consolidating concrete exposed to elevated

Mechanical and physico-chemical characteristics of Selfconsolidating concrete exposed to elevated temperatures H. Fares & S. Rémond & A. Noumowé & A. Cousture L2MGC – EA 4114 – Université de Cergy-Pontoise F-95000 Cergy-Pontoise

ABSTRACT: The aim of this research work is to study experimentally the mechanical and physico-chemical characteristics of self-consolidating concrete (SCC) subjected to high temperature. Four different concrete mixes [three SCC mixes and one vibrated concrete mix (VC)] of different strength categories were studied. At the age of 90 days, specimens were placed in an electrical furnace and heated at a rate of 1°C/min until the target temperature was reached. A maximum temperature of 150, 300, 450, or 600°C was maintained for 1 h. The results showed in particular that the residual compressive strength of SCC increases between 150 and 300°C. SEM observations and image analysis showed that the heating of SCC between 150 and 300°C leads to a decrease in the proportion of anhydrous cement particles. The hydration of anhydrous cement due to water transport into the porosity is therefore certainly responsible of the observed strength increase between 150 and 300°C. 1 INTRODUCTION The use of Self-consolidating concrete (SCC) has considerably developed during the last years and a growing attention is brought to the study of its mechanical properties at hardened state. The mixture proportions of SCC (large paste volume, high content of mineral admixtures, coarse to fine aggregates ratio close to 1 …) in relation with its placing conditions could modify its mechanical behaviour, comparatively to traditional vibrated concrete. The behaviour of SCC subjected to high temperature has in particular to be evaluated. In case of fire, concrete is exposed to high temperature that induces a material degradation: strength decrease, cracking, and in some conditions spalling. Up to now, the effect of elevated temperature has been studied mainly on vibrated ordinary and high performance concretes. The few studies on SCC subjected to high temperature show either a decrease of strength and an increase of the risk of spalling (Vanwalleghem et al. 2003, Annerel et al. 2007, Persson 2004) or a behaviour similar to that of vibrated concrete (CERIB 2001, Bostrom 2003, Etude Feu-Béton 2006). The aim of this study is to analyze the behaviour of SCC subjected to high temperature. Four mixes were investigated: three SCC and one Vibrated Concrete (VC). In this work, we try to explain, through the microstructure and the physico-chemical properties of these concretes, the observed mechanical behaviours.

2 MATERIALS USED AND CONCRETE MIXES

Table1. Concrete mixes

1 m3

SCC 25 SCC 40-1 SCC 40-2 VC 40

CEM I 52.5 N

-

350

420

373

CEM II 32.5 R

328

-

-

-

Limestone Filler

225

130

80

-

Sand 0/4

795

857

818.3

913

Aggregates 4/22.5

745

742

716.5

790

Water

199

200

215

202

Slump (cm)

65

71

70

19

Water/Cement

0.61

0.57

0.51

0.54

Water/Powder

0.36

0.42

0.43

0.54

The four studied mixes are presented in Table 1. Three concrete mixes investigated in our research were developed in the French National Project B@P (PNBAP 2006) (SCC 25, SCC 40 – 1 and VC 40). The characteristics of materials are described in (Fares et al. in press). The concrete mixes contain the same constituents (except the cement that was adapted to the target strength), a similar granular skeleton and a Water/powder ratio equal to 0.36, 0.42, 0.43 and 0.54. The admixture content was determined so as to obtain a slump flow larger than 65 cm for SCC. The SCC 40-2 derives from SCC 40-1. It differs by the paste volume. With these concretes, cylindrical specimens 16x32 cm and prismatic specimens 10x10x40 cm were prepared. After molding, the specimens were covered by a plastic sheet and cured at ambient temperature during 7 days, before being removed from the mould and sealed in watertight bags for storage at ambient temperature. The mechanical tests were carried out, according to the RILEM Recommendations (Rilem 1995) after curing for more than 90 days. 3 TESTS 3.1 Test temperature The specimens were subjected to four different temperature cycles (150, 300, 450 and 600°C) at a heating rate of 1°C/min to the target temperature. Then, the temperature was hold constant during one hour. The heating was then stopped in order to cool the specimens to ambient temperature. The rate of heating refered to the recommendations of the RILEM Technical Committee TC-129 (Rilem 1995).

T (°C)

Heating

600 150 °C 300 °C 450 °C 600 °C Real heating - 300°C

500 400 300 200 100 0 0

Time (h)

5

10

15

20

25

Figure 1. Heating and cooling curves

Figure 1 presents the theoretical evolutions of temperature as a function of time for the four temperature cycles. The real evolution of temperature obtained with a heating up to 300°C is also indicated. The real temperature was very close to the theoretical evolution during heating, whereas the cooling was slower which reduced the risk of thermal shock. 3.2 Mechanical and physico-chemical tests During this study, different tests were performed in order to characterize the behaviour of SCC subjected to high temperature. Residual compressive strength and residual modulus of elasticity have been determined on cylindrical specimens Ø 16x32 cm and residual flexural strength have been measured on prismatic specimens 10x10x40 cm. Water porosity, gas permeability and mass loss due to heating on unheated and heated specimens have also been studied (AFPCAFREM 1997).

Figure 2. Layout of the specimens in the oven

For each mix and each temperature cycle, eleven specimens were stored in the oven. Each batch was composed of four cylindrical specimens Ø 16 x 32 cm, four cylindrical specimens Ø 15 x 5

cm (which were cut in the cylinders Ø 15 x 30 cm) for the permeability tests and three prismatic specimens 10x10x40 cm. All the specimens were stored in the oven in the same conditions in order to have a uniform temperature throughout the specimens. Figure 2 shows the layout of the oven with the specimens. After each thermal test, concrete samples were taken and put in plastic bags in order to carry out physico-chemical characterizations and microscopic observations. Abrasive wet cutting was used in order to prevent damaging the samples. Samples of some centimeters have been cut out of concrete without coarse aggregates. After wet-cutting, the samples used for the SEM observations were dried at 50°C, and coated in cold epoxy resin. When the resin was hardened, the sample could be polished according to the following procedure. First, the sample was ground with abrasive paper SiC. Then, the samples were polished with diamond pastes of decreasing diameter (6, 3, and 1 µm) during 2 minutes for each paste. In order to remove any residual polishing compound, a cleaning with ethanol was carried out (Bentz et al. 1994). The samples were coated with nickel to provide a conductive surface for SEM observations. A Scanning Electron Microscopy LEICA S430i was used. The observations have been carried out with backscattered electrons signal (BEI), which allows an important chemical contrast. This contrast depends on the average atomic number of the observed phases, the clearest phases corresponding to the largest average atomic number (Stutzman 2004). Anhydrous cement appears brightest followed by calcium hydroxide, calcium silicate hydrate, and aggregates. Voids and pores appear dark (Stutzman 2004). In this paper, we present only a part of our study concerning the residual compressive strength and the microstructure. 4 RESULTS AND DISCUSSIONS 4.1 Compressive strength

1.4

Rcθ/Rc20

1.2 1.0 0.8 0.6 SCC 40 - 1 VC 40 SCC 40 - 2 SCC 25 Eurocode 4-1.2 Annex C

0.4 0.2 0.0 0

150

300 Temperature (°C)

Figure 3. Relative residual compressive strength vs. temperature

450

600

Figure 3 presents the variation of the residual compressive strength versus the temperature of heating. Our results are compared with the values proposed by the Eurocode 4.1.2 (Relative residual compressive strength) (Eurocode 2005). The behavior of SCC and vibrated concrete differed significantly. On one hand, for the vibrated concrete (VC 40), the relative compressive strength decreased monotonly. On the other hand, for the three SCC, after a moderate decrease of strength between 20 and 150°C, an important increase in relative compressive strength (of about 25%) was observed between 150 and 300°C. Beyond 300°C, an important decrease in strength was observed for all the mixes. A similar behaviour has already been observed in the literature (Khoury 1992, Kanema 2007). According to Khoury (Khoury 1992), the decrease in strength between 20 and 150°C corresponds to a reduction of the cohesive forces (Van der Waal) between the C-S-H layers. This reduces the surface energy of C-S-H and leads to the formation of silanols groups (SiOH:OH-Si) that presents weaker bonding strength. Dias (Dias et al. 1990) attributed the increase in strength between 150 and 300°C to a rehydration of the paste due to the migration of de-bound water in the pores. Khoury (Khoury 1992) explained the increase in strength by the

loss of a part of the bonds with water in silanols groups, which induces the creation of shorter and stronger siloxane elements (Si-O-Si) with probably larger surface energies. The previous hypotheses do not allow explaining the difference between SCC and the vibrated concrete. Nevertheless, this difference of behavior is not observed systematically in the literature (Etude Feu-béton 2006). Moreover, the increase in compressive strength between 150 and 300°C has already been observed on vibrated concretes (Kanema 2007, Phan et al. 2001). In our case, we observe a very moderate decrease (or no variation) in strength for vibrated concrete. 4.2 Image analysis

SEM

Porosity

Anhydrous phases

20°C

300°C

Figure 4. SEM images, porosity images and anhydrous phases images for temperatures 20 and 300°C (SCC 40-1)

Figure 4 presents the images obtained with SEM on SCC40-1 and the images resulting from image analysis showing porosity and anhydrous phases at 20 and 300°C. An important increase in porosity with the increase of temperature was observed (increase of 65% between 20 and 600°C in average). The increase is not only due to the cracks of samples but also to the deterioration of the cement paste with the departure of bound water.

29

Porosity (%)

27

SCC40-1 - IA

SCC40-2 - IA

SCC25 - IA

VC40 - IA

25

SCC40-1 -Water

SCC40-2 - Water

23

SCC25 - Water

VC40 - Water

21 19 17 15 13 11 9 0

T(°C)

150

300

450

600

Figure 5. Evolution of water porosity (Fares et al., in press) and porosity obtained by image analysis (IA).

Figure 5 presents a comparison between water porosity obtained experimentally and porosity measured by image analysis as a function of temperature. The evolutions obtained by the two experimental methods were similar for each concrete mix. The porosity increased monotonly with the temperature (Persson 2004, Noumowé 1995). The porosity obtained by image analysis has the same order of magnitude, but was generally lower than the water porosity. The difference was certainly linked to a better accessibility of the pores of small size by water in comparison to the resin. Moreover, the SEM observations didn’t allow obtaining a sufficiently fine resolution to quantify the finest porosity. Clearest phases (%)

12 11 10 9 8 7 6 5

SCC 40 - 1 SCC 40 - 2 SCC 25 VC 40

4 3 2 0

T(°C)

150

300

450

600

Figure 6. Evolution of the surface fraction of the clearest phases on the BE images as a function of the temperature

Figure 6 presents the variations of the surface fraction of the clearest phases observed for each concrete as a function of the temperature. The surface fractions of the clearest phases for the three SCC decreased between 20 and 300°C then increased monotonly above 300°C. This fraction increased monotonly with the temperature between 20 and 600°C for the vibrated concrete. Between 20 and 300°C, the clearest phases on BEI correspond essentially to the anhydrous phases of the cement (Fig. 4). Up to 300°C, there is a supplementary hydration of anhydrous cement, due to the important water movements that are produced during the heating between 100 and 300°C. The water movements were due, on the one hand, to the departure of free water (up to about 150°C) (Platret 2002) and on the other hand to the departure of bound water coming from the dehydration of C-S-H. This later created an increase in the porosity of C-S-H, as seen in SEM observations and allowed probably water to attain the anhydrous grains of cement. At 300°C, the anhydrous grains of cement have been rehydrated. Above 300°C, we observed an increase in the surface fraction of the clearest phases. We supposed that these phases didn’t correspond anymore to anhydrous phases, but doubtless to dense phases coming from the decomposition of hydrates. The decrease in the surface fraction of anhydrous phases allowed explaining the results concerning the compressive strength behaviour of concretes exposed to high temperature. During these tests, two different behaviours have been observed: the residual compressive strength of VC decreased monotonically whereas that of SCC presents a significant increase between 150 and 300°C. We explain the increase in compressive strength between 150 and 300°C by a rehydration of anhydrous cement due to water. On the contrary, the VC40 which did not present any strength increase presents also no further hydration. Nevertheless, we observed both increase in strength and increase in porosity for the SCC. So the increase in strength could be explained by the change of microstructure: on one hand, rehydration of anhydrous cement and on the other hand, the creation of shorter and stronger siloxane elements (Si-O-Si) with probably larger surface energies by the loss of a part of the bonds with water in silanols groups. 5 CONCLUSIONS This study deals with the behaviour of SCC exposed to high temperature. The residual mechanical and physical properties of three SCC and one vibrated concrete were determined after different heatings up to 150, 300, 450 and 600°C. Between 20 and 150°C, the compressive strength decreased for all the mixes (SCC and VC) with a small increase in porosity. Between 150 and 300°C, we observed for the SCC, an increase in compressive strength explained by a rehydration of anhydrous cement due to water migration in the pores. Indeed, the experimental results showed that the anhydrous phases decreased between 20 and 300°C. At the same time, the porosity increased with the temperature. Beyond 300°C, all the mechanical properties decreased quickly, and the specimens were very damaged. The main finding of this study is that SEM and BEI results permitted to explain the increase in compressive strength observed at the temperature between 150 and 300°C. REFERENCES AFPC-AFREM sur la durabilité des bétons. 1997. Méthodes recommandées pour la mesure des grandeurs associées à la durabilité des bétons. INSA-LMDC, Toulouse 11-12 déc Annerel E. & Taerwe L. & Vandevelde P. 2007. Assessment of temperature increase and residual strength of SCC after fire exposure. 5th International RILEM Symposium on SCC: 715-720 Bentz D.P. & Stutzman P.E. 1994. SEM Analysis and Computer Modeling of Hydration of Portland Cement Particles. Petrography of Cementitious Materials. ASTM STP 1215. In Sharon M. DeHayes and David Stark Eds. American Society for Testing and Materials. Philadelphia: 60-73. Boström L. 2003. Self-compacting concrete exposed to fire. International RILEM Symposium on SelfCompacting Concrete. Proceedings PRO 33: 863-869. CERIB. 2001. Caractérisation du comportement au feu des Bétons Autoplaçants. Report DT/DCO/2001/29.

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