Controlling cracks caused by various factors is an important issue for concrete engineers in order to construct durable structures. Volume change due to drying and hydration heat is a major factor in concrete cracking. Thermal cracks in wall type structures often penetrate cross-section so serious degradations such as corrosion of reinforcement are caused. Decreasing temperature rise contributes to the restraint of thermal stress at early age, and can be introduced by reducing the unit cement content and using admixtures. Juenger and Siddique [1] reviews recently published literature on concrete properties with supplementary cementitious materials including fly ash, ground granulated blast furnace slag, silica fume, calcined clays and natural pozzolans. This paper demonstrates that the field is far from mature in spite of the fact that supplementary cementitious materials have been used and researched for decades.
One of the most ordinary concrete admixtures as a by-product of other industries is fly ash from coal-fired power plants. Fly ash is generally used as partial replacement of cement in concrete, so hydration heat can be reduced [2]. Clarifying the tensile stress reduction effect of fly ash on crack control can lead to expansion of its use, and active utilization of fly ash can improve the performance of concrete, reduce environmental impact, and contribute to the establishment of a recycling-oriented society.
Numerical simulations are often performed in order to predict thermal cracking. Tensile properties, such as Young’s modulus, strength and creep, are required for an estimation of tensile stress which causes thermal cracking. Tensile Young's modulus is directly dependent on the prediction of tensile stress and is one of the important input data for Finite element method (FEM) analysis. Shen et al. [3] conducted uniaxial tension test using prismatic specimen for uniaxial tensile strength and tensile Young’s modulus at early age. Yoshitake et al. [4] also investigated tensile properties of high volume fly-ash (HVFA) concrete. Young's modulus in Japan is generally a secant modulus obtained from a compression test. Mimura et al. [5] reported that the tensile Young’s modulus based on the direct tension test by using the slender reinforced concrete (RC) specimen is higher than the compressive modulus (secant modulus) at early age. Swaddiwudhipong et al. [6] also investigated tensile behavior of concrete at early age by using the direct tension test. Their test program included ground granulated blast furnace slag and pulverized fuel ash in concrete.
Tensile creep, which is inelastic strain, mitigates the tensile stress caused by restraint of concrete shrinkage including drying and temperature change, so numerical simulation without the relaxation due to creep might induce the overestimation of tensile stress. Shen et al. [7, 8, 9] reported tensile creep behavior at early age. Creep behavior of concrete loaded at early age is greater than that of mature concrete, so tensile stress mitigated by creep at early age is required for accurate FEM simulation of thermal cracks.
In this study, several Young’s moduli obtained from compression test, which can be performed with general equipment, were compared with the tensile Young’s modulus obtained from the direct tension test by using the unique apparatus and specimen. Compressive Young's moduli in the present study were secant modulus and initial tangent modulus. In addition, linear modulus which is taken from a regression line of a compressive stress-strain curve in the range of stresses less than the splitting tensile strength was also evaluated.
The tensile creep tests using the dog-bone shaped specimen of concrete with and without fly ash were conducted at the age of 3 or 7 days, and the loading was sustained for 14 days. The loading stress was constant during the tensile creep test and was set at 30% or 40% of the splitting tensile strength. It should be noted that elastic strain at an early age decreases with the age of concrete. However, the effect of the decrease in elastic strain during creep test has hardly been mentioned in most investigations [10, 11]. This study takes a consideration of stiffness development during creep test to distinguish actual creep and elastic strains based on superposition principle. In order to obtain the tensile mechanical properties development during creep test, splitting tensile strength and Young’s modulus were examined at various ages. The elastic strains during the tensile creep test were also measured by temporary unloading.