INFLUENCE OF IMPOSED COMPRESSIVE STRESS AND SUBSEQUENT SELF-HEALING ON CAPILLARY ABSORPTION AND CHLORIDE PENETRATION INTO UHPFRCC

Transcription

1 INFLUENCE OF IMPOSED COMPRESSIVE STRESS AND SUBSEQUENT SELF-HEALING ON CAPILLARY ABSORPTION AND CHLORIDE PENETRATION INTO UHPFRCC P. Wang (1, 2, 3), X. Yao (4), F.H. Wittmann (1, 5), P. Zhang (1) and T. Zhao (1) (1) Qingdao Technological University, Qingdao, China (2) Jiangsu Research Institute of Building Science, Nanjing, China (3) State Key Laboratory of High Performance Civil Engineering Materials, Nanjing, China (4) Qingdao Building Materials Research Institute, Qingdao, China (5) Aedificat Institute Freiburg, Freiburg, Germany Abstract Ultra-high performance fibre reinforced cementitious composites may be considered to be durable materials as porosity is comparatively low and strength is high. Service life of reinforced concrete structures in contact with seawater or with water containing dissolved aggressive ions such as chloride and sulphate depends essentially on transport properties. Therefore capillary absorption may be considered as an indication for durability and service life of concrete structures. In most cases capillary absorption is measured on small samples prepared in laboratories. Reinforced concrete structures and structural elements in practice, however, are in most cases load bearing. For this reason the influence of an applied load on capillary absorption and on chloride penetration has to be taken into consideration in realistic service life prediction. The influence of an applied compressive load on capillary absorption and on chloride penetration was studied experimentally. It was found that with increasing applied compressive load the rate of capillary absorption increases and the amount of penetrating chloride and the penetration depth of chloride increase significantly as well. Loaded specimens were placed in saturated Ca(OH) 2 solution for 28 days. Self-healing could be observed. Capillary absorption and the rate of chloride penetration are reduced. It is suggested that these obvious effects should be considered in service life prediction to make it more realistic in future. 1. INTRODUCTION For a long time properties of concrete were systematically modified and optimised by changing the size distribution of the aggregates and by changing the water-cement ratio. Within limits properties could be adjusted to specific requirements of designed structures. A 243

2 big step towards higher strength was possible with the development of water reducing agents. In the early nineties of the last century Richard and Cheyrezy [1] developed a novel cementbased material. They realized that due to mechanical incompatibility of the two major components in the composite structure of concrete, i.e. coarse aggregates and hardened cement paste, internal stress distributions limited the theoretically achievable strength. As a consequence they avoided coarse aggregates and choose a size distribution of the fines similar to the size distributions, which were suggested and applied for coarse aggregates in normal concrete. In order to achieve the densest possible structure of the new material fly ash and silica fume were added to fine cement. At the same time these additives facilitate pozzolanic reaction during hardening. In this way cement-based materials can be obtained with very low porosity and compressive strength well above 2 MPa [1, 2]. The high density of the new material which was called reactive powder concrete (RPC) and the homogeneity of the structure were at the origin of high strength. Nowadays RPC is a special material in the group of high and ultrahigh performance fibre reinforced cement-based composites (HPFRCC and UHPFRCC). In both designations the term performance is used instead of strength. This creates the impression that high strength automatically means also high durability. Both high strength and high durability can be achieved by low porosity. Porosity and pore size distribution, however, are influenced by an applied mechanical load. Under the influence of an applied high stress micro-cracks can be formed, which may serve as new pathways for penetration of water and aggressive compounds dissolved in water. In this way originally high durability may be reduced. In this contribution results of different test series will be presented. First capillary absorption of and chloride penetration into undamaged UHFRCC were studied. Then three levels of compressive stress up to 8 % of the compressive strength were applied and capillary absorption and chloride penetration was measured again on identical but pre-loaded samples. Finally samples, which were pre-damaged by compressive load, were placed in saturated Ca(OH) 2 solution to allow self-healing of the load induced micro-cracks. Results obtained will be presented and discussed in what follows. 2. ON CAPILLARY ABSORPTION If a capillary with radius r or a porous material with a given pore size distribution, which may be characterized by an effective radius r eff, gets in contact with a wetting liquid the absorbed mass of liquid as function of time ΔW(t) is given by the following simple equation: W A t (1) In this equation A stands for the coefficient of capillary absorption, which characterizes the porous material and determines the penetrating liquid driven by capillary under-pressure, and t stands for the time of contact of the surface of the porous material with the liquid. A in Eq. (1) is given by the following equation: A r eff cos 2 In equation (2) ψ stands for the water capacity of the porous material, ρ for the density of the penetrating liquid, σ for the surface tension of the liquid, θ for the contact angle at the interface between the liquid and the inner surface of the porous material and η for the (2) 244

3 viscosity of the liquid, while r eff is an effective radius, which represents the wide pore size distribution of the material [3]. If the liquid is capillary absorbed along a horizontal line, gravity can be neglected. Under these conditions the coefficient of capillary absorption A is given directly by Eq. (2). The coefficient of capillary absorption remains constant during the entire sorption process [4, 5]. If, however, the liquid is absorbed along a vertical line, the coefficient of capillary absorption decreases due to the influence of gravity imposed by the mass of the penetrating liquid [5, 6]. In a purely heuristic way the influence of gravity on capillary absorption can be taken into consideration by the following exponential expression [7, 8]: W a[1 exp( b t)] (3) The parameters a and b in Eq. (3) can be determined by fitting the equation with reliable test results. Then the time-dependent coefficient of capillary absorption A(t) can be written as follows: d W At ( ) abexp( b t) (4) dt With Eq. (4) the initial coefficient of capillary absorption at the beginning of the sorption process A i is obtained for t = : A A(t ) ab (4) i If the two parameters a and b are determined by fitting Eq. (3) with suitable experimental data the initial coefficient of capillary absorption can be easily obtained with Eq. (5). This value of A i characterizes capillary absorption of ordinary concrete at least for the first hour after contact with the liquid. 3. PREPARATION OF SPECIMENS For the tests described in this contribution, the composition of the UHPFRCC was first optimized by varying the composition in a systematic way. For the different mixes ordinary Portland cement Type 42.5, fly ash class F with an average diameter of the particles of 5.4 µm and silica fume with and average particle size of.3 µm were used. In addition fine quartz sand with a maximum grain size of.42 mm and steel fibres were added. The final composition of the optimum mix is given in Table 1. Table 1: Optimized composition of the UHPFRCC; mass is given as kg/m 3 Cement Fly ash Silica fume Quartz powder Quartz sand Water SP Steel fibres The fresh mix of UHPFRCC was compacted in steel forms on a shaking table and covered with plastic sheets. After curing for 24 hours in the laboratory at approximately 2 C the prisms with the following dimensions, mm, were further cured in a water bath at 9 C for 72 hours. After cooling down the specimens to room temperature the 245

4 compressive strength was determined in a servo-controlled testing machine. The average compressive strength was found to be MPa. All prisms were loaded at the age of 4 days with a sustained compressive stress for 15 minutes. 3 %, 5 % and 8 % of the compressive strength were chosen for the temporary loading. This corresponds to 38.6, 64.3, and 12.8 MPa respectively. In this way three different degrees of damage were induced to the porous structure of the material. Finally a cube with the dimensions of mm was cut with a diamond saw out of the centre part of each prism. It may be assumed that these parts of the prisms were exposed to a homogeneous compressive stress field. After loading the cubes were further cut parallel to the applied compressive stress into four identical prisms with approximately the following dimensions: mm. These small prisms were finally dried in a ventilated oven at 5 C until equilibrium. The square end faces (5 x 5 mm) and two long faces (5 1 mm) were sealed with self-adhesive aluminium foil. The open molded surfaces of half of the specimens were finally put in contact with water for 24 hours to determine the influence of an applied compressive stress on capillary absorption. The second half were put in contact with 5 % NaCl solution for 28 days. After exposure to chloride solution for 28 days, chloride profiles were determined in these prisms. Layers with a thickness of approximately 1 mm were milled successively from the surface, which was in contact with the salt solution. The chloride content of the powder obtained was determined by chemical analysis. In order to study the potential of self-healing of the damaged material identical specimens were placed in a saturated Ca(OH) 2 solution for 28 days immediately after application of the compressive stress. After self-healing both capillary absorption and chloride penetration were determined experimentally as described above. Determination of capillary absorption and chloride penetration serves in this context as an indication of damage induced by an applied compressive stress and partial recovery of damage by self-healing. 4. RESULTS AND DISCUSSION 4.1 Influence of applied compressive stress on capillary absorption and reduction of induced damage by self-healing Capillary absorption as determined on UHPFRCC before loading and after loading with a compressive stress corresponding to 3, 5, and 8 % of the compressive strength is shown in Fig. 1 as function of the square root of time. As expected the rate of capillary absorption is comparatively high at the beginning but it decreases with time. In an ideal situation, when gravity can be neglected, a linear relationship would be observed. But most important, application of a modest compressive stress corresponding to 3 % of the compressive strength only leads to significant increase of capillary absorption. Both the initial value of the coefficient of capillary absorption A i and the amount of absorbed liquid after long time increase with increasing applied compressive stress. The micro-structure of the material is obviously damaged by the applied load. Due to formation of micro-cracks the porosity of UHPFRCC increases and newly formed pathways facilitate penetration of the liquid. 246

5 Absorbed water, g/m % 5% 8% Absorbed water, g/m %-28d 5%-28d 8%-28d Duration of capillary absorption, h 1/2 Duration of capillary absorption, h 1/2 Figure 1: Capillary absorption as measured on unloaded reference samples and on samples after preloading with 3, 5, and 8% of the compressive strength Figure 2: Effect of reduction of imposed damage by self-healing in saturated Ca(OH) 2 solution for 28 days on capillary absorption Most cement based materials have the potential of self-healing. Samples which were predamaged by application of a compressive stress for 15 minutes were placed in a saturated Ca(OH) 2 solution for 28 days. Then capillary absorption has been determined. Results are shown in Fig. 2. It is obvious that capillary absorption is reduced considerably by selfhealing. This observation can be explained by the high cement content and the presence of components which will react by retarded pozzolanic reaction. Damage induced by compressive load can at least partly be reduced by simple self-healing in water. By now there are more efficient technologies for self-healing such as encapsulation of a healing agent in hollow glass tubes or application of bacteria [1-12]. Capillary absorption can be considered to be a sensitive tool to determine internal damage and self-healing of porous materials. The time dependent coefficient of capillary absorption A(t) as obtained from the data represented in Figures 1 and 2 is plotted in Fig. 3. It is obvious that after one hour of contact of the sample s surface with water already the coefficient of capillary absorption is significantly reduced and finally it approaches zero asymptotically. After contact of the surface with water for 24 hours the rate of water absorption becomes very small. The higher the applied compressive load, the higher will be the coefficient of capillary absorption. In this way the damage induced by compressive load can be quantified. From the data shown in Fig. 1 the parameters a and b in Eq. (3) can be determined by data fitting. Applying Eq. (4) the initial coefficient of capillary absorption can be obtained. The corresponding values of A i are shown in Fig. 4. As can be seen from Fig. 4 A i is more than doubled by application of a compressive stress corresponding to 8 % of the compressive strength of the material. As shown in Fig. 4, the induced damage is reduced again, however, by self-healing during curing in Ca(OH) 2 solution. 247

6 1 1 A i, g/(m 2.h 1/2 ) % 3% 5% 8% 3%-28d 5%-28d 8%-28d Pre-loaded 2 2 Self-healing Linear fit Linear fit Duration of capillary absorption, h 1/2 Stress level, % A i, g/(m 2.h 1/2 ) R 2 =.9854 R 2 =.972 Figure 3: Influence of applied compressive load and self-healing on time-dependent coefficient of capillary absorption (Eq. 4) Figure 4: Influence of applied compressive load and self-healing on the initial coefficient A i of capillary absorption 4.2 Influence of applied compressive stress on chloride penetration and reduction of induced damage by self-healing Chloride profiles were determined on UHPFRCC specimens after one surface (5 1 mm) was in contact with 5 % NaCl solution for 28 days. The obtained chloride profiles are shown in Fig. 5. The undamaged dense material takes up very little chloride in comparison with ordinary concrete [13]. Application of a modest compressive stress of 3 % of the compressive load for 15 minutes, however, causes serious damage in the micro-structure of the high strength material and considerably more chloride is penetrating and at the same time the penetration depth increases. In normal concrete crack growth is hindered by coarse aggregates. This crack arresting mechanism does obviously not exist in UHPFRCC. As shown in Fig. 6 the chloride penetration is noticeably reduced after self-healing in aqueous Ca(OH) 2 solution for 28 days. From Figures 5 and 6 it follows that not only capillary absorption of UHPFRCC is accelerated by an applied compressive stress but also the diffusion coefficient of chloride ions into the water filled pore space increases considerably. These facts have to be taken into consideration in realistic service life prediction. In normal concrete a modest compressive stress of up to 4 % of the strength reduces the diffusion coefficient [15]. This characteristic difference of stress sensitivity of the durability of the material can be explained by the different composite structures and by the mechanical incompatibility of the major components in particular. Durability of this high strength and fine grained material may be possibly significantly improved by water repellent treatment [3, 16]. The behaviour of UHPFRCC under direct tension and under three point bending has also been studied. Results are described in another contribution to this conference [17]. 248

7 Chloride concent, % of mass concrete % 5% 8% Penetration Depth, mm Figure 5: Chloride profiles as determined in undamaged and pre-loaded samples after 28 days of contact with 5 % NaCl solution Chloride concent, % of mass concrete %-28d 5%-28d 8%-28d Penetration Depth, mm Figure 6: Chloride profiles as determined in pre-loaded samples but after 28 days of selfhealing in Ca(OH) 2 solution 5. CONCLUSIONS UHPFRCC is a family of dense cement-based materials with high strength and low porosity. From these basic facts it follows that capillary absorption and chloride penetration of this material are considerably reduced, as compared with ordinary concrete. Application of a modest compressive stress, however, induces remarkable damage into the micro-structure of the fine-grained material and increases porosity. After application of a compressive stress, not higher than 3 % of the strength of the material, both capillary absorption and chloride penetration increase significantly. If the chloride diffusion coefficient is determined on laboratory specimens, durability and service life will be significantly overestimated. Capillary absorption may be considered to be an efficient method to predict and to check durability and service life of reinforced concrete structures. ACKNOWLEDGEMENTS The authors gratefully acknowledge substantial support by Cooperative Innovation Centre of Engineering Construction and Safety in Shandong Blue Economic Zone and by National Basic Research Program, 973-Project, 215CB6551 and by National Natural Science Foundation of China, Project Nr and by Major International Joint Research Project, Project Nr REFERENCES [1] Richard, P. and Cheyrezy, M., Composition of Reactive Powder Concrete, Cement and Concrete Research 25 (7) (1995) [2] Aitcin, P. C., Cements of yesterday and today, Concrete of tomorrow Cement and Concrete Research 3 (2)

8 [3] Wittmann, F. H., Effective Chloride Barrier for Reinforced Concrete Structures in Marine Environment, Basic Research on Concrete and Applications, Proc. of an ASMES Int. Workshop, Wittmann F. H. and Mercier O., editors, Aedificatio Publishers Freiburg, Germany (211) [4] Zhang, P., Wittmann, F. H., and Zhao, T., Quantitative Determination of Capillary Absorption of Concrete, Poc. of an ASMES Int. Workshop, Wittmann F. H. and Mercier O., editors, Aedificatio Publishers Freiburg, Germany (211) 9-2 [5] Zhang, P., Wittmann, F. H., Zhao, T., Lehmann, E., Neutron Imaging of Water Penetration into Cracked Steel Reinforced Concrete, Physica B: Condensed Matter 45 (21) [6] Zhang, P., Wittmann, F. H., Zhao, T., and Lehmann, E., Observation and Quantification of Water Penetration into Frost Damaged Concrete by Neutron Radiography, Restoration of Buildings and Monuments 16 (3) (21) [7] Wittmann, F. H., Wittmann, A. D. A., and Wang, P., Capillary Absorption of Integral Water Repellent and Surface Impregnated Concrete, Restoration of Buildings and Monuments 2 (4) (214) [8] Lu, W. P., Wittmann, F. H., Wang, P., Zaytsev, Y., Zhao, T., Influence of an Applied Compressive Load on Capillary Absorption of Concrete: Observation of Anisotropy, Restoration of Buildings and Monuments 2 (2) (214) [9] Van Tittelboom, K., De Belie, N., Zhang, P., and Wittmann, F. H., Self-healing of Cracks in Concrete, Basic Research on Concrete and Applications, Proc. of an ASMES Int. Workshop, Wittmann F. H. and Mercier O., editors, Aedificatio Publishers Freiburg, Germany (211) [1] Mihashi, H., Kaneko, Y., Nishiwaki, T., and Otsuka K., Fundamental Study on Development of Intelligent Concrete Characterized by Self-healing Capability for Strength, Transactions of the Japan Concrete Institute 22 (2) [11] Sisomphon, K., Copuroglu, O., and Koenders, E. A. B., Self-healing of Surface Cracks in Mortars with Expansive Additive and Crystalline Additive, Cement and Concrete Composites 34 (212) [12] Wiktor, V., Jonkers, H. M., Self-healing of Cracks in Bacterial Concrete, Proc. 2nd Int. Symposium on Service Life Design for Infrastructure, Vol. 2 (21) [13] Zhang, P., Wittmann, F. H., and Zhao, T., Capillary Suction of and Chloride Penetration into Integral Water Repellent Concrete, Restoration of Buildings and Monuments 15 (3) (29) [14] Jiang, F., Wittmann, F. H., Zhao, T., Influence of Mechanically Induced Damage on Durability and Service Life of Reinforced Concrete Structures, Restoration of Buildings and Monuments 17 (1) (211) [15] Jiang, F., Wan, X., Wittmann, F. H., and Zhao, T., Influence of Combined Actions on Durability of Reinforced Concrete Structures, Restoration of Buildings and Monuments 17 (5) (211) [16] Wittmann, F. H., Zhang, P., Lehmann, E., and Zhao, T., Visualization of Frost Damage in Concrete by means of Neutron Radiography, Basic Research on Concrete and Applications, Proc. of an ASMES Int. Workshop, Wittmann F. H. and Mercier O., editors, Aedificatio Publishers Freiburg, Germany (211) [17] Wittmann, F. H., Yao X., Wang, P., Zhang P., and Zhao T., Influence of Imposed Tensile Stress and Subsequent Self-healing on Capillary Absorption and Chloride Penetration into UHPFRCC, (another contribution to this conference) 25