Mehmet Ali Tasdemir

Optimisation of steel fibre reinforced concretes by means of statistical response surface method

Abstract The objective of this research is to optimise the fracture parameters of steel fibre reinforced concretes to obtain a more ductile behaviour than that of plain concrete. The effects of the aspect ratio (L/d) and volume fraction of steel fibre (Vf) on fracture properties of concrete in bending were investigated by measuring the fracture energy (GF) and characteristic length (lch). For optimisation, three-level full factorial experimental design and response surface method were used. The results show that the effects of fibre volume fraction and aspect ratio on fracture energy and characteristic length are very significant.

EVALUATION OF STRAINS AT PEAK STRESSES IN CONCRETE : A THREE-PHASE COMPOSITE MODEL APPROACH

Abstract A wide range of concretes was evaluated to explain the reasons for large strain values of high strength concretes (HSCs) at peak stresses under different loading conditions, such as uniaxial compression, uniaxial tension, bending and torsion. To determine the influences of constituents on the stress distributions at the matrix-aggregate interface, around the aggregate and in the matrix close to the aggregate, concrete was considered as a three-phase composite material consisting of a continuous mortar matrix, model aggregate and the interfacial zone between cement and aggregate. The results obtained show that in normal strength concretes (NSCs) i.e. the hard inclusion case, the elastic mismatch of aggregate and matrix is significant and large tangential, radial and shear stresses occur at the interface. However, in both HSCs and lightweight concretes (LCs), the elastic modulus of the aggregate is closer to that of the matrix, and lower tangential, radial and shear stress distributions occur at the aggregate-matrix interface, resulting in these concretes having a much more uniform stress distribution at the interfaces than NSCs. In both HSCs and LCs, tensile stresses occur at the tips of the aggregate (at the poles in the model) perpendicular to the applied stress, and tangential stresses in the matrix close to the interface or at the aggregate surface are larger than those in NSC ones. These imply that the crack will be forced to go through the aggregate and lower strains will develop in ascending branches of these concretes. Based on the model proposed and on additional microstructural studies, it can be concluded that the levels of strains observed at peak stresses under the different loading conditions are as expected.

Effects of silica fume and aggregate size on the brittleness of concrete

Abstract The effects of silica fume and aggregate size on the softening response and brittleness of high strength concretes were investigated by measuring the fracture energy GF, the characteristic length lch and brittleness index B. Based on the fracture tests and microscopic studies at the aggregate-matrix interface, it was concluded that, in concretes without silica fume, the cement-aggregate interface had a profusion of calcium hydroxide and also much less dense calcium silicate hydrate, hence, the cracks usually developed at this weak interface, i.e. around coarse aggregate. However, in concretes with silica fume, the interfacial zone became stronger, more homogeneous and dense, hence, the cracks usually traversed the aggregates; transgranular type of fracture was observed. In these concretes, the fracture energy decreased dramatically especially for large size of aggregate case and as a result the brittleness index increased significantly.

Influence of Aggregate Type on Mechanical Behavior of Normal- and High-Strength Concretes

Effects of coarse aggregate type on the mechanical properties of both normal- and high-strength concretes (HSCs) were investigated under compressive loading. Basalt, sandstone, and Triassic and Devonian crushed limestone coarse aggregates were used in the concretes. For each coarse aggregate type, 6 concrete mixtures were made with the same portland cement and natural sand. Nominal slump, effective water-cement ratio, and cement content were kept constant in each concrete class. In all mixtures, the grading and maximum particle size were the same. Triassic limestone containing concrete had the highest compressive strength. However, in HSCs, the compressive strength of basalt containing concrete had the highest value. In HSCs, the hysteresis loops of Triassic and Devonian limestone concretes are generally narrower than those of basalt and sandstone concretes. It can be concluded that the irreversible energy up to prepeak stress in compression decreases significantly and the loop becomes narrower with an increase in compressive strength. The brittleness index increases substantially with the compressive strength of concrete.

Mechanical Properties and Rapid Chloride Permeability of Concrete With Ground Fly Ash

The objective of this study was to optimize the design of high-strength, high-volume fly ash concretes. Eight concrete mixtures were prepared using the same batch of ordinary portland cement (OPC) and ground low-lime fly ash. The aggregate grading used in the mixtures of concrete, water-binder ratio, and the maximum particle size of aggregate were kept constant in all concretes, however, the partial replacement of cement by fly ash was varied from 0% to 70% OPC concrete, in steps of 10%. The replacement was on a one-to-one weight basis. At 28 days, there was little reduction in compressive strength up to 40% cement replacement by ground fly ash; then a significant decrease was recorded for the further fly ash dosages. At 56 and 120 days, however, the compressive strength up to 40% cement replacement by fly ash was found to be almost identical to that of the no fly ash concrete, and for one year it was even higher. Beyond 40% replacement, the compressive strength decreased significantly. It was shown that the brittleness index increases substantially with increasing compressive strength of concrete. The results of the rapid chloride penetration tests revealed that high volume ground fly ash concrete had better resistance to the penetration of chloride ions. The mortar phases of these concretes were also prepared. As the dosage of fly ash increased, 2, 7, and 28-day compressive strengths of the mortars decreased. However, at later ages at 56, 120, and 365 days, nearly 40% cement replacement by ground fly ash, the compressive strength of mortar with fly ash was about equal to that of mortar without fly ash. The pozzolanic effectiveness ratio increased with increasing curing time and fly ash content.

Combined Effects of Silica Fume, Aggregate Type, and Size on Post-Peak Response of Concrete in Bending

The influences of silica fume, type, and size of aggregate on the prepeak and postpeak response of high-strength concretes in bending were investigated by measuring the fracture energy, the characteristic length, and brittleness index. Degradation of stiffness and strength were also measured, and a unique focal point was determined using unloading-reloading cycles during the tests. The degradation of stiffness was correlated to the local fracture energy, strength degradation, permanent crack mouth opening displacement (CMOD), and permanent displacement at midspan. It was shown that relations between normalized stiffness, load, local energy, CMOD, and permanent displacement at midspan were independent of the partial replacement of cement by silica fume and of the type and size of aggregate. Based on the fracture tests and microscopic studies at the matrix-aggregate interface, it was concluded that, in both limestone and gravel concretes without silica fume, the cement-aggregate interface had a large amount of calcium hydroxide and also much less dense calcium silicate hydrate; however, in concretes with silica fume, the interfacial zone became stronger, more homogeneous, and dense. In the latter concretes, the fracture energy decreased dramatically, especially when they contained 20-mm maximum size aggregate, and in these concretes, the brittleness index was substantially high. In gravel aggregate concretes with and without silica fume, cracks developed around the aggregates and generally did not traverse them, because of the particle shape and smooth surface; however, in concretes with silica fume, crack surfaces were less tortuous and fracture was in a more brittle manner. In limestone concretes with silica fume, the cracks usually traversed the aggregates and a transgranular type of fracture was observed.

Factors determining the correlations between concrete properties

The factors determining the reliability of the correlations between concrete properties have been investigated. It is assumed that the existence of some resemblance between these properties may affect the strength of the correlations and it is found that the closeness of the sensitivity degrees of the properties to the pore structure of cement paste is the most important factor. Taking into consideration the closeness of dependences of the properties on compositional factors as well as the closeness of their sensitivity degrees, it has been found that it is possible to explain the reliability of the correlations between them with a great degree of confidence.

Sensitivity of concrete properties to the pore structure of hardened cement paste

Coefficients and degrees of sensitivity are introduced to define quantitatively the sensitivity of concrete properties to the pore structure of cement paste. Proposed parameters have been applied to experimental data obtained from 60 different concrete mixtures, measuring eight properties for each mix and the results obtained have been discussed and evaluated.

Investigation of Microstructure Properties and Early Age Behavior of Cementitious Materials Containing Metakaolin

Metakaolin is one of the most attracting gradients of high and ultra high performance concretes which are used in different types of buildings such as nuclear waste storages, impact-resistant military buildings, fire-resistant high-rise buildings and reinforced concrete bridges. The metakaolin induced concretes are very dense and more homogeneous than normal strength concrete. However, these properties cause significant increase in brittleness of hardened concrete and change of volume during the hardening of these materials. In this work, cement pastes were produced at three different water/binder (w/b) ratios of 0.42–0.35 and 0.28 for which cement has been replaced by metakaolin with at different weight fractions of 8%, 16% and 24%. 48 hours volumetric autogenous shrinkage measurements showed that in pastes with 0.42 w/b ratios autogenous shrinkage reduced with increasing metakaolin content. In mixtures with 0.35 and 0.28 w/b ratios, on the other hand, it has been seen that the autogenous shrinkage increased with increasing the amount of metakaolin. Using thermo-gravimetric analysis and mercury porosimetry measurements, it has been found that with increasing metakaolin content the amount of calcium hydroxide decreased and critical pore size reduced. It has been determined that as a result of the pozzolanic reaction the amount of fine gel pores increased and those of the coarser pores significantly decreased at 28 days. One year linear autogenous shrinkage measurements showed that in pastes with 0.42 w/b ratios the autogenous shrinkage increased with increasing metakaolin content, while it decreased in pastes with lower w/b ratios.

Inclusion based Modeling of Concrete with Various Aggregate Volume Fractions

Circular inclusions were arranged with respect to the regular hexagonal symmetry in an infinite plate. The inclusions were all of equal radii and their centers were located at the center of the hexagons. In a representative region of the plate, stress distributions were calculated using the collocation theory under far field uniform loads. It was seen that the results calculated according to the theory were very close to the results obtained by the exact solution in case of single inclusion. The specific fracture energies, tensile strengths and the moduli of Enasticity of concretes were also calculated using the meso-mechanical relationships. It was shown that the hardened cement paste transforms into a more ductile composite (i.e. concrete) as the volume fraction of aggregate is increased without altering the grading.