Influence of Silica Fume on the Long-Term Behavior of Self Compacting Concrete

1. Self compacting concrete (SCC) is an innovative concrete that does not require vibration for placing and compacting. This kind of concrete has been immensely used in the construction for the last decade and it is inevitable to be aware of its behavior, especially time depending deformations such as the creep and shrinkage. In concrete structures deformations due to the creep and shrinkage are several times larger than elastic deformations. Frequently, these deformations cause excessive cracking and deflections or possible failure with an inherent loss in serviceability, durability and long-time safety of concrete structures. Thus, there is an urgent need for a reliable method to predict creep and shrinkage, especially for self compacting concrete structures. In this research, three mix designs were prepared and 21 specimens were fabricated from each mix design. Then the effects of varying percentages of micro silica the on creep and shrinkage of during a period of 250 days, and also on its mechanical properties including compressive strength, tensile strength and modulus of elasticity up to the age of 28 days were investigated.2. Methodology2.1. Materials: The sand used was this study is of river type with sand value of 95% and fitness modulus of 3.1. The gravel used was broken gravel with maximum grain size of 12.5 mm and superficial specific gravity of 2700 kg/m3. Also the micro silica used was with specific gravity of 2200 kg/m3. Ordinary drinking water was used in the mix designs and since limestone powder reduces the interstices between gravels and improves the adhesion of the paste; in this study stone powder was used as filler. Moreover, in order to reach the intended self-compaction, the superplacticizer Sika Viscocrete was applied as a percentage of cement and micro silica.2.2. Testing method and curing: In this paper, three mix designs of self compacting concrete with compressive strength of 300 kg/m2 (SCC 30), 350 kg/m2 (SCC 35) and 450 kg/m2 (SCC 45) were prepared. According to Table 1, in all three types of concrete, only the amount of micro silica was altered, so that its effects on long-term deformations of the concrete including the creep and shrinkage can be studied. Using L-Box test and Slump Flow [1], the properties of fresh SCC was evaluated and after curing the specimens, compressive strength test [2], indirect tensile strength test [3], elastic modulus of elasticity until the age of 28 days [4] and the creep and shrinkage test [5] during 250 days were conducted on the specimens.Table 1. Mix designsComponents SCC 30 SCC 35 SCC 45 ((kg/m3 Cement 280 280 280(Gravel (kg/m3 647 644 638(Sand (kg/m3 971 965 957 (Water (kg/m3 167.3 171 175.4(Microsilica (kg/m3 8.4 14 22.4 (Limestone powder (kg/m3 270 270 270 (Superplacticizer (kg/m3 6.34 6.47 6.65Water-cement ratio 0.58 0.58 0.583. Results from slump flow and L-Box tests are within the allowable range in the European Code [1]. The relation between compressive and tensile strength is presented in Fig. 1. As shown in this Figure compressive and tensile strength increase with a correlation factor of R2=0.902 as the amount of micro silica is increased.According to the results and considering that elastic modulus of concrete is affected by the elastic modulus of its component, modulus of elasticity increases as micro silica and the compressive strength increase. This relation is illustrated in Fig. 2.As it can be seen from Figs. 3-5, all strains have the same trend over the time. In early ages, the strains considerably rise and the slope of the initial segment of the curve, until 50 days, is very steep. Afterwards the increase rate is reduced at old ages nearly to a constant value. 4. The addition of micro silica to SCC mixture improves its mechanical properties and as a consequence, the modulus of elasticity, compressive and tensile strength increase. The relation between indirect compressive and tensile strength of the tested specimens shows that as the compressive strength of SCC increases the tensile strength also increases though at a lower rate. Since the nature of shrinkage and creep is influenced by the same factors, the creep and shrinkage decrease in the specimens as the micro silica proportion is increased.