Effects of aggregate volume fraction on mechanical properties of normal and high strength concrete produced with aggregate from different origins

Concrete is most common used construction material because of easy formation, high resistance to physical and chemical external effects, usage and production practicality and economy properties. Concrete is a particle reinforced composite type and generally it contains cement paste as matrix phase and natural aggregate as distributed phase. Matrix phase is generally auxiliary load bearing phase and particles of distributed phase restricts strain of matrix. Agregate, which constitute 60-80% of concrete by volume, does not chemically react with binder constituents. Bond between aggregate and binder phase is phscical and mechanical. Because of its high fraction, properties of aggregate such as strength, particle density, shape, texture, particle size distribution, significantly affect the concrete properties such as strength, elastic and inelastic properties, durability. Cement paste and aggregate, which are main components of concrete, are brittle materials; there is a linear relationship between stress-strain of these two materials until very high stress level. However, concrete is a semi brittle material and linear relationship between stress-strain turns to non-linear at low stress levels. In addition to composite behavior of material, reason of this is microcracks and voids in cement-based matrix, which are independent from external loads. These crack result from segregation and bleeding and they may be found under coarse aggregate particles and reinforcement bars. However, the large part of crack in concrete is link crack on aggregate-cement paste interface. Concrete shows elastic behavior under low-level loads. However, after the elastic limit, these voids and microcracks start to grow and then lineer relation between stress-strain turns to non-liner and plastic behavior starts. Due to increase on microcracks, Poisson ratio increase significantly at discontinuity limit. After the discontinuity limit, microcracks continues propagate and they form a continuous crack pattern at loosening limit. In addition, volume, which decreases until that level, starts to increase. After that limit, concrete carries the load by friction between the crack surfaces. Cement paste, which has most variable structure, aggregate, which forms the skeleton and aggregate and aggregate-cement paste interface has effects on fracture mechanic of concrete. When short-term compression load applied to concrete, crack propagation starts from bond cracks between aggregate and cement paste. So, particle size distribution, surface texture, shape and volume fraction properties of aggregate, which affects bond cracks, affect the inelastic behavior of concrete. In this work, concrete mixtures were produced with 0,30-0,40-0,50 W/C ratio and 0%, 20%, 40%, 60% and 70% agregate volume fraction. Because of segregation, mixtures with 0,50 W/C and 20% aggregate volume fraction couldn't been produced. 4 aggregate types from different orgins were used; white limestone, black limestone, dolomite and basalt. Compression test was performed on cylindrical specimens with 15 cm diameter and 30 cm height and compression strength, modulus of elasticity, Poisson ratio, discontinuity ans loosening limits and toughness properties were examined. To determine bending tensile strength, three point bending test was performed on 10x10x50 cm prism specimens. Also cube specimens with 10 cm length were cut from bending specimens and splitting and water absorption tests were performed. Test results show that, compression strength has inverse proportion with W/C ratio and direct proportion with aggregate volume fraction. Compression strengths of concrete with 0,30 W/C ratio and low aggregate volume fractions are obtained in line with containing aggregate at an ascending order as, white limestone, black limestone, dolomite and basalt, however, when the aggregate volume fraction is over 60%, the values are close to each other. Strength of paste phase, which is matrix of concrete, decrease with increment of W/C and strength of concrete, which is a composite type. So, effect of aggregate volume fraction on compressive strength decreases and approximate compressive strength values were obtained in spite of increment on aggregate volume fraction on concrete with high W/C ratio. Proportion between modulus of elasticity and aggregate volume fraction is nearly linear. Increment on W/C ratio cause to decrement on modulus of elasticity, but this effect of W/C ratio reduces with increment on aggregate volume fraction and approximate modulus of elasticity values are obtain with different W/C ratios at high aggregate volume fractions. No significant change on Poisson ratio is observed based on aggregate volume fraction for 0,30 W/C ratio, however Poisson ratio generally increase with increment of aggregate volume fraction after 0,40 W/C ratio. Effects of aggregate volume fraction and W/C ratio on discontinuity and loosening limits of concrete are similar with effects on compression strength. Discontinuity and loosening limits of black limestone, dolomite and basalt including mixtures with 0,30 W/C ratio firstly decrease with 20% by volume aggregate replacement but after that they increase with aggregate volume fraction. This decrement continues up to 40% by volume aggregate replacement and similarly they increase with aggregate volume fraction after that level for white limestone aggregate including mixtures. However, these limits are always increase with aggregate volume fraction at 0,40 and 0,50 W/C ratios. Relative compression stress values of concrete mixtures at discontinuity limits are generally between 75-95% for white limestone, 65-85% for black limestone, 85-90% for dolomite and 75-85% for basalt. In addition, relative compression stress values at loosening limits are generally between 85-95% for white limestone, black limestone and basalt, 75-95% for dolomite. Effects of aggregate volume fraction and W/C ratio on fracture-displacement work of concrete are generally similar with effects on compression strength. Nevertheless, a few decrements were observed on fracture-displacement work of mixtures, which are not significantly affected by increment of aggregate volume fraction. Flexural and splitting tensile strengths are generally increase with aggregate volume fraction and decrease with W/C ratio. But, effect of W/C ratio on flexural and splitting tensile strengths is not as significant as aggregate volume fraction. Flexural tensile strengths of black limestone and dolomite including mixtures and splitting tensile strengths of black limestone and basalt including mixtures are greater than other aggregate types. Relation between aggregate volume fraction and water absorption by weight is nearly linear, but parabolic. Increment on aggregate volume fraction and decrement on W/C ratio cause to increment on water absorption. At the end of tests, there is no significant difference depending on aggregate type on water absorption of mixtures with same aggregate volume fraction and W/C ratio.