Mustafa Tokyay

Specimen shape and size effect on the compressive strength of higher strength concrete

Abstract The specimen size and shape effects on the compressive strength of higher strength concrete were investigated on different sized cylinders having constant length-to-diameter ratio ( l d ), different sized cubes, and cylinders with various l d for 40, 60, and 75 MPa compressive strength levels. The apparent strength values of 75mm diameter cylinder and 75 and 100mm cube specimens were lower than those of the larger specimens used. Also, the compressive strength was determined to be insignificantly affected by changing l d as the strength of concrete increases.

Strength prediction of fly ash concretes by accelerated testing

Relationships between standard compressive strength at 7, 28, and 90 days and early strength attained by (1) autogeneous curing, (2) warm water curing, and (3) boiling water curing were obtained and a regression expression to predict the strength of concretes containing high-lime and low-lime fly ashes as partial cement replacement are proposed. The control concretes were designed for 28-day characteristic compressive strengths, f ck 28 5 40, 60, 65, and 70 MPa. All concretes were proportioned to keep the slump at 80‐100 mm. The curing methods used were in accordance with the relevant ASTM and Turkish standards.

Effect of coarse aggregate size on interfacial cracking under uniaxial compression

Abstract The effects of aggregate type and size are important parameters in the formation of interfacial transition zone (ITZ) structure and subsequently in the failure process of concrete. The influence of surface, rigidity and size of aggregates and water/cement (w/c) ratio of the matrix on bond strength at the ITZ and the interrelationship between the bond and the matrix in the failure process of concrete under uniaxial compression were studied. For this purpose a series of experiments were designed and carried out on mortars (with two different w/c ratios) containing single spherical steel aggregates. The ITZ properties and the failure process of concrete were investigated by means of tensile and compressive strength, and stress–volumetric strain measurements. It was observed that the effect of aggregate properties (high modulus of elasticity, smooth surface texture and size) on the weakness of ITZ and the failure process of concrete are of paramount importance for low w/c ratio composites. The effect of reduced bond properties was reflected in lowered critical stress levels for the low w/c ratio composites with larger aggregates.

Comparison of intergrinding and separate grinding for the production of natural pozzolan and GBFS-incorporated blended cements

Abstract A portland cement clinker, a natural pozzolan, and a granulated blast furnace slag (GBFS) were used to obtain blended cements that contain 25% mineral additives. The natural pozzolan, which was softer, was more grindable and granulated blast furnace slag, which was harder, was less grindable than the clinker. Two of the cements produced were obtained by intergrinding and the other two were obtained by separate grinding and then blending. All of the blended cements and the control cement without any additive had the same fineness as 3500 ± 100 cm 2 /g Blaine fineness. During grinding, energy consumption of the mill was recorded and a sample corresponding to each energy level as taken from the mill at regular intervals and particle size distribution was determined. Cements produced were compared for change in particle size distribution during grinding and 1-, 2-, 7-, 28-, and 90-day compressive strengths points of view. Also, interactions between clinker and mineral additive portions of blended cements during intergrinding is highlighted.

Effect of chemical composition of clinker on grinding energy requirement

Abstract Fifteen commercial Portland cement clinker samples with a wide range of chemical compositions were subjected to grinding tests and relationships between grinding energy requirement attained specific surface areas and various chemical parameters were found. Regression analyses of the test results revealed that the energy–fineness relationship is exponential, and grinding energy is correlated with Al 2 O 3 , free CaO, liquid phase contents, silica moduli, and (C 3 S + C 2 S)/(C 3 A + C 4 AF) ratios of the clinkers.

HIGH STRENGTH NATURAL LIGHTWEIGHT AGGREGATE CONCRETE WITH SILICA FUME

High strength lightweight aggregate concretes are usually produced using special artificial aggregates together with mineral and chemical admixtures. Using natural lightweight aggregates instead of processed artificial aggregates can significantly reduce the cost of such concretes. Turkey has rich reserves of volcanic tuff and pumice stones. In Turkish standards highest strength classes for lightweight tuff and pumice concretes are 30 and 16 MPa, respectively. In this research, selected samples of these lightweight rocks were used to produce high strength lightweight aggregate concretes. The binding medium was made of portland cement, silica fume and superplasticizing admixture. For each concrete mixture properties such as workability, unit weight, compressive strength at various ages, as well as splitting tensile strength, modulus of elasticity and thermal conductivity values were determined to find the optimum quantities of materials to be used. Tests show that it is possible to produce a natural lightweight aggregate concrete with a 28-day compressive strength of 55 MPa, a dry unit weight in the range of 1700-2100 kg/m cubed and a coefficient of thermal conductivity value of about 0.55 W/m square K. Mathematical equations based on experimental results were obtained by regression analyses. The equations are useful for optimizing concrete mixtures for specified unit weight and compressive strength.

EFFECTS OF THREE TURKISH FLY ASHES ON THE HEAT OF HYDRATION OF PC-FA PASTES

Abstract Effects of the type and amount of fly ash substitution on the heat of hydration of portland cement-fly ash pastes were investigated. Three Turkish fly ashes were used. One of them was a high-calcium and the other two were low-calcium fly ashes. The specimens contained 0, 10, 20, and 40% fly ash by weight of portland cement. The tests were carried out as described in ASTM C 186 however one separate set of specimens were first subjected to an early external temperature of 67±2°C for six hours followed by the standard temperature until time of test. The results revealed that the low-calcium fly ashes, regardless of their type, reduce the heat evolution when used for partial cement replacement. The high-calcium fly ash, on the other hand, does not produce significant changes in the heat of hydration.

INTERFACIAL FAILURE IN STEEL FIBER REINFORCED POLYSTYRENE IMPREGNATED MORTAR

ABSTRACT Interfacial bond strength, failure modes, and pull-out behavior in steel fiber reinforced plain and polystyrene impregnated mortar have been studied using single-fiber pull-out tests. In plain mortar cohesive failure occurred in the calcium hydroxide (portlandite) layer adjacent to the fiber/matrix interface; matrix cracking was also observed. In contrast, polystyrene impregnated mortar exhibited adhesional separation at the interface, with the matrix being intact. An adhering layer of matrix material covered the relatively rough steel wire surfaces in unimpregnated mortar specimens whereas the fiber surfaces were clean in polystyrene impregnated mortar specimens. A fourfold increase in the interfacial bond strength has been achieved by polymer impregnation. Impregnated specimens also showed a slower decay in fiber frictional shear traction during pull-out; pull-out work improved by 640 percent.