Bilal Baz

Georges Aouad

Sébastien Remond

Effect of the Printing Method and Mortar's Workability on Pull-Out Strength of 3D Printed Elements

Keywords:

3D Printing of Concrete is gaining more attention with time as an alternative method for construction for its high degree of freedom. Until now, most of 3D printed elements are preprinted then moved to their designated locations. The most practical method for moving printed elements is lifting them by means of implemented anchors. However, due to the nature of this construction method, it does not allow for any type of vibration, also due to the use of a special type of concrete mix, that do not flow by itself, there are still a lot of queries concerning the adherence of concrete with steel bars. The objective of this paper is to characterize the bond between steel and printed mortars as a function of mortar's workability and printing method.

Pull-out tests of an 8mm steel bar embedded in either printed or non-printed mortars of varying workability have been performed after 3 days of casting. It is found that the workability of the ink does not affect the pull-out strength, neither the printing method nor layers direction affect the pull-out strength in respect to the steel bar.

1-Introduction

3D printing (3DP) is an additive manufacturing process (AM) defined by the ASTM for being ''the process of joining materials to make objects from 3D model data, usually layer upon layer" [START_REF]F2792-12a -Standard Terminology for Additive Manufacturing Technologies[END_REF].

3D printing has become one of the fastest growing technologies, and it took place in the everyday life. It was introduced to all kinds of manufacturing industries [START_REF] Berman | 3-D printing : The new industrial revolution[END_REF], medical applications [START_REF] Lam | Scaffold development using 3D printing with a starchbased polymer[END_REF], and food preparation [START_REF] Wegrzyn | Food layered manufacture: a new process for constructing solid foods[END_REF]. It is a sophisticated computer modeling technology, where physical objects are created using an automated process based on CAD models [START_REF] Berman | 3-D printing : The new industrial revolution[END_REF].

Since its emergence, 3D printing of concrete materials imposed a lot of challenges and opportunities to the construction sector [START_REF] Buchli | Digital in situ fabrication -Challenges and opportunities for robotic in situ fabrication in architecture , construction , and beyond[END_REF] [START_REF] Labonnote | Additive Construction : state of the art , challenges and opportunities[END_REF]. Different additive manufacturing methods have been developed, and they rely on different systems [START_REF] Lim | Developments in construction-scale additive manufacturing processes[END_REF]. The most commonly used methods are the Extrusion-Based systems, Powder-Based, or D-Shape techniques [START_REF] Khoshnevis | Automated construction by contour crafting -Related robotics and information technologies[END_REF] [9] [START_REF] Le | Hardened properties of high-performance printing concrete[END_REF]. First, the Extrusion-Based systems consist of extruding a cementitious material from a nozzle mounted on a robot or a gantry frame, and it prints the structure by depositing concrete filaments, having a constant cross section, on top of each other [START_REF] Nematollahi | Current Progress of 3D Concrete Printing Technologies[END_REF] [START_REF] Le | Hardened properties of high-performance printing concrete[END_REF]. Whereas, the Powder-Based technique is capable of making complex shapes or structures by jetting selectively a liquid binder through a nozzle on an existing layer of printable powder to bind the particles together. Then, the remaining non-bonded particles are removed by means of a de-powdering process [START_REF] Xia | Printability , accuracy and strength of geopolymer made using powder-based 3D printing for construction applications[END_REF] [START_REF] Lowke | Particle-bed 3D printing in concrete construction -Possibilities and challenges[END_REF]. The Powder-

Based technique is mostly used as an off-site process and that suits small scale building components [START_REF] Nematollahi | Current Progress of 3D Concrete Printing Technologies[END_REF] [START_REF] Le | Hardened properties of high-performance printing concrete[END_REF]. For this research, the Extrusion-Based technique is adopted. With all the techniques mentioned earlier, the incorporation of steel bars is hard to be done properly in a 3D printed element. This concern represents an evident obstacle for the maturity of 3D printing in the construction field [START_REF] Salet | 3D concrete printing -a structural engineering perspective 3D Concrete Printing -A Structural Engineering Perspective[END_REF] [START_REF] Asprone | Rethinking reinforcement for digital fabrication with concrete[END_REF]. Many alternative attempts have been performed to provide sufficient ductility for 3D printed elements. For example, fibers can be used in the 3D printing of concrete elements as a reinforcing agent [START_REF] Nematollahi | Effect of polypropylene fibre addition on properties of geopolymers made by 3D printing for digital construction[END_REF] [16] [START_REF] Hambach | Properties of 3D-printed fiber-reinforced Portland cement paste[END_REF] [START_REF] Panda | Anisotropic mechanical performance of 3D printed fiber reinforced sustainable construction material[END_REF] [START_REF] Farina | On the reinforcement of cement mortars through 3D printed polymeric and metallic fibers[END_REF]. However, fibers cannot be always used as a replacement for the structural steel because it is limited in terms of strength and ductility [START_REF] Asprone | Rethinking reinforcement for digital fabrication with concrete[END_REF]. In most cases, to obtain the optimal structural performance, steel reinforcement has to be incorporated in order to improve the physical and mechanical properties of the 3D printed components [START_REF] Mata-Falcon | Exploiting the Potential of Digital Fabrication for Sustainable and Economic Concrete Structures[END_REF]. Therefore, structural steel will improve the overall strength of the component and its integrity [START_REF] Panda | Current Challenges and Future Perspectives of 3D Concrete Printing[END_REF]. Several attempts have been taken by different companies and research institutes. For example, HuaShang Tengdam started by reinforcing 3D Printed structures by printing over an actual structural steel cage, where the printer has a fork shaped nozzle that eject concrete simultaneously on both sides of the steel cage [START_REF] Nematollahi | Current Progress of 3D Concrete Printing Technologies[END_REF] [START_REF] Bos | Additive manufacturing of concrete in construction: potentials and challenges of 3D concrete printing[END_REF]. Apis Cor and Win Sun introduced the reinforcement to their printed elements by producing a permanent 3D printed formwork and then placing inside of it the Rebar. Then after, the printed formwork is filled by conventional casted concrete [START_REF] Schutter | Vision of 3D printing with concrete -Technical, economic and environmental potentials[END_REF]. Other novel methods of reinforcement are also applied such as the Mesh-Molding method, where the whole system is robotically printed and assembled, including the steel mesh [START_REF] Hack | Mesh Mould: Robotically Fabricated Metal Meshes as Concrete Formwork and Reinforcement[END_REF], and the Sparse Concrete Reinforcement In Meshworks (SCRIM) that joins concrete printing and textile reinforcement meshes to produce a 3D printed reinforced element [START_REF] Ayres | SCRIM -Sparse Concrete Reinforcement in Meshworks[END_REF].

Besides, whatever the method used is, an important issue concerns the link developed between concrete and steel, because the ink used for printing is a particular material. This concrete material is different than any other one since it is allowed neither to flow by itself, nor to be vibrated. Thus, the performance of the link between steel and printed concrete has to be verified, especially when working with materials that have different workabilities. A stiff printable material could end-up with a bad wrapping of the steel bar, while a more workable material would help in a better covering of all the surface of the bar leading to a better link by eliminating the chance of unintended voids. For the moment, regardless of the lack of ductility in 3D printed elements and its consequences on the structural behavior, the issue of putting reinforcement bars into printed concrete is certainly not mature enough to be fully addressed [START_REF] Bos | 3D Printing Concrete with Reinforcement[END_REF] [START_REF] Bos | Experimental exploration of metal cable as reinforcement in 3D printed concrete[END_REF]. However, lifting printed elements and transporting them need an immediate solution, since until now most of the 3D printed elements are pre-fabricated then transported to their final destination, and eventually linked to an existing structural element. This paper deals with this specific question, by studying the effect of incorporating a steel bar inside the element during the printing process on the pull-out capacity at early age. This paper particularly investigates the bond strength generated between printed mortar and steel based on the workability of the used mortar. So the main objectives are:

-First, develop a conceptual method for measuring the bond between steel and printed mortar.

-Second, understand how the printing method and the layers direction with respect to the steel bar affect the pull-out strength at early age.

-Third, understand the effect of workability of the mortar on the bond developed with steel bars. Section 2 first introduces the mixes used, in addition to the appropriate protocols for the evaluation of the fresh and hardened properties of the materials. Then after, a detailed description of the pull-out test used for evaluating the bond generated between concrete and steel is presented. Section 3 presents all the experimental results for both material properties and pull-out test.

2-Materials and Methods

2.1-Material Properties

There are two prime specifications for a mortar to be considered printable. Precisely, these characteristics are extrudabuility and buildability. Extrudability is the ability of a material to be workable and flowable enough to be pumped and printed without blocking the nozzle or the conduits. Buildability requires a fast setting and stiff material that can preserve its shape after being printed, and withstand the load coming from superposed layers. These requirements lead to a completely opposite performance when compared to the commonly used material that has the tendency to flow by itself when pumped, such as self-leveling concrete. The contradiction appears when a Zero slump material has to be flowable enough to be used for concrete printing.

That is why the material's properties influence the link between steel and concrete. In this study, four different printable mixes have been manufactured.

The developed mixes consist of ordinary Portland cement CEM I (PA L 42.5) from Holcim-Lebanon, Silica Fume from HOLDERCHEM-Lebanon and a high range water reducer poly carboxylate (PCE) based Super-Plasticizer. The sand used consists of crushed limestone with a particle size ranging between 0 and 1.5mm (Table . 1). In order to vary the mortar's workability, the super plasticizer contents were changed, except for mix4 where the water to binder ratio (W/B) was also increased. The mixing process and testing method of the mixes are done according to Khalil et al [START_REF] Khalil | Use of calcium sulfoaluminate cements for setting control of 3D-printing mortars[END_REF]. In order to test the workability, a flow test is done for all mixes as per the Standard Test Method for Flow of Hydraulic Cement Mortar (ASTM C1437-15). In this standard, the spreading of a material is measured after 25 chocks on the flow table. In addition, the extrudability and buildability properties are tested by printing manually the largest number of superposed layers having a straight wall shape for each mix using a lab gun device having a circular nozzle of 1cm diameter previously used by El-Cheikh et al. [START_REF] Cheikh | Numerical and experimental studies of aggregate blocking in mortar extrusion[END_REF] and Khalil et al. [START_REF] Khalil | Use of calcium sulfoaluminate cements for setting control of 3D-printing mortars[END_REF]. The compressive strength of all mixes has been measured at 3 and 28 days. This is done to determine whether the workability of the ink used has a major influence on the strength of the material when printed or not. Tests are conducted over a set of 4x4x16cm beams. Two sample categories are tested for each mix design. The first category consists of specimens simply poured in the appropriate molds in 2 layers. According to the standard placing method (ASTM C348), each layer is struck 60 times using a jolting table. The next set of samples consists of elements printed manually inside the same molds. In this case specifically, the molds are used only to produce perfectly plane surfaces so the compressive strength test can be performed accurately.

Each printed beam is made out of 4 layers, 1cm thick each (Fig. 1), using a lab gun device having a rectangular nozzle of 1cmx3cm cross section similar to the device used by Sanjayan et al. [START_REF] Sanjayan | Effect of Surface Moisture on Inter-Layer Strength of 3D Printed Concrete[END_REF] to study the effect of surface moisture on inter-layer strength of 3D printed concrete and Marchment et. al [START_REF] Marchment | Effect of Delay Time on the Mechanical Properties of Extrusion-based 3D Printed Concrete[END_REF] when studying the effect of delay time on the mechanical properties of extrusion-based 3D printed concrete. The layers are printed successively without any time gap, in order to avoid the formation of cold joints that affect the mechanical properties of the printed samples. The samples are kept for 24h in the molds, then they are de-molded and put to cure in a fully humid environment (RH=100%, 23°C) until the testing day. After then, all samples are tested under compression at a load rate of 2.5kN/sec according to ASTM C349. However, the printed beams are tested in two directions. The applied load is either parallel or perpendicular to the printed layers (Fig. 1).

2.2-Pull-Out test

The pull-out test is generally applied to study the bond between steel and concrete. This research aims to specifically study the bond between a steel bar and the newly developed printable mixes.

The reinforcing steel bars used in the pull-out experiments are T8 bars with 8.15mm nominal diameter, a yield strength "Fy" equal to 501Mpa and an ultimate tensile strength capacity "Fu" of 583Mpa.

It is well known that the bond strength developed between steel and concrete is directly related to the compressive strength of the material [START_REF] Bos | 3D Printing Concrete with Reinforcement[END_REF]. However, the failure mode of a sample differs between confined and unconfined boundary conditions. In the case of an unconfined sample, a concrete splitting failure occurs resulting from the longitudinal propagated cracks coming from the bar's wedging action over the surrounding concrete. In confined condition, the failure mechanism is generally governed by a de-bonding, resulting from the shearing action of the bar's ribs over the concrete, and therefore a slipping failure of the steel bar happens [27] [32]. For the purpose of this study, a bar slipping failure concerns us the most. In addition, there are many other factors that affect the failure mechanism such as the volume of concrete around the bar, the surface condition of the bar and its geometry [START_REF]ACI 408R-03 Bond and Development of Straight Reinforcing Bars in Tension[END_REF].

In general, there are three major mechanisms that are resisting the pull-out of a conventional ribbed bar. These are mainly adhesion, dilatancy and friction. The combination between adhesion and dilatancy creates the bond resistance occurring before failure, and friction produces the resistance after failure [START_REF] Haskett | Local and global bond characteristics of steel reinforcing bars[END_REF].

2.2.1-Specimens preparation

The pull-out samples consisted of 16x16x20cm concrete cubes with an effective embedment depth of 8cm for the steel bar. The adopted sample's dimensions satisfy our aim for having a slipping failure of the bar, not a splitting failure of concrete cube nor a steel rupture. Two different types of samples are presented. The first set of samples consists of conventionally casted elements, and the second set consists of printed elements over a steel bar with different printing methods, both to be presented after. In order to cast the samples, a prismatic mold is made of wood with an open top. Because of the downward protruding reinforcing bar, the bottom cap is drilled at the middle to let the bar pass through (Fig. 2). In addition, an adhesive tape is applied on the free part of the anchor, for a depth of 1cm in order to break the adhesion, to set the exact embedment length and to decrease the stress concentration at the top of the anchor. Moreover, a stability support is added to the mold at its bottom to ensure that the bar remains rigid and vertical at all times, letting the applied forces be purely tensile. 24h after casting, the specimens are de-molded and placed in a 100% humid environment at 23°C until the day of testing. To proceed with the pull-out test of the printed elements, the same volume was adopted (16x16x20cm). For the printed specimens, the main variables that have to be defined are the layer's printing method and orientation with respect to the steel bar. This means that whether the layers are parallel or perpendicular to the steel bar, and if it is feasible to insert the reinforcing steel bar after printing and before the material's setting.

In this paper it was decided to print directly over the steel bar, which can serve the objectives more, knowing that there are different methods for incorporating steel bars in 3D printed elements, for example, printing steel bars simultaneously with concrete [START_REF] Mechtcherine | 3D-Printed Steel Reinforcement for Digital Concrete Construction -Manu-Facture, Mechanical Properties and Bond Behaviour [Under Review[END_REF], printing over an actual steel cage, applying an external steel system after printing, and many other methods [START_REF] Nerella | Incorporating reinforcement into digital concrete construction[END_REF].

In this paper, printing is done using a lab gun device having a circular nozzle of 1cm diameter similar to the device used by El-Cheikh et al. [START_REF] Cheikh | Numerical and experimental studies of aggregate blocking in mortar extrusion[END_REF] and Khalil et al. [START_REF] Khalil | Use of calcium sulfoaluminate cements for setting control of 3D-printing mortars[END_REF], simulating the printer's work. Each layer has a 1cm thickness. To print parallel layers to the steel bar, two different methods were adopted, and to print perpendicular layers, two other different techniques were also used. Fig. 3 shows a schematic description of the printing methods. The first perpendicular method (PerpM1) consists of printing layers in a circular manner all around the bar using a single gun, while the other method (PerpM2) is done by printing 2 adjacent layers at once surrounding the bar using 2 guns simultaneously. As for the parallel layers, also two different methods were used. The first method (ParaM1) consists of using two guns simultaneously in line along the steel bar. The bar is placed between the two devices so the printed layers can cover the whole perimeter. The second method (ParaM2) is made by first, printing the bed layer, then, simply placing the steel bar on top of it. The bar is slightly pushed down in a way that half of its diameter is merged in the layer below, after that, a second layer is printed over it.

Regarding the reversed scenario where the printing takes place first, the layers are printed similarly to PerpM1, and then the steel bar is inserted directly after printing without any time gap. The insertion of the bar is done manually and very delicately with the help of a stabilization support to ensure a vertical penetration without perturbing the surrounding concrete.

The standoff distance of the nozzle is approximately 0.5cm apart of the bar, and the printed part exceeds the bar by 6cm to ensure a full and strong coverage. Thus, the overall printed part is 15cm long including the adhesive tape. For all samples and conditions, each printed element is placed inside a 16x16x20cm mold after 24h and a different mortar mix is used to fill up the remaining volume and ensure a strong encapsulation of the printed segment. It is not mandatory to use the printable mix to fill the mold since this research is only concerned by the link between the steel bar and the printed material, knowing that any failure at the joint between the two types of material would never happen. Fig. 4 shows an example of how the printed sample looks like before and after being wrapped by the filling mortar.

As for the non-printed samples, they are produced by pouring and vibrating printable concrete around the steel bar, inside a small mold. After de-molding, the smooth surfaces of the elements are scraped to ensure a better link with the filling material providing confinement (Fig. 5).

2.2.2-Test Set Up Preparation

The machine used to perform the pull-out test is a UTM machine. A customized steel setup is developed to serve the pull-out test (Fig. 6). This setup aims to hold the pull-out specimens inside.

First, the system is made out of two large steel plates with 3cm thickness and 32x32cm surface area. Both plates are connected by means of bolts so the space between them can be fixed according to the specimen's size. The system forms a box with open sides. The top plate is grooved straight to the middle so it allows the steel bar coming out of the specimen to slide in.

The bottom plate is drilled at its center and connected to a steel bar that is held by the testing machine. After that, the sample is placed inside the setup and the steel bar that has to be pulled out is held by the UTM clamps. A rubber pad is placed between the specimen and the top plate of the setup in order to transfer a uniform stress distribution over the surface of the specimen in case of any making defects. The loading rate is set to be 1KN/sec, falling within the allowable range defined by the ASTM C234 standard.

3-Results

3.1-Material Results

All mixes satisfy the Printability characteristics, even for different workabilities. All freshly printed elements failed by buckling, but the obtained results showed a good correlation between the maximum number of layers and the material's workability (Table . 2). Fig. 7 and Fig. 8 show the compressive strength at 3 and 28 days respectively for all samples whether printed or not. The results for the non-printed samples showed approximately the same resistance for mix1, mix2 and mix3 at 28 days since they have almost the same compositions, while mix4 gave a lower resistance due to its higher W/B ratio. But when talking about 3D

printing, an additional factor comes into play and influences the compressive strength of the elements, which is the direction of the printed layers whether parallel or perpendicular to the load imposed [START_REF] Paul | Fresh and hardened properties of 3D printable cementitious materials for building and construction[END_REF]. Indeed, the results of this research confirm that the 3D printed specimens are anisotropic elements.

The results showed that at 28 days the non-printed samples dominated, followed by the parallel samples that gave better results than the perpendicular ones. This happened because nonprinted samples contain less voids than the printed ones due to the external vibration. However, printed samples contain voids mostly located between layers, this justifies the fact of having lower compressive strength than non-printed samples. But then, the voids found between layers are more vulnerable when the printed samples are loaded perpendicularly to the layers. This is said because these voids show up in a series form of weak points causing failure as soon as the chain breaks down. This results in having a lower resistance when compared to the samples loaded parallel to the layers. As stated earlier, the compressive strength of non-printed samples were almost the same for the first three mixes while being lower for mix4. In general, the variation between printed and non-printed elements diminishes as the workability of the mix increases, since the printed layers have higher tendency to merge together, decreasing the effect were identical even for mix4. These results were in agreement with the results obtained by Feng et al. [START_REF] Feng | Mechanical properties of structures 3D printed with cementitious powders[END_REF] and Nerella et al. [START_REF] Nerella | Studying printability of fresh concrete for formwork free Concrete on-site 3D Printing technology[END_REF]. Even though, based on the research done by Koker [START_REF] Koker | Manufacturing processes for engineered cement-based composite material products[END_REF], it was found that the mechanical strength of extruded materials could be greater than the simply casted elements, and this is certainly due to the extra pressure exerted on the material in its fresh state reducing the voids inside the extruded layer itself which will end-up having a denser matrix.

However, this is not always the case for 3D printed elements since many other factors come into play, mainly the nozzle shape, nozzle standoff distance and printing speed [37][41].

Regarding the compressive strength of the elements tested at 3 days, all results were almost the same for all mixes and loading directions. The variation between results cannot be clearly seen, thus a significant conclusion could not be drawn. Compressive Strength at 3 Days

3.2-Pull-Out Results

Table 3 presents the pull-out test results for all mixes and conditions. All numbers show the bond stress corresponding to the actual force applied, calculated using the following equation: τ = . . (Eq. 1)

Where: τ = experimental bond stress (MPa) F = ultimate axial tension force (N) db = nominal rebar diameter (mm) le = embedment length (mm)

In addition, Table 3 includes the average bond stress and standard deviation for each condition individually. Although, Fig. 9 shows how the relative pull-out strength (compared to non-printed sample) varies according to the printing method and the variation of workability for each mix.

Based on these results, it can be clearly seen that the pull-out strengths of most printed samples are close enough to the results of the non-printed ones relative to each mix. This is true except for the case where the bar is inserted after the printing takes place. It can be said that all mixes with different workabilities gave more or less the same pull-out results for all printing methods and conditions. This can be clearly indicated by the standard deviation bars shown on the curves of Fig. 9.

All standard deviations for pull-out results corresponding to either printed or vibrated samples are almost equivalent, altering in the same extent. Namely, all results fall in the same array, thus they also confirm that the manual printing technique adopted for this research is a reliable and consistent method.

Concerning the case of a steel bar inserted after printing, this method is neither practical nor efficient. Even though, as the workability of the material increases, the bond with the steel bar increases, because a more flowable material would fill more voids caused by the insertion of the bar. However, the relative pull-out strength of a steel bar that is inserted after printing is still behind when compared to any other method adopted in this study. In other words, whatever the mix properties were, the irregularities (voids, entrapped air, etc.) created around the inserted bar are more severe than any other case (Fig. 10).

Besides, according to the compressive strength results of printed samples, perpendicular samples showed the lowest bearing capacity for all mixes. Even though, this weakness did not affect the resistance of the perpendicular printed layer against the stresses generated by the pull-out forces when the layers are perpendicular to the steel bar.

In this study, all printing methods reflect different approaches for 3D printing. Yet, PerpM1 and ParaM2 are considered as the most suitable printing methods because they are the most practical, therefore reducing any chance for manufacturing defects such as voids formation.

However, from a global point of view, the manual printing method adopted in this paper is relatively different than what can be done using an automated 3D printer. But stills, even when using a robotized 3D printer, a large scatter between samples coming from different sources will always shows up, because all the developed 3D printing techniques until now have not been standardized and do not follow a predefined procedure for reproducibility. They majorly depends on the piloting of the operator and the material properties. In other words, 3D printing technology in all its aspects and applications is still a subjective technique until being standardized.

4-Conclusion

A pull-out test has been conducted on conventionally mold-cast and printed samples under different conditions. The factors that were studied in this research are the effect of workability on the link developed between steel and concrete, and the effect of the printing method and layers direction on the pull-out strength of a steel anchor. In addition, the compressive strengths of the different mortars used were tested for printed and conventionally mold-cast conditions under different loading directions (parallel and perpendicular to the printed layers). Basically, this study figured out four different mixes with different workabilities and five different printing methods. So, the following conclusions are drawn:

• Non printed samples dominated when tested at 28 days in regards to the printed ones.

However, perpendicular samples always showed the lowest compressive strengths.

• Printed samples act more homogeneously when increasing the workability of the mortar used. In other words, the compressive strength of parallel printed samples converge toward those of non-printed elements as the workability increases.

• The concept of printing over an actual steel bar has been shown to be feasible and effective.

• 3D printed elements gave more or less the same pull-out strength as for non-printed elements, except for the case where the steel bar is inserted after printing.

• The printing direction did not majorly affect the capability of the element to withstand tensile loads applied over the steel bar.

• PerpM1 and ParaM2 were the most qualified printing methods because of their practicalities. Undeniably, all other methods were also acceptable.

• Based on the results of this research, as long as the material is considered printable and independently of the mix's workability, a strong link between concrete and steel will still be developed, and the pull-out strength would still be significant.

At the end, all the results and conclusions are based on tests done over manually printed samples. In a next step, the same study will be conducted over samples fabricated using an automated printer to validate more these results and conclusions. Simultaneously, different variables are going to be introduced and different parameters will be tested apart of the link between steel and concrete. Certainly, one of the targeted subjects to be investigated is the effect of the layer's geometry on the failing mode of pull-out specimens.

Figure 1 (

 1 Figure 1 (top) printed beam (bottom left) Perpendicular loading (bottom rigth) Parallel loading

Figure 2

 2 Figure 2 Casting molds for Pull-Out samples

Figure 3

 3 Figure 3 Different printing methods

Figure 4 8 Figure 5

 485 Figure 4 Printed Pull-Out sample before (left) and after (right) being confined

Figure 6

 6 Figure 6 Pull-Out test set up

Figure 7

 7 Figure 7 Compressive Strength at 3 Days

Figure 9 Figure 10

 910 Figure 9 Pull-Out strength variation for all mixes

Table 1

 1 Mixes compositions

		OPC (g)	SF (g)	Sand (g)	W/B	SP (%)
	Mix1	614.47	68.28	850	0.4	0.26
	Mix2	614.47	68.28	850	0.4	0.36
	Mix3	614.47	68.28	850	0.4	0.4
	Mix4	614.47	68.28	850	0.45	0.4

Table 2

 2

	Workability Characteristics
		Spreading (cm)	Maximum number of Superposed layers
	Mix1	14.5	22
	Mix2	16	16
	Mix3	18	12
	Mix4	20	9

Table 3

 3 Pull-Out results

			Mix1		Mix2		Mix3		Mix4
		T(MPa) Av. (S.D.) T(MPa) Av. (S.D.) T(MPa) Av. (S.D.) T(MPa) Av. (S.D.)
		15.1		15.5		14.9		14
	Non-Printed	15.2 14.9 15.6	15.23 (0.27)	15.8 14.6 15.5	15.33 (0.52)	15.8 14.8 14.8	14.91 (0.54)	12.8 13.6 14	13.6 (0.57)
		15.4		X		14.3		X
		14.1		15		14.3		11.7
		15.5		15.4		15.2		12.3
	PerpM1	13.5	14.37 (0.73)	15.1	14.98 (0.3)	14.5	14.87 (0.68)	12.1	12.19 (0.45)
		14.3		14.8		15.9		12.9
		14.5		14.6		14.4		12
		11.8		14.6		14.8		15
		12.9		14.3		14.2		14.6
	PerpM2	10.2	11.98 (1.09)	13.8	14.29 (0.41)	15.1	14.32 (0.73)	13.9	14.34 (0.47)
		12.2		14		14.3		13.9
		12.8		14.8		13.2		14.3
		13.8		14.2		12.4		11.2
		14.8		14.8		12.3		11.3
	ParaM1	12.5	13.49 (1.26)	14.6	14.57 (0.58)	12.5	12.34 (0.15)	13.2	10.95 (1.51)
		14.5		15.4		12.4		9.5
		11.9		13.9		12.1		9.6
		14.6		14.2		15.2		15.1
		15.4		12.5		15.5		15.6
	ParaM2	15.4	15.0 (0.4)	13.3	13.69 (0.78)	15.6	15.35 (0.29)	14.8	15.13 (0.34)
		15		14.2		14.9		14.8
		14.6		14.3		15.5		15.3
		1.9		2.6		3.6		9.8
	Bar After	3.4 5.2 4.2	3.68 (1.21)	5.9 2.4 3.7	3.68 (1.39)	5.8 5 6.1	5.01 (1.01)	7.5 7.3 7.9	8.2 (1.0)
		3.6		3.7		4.5		8.5

Acknowledgments

Financial Supports by CEDRE "Conférence économique pour le développement, par les réformes et avec les entreprises" are greatly appreciated.