Journal of Materials in Civil Engineering

The structural performance of reinforced concrete elements is related to the interface behavior of rebars to concrete. In the last decade several research works were carried out to investigate the bond between fiber-reinforced polymer (FRP) rebars and concrete, however some aspects need further studies in order to obtain reliable design indications. In this paper the analysis of bond was performed referring to different kinds of FRP rebars and some varying influential parameters (surface treatment, kinds of fibers, and kinds of test). Results obtained show the role of the investigated parameters on bond stress law; in particular the surface treatments involve different transfer mechanisms passing from simple chemical adhesion and friction, for sanded rebars, to a relevant contribution of mechanical interlocking for deformed rebars. The kind of test utilized influences the most significant parameters of the bond stress–slip law and in different ways depending on the kind of rebars. Finally the kind of fibers causes a variation of interface stiffness. In addition a calibration of experimental bond–slip laws was performed on the basis of available theoretical models and the results are discussed.

Plain shotcrete and fiber-reinforced shotcrete (FRS) are commonly employed to produce layers or linings with large surface/volume ratios. Restrained shrinkage cracking is hence an important concern from both structural and durability aspects. In this investigation, a new testing procedure is adopted to study the shrinkage cracking behavior for plain shotcrete and four different types of FRS (polypropylene, steel, polyvinylalcohol, and hybrid). To introduce shrinkage restraint, specimens cut from a shotcrete panel are bonded to a steel fixture. By measuring the shrinkage strain of the restrained specimen at different times, the occurrence of cracking can be easily observed from a sudden “jump” in the strain values. To find the actual strain induced on the restrained specimen, unrestrained members were also monitored to obtain the free shrinkage. For various plain and reinforced shotcrete mixes, the induced strain when cracking occurs is found to follow an order that is generally consistent with physical intuition.

A total of 160 different hot mix asphalt (HMA) combinations of binders, aggregates, and reclaimed asphalt pavement (RAP) were compacted and tested using indirect tension stress relaxation at two temperatures (5 and $22°\mathrm{C}$ ). Two relaxation characteristics from a power law fit through the data were used to define the effect of RAP on the HMA binder-related properties: the initial modulus at $1\mathrm{s}$ (regression constant) and the curvature coefficient (regression exponent). The results showed there is a nonlinear relationship between both the initial modulus and the curvature coefficient and the percent RAP from 0 to 100% RAP. A linear relationship can only be obtained between the properties and the percent RAP between 0 and 50%. There is little change in either the initial modulus or curvature coefficient for HMA mixes with 50 or more RAP. When three tests per sample are averaged, the coefficient of variation is 18 and 7% for the initial modulus and curvature coefficient, respectively.

Nowadays, asphalt cement is frequently modified by blending it with various polymers in order to obtain a material with improved engineering properties. In number of studies it has been shown that ethylene-vinyl-acetate (EVA) and styrene-butadiene-styrene (SBS) copolymers can enhance the rheological properties of conventional asphalt at relatively wide range of temperatures. It is believed that when polymer is added to the base asphalt it absorbs low molecular weight oil fraction of asphalt and swells, thus forming a three-dimensional network structure. The properties of this structure determine the rheological behavior of polymer-modified asphalt. Asphalt modified by SBS and EVA copolymers (concentrations from 2 to 8% by weight) was investigated in dynamic mechanical analysis and interrupted shear flow tests. The interrupted shear test involves two phases of shearing (stress growth and flow relaxation in between) with varying rest periods between the two shear phases. All the tests were performed at $60°\mathrm{C}$ in strain control rotational viscometer. The destruction and reformation of the internal network structure of polymer-modified asphalts can be demonstrated in this test.

This paper presents a variability study of several engineering properties of tire-derived aggregate based on a comprehensive literature survey of experimental test programs. The dry compacted unit weight, cohesion intercept, friction angle, constrained modulus, and modified secondary compression index were evaluated and compared to the engineering parameter variability of natural soils. A series of regression analyses were performed to investigate the presence and significance of scale-dependency. The results of the variability analysis indicate that unit weight has the lowest value of coefficient of variation (COV) whereas the shear strength parameters, constrained modulus, and compression index have COV values that are substantially higher. Regression analyses indicated that unit weight and constrained modulus showed the greatest sensitivity to changes in maximum tire particle size. A nonstatistical investigation was used to further investigate the variability and scale-dependency of the shear strength parameters. Using Mohr-Coulomb failure criterion and assuming that cohesion is negligible, the analysis showed a scale-independent relationship which is consistent with the statistical findings for cohesion and friction angle.

An experimental program was undertaken to study the effects of polyester fiber inclusions and lime stabilization on the geotechnical characteristics of fly ash-soil mixtures. An Indian fly ash was mixed with expansive soil in different proportions. The geotechnical characteristics of fly ash-soil specimens, lime-soil specimens and lime-fly ash-soil specimens mixed with different proportions of randomly oriented fibers were investigated. Lime and fly ash were added to an expansive soil at ranges of 1–10% and 1–20%, respectively. Test specimens were subjected to compaction tests, unconfined compression tests and split tensile strength tests. Specimens were cured for 7, 14, and $28\text{days}$ after which they were tested for unconfined compression tests and split tensile tests. Based on optimum values obtained for lime and fly ash, tests were conducted on test specimens prepared from fly ash-expansive soil- lime-fiber mixture after $28\text{days}$ of curing. Samples were tested with 0, 0.5, 1.0, 1.5, and 2% plain and crimped polyester fibers by dry weight. Based on the favorable results obtained, it can be concluded that the expansive soil can be successfully stabilized by the combined action of fibers, lime, and fly ash.

Various prediction models have been developed to predict the creep and shrinkage in concrete. Experimental studies to measure the creep and shrinkage in concrete have also been carried out by researchers and reported in the literature. RILEM has compiled all these experimental studies which are stored in a computerized data bank. This paper briefly describes some of the recent models which are based on the extensive research and experimental studies. Creep and shrinkage have been predicted up to 5,000 days of observation by the ACI-209R-82 model, the B3 model, the CEB-FIP model code 1990, and the GL2000 model for various grades of concrete. Predicted values of creep and shrinkage were compared with the experimental results of Russell and Larson, in 1989, as well as the RILEM data bank. Prediction of creep and shrinkage by GL2000 model is found to be the closest to the experimental results.

Three types of stress-strain relations of high-strength concrete under triaxial compression, axial stress–axial and lateral strain, octahedral normal stress–volume change, and octahedral shear stress-strain curves, are studied in this paper. The influence of different load paths is also addressed. The Saenz equation is found to be appropriate to represent the octahedral shear stress-strain relation of high-strength concrete under low confining pressures. A relation between peak octahedral shear stress and peak octahedral shear strain is developed, which can be used to define the peak point on the curve from the peak stresses obtained from the stress failure surface. The high-strength concrete is found to experience degradation in elastic and shear modulus in cyclic loading under triaxial compression.

The equivalent periodic eigenstrain method is used to evaluate the effective elastic stiffness of periodic masonry structure. An Eshelby tensor, for a unit cell of the periodic masonry structure, is derived analytically, and is combined with a strain energy approach to formulate the effective stiffness of the masonry. The new homogenization scheme is simple, one step, and closed form. The model described the periodicity and microstructure details of brick and mortar precisely and is compared to other analytical models. The improved accuracy of model prediction is validated through a finite-element simulation reported in literature.