increasing polishing (Table 4).

The standard deviations obtained in the case of mirror polishing are relatively larger than the ones obtained for the other formwork surface The vertical resolution of the instrument under the measuring conditions and the values of the surface parameters measured are of the same order of magnitude: 5 nm for the vertical resolution of the interferometer and 16 nm for the Sa (F17 M). The dispersion of the values of the surface parameters may result from the limitations of the interferometric measuring equipment, in the case of a highly reflective surface.

The characterization of surfaces in civil engineering remains an intricate subject, as reported in detail in the work of Sadowski and Mathia [START_REF] Sadowski | Multi-Scale Metrology of Concrete Surface Morphology: Fundamentals and Specificity[END_REF]. The morphological characteristics of the surface can be evaluated objectively from the analysis of the surface parameters. Influence of the surface morphology in the context of formwork has been investigated in various studies [START_REF] Courard | Influence of the Reuse of OSB and Marine Plywood Formworks on Concrete Surface Aesthetics[END_REF][START_REF] Vanhove | Prediction of the Lateral Pressure Exerted by Self-Compacting Concrete on Formwork[END_REF][START_REF] Libessart | Correlation between Adhesion Energy of Release Agents on the Formwork and Demoulding Performances[END_REF][START_REF] Spitz | Quantitative and Representative Adherence Assessment of Coated and Uncoated Concrete-Formwork[END_REF][START_REF] Horgnies | Superhydrophobic Concrete Surfaces with Integrated Microtexture[END_REF]. They essentially described the surface morphology of the formwork by means of the surface profile parameter Ra, sometimes the surface profile parameter Rt. A few authors extend the characterization to other parameters [START_REF] Spitz | Quantitative and Representative Adherence Assessment of Coated and Uncoated Concrete-Formwork[END_REF][START_REF] Garbacz | Characterization of Concrete Surface Roughness and Its Relation to Adhesion in Repair Systems[END_REF][START_REF] Sadowski | The Effect of Surface Treatment on the Microstructure of the Skin of Concrete[END_REF][START_REF] Hoła | Usefulness of 3D Surface Roughness Parameters for Nondestructive Evaluation of Pull-off Adhesion of Concrete Layers[END_REF]. The surface profile parameters are not sufficient to represent the three dimensions of the surface. Areal parameters, such as Sa, can be assessed thanks to interferometry. The characteristics of a surface are then no longer evaluated by studying profiles, i.e. in 2D (height information on a profile), but by analyzing an area integrating the 3D information of the surface [START_REF] Blateyron | The Areal Field Parameters[END_REF]. This 3D analysis is beneficial even though the strict quantitative morphological approach cannot be considered as sufficient.

Spitz et al. [START_REF] Spitz | Quantitative and Representative Adherence Assessment of Coated and Uncoated Concrete-Formwork[END_REF] also studied the adhesion between metal or polymer-coated formwork and concrete. They evaluated the value of the parameters Sa, Sq, Sdr, Vvc and Vvv for each type of formwork by interferometric analysis. In the case of a mild steel formwork, Sa was equal to 0.9 ± 0.1 µm over an area of of 4*4 mm². In comparison, this Sa value is close to that of an F17 I intermediate polished stainless steel (Sa=0.112 ± 0.013 µm). The work of Magnoni et al. [START_REF] Magnoni | Correlation between the Scratch Resistance of UV-Cured PUA-Based Coatings and the Structure and Functionality of Reactive Diluents[END_REF] confirmed a close Sa value for a surface composed of the same stainless steel F17. They measured a Sa of 4.8 µm on a surface of 1*1mm² with an optical profilometer. In this work, Sa is approximately equal to 4.1 µm for a surface of 0.6*0.5 mm² measured with the interferometer (Table 4).

Interferometric measurements on the C20C27 coated formwork show a decrease of the value of all the parameters compared to the F17 R formwork surface (Table 4). Similarly, to the polishing, the coating induces a concentration of the height values to a common value and a reduction of surface area offered in contact, relatively to the F17 R reference surface. Surface morphology of formworks coated with other polymers (polypropylene PP and polytetrafluoroethylene PTFE) was also characterized by Spitz et al. [START_REF] Spitz | Quantitative and Representative Adherence Assessment of Coated and Uncoated Concrete-Formwork[END_REF]. The value of the parameters was higher than the one assessed on C20C27. For example, the surface texture parameters Sa, Sq and Sdr were evaluated on a PP coating and were equal to 0.8 ± 0.1 µm, 1.0 ± 0.1 µm, and 1.5 ± 0.2 % respectively. On the C20C27 coated formwork, the same parameters are equal to 0.2 ± 0.06 µm, 328 ± 72 nm i.e. 0.3 ± 0.06 µm, and 0.01 ± 0.01 % (Table 4).

Two types of modification of surface texture of the F17 R reference formwork are analyzed: polishing and coating. Both of the modifications induce a reduction of the surface texture parameters (Table 4). This reduction can be also measured by a percentage of reduction in comparison with the F17 R reference formwork surface (Table 5). This percentage is calculated considering the mean value of the parameter, the standard deviation, the minimal and maximal (mean minus/plus standard deviation) values. Table 5: Minimal and maximal percentages of reduction of the surface texture parameters due to polishing or coating, relatively to F17 R formwork surface.

The mirror polishing is the surface texture modification that allows to get the greater reduction percentage in comparison to the intermediate polishing and coating (Table 5). That is confirmed for all the surface texture parameters. All parameters included, the mirror polishing conducts to reduction percentage by at least 99 %. The Sa parameter can be reduced by 99.3 % and by up to 99.8 % for example. The intermediate polishing can lead to a percentage reduction by at least 92 % (% min Sdr) while the coating permits to reduce the percentage by at least 88 % (% min Vvc), all parameters included. The reduction of the parameters tends to be less important for the coated formwork surfaces than the mirror or intermediate polished ones. Moreover, the surface texture parameters of the coated surface and the ones of the intermediate polished surface have a tendency to be close.

Wettability and surface energy

The wettability and the surface energy of the C20C27 coated formworks and the polished formworks, F17 M and F17 I are reported in Table 6.

The values of contact angle on the F17 R formwork surfaces are more dispersed than the ones of the coated and polished surfaces. No difference can be thus highlighted between the reference surfaces and the others, considering standard deviation. When water is used, the mirror polished surface appears to be more hydrophilic than the intermediate polished surface. The values of the contact angle for the F17 M and the F17 I formwork surfaces are 67.3° ± 9.4°, 84.5° ± 6.9° respectively. A difference of contact angles between the F17 M and the F17 I formwork surfaces also exists when glycerol and ethylene glycol are used as measurement liquids.

Goossens [START_REF] Goossens | Modélisation du processus de polissage: identification des effets et des phénoménologies induits par l'usinage abrasif[END_REF] also observed a decrease of the contact angle with the decrease of the topographic characteristics of a stainless surface (316 L type) which was polished with different grit sizes. The contact angles of the polished surfaces were equal to 93.5°, 89.5° and 79.1° when the values of the Ra parameter varied from 0.58 µm, 0.1 µm to 0.04 µm respectively. Various factors can affect the value of contact angles of steel surfaces, especially stainless surfaces. The type of steel, the environmental conditions of measurement (relative humidity, temperature), the eventual treatment of the surface (polishing, thermal treatment for example), the procedure of cleaning or also the time between the cleaning/treatment and measurement are reported [START_REF] Kim | Aging Effect on the Wettability of Stainless Steel[END_REF][START_REF] Hedberg | Correlation between Surface Physicochemical Properties and the Release of Iron from Stainless Steel AISI 304 in Biological Media[END_REF][START_REF] Mantel | Influence of the Surface Chemistry on the Wettability of Stainless Steel[END_REF]. Kim et al. [START_REF] Kim | Aging Effect on the Wettability of Stainless Steel[END_REF] measured contact angles on a stainless surface varying from 80° ± 3.3° to 51° ± 2.9 according to the treatments and storage conditions.

The values of contact angle do not permit to distinguish the C20C27 and the F17 R formwork surfaces (Table 6). In the study of Spitz et al. [START_REF] Spitz | On Functional Signatures of Bare and Coated Formwork Skin Surfaces[END_REF] coated and mild steel formwork surfaces had different contact angle values. On the coated formwork (polypropylene PP), the value of contact angle was 71 ± 1° with water and 80 ± 2 ° with glycerol. Horgnies and Chen [START_REF] Horgnies | Superhydrophobic Concrete Surfaces with Integrated Microtexture[END_REF] measured a contact angle of 87 ± 3° with distilled water on a PVC formwork surface. Although the formwork surfaces are composed of different types of polymers (PP or PVC) the values of contact angle are close to the ones of the C20C27 formwork surfaces.

The surface energies assessed on the different formwork surfaces are summarized in Table 6. The surface energy of the F17 R formwork is equal to 27.7 ± 5.1 mN/m by application of Owens-Wendt model. The standard deviation is relatively high for this surface and distinction with the other formwork surfaces cannot be done. The surface energy of F17 R stays lower to the one of the F17 M and it is close to the ones of the F17 I and C20C27 surfaces. Indeed, the surface energy of the F17 M surface is 33.4 ± 3.8 mN/m and the ones of the F17 I and C20C27 are respectively 24.1 ± 3.6 mN/m and 25.7 ± 0.6 mN/m. The F17 M formwork surfaces can be distinguished from the F17 I and the C20C27 formwork surfaces due to their surface energies. The F17 I and the C20C27 formworks surface cannot be differentiated. It can be noted that the surface energy of the C20C27 surfaces is closer to the one of the F17 M surfaces than the one of the F17 I surfaces is.

These values of surface energy are consistent with the one measured on mild steel or coated formwork surfaces from the study of Spitz et al. [START_REF] Spitz | On Functional Signatures of Bare and Coated Formwork Skin Surfaces[END_REF]. The surface energy was 15.6 mN/m for mild steel surface, 8.5 mN/m for PP coating and 12 mN/m for PTFE coating.

Influence of the surface characteristics on the demolding efforts

The demolding forces are assessed for the different types of formwork surfaces: reference, polished, oiled and coated by use of the demolding test with pre-crack. The mean force for each type of formwork surface is reported in Table 7. Table 7: Force values at demolding according the type of formwork surface.

Surface morphology and demolding force

The mirror and the intermediate polishing tend to reduce the mean force of demolding, relatively to the F17 R reference formwork. The demolding force is 25 ± 20 N and 108 ± 22 N for the mirror and intermediate polished surfaces whereas it is 145 ± 47 N for the reference surfaces (Table 7). There is a clear influence of the polishing: the demolding efforts are significantly reduced by the mirror polishing. The same parameter can be reduced from 96.6 % to 97.8 % between the F17 R surface and the F17 I polished surface (Table 5). However, the minimal and maximal reduction percentages of the demolding force can vary from 54.2 % to 97.5 % for the F17 M surfaces and from 0 % to 54.8 % for the F17 I surfaces. The mean force can be reduced by 82.8 % and 25.1 %, for the F17 M and F17 I surfaces respectively (Table 8). 8: Reduction percentages of the demolding force depending on the type of formwork surface, relatively to the F17 R reference surfaces.

The dispersion of the recorded values induces an overlapping of the standard deviations (Figure 4) and explains why the decrease in force to detach the cement from the F17 R surfaces and the F17 I surfaces may be zero.

The difference in force values between the F17 I and the F17 M surfaces is noteworthy. Although the decrease in the surface texture parameters is not as consequent as it is between the F17 R and F17 M surfaces, the decrease of the force is significant when moving from the F17 M surfaces to the F17 I surfaces (Table 5 andTable 8). Considering the Sa parameter, it can be reduced from 76.1 % to 93.4 % (Table 5) between the F17 M and the F17 I surfaces, while the demolding force can decrease from 48.1 % to 96.3 % between these two surfaces F17 M and F17 I (Table 8).

The demolding efforts decrease with the decrease in the developed interfacial area. The parameter Sdr is an indicator of the possibility of contact with fresh cement paste. The fresh cement paste comprises cement phases at different times of hydration: anhydrous phases and others in the process of hydration. The dimensions of these phases are variable: from a few nanometers to a hundred micrometers [START_REF] Berodier | Impact of the Supplementary Cementitious Materials on the Kinetics and Microstructural Development of Cement Hydration[END_REF][START_REF] Bazzoni | Study of Early Hydration Mechanisms of Cement by Means of Electron Microscopy[END_REF][START_REF] Hernandez | Petrographic Characterization of Portlandite Crystal Sizes in Cement Pastes Affected by Different Hydration Environments[END_REF][START_REF] Asgari | Effect of Water and Nano-Silica Solution on the Early Stages Cement Hydration[END_REF]. The arrangement of these cementitious structures is variable as well: particle organization in the form of globular aggregates, or crystals. A compatibility of forms between the morphology of the formwork surface and the cementitious elements can emerge. This structural compatibility corresponds to the mechanical anchoring. The results indicate that the mechanical anchoring is greater in the case of the F17 reference surfaces than it is for the F17 M polished surfaces. The same conclusions appear with the analysis of the Vvc and Vvv parameters. The Vvc and Vvv parameters allow for an assessment of the open and available spaces on the formwork surface. The reduction of the demolding force follows the reduction of these parameters. In the case of the F17 I polished surfaces, even though a reduction of the surface texture parameters is noticed, the reduction of the demolding force is not automatic.

There is a clear reduction of the surface texture parameters between the C20C27 surfaces and the F17 R surfaces: Sa can be reduced from 91.0 % to 95.5 % (Table 5). However, the reduction of the demolding force is not automatic. Indeed, the reduction of the force between the two types of surface is 61 % at maximum or zero (Table 8). A dispersion of the force values exists (Figure 4 b), even though the mean force is reduced in the case of the coated surfaces relatively to the reference surfaces.

The decrease in surface texture parameters observed on the coated surfaces, relatively to the reference surfaces, can influence the mechanical anchoring in a similar way the polishing does.

Wettability, surface energy and demolding force

The F17 M surfaces present the greatest surface energy (Table 6 and Figure 5). For both of the polished surfaces, F17 M and F17 I, the reduction of the contact angles, the increase of the surface energy and the decrease in the demolding force are associated. There is no overlapping of the standard deviations for the surface energy as well as the demolding force. Despite the decreasing trend of the demolding force and of the surface energy with the intermediate polishing, there is no clear thermodynamic characterization that distinguishes the intermediate polished surface from the reference surface. A similar trend can be noted for the coated surfaces. Even though, the demolding force (mean value) tends to decrease, the wettability and the surface energy of the C20C27 cannot be differentiated from the thermodynamic properties of the F17 R surfaces (Table 6 and Figure 5).

1.6

Surface characteristics and demolding force

Distributional characteristics of demolding forces

The reduction of the surface texture parameters tends to reduce the demolding force. However, this reduction is not automatic for the F17 I and the C20C27 formwork surfaces. Considering these surfaces, the mean force is reduced but the dispersion of the force values leads to an overlapping of the standard deviations. A decrease in the contact angles and an increase in the surface energy accompany a decrease in the force values, only for the polished formwork surfaces F17 M and F17 I. The evaluation of the demolding force by use of the pre-crack test allows to observe that the effect of the demolding oil is similar to the one of the mirror polishing. In order to analyze in details the distribution of the force values for each formwork surface, the results are presented on Table 9 and Figure 6. The F17 R and the F17 I surfaces show the largest and the smallest interquartile ranges respectively: 65 N and 28 N. The Figure 6 shows that the three treatments: oil application, polishing and C20C27 coating, tend to reduce the demolding force value relatively to the reference surface. The mirror polishing permits to obtain the lower force values. The demolding oil also induces a significant decrease in the force values. Indeed, in the case of the F17 O surfaces, the force values of the interquartile range are between 27 N and 60 N, that represents the half of the whole force values. By comparison, the interquartile range of the F17 R surfaces comprises force values from 109 N to 173 N. For the F17 I surfaces, the interquartile range concentrates force values from 97 N to 125 N while the force values of the interquartile range are between 83 N and 112 N for the C20C27 surfaces. Their effect on the force reduction is lower compared to the one of the oil or the mirror polishing, but notable.

The influence of the type of formwork surface during the demolding was studied by Spitz et al. [START_REF] Spitz | Quantitative and Representative Adherence Assessment of Coated and Uncoated Concrete-Formwork[END_REF][START_REF] Spitz | On Functional Signatures of Bare and Coated Formwork Skin Surfaces[END_REF]. Authors analyzed surface texture parameters, adherence stress and surface energy for different types of formwork surfaces: mild steel, Zn/Al coated steel, PP coating and PTFE coating. The Table 10 presents the stress calculated from the mean demolding force obtained by use of the pre-crack test with the reference, oiled, polished and coated formwork surfaces.

The PP and PTFE coated surfaces were the formwork surfaces with the highest surface energies: 8.5 mN/m and 12 mN, respectively while the surface energies were 15.6 mN/m and 20.6 mN/m for the mild steel and Zn/Al coated steel surfaces [START_REF] Spitz | Quantitative and Representative Adherence Assessment of Coated and Uncoated Concrete-Formwork[END_REF]. The stress assessed between the coated surfaces and the concrete were higher than the ones evaluated for the metallic surfaces. These results highlighted the capillary effect arisen at the interface with smooth and hydrophobic surfaces. Adherence stress between metallic formwork and mortar, at one day of hydration, was assessed in the study of Mazkewitsch and Jaworski [START_REF] Mazkewitsch | Adhesion between polymers and concrete / Adhésion entre polymères et béton: Bonding • Protection • Repair / Revêtement • Protection[END_REF]. The adherence stress belowed to 0.047 MPa with the application of release agent on the metallic formwork while it was 0.185 MPa without. The F17 O induces a stress similar (Table 10) to the one measured by Mazkewitsch and Jaworski [START_REF] Mazkewitsch | Adhesion between polymers and concrete / Adhésion entre polymères et béton: Bonding • Protection • Repair / Revêtement • Protection[END_REF]. Besides, the stress value obtained for the F17 R, the F17 I and the C20C27 surfaces are in the same order of magnitude (Table 10) that the stress measured with the use of a metallic formwork without release agent. In the study of Mazkewitsch and Jaworski [START_REF] Mazkewitsch | Adhesion between polymers and concrete / Adhésion entre polymères et béton: Bonding • Protection • Repair / Revêtement • Protection[END_REF], when using a plastic formwork with glass fibers, the adherence stress was 0.021 MPa and it was 0.009 MPa for the plastic formwork with Fluor. This latest value is closed to the stress value assessed for the F17 M surfaces.

Synthesis

Adhesion between the formwork and the cement paste or concrete is partly the result of mechanical anchoring. The physical and chemical conditions at the formwork/cement interface also contribute to the creation of bonds between the formwork elements and the cement phases.

Interlocking of the elements of the cement paste in the formwork asperities can occur during casting, curing and demolding, and thus leading to mechanical anchoring. When casting, formwork surface textures can modify the water flow conditions and interactions with the finest elements of the cement paste. Anhydrous cement phases, partially hydrated and hydrated cement phases can be entrapped in the formwork surface cavities due to a potential correspondence of geometry and dimensions [START_REF] Berodier | Impact of the Supplementary Cementitious Materials on the Kinetics and Microstructural Development of Cement Hydration[END_REF][START_REF] Bazzoni | Study of Early Hydration Mechanisms of Cement by Means of Electron Microscopy[END_REF][START_REF] Hernandez | Petrographic Characterization of Portlandite Crystal Sizes in Cement Pastes Affected by Different Hydration Environments[END_REF][START_REF] Asgari | Effect of Water and Nano-Silica Solution on the Early Stages Cement Hydration[END_REF]. Indeed, when cement is cast and put in contact with the formwork surface texture, the growth of hydrates is in progress, and some hydrates can be unevenly located on the surface of the anhydrous phases [START_REF] Berodier | Impact of the Supplementary Cementitious Materials on the Kinetics and Microstructural Development of Cement Hydration[END_REF][START_REF] Nicoleau | A New View on the Kinetics of Tricalcium Silicate Hydration[END_REF]. The growth of hydrates, such as CSH and CH, depends on time, space availability, paste composition (anhydrous phase such as alite, belite, calcareous filler) and available water [START_REF] Bazzoni | Study of Early Hydration Mechanisms of Cement by Means of Electron Microscopy[END_REF][START_REF] Jennings | Refinements to Colloid Model of C-S-H in Cement: CM-II[END_REF][START_REF] Galmarini | Atomistic Simulation of Cementitious Systems[END_REF]. The surface texture parameters Sa, Sq, Sdr, Vvc and Vvv constitute surface indicators of the area and the volume forming potential contact zones for cement paste and its anhydrous and hydrated phases.

The surface texture parameters of the F17 R formwork surfaces, which are the highest, favor the mechanical anchoring and take part to the increase in the demolding efforts, relatively to the other formwork surfaces. Indeed, for the F17 R formwork surfaces, the mean value of Sa is in the order of 4000 nm, i.e. 4 µm and Sq is in the order of 5000 nm, i.e. 5 µm. The mean values of Vvc and Vvv are in the order of 5000 nm 3 .m -² (being 5000.10 -3 µm 3 .µm -²) and 700 nm 3 .nm -² (being 700.10 -3 µm 3 .µm -²) respectively. The particle size analysis of the cement powder defines the dimension of the smallest particle as 0.166 µm and a secondary mode at 0.6 µm, even though the main mode is 18.7 µm. The cement particles present dimensions of the same order of magnitude as the mean value of the arithmetic mean height and as the root mean square height of the F17 R formwork surface. In addition, the void space available at the formwork surface also allows these finer particles to lodge themselves in the reliefs of the formwork surface. Without assessment of the occurrence, anhydrous cement grains, as well as partially hydrated grains and hydrated cement phases, can fit into the texture of the formwork surface. The F17 I formwork surfaces and the C20C27 formwork surfaces are characterized by surface texture parameters compatible with the cement particles dimensions, but to a lesser extent. This is consistent with the decrease of the demolding force observed when the formwork surface used is from F17 R type to F17 I and C20C27 type, and even more significant for the F17 M type.

The release agent has proven its efficiency in reducing formwork/cement adhesion. The results obtained in the case of the F17 O formwork surfaces are thus coherent. Various studies [START_REF] Bouharoun | Effect of a Superplasticizer on the Properties of the Concrete/Oil/Formwork Interface[END_REF][START_REF] De Brito | Evaluation of the Technical Performance of Concrete Vegetable Oil Based Release Agents[END_REF][START_REF] Libessart | Influence of the Type of Release Oil on Steel Formwork Corrosion and Facing Aesthetics[END_REF][START_REF] Djelal | Comprehension of Demoulding Mechanisms at the Formwork/Oil/Concrete Interface[END_REF] detailed the action of the release agent in the segregation of the concrete and the formation of its finishing. The presence of a release agent at the interface contributes to create a film of water and soap, depending on the agent type. By forming a chemical and physical barrier, the release agent prevents the fine elements of concrete from trapping into the surface formwork asperities and therefore lowering the mechanical anchoring.

Wettability and surface energy results emphasize the efficiency of the F17 M formwork surfaces. The combination of their thermodynamic and geometric characteristics makes the F17 M surfaces the most effective treatment for reducing demolding efforts. The smoothest topography, the highest wettability and surface energy of the F17 M surfaces promote the formation of a film of water at the interface formwork/cement. This film decreases the interaction between cement phases and the formwork hollows. Hydration of cement phases can be thence impacted. The C20C27 formwork surfaces tend also to diminish the demolding effort and to be closer to the F17 M surfaces than the F17 I are. These features, near those of the F17 M, permit the C20C27 to offer similar physical and chemical conditions at the interface with cement, acting on hydration of the cement phases.

Conclusion

The work presented aims to develop an innovative formwork process. To this end, the influence of the surface quality of the formwork on cement/formwork adhesion is analyzed. The formwork surface quality is modified by different treatment: polishing and deposition of a polymer coating. An original experimental device for assessing the unmolding forces at 24 hours is designed. The different surface formworks are characterized by interferometry for the description of their geometry and by measurement of contact angle for the evaluation of their thermodynamic conditions. The surface parameters were correlated with the polishing procedure.

The polishing, mirror or intermediate, reduces the surface texture parameters compared to the reference surface. The coating can also permit a decrease in the surface texture parameters. The analysis of the wettability and the surface energy indicate that the different formwork surfaces present similar thermodynamic conditions. The F17 M surfaces tend to distinguish from the F17 I ones due to their contact angles and surface energy values.

The polishing mirror seems to gather the surface characteristics allowing the highest performances in demolding. For laboratory tests performed with small dimensions samples (surface area: 541 mm 2 ), the modification of the formwork surface morphology provides a relevant solution. At the scale of the formwork panel (2.40 * 2.80 m 2 ), this solution seems to not be appropriate due to placing requirements and surface quality evolution by utility scratches. Moreover, using an extremely smooth formwork surface, the capillary forces could become the dominating parameters. In conclusion, the C20C27 coating constitutes a promising solution for reducing the demolding efforts.

The impact of this coated solution has to be now analyzed in view of its impact on the cement surface quality. Indeed, the contact with formwork surface influences the formation of the structure skin, as well as the development of its physical-chemical and mechanical properties. Moreover, the need of concrete surface characterization, from both an aesthetic and functional point of view is growing, as the study by Sadowski and Mathia [START_REF] Sadowski | Multi-Scale Metrology of Concrete Surface Morphology: Fundamentals and Specificity[END_REF] reminds it. The emergence of new techniques of surface characterization coupled with the multiplication of parameters can complicate the study of cement or concrete surface morphology. Moreover, cement, as concrete, comprises elements of various dimensions and physical-chemical states which impact its surface analysis. Therefore, the characterization of the cement surface after demolding with the C2027 solution will have to embrace the morphological, chemical and mechanical aspects.

Figure 1 :

 1 Figure 1: Experimental device for assessing demolding forces: a) photograph of the device; b) front view [19]; c) top view [19]; d) experimental working curve

Figure 2 :

 2 Figure 2: Structure of the C20C27 formwork coating, double-layer polymer solution: a) schematic representation; b) Observation of the coating by optical microscopy

  Figure 3 a). The F17 M mirror formwork surfaces have no scratch whereas some can be seen on the F17 I intermediate formwork surfaces (Figure 3 a and b).

Figure 3 :

 3 Figure 3: Observations of formwork surface [19]: F17 R reference surface, I: F17 I intermediate polished surface, M: F17 M mirror polished surface, a) macroscopic scale and b) interferometric mapping of surface texture At the reference scale, values of the surface texture parameters indicate a modification of the surface morphology due to the mirror polishing and the intermediate one (Table4).

  The pre-crack tests using the mirror polished surfaces lead to low values, in the order of 10 N. These values were too weak to be taken into account with regard to the measuring accuracy. Only one value could be recorded and it is equal to 25 N. It appears that the force tends to decrease with the reduction of the surface texture parameters. The decrease of the force is sharper between the mirror and intermediate polished surfaces than it is between the intermediate polished surfaces and the reference surfaces (Figure4 a).

Figure 4 :

 4 Figure 4: Evolution of the demolding force and of the surface texture parameters according to the formwork surface type: a) Case of the polished surface; b) Case of the coated surfacesTaking into account the standard deviation, the decrease in all the surface texture parameters is almost identical between the reference surface and the polished surfaces, both mirror and intermediate. The Sa parameter can decrease from 99.3 % to 99.8 % between the F17 R surface and the F17 M polished surface. The same parameter can be reduced from 96.6 % to 97.8 % between the F17 R surface and the F17 I polished surface (Table5). However, the minimal and maximal reduction percentages of the demolding force can vary from 54.2 % to 97.5 % for the F17 M surfaces and from 0 % to 54.8 % for the F17 I surfaces. The mean force can be reduced by 82.8 % and 25.1 %, for the F17 M and F17 I surfaces respectively (Table8).

Figure 5 :

 5 Figure 5: Evolution of the demolding force and of the surface energy according to the type of formwork surface

F17

Figure 6 :

 6 Figure 6: Box plot of the demolding forces assessed by use of the pre-crack test with different types of formwork surfaceThe F17 R reference surfaces present the most dispersed distribution while the F17 I surfaces have the smaller range (Figure6). The range of the distribution of the F17 R surfaces is 231 N and 73 N for the F17 I surfaces. The ranges of the F17 O and the C20C27 are 104 N and 113 N, respectively. Moreover, the distributions of the demolding force values are skewed for the F17 O and F17 I formwork surfaces compared to the F17 R and the C20C27 formwork surfaces that have more symmetric distributions. For the F17 O distribution, the force values tend to be concentrated on the lower values whereas for the F17 I distribution, the force values are concentrated on the upper values. Indeed, the Figure6shows that 75 % of the force values of the F17 O surfaces are between 11 N and 60 N (Q3) and 75 % of the force values of the F17 I surfaces vary from 97 N (Q1) and 133 N. The fact that F17 I formwork surface is manufactured could explain the concentration of the force values. The dispersion of the force values can also be observed with the interquartile range.

Table 1 :

 1 Characteristics of the release agent used for the F17 O formwork surfaces

	Aspect	liquid
	Density at 20°C	0,85
	Flash point	65° C
	Colour	Yellow straw
	Viscosity at 20°C	56 seconds

1.2.2 Polishing procedures of the reference formwork

	Two types of polished formwork surfaces are prepared from reference plates (F17 R) by automatic polishing.
	One type gathers surfaces qualified as mirror-like or mirror, named F17 M. The other type encompasses
	polished surfaces called intermediate F17 I.

Table 3 :

 3 Total of demolding tests with pre-crack according the type of formwork.A large number of tests is done to deeper analyze the behavior of the interface between a real formwork surface and cement paste. The repetition of tests allows to observe behavior trends. Especially as experimental work on cement can include scattered data, non representative occurrences. The performance of the reference surface, F17 R, is compared with other types of surface. These ones are fabricated in laboratory, in few quantity and non-commercialized. The test results are analyzed through the box-plot statistic tool. Thus the performances of each type of surface is analyzed while including the number test variability.

Table 4 )

 4 .

	Formwork surface	Sa (nm)	Sq (nm)	Sdr (%)	Vvc (nm 3 .nm-²)	Vvv (nm 3 .nm-²)
	F17 M mirror	16 ± 8	32 ± 15	0.01 ± 0.01	18 ± 9	5 ± 3
	F17 I intermediate	111 ± 11	153 ± 33	0.63 ± 0.09	152 ± 14	22 ± 5
	F17 R reference	4064 ± 488	4920 ± 593	11.08 ± 1.70	4994 ± 893	647 ± 105
	C20C27 coated	264 ± 60	328 ± 72	0.01 ± 0.01	388 ± 90	37 ± 12

Table 4 :

 4 Values of the surface texture parameters for the different types of surface texture.

Table 6 :

 6 Contact angles and surface energies measured on the coated and polished formwork surfaces.

	Formwork surface		Contact angle (°)		Surface energy (mN/m)
		Distilled water	Glycerol	Ethylene Glycol	Owens -Wendt model
	F17 M	67.3 ± 9.4	73.2 ± 5.3	57.7 ± 3.4	33.4 ± 3.8
	F17 I	84.5 ± 6.9	89.6 ± 5.0	69.3 ± 2.3	24.1 ± 3.6
	F17 R	83.3 ± 14	84.6 ± 5.8	55.8 ± 10.2	27.7 ± 5.1
	C20C27	80.6 ± 4.0	81.0 ± 4.7	56.6 ± 3.9	25.7 ± 0.6

Table 9 :

 9 Characteristic values of the distribution of the demolding force (N) for the different types of surface.

		R	F17 O	F17 M	F17 I	C20C27
	1st Quart. (1e Q)	109	27		97	83
	Minimum	29	11		60	38
	Median	146	46		113	95
	Mean	145	46	25	108	98
	Maximum	259	115		133	151
	3rd Quart. (3e Q)	173	60		125	112

Table 10 :

 10 Force and stress obtained by use of the pre-crack test for different types of formwork surfaces.

Acknowledgements

This work is carried out within the framework of the FUI 18 ERGOFORM project which brings together four partners: the companies Hussor and Lormac, as well as two research laboratories, the Laboratory of Photochemistry and Macromolecular Engineering (LPIM) and the research institute of Engineering, Computer Science and Imaging (ICube). The authors would like to thank the project partners, the Fibres Energivie and Matéralia competitiveness clusters, which approved the project, as well as the Alsace Champagne-Ardenne Lorraine region, the EuroMétropole de Strasbourg and the Banque Publique d'Investissement France.

Declaration of interest

None.