Ahmed Elshami

Stéphanie Bonnet

Abdelhafid Khelidj

Latefa Sail

Effectiveness of Corrosion Inhibitors in Simulated Concrete Pore Solution

Keywords: Concrete, Corrosion, Chloride, Carbon Steel, CNI, AMA and MFP

published or not. The documents may come

Effectiveness of corrosion inhibitors in simulated

concrete pore solution

Introduction

Minimising the direct and indirect costs of marine structure maintenance and maximising the available capacity is critical to the economic competitiveness of an expanding European Union. The use of corrosion inhibitorsa developing technology -as part of a repair strategy may provide a cost effective solution. A bibliography of over 180 references was assembled.

A detailed review was made of material published in the last fifteen years. The review embraced some aspects of CNI, AMA and MFP based inhibitors, used in new construction and repair methods based on cementitious mortars but the main focus was on surface applied corrosion inhibitors. Several studies have been conducted to assess the effects of chloride ions in simulated pore solution. A summary of available literature was given to the most commonly used inhibitors such as CNI, AMA and MFP in artificial concrete pore solutions.

In tests with calcium nitrite, sodium monoflurophosphate and ethanolamine based inhibitive treatments [START_REF] Monticelli | A study on corrosion inhibitors for con-crete application[END_REF] [START_REF] Monticelli | A study on corrosion inhibitors for con-crete application[END_REF] observed moderate reductions in the overall rates of corrosion after the inhibitive treatments. To ensure this effectiveness the ratio between inhibitors and chloride ions has to be relatively high (approximately 1) (Elsener et. al. 1999) [START_REF] Elsener | Migrating corrosion inhibitor blend for reinforced concrete: Part 1 -Prevention of corrosion[END_REF]. [START_REF] Mammoliti | Corrosion inhibitors in concrete. Part II: effect on chloride threshold values for corrosion of steel in synthetic pore solutions[END_REF] [START_REF] Mammoliti | Corrosion inhibitors in concrete. Part II: effect on chloride threshold values for corrosion of steel in synthetic pore solutions[END_REF] have shown that calcium nitrite addition into simulated pore solution at a level of 0.1% was able to repassivate pre-corroded steel in 1% chloride. When the chloride was added to the simulated pore solution already containing 0.1% calcium nitrite, the increase in chloride up to 1.5-2.0% did not change the potential or the corrosion rate significantly [START_REF] Ngala | Corrosion inhibitor systems for remedial treatment of reinforced concrete. Part 1: Calcium nitrite[END_REF] [START_REF] Ngala | Corrosion inhibitor systems for remedial treatment of reinforced concrete. Part 1: Calcium nitrite[END_REF]. However, CNI in solutions of very high pH value the concentration of Ca 2+ ions is strongly limited by the solubility product of Ca (OH) 2 . The presence of CNI in the pore solution leads to precipitate calcium hydroxide (and consequent reduction in pH) [START_REF] Li | The microstructure and sulfate resistance mechanism of high-performance concrete containing CNI[END_REF] [START_REF] Li | The microstructure and sulfate resistance mechanism of high-performance concrete containing CNI[END_REF]. [START_REF] Kawamura | Pore solution composition and electrochemical behavior of steel bars in mortars with nitrite[END_REF] [START_REF] Kawamura | Pore solution composition and electrochemical behavior of steel bars in mortars with nitrite[END_REF] measured the [Cl - ]/ [NO 2 -] ratio both in pore solution and in mortar specimens. The free chloride concentration in the pore solution decreased and the OH -content increased with the time of aging. The [Cl -]/[NO 2 -] ratio remained fairly constant and achieved a value of about 0.8 for 0.5% of admixed NaCl and 1% of sodium or calcium nitrite. The best inhibiting capacity was noted when the inhibitor was introduced in the solution before the contamination with chlorides, what gave a reduced effect especially when chlorides were present in the simulated pore solution [START_REF] Benzina Mechmeche | Investigation of the early effectiveness of an amino-alcohol based corrosion inhibitor using simulated pore solutions and mortar specimens[END_REF] [START_REF] Benzina Mechmeche | Investigation of the early effectiveness of an amino-alcohol based corrosion inhibitor using simulated pore solutions and mortar specimens[END_REF]. In salt solution the AMAbased inhibitor delayed the initiation of corrosion and reduced the Icorr (when the NaCl concentration is 0.3%, Icorr was 0.45 and 0.1µA/cm 2 for the control and the inhibitor samples, respectively) [Heiyantuduwa et al,2003] [START_REF] Heiyantuduwa | The effectiveness of corrosion inhibitors in concretes subjected to chloride attack and carbonation[END_REF]. AMA inhibitors are best used to extend (or help to achieve) the required service life by deferring the initial time to depassivation, and/or through reducing the rate of corrosion once corrosion is propagated, or retard incipient action (ring anode) [START_REF] Li | In situ leaching investigation of pH and nitrite concentration in concrete pore solution[END_REF]. Passivation due to the amino inhibitor is reached by the formation of an adsorbed layer on the steel surface. When the critical concentration ratio is not reached, the layer is partly destroyed, and the corrosion damage is rather localized.

When the stabilization of the passive state is reached due to the inhibitor and subsequently the concentration of the inhibitor is reduced, the initiation of corrosion might reoccur (the ratio of concentrations inhibitor/Cl -is critical also in such cases) [START_REF] Mammoliti | Corrosion inhibitors in concrete, Part II: Effect on chloride threshold values for corrosion of steel in synthetic pore solutions[END_REF]. The sooner the inhibitor is introduced after corrosion propagation the more effective it is [START_REF] Olek | Influence of corrosion inhibitors on the electrochemistry of corrosion processes of reinforcing steel in simulated pore solutions[END_REF]. The long-term concentration of the inhibitor near the reinforcement, which may decrease over time due to leaching and evaporation, is a crucial parameter for the effectiveness of the inhibitor as part of a repair strategy [START_REF] Short | Effect of corrosion inhibitors on pore solution chemistry of hardened cement pastes[END_REF]. Monoflurophosphate seems slightly efficient in simulated pore solution and in mortar and higher MFP concentration being slightly better (0.05, 0.1 and 0.5 M MFP and 0.5 M NaCl) [Alonso, 1996] [13]. The effectiveness of MFP increases with the inhibitor concentration in alkaline solution [Hope and Thompson, 1995] [14] and the minimum efficient ratio MFP/Cl -(0.5 M NaCl and 0.5 M MFP) is reported to be 1-1.5 [Lafave et al, 2002, Palmer andMalric, 2000] [15,16].The aim of this work is study of the basic mechanism of inhibitors for corrosion control of reinforcement in concrete by tests in simulated concrete pore solution.

Experimental program

This part consists of two methods based on influence of (CNI, AMA and MFP) inhibitors firstly, in simulated concrete pore solution and secondly, influence of AMA in extracted cement solution because AMA is claimed to inhibit corrosion by penetrating concrete and adsorbing on the metal surface more than CNI and MFP. The demonstrate AMA in extracted cement solution is new applied to evaluate the influence of inhibitors in blended cement extract solution. The laboratory tests in simulated concrete pore solution included exposure of steel specimens in pore solution with analysis of corrosion damage and passive layers; exposure of steel specimens in pore solution with chloride and analysis of corrosion damage; determination of potentiodynamic polarization curves in simulated pore solution; determination of electrochemical impedance spectra in simulated pore solution.

Materials and methods

In order to achieve the goals of this study several laboratory tests in simulated concrete pore solution were performed. These were as follows:

• Exposure of steel specimens in solution with analysis of corrosion damage and passive layers,

• Exposure of steel specimens in artificial pore solutions with Cl -and analysis of corrosion damage,

• Determination of open circuit potential E OC , potentiodynamic polarization curves in simulated pore solution,

• Determination of electrochemical impedance spectra in simulated pore solution,

• Influence of AMA inhibitors in blended cement extract solution Corrosion inhibitors, CNI, AMA and MFP were added to the different solutions separately. The dosage of CNI is 300 ml/l and the dosage of AMA is 4% (v/v) which is equivalent to (40 ml/l). In this work the dosage of MFP is 2% equivalent to (20g/l) and which is below the recommended amount (5%) to check the inhibition effect in the corrosion process. Experiments were conducted using carbon steel bars, and with a diameter of 8 mm), reference steel. The inhibited steel surface was approximated as a cylinder with a height of 2 cm corresponding to a surface area of 5.02 cm 2 . Each sample was allowed a period of 24 hours to stabilize in the solution before applying the test. Firstly, a saturated calcium hydroxide solution (noted S1) has been used to simulate the aqueous alkaline content of the (non-carbonated) concrete pore solution, with an approximate pH of 13. Secondly to simulate the aqueous phase of a concrete contaminated with chloride, was used to obtain an electrolyte designated by S2 (see Table 1). This contains Ca(OH) 2 saturated solution and NaCl . This chloride content is higher than critical threshold by approximately 20 times, because most of the existing research generally agrees that the Cl -/OH -threshold ratio for carbon steel is < 1.The initial pH measured was 13, but, it dropped to a value of 12.5±0.05 due to the chloride addition and possible carbonation from air during poring into the corrosion cells [START_REF] Nedal | Comparative Study of the Corrosion Behaviour of Conventional Carbon Steel and Corrosion Resistant Reinforcing Bars[END_REF] [START_REF] Nedal | Comparative Study of the Corrosion Behaviour of Conventional Carbon Steel and Corrosion Resistant Reinforcing Bars[END_REF]. The range was 3.5% for chloride concentration. In some experiments the inhibitor was added to the simulated pore solution (S1) at the very beginning, when the steel surface was still clean, to investigate delay of time to depassivation. In other experiments the inhibitor was added the simulated pore solution (S2) after the corrosion already started to investigate the influence on corrosion rate control. The results of the measurements obtained in simulated pore solution represent the basis for further investigations in concrete.

Extracted cement solution

This part is concentrated on the influence of AMA in extracted cement solution. Pore solution extract for a concrete mix of water to cement ratio of 0.4 is (Hydroxyl OH -,700 mmol, Sodium Na + , 192 mmol , Potassium K + , 592 mmol, Sulphate SO4 -2 , 44 mmol, Calcium Ca +2 , 2 mmol). The pH found in extracted cement solution is 13.7. The use of AMA in extracted cement solution is new applied in our laboratory. The inhibition efficiency of the AMA to the surface of six steel (repeatability for one test) placed in extract solutions more or less corrosive and simulating more or less degraded concrete was investigated through electrochemical methods Figures [START_REF] Monticelli | A study on corrosion inhibitors for con-crete application[END_REF]. Free corrosion potential measuring E OC , Cyclic polarization test and potentiodynamic scan (GC) were applied after 1 day and 2days up to 8 days of immersion. Experiments were conducted using carbon steel bars, and with a diameter of 8 mm), reference steel.

Electrochemical Measurements

In order to apply the open circuit potential (Eoc) of a metal, the potentiodynamic scan or the cyclic polarization scan, three electrodes are immersed in the testing solution and connected to a Biologic Ec-Lab potentiostat a device that is used to apply on over potential and record the induced current. Potentiodynamic scan and cyclic polarization are similar to the Cyclic polarization test. These tests were applied after 1 day and 2days up to 8 days of immersion.

The data obtained for each time from six steel (repeatability for one test). The electrochemical impedance is then automatically calculated from this data in lab measurements. The experiment is controlled by data logging system and analysis is done using software provided by the manufacturer.

Open Circuit Potential Measurements EOC

The free corrosion potential is a good indication of the metal's tendency to corrode in a certain solution. Having a relatively fixed (not drifting) potential is an indication that the sample has stabilized in the solution. The test parameters are:

• The total time: this is the test duration in seconds. The sample's potential was measured against the Saturated Calomel Electrode (SCE) for duration of 60 seconds.

• The sample period: this parameter determines the spacing between data points in seconds. The sample period was 1 second.

• The stability: that is used to tell the system the definition of a stable potential.

When measuring the open circuit potential, if the drifting rate falls below the stability this will result in terminating the experiment immediately. This parameter has units of mV/sec. The stability was set to be zero mV/sec. This means that the test will be terminated only when the total time ends.

• Sample Area: the sample surface area in (cm 2 ) that is immersed in the solution.

. Potentiodynamic Scans

The potentiodynamic scan was used to measure corrosion current density, and then corrosion rate was calculated. For every sample, the potentiodynamic scan was applied to determine the corrosion current density icorr at that time. The test parameters are:

•Initial E: is the starting point for the potential sweep in Volts. The initial potential E was -100 mV vs. E OC .

•Final E: is the ending point for the potential sweep in Volts. The final potential E was 1000 mV vs. E OC . This scan range (E OC from -100 to + 1000 mV) enabled the estimation of icorr without destroying the sample. Testing further anodic potentials would force the sample to corrode (i.e., destructive testing).

•The sample period: the sample period was 1 second.

•Scan Rate: is the speed of the potential sweep during data acquisition. Its unit is mV/sec.

Very high scan rates lead to unreliable data; however, very low scan rate elongate the test period. The applied scan rate was 0.1 mV/sec. ˖Density: is the density of the metal tested in g/cm 3 . This parameter is used for corrosion rate calculation.

•Equivalent Weight: is the theoretical mass of metal that will be lost from the sample after one Faraday of anodic charge has been passed. This parameter is used in corrosion rate calculations.

•Initial Delay: this option is used to allow the open circuit potential of the sample to stabilize prior to the potential scan. The delay time is the time that the sample will be held at its open circuit potential E OC prior to the scan. The delay may stop prior to the delay time if the stability criterion for E OC is met. The delay time parameter is active only if the initial delay is turned on. This option was turned off in all the experiments.

•Extrapolation of the corrosion current density icorr was made with the help of potentiostat Analyst software.

Cyclic polarization

Each sample was allowed a period of 24 hours to stabilize in the solution before applying the test. The test parameters are:

•Initial E: is the starting point for potential sweep; initial Ewas -0.025 V vs. E OC .

•Final E: is the ending point for potential sweep; final E was +0.025 V. The forward scan rate was 0. 5 mV/sec. The test is used mainly to investigate the metal"s tendency to pitting corrosion in a certain environment and this ratio may be reach to -600 mmV to +600 mmV according to aggressive environment.

•Apex E: is the end potential for the anodic (upward) scan. Apex E (Erev) is one of two conditions that will terminate the forward sweep and initiate the reversal scan if it is reached and the Apex I is not exceeded yet. Apex E was one V vs. E OC .

•Apex I: is the corrosion current density that is indicative of pitting initiation and it is one of two conditions that will terminate the forward sweep and initiate the reversal scan if it is exceeded by the absolute current of the sample. The Apex I was 1 µA/cm 2 .

Electrochemical Impedance Spectroscopy (EIS):

Literature on the applications of EIS shows that it has great advantages over other techniques in studying the passive film formation providing the use of low frequencies of AC signal [Pruckner, 2001] [18]. The test parameters are:

Initial Frequency: is the starting point for the frequency sweep during the data acquisition phase. The frequency unit is Hertz. The initial frequency was 100 kHz.

Final Frequency: is the ending point for the frequency sweep during the data acquisition phase. The final frequency was 100 mHz for cathodic electrochemical treatment and 50 mHz for inhibitor treatment. Testing lower frequencies would make the test duration too long.

Points/decade: is the number of data points in each decade frequency. The value used was the default value of 70 -90 points per decade.

AC Voltage: is the amplitude of the applied AC (alternated current) signal to the working electrode. The unit is in root mean square (rms) mV. The excitation AC signal should be small in order to keep in the linear region (i.e., the AC current response will be in the same frequency but with phase shift) and not to destroy the sample. The applied AC voltage was 10 mV rms.

DC Voltage: is the applied DC (direct current) voltage on the working electrode. The purpose of the experiment was to study the properties of the passive film that is formed naturally on the metal surface. Thus, samples were held at their free corrosion potential EOC (i.e., DC Voltage = 0.0 vs. EOC).

Estimated Z: is a rough estimate of the cell's impedance at the Initial Frequency entered. This value is used to minimize the number of trials that the system operates to calculate the cell's impedance at the first data point. After the first point, this value is not important as the system uses the last estimated value for Z to calculate the new Impedance with the new frequency.

Figure 2a shows a schematic of the equivalent circuit R(QR) for non treated steel used in EIS data fitting. The proposed electric equivalent circuit Rel+Cf/Rf+Cdl/Rct for steel treated with inhibitors by [START_REF] Nedal | Comparative Study of the Corrosion Behaviour of Conventional Carbon Steel and Corrosion Resistant Reinforcing Bars[END_REF]Pruckner 2001 [17, 18] in their work to study passive film formation on carbon steel in alkaline solutions is given in Figure 2b. This circuit consists of the solution ohmic resistance Rel connected in series to two loops. The first loop represents the double layer capacitance Cdl and the charge transfer resistance Rct. The other loop represents the passive film formed on steel surface in the high alkaline solutions where Cf and Rf are the faradic capacitance and the ohmic resistance of the film, respectively. It is believed that this circuit introduces a reasonable explanation of the ongoing electrochemical process on the metal surface immersed in a high alkaline solution.

Since the capacitance (c) equals the charge built up in the capacitor (q) divided by the potential (V) (the free corrosion potential in this case), a metal in a passive state (i.e. less metal dissolution-less charge transfer between the metal surface and the electrolyte) will have less capacitance than an actively corroding metal. In other words, for a constant exposed area an active corrosion process will lead to an increase in the total charge transfer, which leads to increased film capacitance [Nedal, 2009,Sahoo andBalasubramaniam 2008] [17, 19].

This electrochemical impedance of the system presented in the following equations.

The impedance Z of a resistor equals:

Z = R [5.1]
Where R is the resistance in ohms.The impedance of a capacitor equals:

Z = JWC 1 [5.2]
Where C is the capacitance in farads, j is 1  and w is the angular frequency w= 2πf.

The total impedance of a system Ztotal when the components are connected in series equals:

Ztotal =Z1+Z2 [5.3]
The total impedance of a system Ztotal when the components are connected in parallel equals:

2 1 2 1 total Z Z Z Z Z   [5.4]
Thus, the total impedance equals: A capacity film reduction (Cf) that leads to a film thickness increase (equation (5.6))

) dl C . w . j ( ct R + 1 ct R + ) f C . w . j ( f R + 1 f R + el R = total Z [ 5
expressed as A/d r ε ε = f C  [5.6]
Where ο ε is the vacuum permittivity (8.85×10-14 F cm-1), r ε is the dielectric constant (≈1), A the active surface and d the thickness of film.

SEM and EDS analysis.

To characterize the influence of coating in protection of steel from corrosion, SEM reveals the morphology and composition of film on the surface of steel created by inhibitive treatments in simulated concrete pore solution. The overall analysis SEM and EDS has identified the elements on surface of steel treated by inhibitors.

Results and discussion

Results of Carbon Steel Immersed in Synthetic Solutions Emulating Concrete

Admixed with Inhibitors Open circuit potential.

Open circuit potential

Corrosion current evolution

The corrosion currents obtained from applying cyclic polarization curves for S1+CNI, S1+AMA and S1+MFP are illustrated in 3. Each point of the corrosion current in this figure is the average of six points. After addition of inhibitor, the data shows lower corrosion currents with time and after 8 days the corrosion current range is from 0.12 to 0.46 μA/cm² for S1+CNI, S1+AMA respectively. For rebar S1+MFP, corrosion activity after 8 days is (3.5 μA/cm 2 ). In cases, S1+CNI, S1+AMA, the corrosion activity is constant with time. For S1 without inhibitor and MFP the current also is almost constant and more than 1 μA/cm 2 corresponding to high rate corrosion. So the corrosion process is probably slowed down but not stopped with S1+MFP. The order of Icorr from high to weak is: S1 = S1+MFP, S1+AMA >S1+CNI.

EIS measurements in pore solution

The electrical parameters (C dl ,Rc t ) obtained through fitting EIS data, using the electric equivalent circuit R(QR) and R el +C f /(R f +C dl /R ct ), are listed in (Table 4). The values of Epit in table 4 illustrate that inhibitors increase the pitting resistance of the metal. EC-Lab software was used in data fitting. The mathematical method used was Levenberg-Marquardt method.

Figure 5 shows a Nyquist plot of the impedance of carbon steel immersed in fresh concrete pore solution with various amounts of inhibitors added to the solution in a period of 8 days. It can be noticed that the low frequency portion of the impedance spectra (on the right hand side) increases with presence of inhibitor in the solution as a sign of passivity does not break down. Figure 5 shows the main conclusions for all types of inhibitors, the order of inhibition is: S1+AMA > S1+CNI >S1+MFP >S1 according to the values of R ct .

In the Nyquist impedance two time constants (one at high frequency HF and another at low frequency LF) for treated steel, and thus the capacitive loop observed in the Nyquist plots is consisted of two non-decoupled capacitive loops just for CNI. For the carbon steel film, the double layer capacitance C dl < 2.5 μF/cm 2 shows that a small part R ct of the inhibited steel surface is involved in the electrochemical reactions at the steel/solution interface [START_REF] Simescu | Corrosion behaviour in alkaline medium of zinc phosphate coated steel obtained by cathodic electrochemical treatment[END_REF]] [START_REF] Simescu | Corrosion behaviour in alkaline medium of zinc phosphate coated steel obtained by cathodic electrochemical treatment[END_REF]. The corrosion reaction occurs only on a very small fraction of the total inhibitor steel area.

Basically, the passive film has a high resistivity with inhibitors; however, it decreased with reference specimens in the solution S1. The inhibitor layer of iron oxides/hydroxides surface film seems to cover the entire surface blocking the active sites and the total impedance of the system increases continuously. Thus, the increase in the sum of the charge transfer resistance and the film resistance (R ct + R f ) was observed in Table 4. The faradic capacitance of the passive film Cf decreased with inhibitors addition to the solution due to the decrease of charge transfer at the metal/electrolyte interface and reduction of capacity film (C f ) that leads to a film thickness increase. It is remarkable that the film capacitance MFP decreased by about six times than AMA and by three times than CNI. R f and C f are absent in diagrams corresponding to reference specimens immersed in alkaline solution, during only 8 days, due to the 8 days is not enough formation film on the surface of steel and also absence of inhibitors.

Cyclic polarization curve

It can be noticed that the metal had a passive region (decreased applied over potential and current density) that started immediately after the Tafel area (E OC ± 50 mV) for CNI and between -220 and -400 mV for all inhibitors added in S1 and continued until E pit was reached Figure 6. The results of E pit are shown in Table 4. In these solutions, the hysteresis loop was determined but Erev seems adherent because corrosion inhibitors are not used to totally stop corrosionand E pit , which is an indication that the metal is repassivate and able to resist pitting corrosion at these inhibitors types. In addition, the recorded i corr of MFP in reverse scan is higher than other inhibitors. Perhaps a longer exposure time in that solution with MFP was needed to increase the pitting resistance of the metal.

Surface analysis with SEM a). SEM of carbon steel in S1+MFP

Figure 7b shows the SEM on the surface of the samples immersed during

b). SEM of carbon steel in S1+AMA

The surface of the samples immersed during 1 week in solutions with inhibitor is depicted in Figure 7c. The one zoom images shows the presence of an oxide film due to displacement of the first hydrated monolayers of the iron oxide film. [START_REF] Smialowska Szlarska | An electrochemical and ellipsometric investigation of surface films grown on iron in saturated calcium hydroxide solutions with or without chloride ions[END_REF] [START_REF] Smialowska Szlarska | An electrochemical and ellipsometric investigation of surface films grown on iron in saturated calcium hydroxide solutions with or without chloride ions[END_REF] on the other hand, showed that the passive films formed in alkaline solutions are closer to an iron oxihydroxide (FeOOH) than to γ -Fe 3 O 4 or Fe 2 O 3 [START_REF] Smialowska | Activation of the iron surface to hydrogen absorption resulting from a long cathodic treatment in NaOH solution[END_REF] [START_REF] Smialowska | Activation of the iron surface to hydrogen absorption resulting from a long cathodic treatment in NaOH solution[END_REF]. The uniform film deposited over the surface and practically there are no signs of corrosion. The film becomes much thicker, more uniform and amino compounds are present on its surface. This behaviour is according to the literature, which suggests that the organic inhibitors usually adsorb on the surface leading to the formation of a thin adsorbed layer [START_REF] Jamil | An electrochemical and analytical approach to the inhibition mechanism of an amino-alcohol-based corrosion inhibitor for reinforced concrete[END_REF]] [START_REF] Jamil | An electrochemical and analytical approach to the inhibition mechanism of an amino-alcohol-based corrosion inhibitor for reinforced concrete[END_REF].

The SEM observations show that the (N-CH bonds) is compact well crystallized and covers completely the steel surface. The film observed in big area characterized by eggs shape or cotton-balls shaped due to its big thickness. These positive charged groups are likely to form at the expense of inhibitor interaction with water molecules (from hydrated layers compounds) on the iron surface [Jamil et al, 2003] [25]..

c). SEM of carbon steel in S1+CNI

The results of influence of CNI on microstructure are shown in Figure 7d. A stable passive layer of γ -FeOOH is formed on the surface of steel. This film is insoluble in aqueous alkaline solution and creates a solid layer on steel surface blocking the transport of ferrous ions into the electrolyte. As can be seens from Figure 7d

Open circuit potential

Figure 8 shows the open circuit potential in case of pore solution admixed with inhibitors under amount of chlorides 35 g/l. The rest potential value for S2 decreases significantly after 8 days (due to the presence of chloride). A drop in the corrosion potential was happened by other researchers as an indication of reaching the critical chloride limit [START_REF] Hausmann | Steel corrosion in concrete -How does it occur[END_REF]Li andSagues, 2001] [27,28]. For S2+MFP and S2+AMA the rest potential has approximately the same value and increases with time. This result due to two reasons: the first reason is the competition between chloride and the film of inhibitor which leads to increase resistivity of the film .the second reason is that protonated amino group's bind chloride so the concentration of chloride required for initiation of corrosion. This is an important feature when the inhibitor is used in concrete structures, since it may help to increase the Cl -/OH - threshold ratio, which is one of the more important parameters in controlling the corrosion in reinforced concrete structures [Jamil et al, 2003] [25]. AMA is recommended as complementary technique for reduction in time of regeneration of the passivity is obtained.

CNI is not affected by presence of chloride see Figure 8 3

.2.2 Corrosion current evolution

EIS measurements in pore solution (Initiation of corrosion (prevention of corrosion))

In this group of experiments the inhibitor was firstly added to the pure simulated pore solution, and afterward chlorides were successively added. The main goal of these experiments was to investigate whether the inhibitor additionally stabilizes the passive state of clean, already passivated steel surface. At the beginning, it can be seen in Figure 10 that the inhibitor increased the total electro-chemical impedance of the system, as well as changed the profile of the spectra. The significant reduction of the total impedance was observed when the chloride concentration in pore solution reached a rather high value. From the measured spectra presented as Nyquist plots it can be seen that the inhibitor considerably changed the shape of the spectrum. It can be therefore concluded that the structure of the passive layer due to the inhibitor differs from the layer in pure pore solution. The electrical parameters are listed in (Table 5) at 8 days. EIS data for S2 containing CNI, AMA and MFP aged 8 days in the same aggressive solution. Calcium nitrite is an anodic inhibitor. It functions by oxidizing corrosion product -ferrous ions -to ferric ions that precipitate in the alkaline solution of the concrete and form a protective layer on the reinforcement. The precipitate functions as a film repair because the ferric ions are insoluble in aqueous alkaline solutions and block the transfer of ferrous ions into the electrolyte. This is data due to competition between the formation and local destruction of the film obtained by the slow inhibitor dissolution. This classical corrosion mechanism leads to the diminution of R ct and to the increase of the double layer capacitance C dl while R f increases in comparison with S1. This evolution corresponds to the electrolyte diffusion into the film pores. The order of inhibition is: S2+CNI >S2+AMA >S2+MFP >S2 according to the values of R ct . with inhibitor and the S2 without inhibitor. It can be noticed that the pitting potential for CNI equal to +200 mV vs. SCE and this help the metal to repassivate (heal) when the pitting potential is increase.

. Cyclic polarization curve

The increase of the passive current density for the film resistance is suggesting the presence of a protective homogeneous layer on the steel surface. S2+MFP lead to the anodic formation of a passive layer which contributes to the decrease of chloride aggressiveness. A drop in polarization resistance is an indication of reaching the critical chloride limit [START_REF] Trejo | Accelerated chloride threshold testing -Part II: Corrosion-resistant reinforcement[END_REF][START_REF] Trejo | Accelerated chloride threshold testing -Part II: Corrosion-resistant reinforcement[END_REF]Qian et al., 2008] [31,32].

Perhaps a longer exposure time of MFP in the solution S2 was needed to increase the pitting resistance of the metal. At higher chloride concentrations of reference specimen S2, the pitting potential E pit decreased to be -400 mV vs. SCE, and the hysteresis loop did not close.

The results of E pit are shown in Table 5. This is an indication of the breakdown of the passive film and that the metal was not able to repassivate (was not able to heal) at these chloride concentrations. This behaviour can be explained by the competitive action of both Cl -and OH -ions in the solution [Naceur et al, 2007] [33].

The charge transfer and film resistance

Figure 12 and Figure 13 show an increase in the charge transfer resistance (R ct ) and film resistance (R f ) for all inhibitor more than the refrence. However, R f is equal to zero in Ref steel due to absence of inhibitors even with and without of Cl -ions in the solution, this behaviour may be due to the time (8days) no sufficint for formation of film on the surface of steel. The order of inhibition when immersed in S1 is: AMA >CNI >MFP >Ref steel according to the values of Rct, this meaning that AMA is more active in absence of chloride.The order of inhibition is: CNI >AMA >MFP >Ref steel according to the values of R ct and R f when immersed in S2, this meaning that CNI is more active in presence of chloride.

Basically, the passive film has a higher resistivity with CNI, AMA and MFP than the reference in pore solution S2 due to competition between chloride and the film of inhibitors which act as mechanical barrier against corrosion. MFP seems slightly efficient in simulated pore solution and slightly lower with NaCl.

Surface analysis with SEM a). SEM of carbon steel in S2+MFP

The SEM shows that the pitting was observed on the sample Figure 14b. The brighter surface in SEM microphotographs exhibit pits that appeared darker in optical microphotographs. The pits appeared to occur only on sharply faceted grains. This indicates the accumulation of salts of phosphorus, chloride and calcium in the pits. The cracks have higher phosphorus content and exhibited the deposition of corrosion products.

b). SEM of carbon steel in S2+AMA

In figure 14c the SEM observations show the presence of a great amount of corrosion products like balloons swell and spread over the entire layers of the film surface.

It is suggested [Smialowska et al.1985 and1981] [23,24] that the presence of chlorides in the solution changes the composition, thickness, and density of the passive film. Some author's report that the film formed in the presence of chlorides becomes thicker, but more porous and hydrated, loosing its protective character [START_REF] Jamil | Corrosion behaviour of reinforcing steel exposed to an amino alcoholbased corrosion inhibitor[END_REF]] [START_REF] Jamil | Corrosion behaviour of reinforcing steel exposed to an amino alcoholbased corrosion inhibitor[END_REF].

The negatively charged ions (chloride ion) could reflect an interaction with the protonated amino groups.

c). SEM of carbon steel in S2+CNI

In figure 14d the SEM observations show the presence of a white precipitate formed over surface. This precipitate was analysed using EDS (X-ray spectroscopy) and found to be predominantly calcium and oxygen, suggesting that it is most likely Ca(OH) 2 . However, full protection depends greatly on the concentration of aggressive ions such as the chloride ion.

The free chloride concentration in the pore solution admixed with CNI decreased and the OH - content increased with the time of aging.

CNI is highly soluble in neutral solution. However, in solutions of very high pH value the concentration of Ca 2+ ions is strongly limited by the solubility product of Ca(OH) 2 . The presence of CN in the pore solution causes part of its Ca 2+ ions to react with the OH -ions in the solution to precipitate calcium hydroxide [START_REF] Li | The microstructure and sulfate resistance mechanism of high-performance concrete containing CNI[END_REF] [5].

Corrosion behaviour in solution extract of blended cement.

Results of Carbon Steel Immersed in Solutions Extract of blended Cement Admixed with AMA

In the recent years, there are few studies using AMA in advanced types of blended cement concrete. So this new study focused only on using ethanolamine in extract pore solution of blended cement. For ECC, after eight days, the rest potential values are higher until 6 days then decrease. This means that the corrosion conditions are in risk range.

a). Open circuit potential

b). Corrosion current evolution

The corrosion currents are illustrated in Figure 16. After addition of inhibitor, the data shows lower corrosion currents with time and after 8 days the corrosion current range is from 4.5 to 3.9 μA/cm² for EC+AMA. It is to be noticed that this corrosion current constant with tendency of steel to corrod, which could be considered as an efficiency indicator for the inhibitor treatment according to RILEM studies [START_REF] Andrade | Test method for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method[END_REF] The electrical parameters obtained through fitting EIS data, using the electric equivalent circuit R(QR) and R el +C f /(R f +C dl /R ct ), are listed in Table 6. From these diagrams low values of C f and C dl are recorded. The increase in the sum of the charge transfer resistance and the film resistance (R ct + R f ) was observed was adding of AMA.

For ECC, an increase in the sum of the charge transfer resistance and the film resistance (R ct + R f ) was observed with AMA. It is remarkable that the film capacitance of AMA increased by about three times from the beginning in EC of the experiment AMA (C f 27.6 μF/cm 2 ) until a chloride concentration in the solution ECC of 35g/l (C f 80 μF/cm 2 ). AMA has great corrosion protection ability although it induced more chloride ion into the OPC concrete than BCC. This corrosion protection might be greatly attributed to the organic part in AMA around the rebar.

The low frequency portion of the impedance spectra decreases in solution extract of cement in presence of chloride ion in the solution.

Conclusions

From the results presented in this paper it can be concluded that:

 Calcium nitrite (CNI), sodium monoflurophosphate (MFP) and ethanolamine (AMA) based inhibitors showed reductions in the overall rates of corrosion after the inhibitive treatments.

 In alkaline solution with or without chloride, the CNI steel sample is more resistant than carbon steel alone and causing passivation of steel in concrete. Thus, a dense and protective layer is formed.

 The laboratory study in simulated pore solution proved that all tests in simulated concrete pore solution proved that the amino alcohol inhibitor can reduce corrosion: the most important parameter is the ratio of inhibitor/chloride concentration,  if the steel reinforcement is heavily corroded the critical concentration ratio is strongly dependent on the steel surface conditions: after the initiation of corrosion this ratio must be quite high (e.g. 1:1) for the retardation of corrosion and if the steel is heavily corroded complete repassivation is practically impossible,  the efficiency of the inhibitor in solution with higher pH (simulated concrete pore solution without chlorides) cannot be exactly assessed, because the inhibitor shows a buffering effect,  passivation due to the inhibitor is reached by the formation of an adsorbed layer on the  steel surface,  when the critical concentration ratio is not reached, the layer is partly destroyed, and the corrosion damage is rather localized (localized corrosion spots can be relatively deep),

 when the stabilization of the passive state is reached due to the inhibitor and subsequently the concentration of the inhibitor is reduced, the initiation of corrosion might reoccur (the ratio of concentrations inhibitor/Cl-is critical also in such cases),  the sooner the inhibitor is introduced after corrosion propagation the more effective it is,  it can be expected that the long-term concentration of the inhibitor near the reinforcement, which may decrease over time due to leaching and evaporation, is a crucial parameter for the effectiveness of the inhibitor as part of a repair strategy  Na 2 PO 3 F hydrolyses into the pore solution simulating the concrete to form phosphate and so anodic formation of passive layer contributes to the decrease of chloride aggressiveness.

 The corrosion currents measured for solution extract of blended cement were significantly higher than those obtained at the same chloride concentration in the mortar and pore solution.It was found that the corrosion currents of carbon steel embedded in mortar immersion in NaCl at 3, 5%, would decrease due to the decrease in the degree of pore saturation PS after 28 days. Regardless in the presence of chloride content, the corrosion current was as low as 1, 5 µA/cm 2  .

  resistance formed in alkaline solution Rct: charge transfere resistance The other loop represents the passive film formed on steel surface in the high alkaline solutions where Cf and Rf are the faradic capacitance and the ohmic resistance of the film, respectively.

Fig. 3

 3 Fig.3shows the open circuit potential (also referred to the rest potentials) test obtained from

Figure 4 .

 4 According to RILEM studies [Andrade et al. 2004] [21], 4 ranges of corrosion activity can be distinguished from negligible, to weak, to moderate and up to high Table.

Figure 9

 9 Figure9shows the corrosion current density (i corr ) obtained from carbon steel samples

Figure 11

 11 Figure 11 represent the potentiodynamic polarization curves of steel in S2 solution without

Figure 15 shows

 15 Figure15shows the open circuit potential tests for carbon steel samples immersed in solution

Figure 17

 17 Figure17shows a Nyquist plot of the impedance for carbon steel samples immersed in

  1 week in solutions S1 with MFP inhibitor. The inhibiting action of sodium phosphate showed that phosphate compounds form a protective layer on the steel surface when immersed in alkaline solution without chlorides. A passive layer of Fe 3 O 4 , γ Fe 2 O 3 and FePO 4 .H 2 O results of hydrolysis Na 2 PO 3 F in aqueous and neutral media to form orthophosphate and fluoride [Alonso et al,

1996]

[START_REF] Alonso | Na 2 PO 3 F as inhibitor ofcorroding reinforcement in carbonated concrete[END_REF]

. But phosphate compounds characterized by spherical glassy plate of sand rose, lamellar shapes crystal exists on the surface of steel.

  ][START_REF] Andrade | Test method for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method[END_REF]. In case of EC and EC+AMA the corrosion activity is constant. In case, of extract solution ECC without inhibitor, under amount of chlorides 35 g/l gives corrosion activity increase with time and those obtained from extract solution with inhibitor ECC+AMA decrease due to presence of amino inhibitors and this results agree with[Qian et al, 2008] [32]. The ECC behaviour can be explained by lowering of pH by competitive action of both Cl -and OH -ions in the solution (see equation[8.3]).

	Fe (Cl 6 ) -3 + 6 H 2 O = Fe (OH) 3 + 3 OH + 6 HCl	[8.3]
	c). EIS measurements in pore solution


A Djerbi Tegguer

S Bonnet

A Khelidj

V Baroghel-Bouny

Effect of uniaxial compressive loading on gas permeability and chloride diffusion coefficient of concrete and their relationship

Keywords: Diffuse damage, Damage variable, Chloride Diffusion, Durability, Microcracks, Gas Permeability, High performance concrete, Time lag

Knowledge of the transport properties of damaged concrete in marine environments is essential for predicting its durability. The objective of this study was to fill this gap by correlating the change in permeability and chloride diffusivity with an increasing uniaxial load on ordinary concrete (OC) and high performance concrete (HPC). Concrete cylinders were induced microcracks by mechanical uniaxial compression between 60% and 90% of the ultimate strength to get diffuse damage. The damage variable of specimens was evaluated by elastic stiffness degradation and ultrasound pulse velocity. After unloading intrinsic gas permeability was measured using a constant head permeameter, the chloride migration coefficient was evaluated by migration test in steady state conditions, with the same concrete specimen. The damage variable of specimens showed correlation with gas permeability and chloride diffusion of concrete in this experiment. A linear correlation was obtained between intrinsic permeability coefficient and chloride diffusion coefficient depending on the damage variable, specific for each concrete type (OC and HPC).

Introduction

Predictions of service life for both new and existing concrete structures has assumed increasing importance as the incidence of damage due to reinforcement corrosion increases.

Sustainability indicators of concrete such as permeability and diffusion are the main transport properties for concrete, because it governs the ingress of water, oxygen, carbon dioxide, and chlorides which are necessary for the electro-chemical reaction of corrosion. Extensive work has been carried on over the past decades to understand transport properties of concrete, and numerous service life prediction models have been introduced. The disadvantage of these models is that all predictions are carried out considering a perfect, uncracked concrete [START_REF] Maage | Service life prediction of existing concrete structures exposed to marine environment[END_REF][START_REF] Jung | Predicting the remaining service life of land concrete by steel corrosion[END_REF][START_REF] Zhang | Accelerate life test of concrete in chloride environment[END_REF].

Concrete structures near marine environments are subjected to a combination of chloride and load resulting in both macro and microcracking. The presence of cracks can significantly modify these transport properties of concrete. Microcracks that are discrete and well distributed will influence transport properties in very different manner compared to visible connected localized macrocracks [START_REF] Picandet | Crack effect on gas and water permeability of concretes[END_REF][START_REF] Aldea | Effect of cracking on water and chloride permeability of concrete[END_REF][START_REF] Olga | Influence of cracks on chloride ingress into concrete[END_REF][START_REF] Tognazzi | Couplage fissurationdégradation chimique des matériaux cimentaires: premiers résultats sur les propriétés de transfert[END_REF][START_REF] Djerbi | Influence of traversing crack on chloride diffusion into concrete[END_REF][START_REF] Kermani | Permeability of stressed concrete[END_REF][START_REF] Sugiyama | Effect of stress on Gas permeability in concrete[END_REF][START_REF] Picandet | Effect of axial compressive damage on gas permeability of ordinary and high-performance concrete[END_REF][START_REF] Choinska | Effects and interactions of temperature and stress-level related damage on permeability of concrete[END_REF][START_REF] Sugiyama | Permeability of stressed concrete[END_REF][START_REF] Hr | Influence of microcracking on the mass transport properties of concrete[END_REF][START_REF] Saito | Chloride permeability of concrete under static and repeated loading[END_REF][START_REF] Cc | Microcracking and chloride permeability of concrete under uniaxial compression[END_REF][START_REF] Zhou | Characterizing the effect of compressive damage on transport properties of cracked concretes[END_REF]. In the case of localized damage it has been shown that gas permeability of concrete measured on disks fractured using a splitting tensile test increases with crack opening displacement COD cubed [START_REF] Picandet | Crack effect on gas and water permeability of concretes[END_REF]. Concerning chloride diffusion measured by migration test, a linear variation was obtained between the diffusion coefficient through the crack and crack width, this coefficient was not dependent on material parameters and becomes constant when the crack width was higher than 80 µm, its value is the diffusion coefficient of chloride in a free solution [START_REF] Djerbi | Influence of traversing crack on chloride diffusion into concrete[END_REF].

In comparison, the studies of diffuse damage obtained by compression load have shown a marked threshold effect which called percolation threshold, regardless of concrete studied and the experimental protocol adopted to measure the transport properties and crack of concrete [START_REF] Kermani | Permeability of stressed concrete[END_REF][START_REF] Sugiyama | Effect of stress on Gas permeability in concrete[END_REF][START_REF] Picandet | Effect of axial compressive damage on gas permeability of ordinary and high-performance concrete[END_REF][START_REF] Choinska | Effects and interactions of temperature and stress-level related damage on permeability of concrete[END_REF][START_REF] Sugiyama | Permeability of stressed concrete[END_REF][START_REF] Hr | Influence of microcracking on the mass transport properties of concrete[END_REF][START_REF] Saito | Chloride permeability of concrete under static and repeated loading[END_REF][START_REF] Cc | Microcracking and chloride permeability of concrete under uniaxial compression[END_REF][START_REF] Zhou | Characterizing the effect of compressive damage on transport properties of cracked concretes[END_REF]. If the crack network is not connected, these transport properties are related to the concrete itself, such as the porosity of concrete and interconnectivity of the pore system.

Beyond the percolation threshold cracked concrete its related to crack properties, the gas permeability changes very significantly with the increase of damage level: an increase in permeability with several orders of magnitude for damaged concrete can be observed [START_REF] Tognazzi | Couplage fissurationdégradation chimique des matériaux cimentaires: premiers résultats sur les propriétés de transfert[END_REF][START_REF] Kermani | Permeability of stressed concrete[END_REF][START_REF] Sugiyama | Effect of stress on Gas permeability in concrete[END_REF][START_REF] Picandet | Effect of axial compressive damage on gas permeability of ordinary and high-performance concrete[END_REF][START_REF] Choinska | Effects and interactions of temperature and stress-level related damage on permeability of concrete[END_REF][START_REF] Zhou | Characterizing the effect of compressive damage on transport properties of cracked concretes[END_REF], while chloride penetration is much less affected by cracks [START_REF] Sugiyama | Permeability of stressed concrete[END_REF][START_REF] Hr | Influence of microcracking on the mass transport properties of concrete[END_REF][START_REF] Saito | Chloride permeability of concrete under static and repeated loading[END_REF][START_REF] Cc | Microcracking and chloride permeability of concrete under uniaxial compression[END_REF].

Existing experimental data provide some correlation between the material degradation and the permeability of concrete. The studies done by Picandet et al. [START_REF] Picandet | Effect of axial compressive damage on gas permeability of ordinary and high-performance concrete[END_REF] and Zhou et al. [START_REF] Zhou | Characterizing the effect of compressive damage on transport properties of cracked concretes[END_REF] show a correlation between the gas permeability and damage variable evaluated from uniaxial compression test. Moreover very little data are available on the relationship between permeability and chloride diffusion of concrete damaged by uniaxial compressive loading.

The real contribution of this work is to investigate simultaneously the altered diffusivity and the altered permeability by uniaxial damage in order to find the correlation between these two parameters: chloride diffusion coefficient and the gas permeability. This correlation will be established by introducing damage variable resulting from a reduction in stiffness of the concrete damaged by sustained uniaxial loading to get diffuse damage. One objective of this research was conducted to achieve further information on the influence of microcracks on the gas permeability and chloride diffusion of ordinary concrete (OC) and high performance (HPC). Concrete cylinders were loaded under uniaxial compression between 60% and 90% of the ultimate strength. The time of sustained load is 2 h for each load level, added 1h and 30 min for high load level. A damage variable can be obtained by static and dynamic method.

After unloading intrinsic gas permeability was measured using a constant head permeameter, the chloride migration coefficient was evaluated by migration test in steady state conditions, with the same concrete specimen. These measurements allow the comparison between chloride diffusion and gas permeability for similar concrete at similar load levels.

Experimental program

Specimen Preparation

Two concrete mixes were made with the same cement CPA-CEM I 52.5: one ordinary concrete (OC) with a water/cement ratio of 0.49 and high performance with a water/cement ratio of 0.32 (HPC), (see Table 1). Concrete cylinders of length 22 cm and diameter 11 cm were prepared from a single batch for each mix. The concrete mixtures were cast in steel molds and compacted using a mechanical vibrator. After casting the cylindrical specimens were stored in a room maintained at 20°C and about 95% relative humidity (RH) for 24 hours, and were cured in water at 20°C for 1 year. They were stored in air-conditioned room (20°C and RH 50  5%) until testing. The characteristics of these concretes ageing 1 year are shown in Table 2. The open porosity was measured by water saturation.

All concrete specimens were tested under an axial compressive loading condition (see Fig.

1.

). After unloading concrete discs of 5 cm in thickness were cut off from the central portion of the cylindrical specimens with a diamond blade saw (see Fig. 2.). These discs were sealed with two epoxy resin coats in order to ensure one-dimensional gas and chloride flow through the discs. Drying the discs before proceeding to any gas permeability test is necessary [START_REF] Picandet | Effect of axial compressive damage on gas permeability of ordinary and high-performance concrete[END_REF][START_REF] Choinska | Effects and interactions of temperature and stress-level related damage on permeability of concrete[END_REF][START_REF] Zhou | Characterizing the effect of compressive damage on transport properties of cracked concretes[END_REF][START_REF] Jacobs | Permeability to gas of partially saturated concrete[END_REF][START_REF] Nagataki | Effect of heating condition on air permeability of concrete at elevated temperature[END_REF]. In this study, all disc specimens are oven-dried at 60°C to constant weight. This procedure has taken 2 months for OC and 3 months for HPC. Then, they are cooled for 48h in a desiccator at 20°C before being tested.

The concrete sample preparation such as cutting and drying can affect the microstructure of concrete. However this additional damage does not affect the evolution of transport properties versus uniaxial damage, since the same additional damage was applied on the concrete sample for each damage level.

After permeability test the concrete samples were then vacuum saturated in order to measure the chloride diffusion. These were placed in vacuum container with a 2.5 kPa pressure for 4 h.

Then, with the vacuum pump still running, the container was filled with water saturated with NaOH (0.025 mol/l) and KOH (0.083 mol/l) in order to immerse the specimen. The samples were saturated with alkaline salt solutions to prevent leaching of concrete. The vacuum was maintained for 24 h before allowing the air to enter into the container [START_REF]Méthodes recommandées pour la mesure des grandeurs associées à la durabilité[END_REF]. After this saturation procedure, chloride migration test was driving on damaged and undamaged concrete.

Uniaxial damage

Concrete cylinders were tested under uniaxial compression between 60% and 90% of the ultimate strength measured at 1 year called f c (see Table 2). An hydraulic press of 2400 kN capacity was used for this test. On each cylinder, the longitudinal strain is measured using an extensometer cell equipped with three linear variable displacement transducers (LVDT), with a range agreeing within 0.5 mm and an accuracy of 1 µm, (see Fig. 1.). The transducers are laid every 120° intervals to take into account any asymmetric longitudinal strain. The concrete cylinders were loaded progressively for predetermined load levels. Each load level was being sustained for varying periods up to two hours [START_REF] Mazzotti | Non linear Creep, Poisson's Ratio, and Creep-Damage interaction of Concrete in Compression[END_REF]. The short duration of tests was required to avoid the broken of measurement system in the case of high load level (90% f c ). These displacements are recorded during the loading and unloading phase until the recovery was negligible. The average longitudinal displacement is calculated with the three LVDT measurements.

The data required for the assessment of the mechanical properties consists of the axial stress and longitudinal strain. The evaluation of the variation of elasticity modulus is used to calculate damage variable which is an indication of the degree of microcracking. A damage parameter d in damage mechanics can be defined from relative ratio of modulus in elasticity [START_REF] Lemaitre | Damage measurements[END_REF], as in Eq. ( 1):

0 0 E d E E   (1)
Where E 0 is the initial modulus of elasticity (Table 2), and E is the final modulus of elasticity obtained for damaged concrete.

The modulus of elasticity can be determined by a static and dynamic evaluation. The static modulus of elasticity is usually calculated with (strength-strain) curve from a plasticfracturing model [START_REF] Mazars | Application de la mécanique de l'endommagement au comportement non linéaire et à la rupture du béton de structure[END_REF]. Before each stress level, the initial modulus of elasticity of each cylinder was obtained with the slope of the loading curve calculated from 5% to 30% of the ultimate strength. The final modulus of elasticity for damaged concrete is calculated with the unloading curve also from 5% to 30% of the ultimate strength. . The dynamic modulus of elasticity of concrete can be determined non destructively using resonance tests based on measuring the fundamental flexural and torsional frequencies of concrete specimens using a 'Grindosonic' apparatus [START_REF] Allison | Non-destructive determination of Young's modulus and its relationship with compressive strength, porosity and density[END_REF]. This last were carried out on concrete cylinders before loading to obtain E 0 and immediately after unloading to obtain E.

Gas permeability test procedure

An apparatus known as the CEMBUREAU permeameter was used for the determination of permeability [START_REF] Kollek | The determination of the permeability of concrete to oxygen by the Cembureau methoda recommendation[END_REF]. This is a constant head permeameter and nitrogen gas is used as the permeating medium. A pressure difference up to 0.3 MPa can be applied to the specimens in the pressure cells which are sealed by a tightly fitting polyurethane rubber pressing under high pressure (0.7 MPa) against the curved surface. Permeability measurements were made in an air-conditioned room (20  1°C and RH 50  5%). Each disc was tested with five differential pressures: 0.05, 0.1, 0.15, 0.2 and 0.3 MPa. The volume flow rate through the specimens is measured by means of a soap bubble flow meter. After initiating the percolation of nitrogen through a specimen at a given applied pressure, sufficient time (varying from 40 minutes to several hours) is provided for the establishment of steady state flow before an actual measurement is taken. This condition is verified by taking two measurements separated by a 15 minute time interval. If the two values differ by less than 3%, a steady state flow condition is assumed to be achieved.

The apparent coefficient of permeability k A (m²) is calculated from the Hagen-Poiseuille expression Eq. ( 2) for laminar flow of a compressible fluid through a porous body under steady state conditions [START_REF] Dhir | Near surface characteristics of concrete: intrinsic permeability[END_REF].

) P (P P L 2µ A Q k 2 atm 2 i atm A   (2)
Where L is the thickness of the sample (m), A is the cross-sectional area (m²), Q is the gas flow (m 3 /s), µ is the coefficient of viscosity (1.78.10 -5 Pa.s for nitrogen gas at 20°C), P i is the applied absolute pressure or inlet pressure (Pa), and P atm is the atmospheric pressure (Pa).

In fact, the gas percolation through a fine porous body like concrete, can be regarded as resulting from two flow modes: viscous flow and slip flow or Knudsen flow. Various methods for the calculation of non-viscous flow exist. The most widely used is the relation proposed by Klinkenberg Eq. (3) introducing the concept of an intrinsic coefficient of permeability k V relative to viscous flow only [START_REF] Klinkenberg | The permeability of porous media to liquid and gases, Drilling and Production Practice[END_REF].

          m V A P b 1 k k (3)
Where P m is mean gas pressure, P m = (P i + P atm )/2, b is the Klinkenberg coefficient (Pa) which is function of the porous body and the infiltrated gas, and k V is the limiting value of gas permeability when the mean pressure P m tends towards infinity. The method of determination of k V consists in measuring k A at different pressures (P i ) and plotting it against the inverse of the mean pressure (1/P m ). The slope of the line leads to the empirical Klinkenberg coefficient b and the origin leads to the intrinsic permeability.

Steady state migration test

Since diffusion experiments are time-consuming, steady state migration tests were developed to accelerate chloride ions through the concrete [START_REF] Andrade | Calculation of chloride diffusion coefficient in concrete from ionique diffusion measurements[END_REF][START_REF] Truc | A new way for determining the chloride diffusion coefficient in concrete from steady state diffusion test[END_REF][START_REF] Luping | Chloride transport in concrete-Measurement and predictions[END_REF]. Each specimen is placed between the two compartments of a cell where flat silicone circular seals ensure that the system is leaktight (Fig. 3.). The solutions were made with NaOH (0.025 mol/l) + KOH (0.083 mol/l) in upstream and downstream compartment. NaCl (0.513 mol/l) was added in the upstream solution. A 12 V was applied between the sides of the concrete sample and the test was carried out at temperature T=20±5°C. The downstream solution was titrated with silver nitrate (0.05M). As the flux becomes constant, Nernst-Planck's relation allows to deduce the value of the diffusion coefficient, as seen in Eq. ( 4):

    x cν c RTL zF D x c D x J e e e       (4)
where D e is the diffusion coefficient of concrete (m²/s), c is the chloride concentration of the upstream compartment (mol/m 3 ) assumed to be constant, J(x) is the flux of chloride ions (mol/(m 2 s)), z is the chloride ion valency (z=1), F is the Faraday constant (F = 96480 J/(V.mol)), e is the actual potential drop between the surfaces of specimen (V), R is the gas constant (R=8.3144 J/(mol.K)), T is the absolute temperature (K) and ν(x) is the velocity of the solute (m/s). If the concrete is saturated, the velocity of the solute can be neglected. Since the potential drop is ≥ 10V, ions migrate as a result of the electrical field rather than of the concentration gradient. This ensures that the diffusion flow can be neglected in the experiments as shown by Andrade [START_REF] Andrade | Calculation of chloride diffusion coefficient in concrete from ionique diffusion measurements[END_REF]. Eq. ( 4) can be simplified and then Eq. ( 5) is obtained:

J zF RT c L D e e  (5)
To establish a steady state, the concentration gradient must be constant during the test.

Therefore we have to renew the upstream and downstream solutions frequently. The evolution of current during testing showed a small increase at the beginning of the test and then a stabilisation for all concretes.

Results and discussion

Effect of uniaxial damage on the strain results

Strain is the first criterion for damage evaluation of concrete. For each load level maximal strain ε max under loading and residual strain ε res which was recorded 30 min after unloading were evaluated. Figure 4 presents the evolution of maximal strain ε max versus load level. The results put in evidence that the maximal strain obtained at 2 hours increased regularly with the level of applied load for both concretes. This might be explained by the development of internal microcracking of concrete. It was generally accepted that cracking of concrete subjected to compressive loading may take the form of one of the following three ways: bond cracks, mortar cracks and aggregate cracks. Bond cracks are the microcracks in the interfacial transition zone (ITZ) between the cement paste and aggregate. In facts bond cracks may develop even before any loading is placed on the concrete due to settlement of fresh concrete, hydration of cement paste, drying shrinkage and carbonation shrinkage. Mortar cracks are microcracks which can be found in the matrix between aggregates. Theses cracks begin after the bond cracks have started and it is these type of cracks that lead the failure of concrete. As the load increases they begin to propagate throughout the matrix extensively combining with other mortar cracks as well as bond cracks. Aggregates cracks are cracks formed through the aggregate.

Compressive load below 30%f c will not cause microcracks and the concrete behave as an elastic material. At 30%f c to 60%f c the bond cracks increase in length, width and number as the load increases (as reported in [START_REF] Cc | Microcracking and chloride permeability of concrete under uniaxial compression[END_REF][START_REF] Smadi | Microcracking of high and normal strength concretes under short and long term loadings[END_REF]). However at 60%f c and above the crack system becomes unstable and causes the strain to increase. In addition to bond cracking, mortar microcraking begins to become interconnected and form "continuous cracks" as classified by Shah et al. [START_REF] Shah | Critical Stress, Volume Change and Microcracking of Concrete[END_REF]. At 90%f c the maximal strain increased with the increase of the time load.

This might be explained by the increase of the crack size due to the expansion of the cylinder and the increase of longitudinal displacement. This evolution confirms that the cracking pattern is developing during the loading phase only.

Microcracking was investigated by a videomicroscope with 200 times magnification and an oblique light was used to observe the cross section of the most damaged discs (see Figure 5).

Crack width and length are than smaller than 3µm and 6 mm, respectively. The specimens of OC and HPC group show clearly different crack. In OC cracking takes place in the ITZ and mortar; however HPC does not develop significant bond cracks than in corresponding of OC.

This may be partially explained by the elimination of the region of connected pores that were a common feature of the ITZ of OC, but also confirms the improvement of the strength of the interfacial zone in the HPC. HPC has a lower w/c, resulting in stronger cement paste. As a consequence, high strength concrete has stronger interfacial zone, which is due to a reduction in excess bleeding. This leads the development of aggregate cracks in HPC. It is presumed that these aggregates experienced in previous study leading to a mode of fracture analogous to that which occurs in a splitting tensile test [START_REF] Djerbi | Influence of traversing crack on chloride diffusion into concrete[END_REF].

The residual strain ε res evaluated after unloading increase linearly with the increase of maximal strain ε max for both concretes (see Figure 6). The microcracking patterns varied among the mixes for each load level the maximal strain under loading of HPC is higher than OC but the residual strain is lower. This implies a partial closure of the microcracks of HPC concrete after unloading which is due to a more elastic response of HPC than OPC (see Table 2). This results can also be interpreted by the development of combined cracks, consisting of combinations of bond and mortar cracks, which is significant in OC than HPC (as observed by Carrasquilio [START_REF] Carrasquilio | Microcracking behavior of high strength Concrete Subject to Short Term Loading[END_REF] and Nemati [START_REF] Nemati | Analysis of Compressive Stress-Induced Cracks in Concrete[END_REF]). This can be attributed to the fact that HPC is more brittle than OC. As a result, when fracture occurs in HPC, the crack propagation is associated with less combination of bond and mortar cracks.

Increase permeability versus strain

The results put in evidence that the intrinsic permeability coefficient k V of OC and HPC increased with the increase of the residual strain, and HPC k V values were lower than the OC values (Figure 7). This might be explained by the progressive change in the internal structure of the specimen due to the increasing of load level. Concrete is a capillary porous material and its permeability is closely related to its microstructure. The HPC porosity is lower than OC porosity (see Table 2) and thus the HPC gas permeability is lower than OC permeability, the average k V values for the virgin specimen, were 2,57 10 -17 m 2 /s for OC and 1,11 10 -17 m 2 /s for HPC. Once there are micro cracks in the concrete caused by uniaxial damage, there would also be favourable passages for gas flow, This can be explained by the gas flow through the pores and additionally through cracks in the case of cracked concrete. Therefore, higher residual strain would result in higher gas permeability as obtained in figure 7.

Relationship between increase in gas permeability and damage value

The damage coefficient, d, which is a measure of the stiffness degradation, can be calculated using (Eq. 1). For these load levels (60%f c to 90%f c ), damage coefficients approximately from 0.015 to 0.16 were obtained (see Figure 8). The damage coefficient of OC and HPC increased with the increase of the load levels; this is due to the applied load which produces the degradation in the materials shown by a continuous reduction in the elastic modulus and appearance of the residual strain. The damage coefficient evaluated by static or dynamic method of OC is higher than HPC for the same load level. This result confirms that the microcracking patterns of OC is more significant than HPC. There is a relationship between the damage variable measured by the static method and that measured by the dynamic method with a correlation coefficient R 2 =0.95 (Figure 8). This linear relationship shows that the damage obtained by the dynamic method is slightly higher than that obtained by the static method. This linear trend which is non material dependent should be confirmed with others cementious materials before being used to get damage rate with this very useful non destructive technique.

The relative permeability of cracked concrete k v (d)/k v0 is defined as the ratio between the permeability coefficient of cracked concrete k v (d) and the permeability coefficient of uncracked concrete k v0 . It can be related to the damage variable "d". The increase in permeability relative permeability k v (d)/k v0 with the damage coefficient evaluated by dynamic method is illustrated in Figure 9. k v (d)/k v0 tended to increase slightly beyond the damage coefficient of 0,03 for both concretes. This range of damage coefficient corresponds to the observed load level which was found 70%f c for OC and HPC. An exponential curve for the relative increase of gas permeability was obtained as a function of damage coefficient. A similar relation was obtained by Picandet et al. [START_REF] Picandet | Effect of axial compressive damage on gas permeability of ordinary and high-performance concrete[END_REF] using cyclic compression test. This relation was obtained on the range of d < 0.15. However, with the increase of damage, d > 0.15, different relations should be adopted for homogenous crack pattern and fracturedominating pattern otherwise the impact of damage extent on permeability increase can be overestimated [START_REF] Zhou | Characterizing the effect of compressive damage on transport properties of cracked concretes[END_REF].

Effect of uniaxial damage on chloride diffusion

After the measurements of the gas permeability on virgin and cracked specimen, these specimens were then saturated in basic solution in order to measure the chloride diffusion.

The accumulation of chloride ions in the downstream compartment is shown as a function of time for OC and HPC in Figure 10. The amount of chloride ions that had flowed through the thickness of the concrete specimen was recorded and plotted versus time and for different load level. Two distinct phases with respect to the change of chloride concentration are acknowledged. The first is a phase where chloride ions are in the process of migrating through saturated pores in concrete and hence have not yet reached the anode compartment. This phase is normally called a transition period and the migration of chloride ions is under non steady-state conditions. It is apparent that transition period was dependent on load level and on the concrete mixes. The second phase is called the steady-state period where the increase of chloride concentration becomes constant with elapsed time, providing a constant flux of chloride ions, the flux J is calculated with the constant slope of a regression line. These slopes were used to calculate the diffusion coefficient of chlorides ions using Eq. ( 5). The time to penetrate through the specimen is called lag time (T lag ), which is the intersection point of the line with the X-axis [START_REF] Djerbi | Influence of traversing crack on chloride diffusion into concrete[END_REF]. The results showed that for virgin concrete the chloride diffusion coefficient increased with the increase of the porosity, the average diffusion coefficients for the virgin specimen, were 1,88 10 -12 m 2 /s for OC and 0,67 10 -12 m 2 /s for HPC. For the applied load of 90% f c for 2 hours, the diffusion coefficient were 5,74 10 -12 m 2 /s for OC and 1,28 10 -12 m 2 /s for HPC. Figure 11 presents the variation of time lag versus the diffusion coefficient.

The results show that for virgin concrete T lag of HPC are higher than for OC. A higher T lag may result from a lower porosity. This is an effect of the material. T lag depends on the porosity of concrete and chloride binding [START_REF] Coussy | Non-linear binding and the diffusion-migration test[END_REF]. However, for cracked specimen it appears that T lag decreased with the diffusion coefficient D e increasing. A linear correlation was obtained between the diffusion coefficient D e and the time lag T lag . It can be seen that the slope for HPC is greater than for OC. This was due probably to chloride ion penetration from microcracks to the virgin concrete, which was considered to occur more easily in OC concrete compared with HPC as OC porosity is higher than HPC porosity. This suggests that the effect of cracking on the time it takes chloride to penetrate the sample is greater for OC than for HPC.

Increase chloride diffusion coefficient versus strain

The chloride diffusion coefficients evolutions of OC and HPC versus a residual strain are reported in Fig. 12. It seems that the diffusion coefficient less sensitive than gas permeability to the residual deformation obtained. The variation of chloride diffusion coefficients begins for residual deformations in excess of 100 µ/m corresponding to loads greater than 70%f c for OC and 80% f c for HPC.

Relationship between increase chloride diffusion coefficient and damage value

The relative diffusivity of cracked concrete D e (d)/D e0 is defined as the ratio between the diffusion coefficient of cracked concrete D e (d) and the diffusion coefficient of uncracked concrete D e0 . It can be related to the damage variable "d". Figure 13 presents the evolution of relative diffusivity of cracked concrete with damage value assessed by the dynamic method.

This evolution which is non material dependant is consistent with an increasing more marked

for OC which has a diffusion coefficient of uncracked concrete larger than the HPC. The relative diffusivity of OC for the high load level is about 2.5 while it's about 1.9 for HPC.

From Figure 13 the relationship between these two parameters can be expressed by a linear variation with a correlation coefficient R 2 =0.94 and also can be written as the exponential function with a correlation coefficient R 2 =0.98. This exponential function is similar to that obtained for the gas permeability with different empirical coefficients. This allows to develop an empirical relationship between these two transfer parameters, even if they are not corresponding to the same transport mechanisms.

Relationship between increase the gas permeability and chloride diffusion coefficient

Gas permeability and diffusivity are governed by two transport modes. The variation of these two parameters depends on the microstructure of concrete and cracking that can be characterized by the damage. According to the equations presented in Figure 9 and Figure 13, we find that the gas permeability and diffusion coefficient show a same trend based on the damage variable. This allows us to establish a relationship between these two transfer parameters. Gas permeability is measured on a dry sample as the diffusion coefficient is measured on the same sample in saturated state by migration test under steady state condition.

The results show that there is a linear relationship between the evolution of these two parameters depending on the damage variable, specific for each concrete type (Figure 14), with a correlation coefficient R 2 = 0.98 for OC and R 2 = 0.99 for HPC. increases by a factor of 7 while D e (d)/D e0 increases by a factor of 1.9. This tendency was already observed in previous studies, the gas permeability changes very significantly with the increase of damage level [START_REF] Tognazzi | Couplage fissurationdégradation chimique des matériaux cimentaires: premiers résultats sur les propriétés de transfert[END_REF][START_REF] Kermani | Permeability of stressed concrete[END_REF][START_REF] Sugiyama | Effect of stress on Gas permeability in concrete[END_REF][START_REF] Picandet | Effect of axial compressive damage on gas permeability of ordinary and high-performance concrete[END_REF][START_REF] Choinska | Effects and interactions of temperature and stress-level related damage on permeability of concrete[END_REF], while chloride penetration is much less affected by cracks [START_REF] Sugiyama | Permeability of stressed concrete[END_REF][START_REF] Hr | Influence of microcracking on the mass transport properties of concrete[END_REF][START_REF] Saito | Chloride permeability of concrete under static and repeated loading[END_REF][START_REF] Cc | Microcracking and chloride permeability of concrete under uniaxial compression[END_REF].

It is noted that the relative permeability increases linearly with the relative diffusion coefficient, with a correlation coefficient R 2 = 0.98 for OC and R 2 = 0.97 for HPC. This relation could be assimilated to a no material dependant linear relationship. This result has to be verified with other materials tested with different curing history and test conditions. This linear relationship could be useful to avoid experimental test in the case of safety assessment of existing marine structures which are not so aged. Since the building of Vasco Gamma Bridge in Lisbon (1995)(1996)(1997)(1998), the durability criteria are requested for structures design in Europe [START_REF] Baroghel-Bouny | Concrete design for structures with predefined service lifedurability control with respect to reinforcement corrosion and alkali-silica reaction state-of-the-art and guide for the implementation of a performance-type and predictive approach based upon durability indicators[END_REF]: gas permeability and chloride diffusion coefficients are evaluated for non damaged material before being used in the structure. For safety assessment of these structures, only damaged gas permeability has to be measured to evaluate the relative diffusion coefficient of cracked concrete and so the damaged diffusion coefficient. The gas permeability measurement is not time consuming compared to the chloride migration test.

Moreover this test allows keeping a non contaminated concrete specimen which could be used for others concrete characterisation: the coring is so limited. But this linear relationship between relative gas permeability and relative diffusion coefficient has to be established with other concrete mixing and also in the case of more advanced damage rate before being used for safety assessment. Unfortunately this study covers a very limited damage rate: maximal d value is about 0.15.

The increase obtained for these two transfer parameters is more important for OC than for HPC, this result confirms that micro cracking obtained with diffuse damage at same load level impact on the transfer parameters for concrete which have higher porosities or higher crack density as OC [START_REF] Zhou | Geometry of crack network and its impact on transport properties of concrete[END_REF].

Conclusions

From the results presented in this paper it can be concluded that:

 The damage of concrete can be obtained using a uniaxial compression test. For loads above 60%f c , the maximal strain ε max and residual strain ε rés increase with increasing load and time load for both concretes OC and HPC. This variation can be interpreted by the existence of microcracks and their evolution according to the applied load.

 There is a linear relationship between the damage variable calculated with the modulus obtained by the static method with the strength-strain curve and that calculated with the modulus measured by the dynamic method. This method is non destructive and very useful by comparison with the static method but the modulus obtained are less representative of the material. This linear relationship should be verify with other concrete but could be very useful to get the static damage variable with the grindosonic apparatus.

 The intrinsic permeability coefficient k V of OC and HPC increased with the increase of the residual strain, OC shows higher values. This can be explained by the gas flow through the pores and additionally through cracks in the case of cracked concrete with diffuse damage at the same load level.

 Results show that there is an exponential variation between the increase permeability k v (d)/k v0 and damage variable "d", for both types of concretes OC and HPC. There is a also a single relationship between the ratio D e (d)/D e0 and the damage variable "d".

 The diffuse damage obtained by uniaxial compression test in pre-peak affects the diffusion of chloride ions. This damage causes an increase in the diffusion coefficient D e and a decrease in the time lag T lag . There is a linear correlation between these two parameters; this relationship is specific for each of concrete type.

 The time lag reduction T lag between virgin and most cracked concrete is more marked for HPC which has a T lag of virgin concrete higher than the OC. This was due probably to chloride ion penetration from microcracks to the virgin concrete, which was considered to occur more easily in OC concrete compared with HPC as OC porosity is higher than HPC porosity. This essentially means that the effect of cracking on T lag is more important for dense materials with a higher T lag for the uncracked material.

 A linear correlation is obtained between intrinsic permeability coefficient k V and diffusion coefficient D e depending on the damage variable, specific for each concrete type (OC and HPC).

Figure 15 shows

 15 Figure 15 shows the variation of the k v (d)/k v0 according to the ratio D e (d)/D e0 for OC and

 Gas permeability is more sensitive of concrete damage than chloride diffusion: in the case of OC k v (d)/k v0 increases by a factor of 10 while D e (d)/D e0 increases by a factor of 2.5.

For high-performance concrete k v (d)/k v0 increases by a factor of 7 while D e (d)/D e0 increases by a factor of 1.9.