Journal of Materials in Civil Engineering

An experiment was conducted to observe the real distribution of local microstrains at different orientations in the layers of mortar that separate aggregates in concrete material. The relationship between the microstrains at different locations with different orientations and the macroscopic strain tensor was also investigated. Other microstructural features such as the normal microstress-microstrain relation and the crack patterns were observed. Five specimens were tested under uniaxial compressive loading. Three specimens had 5-mm-thick mortar layers between the hard inclusions, and the other two specimens had 10 mm layers. The aggregate were simulated by rounded granite cylinders embedded in mortar. Both mechanical and strain gauges were used to measure the local microstrain. A testing machine with very high stiffness was used. Although the strains at the microlevel are highly scattered and irregular, the average microstrain in all mortar layers in the same direction, but at different locations, can be considered as the resolved components of the macroscopic strain tensor.

The various approaches to the problem of drying creep of concrete and their limitations are analyzed. The simple phenomenological approach describing extra deformation of concrete under drying creep as a sum of shrinkage-induced creep and creep-induced shrinkage is suggested. It is shown that the first component is linearly proportional to free shrinkage strain, but behaves as an ordinary creep component, i.e., it varies linearly with stress. The second one is linearly proportional to basic creep, but should depend on the intensity of the shrinkage process in the material. The test results of concrete on drying creep under tension are presented. It is shown that in the initial period when creep-induced shrinkage is dominating, the total time-dependent strain is less than a sum of basic creep and free shrinkage, as opposed to the compression case; but later on, shrinkage-induced creep becomes dominant, and the total strain becomes larger than the previous sum, similar to the compression case. The microcracking in concrete skin, as one of the known explanations of extra drying creep deformation, is called into question.

The effectiveness of silica-fume concrete in resisting damage caused by corrosion of embedded steel has been investigated using an accelerated impressed voltage-testing setup. The silica-fume concrete included 0, 10, 15, 20, and 25% silica as equal replacement of ordinary portland cement. Concrete samples were cured in either fresh water or 4% NaCl saline water for a 7- or 28-day curing period. Samples were saturated surface-dry at the beginning of the accelerated-corrosion testing to simulate actual conditions of superstructures. A susceptibility-to-corrosion (STC) index was calculated from test results. The setup and index have the potential of being used in a concrete-mix design approach that directly considers exposure to corrosive environments. The performance of silica-fume concrete in resisting damage caused by corrosion was improved. This improvement was optimal at a silica-fume dosage of 15%, and found to be several times better than the control mix. It was most significant at 28-day curing, when pozzolanic action had a chance to take place. Electrical resistivity was higher for silica-fume concrete, but alone could not characterize the concrete's resistance to corrosion damage.

A test setup and adequate instrumentation were developed to record the tension properties of high-strength concrete, including the postpeak softening response. The direct uniaxial tension tests were performed under a strain-controlled mode through a close-loop testing machine. The splitting tensile strength and modulus of rupture were also recorded conforming to standard ASTM test procedures. Test results revealed that high-strength concrete exhibits a more brittle and stiffer behavior with a large initial modulus of elasticity and a more sharply descending branch of the stress-deformation curve beyond the peak load. The unique softening behavior and the more brittle nature of high-strength concrete were expressed in terms of a stress-displacement (crack width) diagram and fracture energy. The fracture energy of high-strength concrete is estimated to be about five times the area under the ascending portion of the stress-deformation curve, compared to a corresponding value of 10 estimated for normal-strength concrete. Based on the test results, a constitutive relationship is recommended for the behavior of high-strength concrete in tension, including postpeak softening response.

Since the mid-1980s, polymer-modified asphalt concrete mixtures have been widely used to minimize rutting failures of flexible pavements. This research investigates the permanent deformation behavior of unmodified asphalt concrete mixtures as compared to the behavior of asphalt concrete mixtures modified with polyethylene. The rutting resistances of the mixtures at 40°C and 60°C are evaluated in the laboratory using the repeated-load permanent-deformation test. The data from the repeated-load permanent-deformation tests are used in the VESYS model to predict the rutting development of typical pavement structures built with both modified and unmodified mixtures. The analysis of the VESYS results indicate that the polyethylene-modified mixtures exhibit higher resistance to rutting than the unmodified ones. In addition, the layer coefficients of the polyethylene-modified mixtures were 75–85% higher than the layer coefficients of the unmodified ones.

The interfacial bond properties of two carbon fibers, having diameters of 10 μm and 46 μm, were tested for cement matrices of different water-to-binder ( w /b) ratios and silica-fume contents. The pullout test was conducted using a special technique that prevented the brittle fiber from breaking during specimen handling. An environmental scanning electron microscope (ESEM) was used to determine the nature of the fiber-matrix bond. The results show a friction-based bond mechanism for the fiber of 10 μm, with a bond strength of 0.5 MPa for the high w /b matrix and without silica fume. Densifying the matrix by lowering the w /b ratio or by using silica fume improved the bond by 50–100%. For the larger-diameter fiber, silica fume had a very strong effect on the bond (an increase of 370–670%). It appears that long grooves along the fiber surface creates a mechanical anchorage, which is strongly affected by the presence of silica fume.

Recycled polyethylene terephthalate (PET), mainly recovered from used plastic beverage bottles, can be chemically modified to produce unsaturated polyester resin. In turn, the polyester resin can be mixed with fly ash to produce polymer mortar (PM). PM made with recycled PET and fly ash can be utilized very effectively in construction applications. The use of the PM could help save energy and cost, and alleviate an environmental problem. The effect of the amount of fly ash on the glass transition temperature, degradation temperature, tensile peak strength, and tensile modulus of the PM has been extensively investigated. Models were also developed using multiple regression analysis to predict the strength and modulus of PM using various percentages of fly ash at various temperatures. This evaluation is not only important from the standpoint that resins based on recycled PET and fly ash fillers are being used, but also because very little research has been done in this area with virgin materials.

In an experimental investigation, the minimum length of curing of silica fume concrete was determined and compared with that of plain portland cement concrete and fly ash concrete. Four silica fume concrete mixes with silica fume contents of 5%, 10%, 15%, and 20% of the total cementitious material content were prepared along with one plain portland cement concrete mix and one fly ash concrete mix. After demolding, the concrete specimens from each mix were moist cured in a fog room for six different periods of time: 0, 1, 3, 7, 14, and 28 days. The compressive strength determinations were made after each curing period. From the regression equations developed in this study, relating the strength and curing time, and the guidelines of ACI 308-81(86) the minimum length of curing of silica fume concrete mixes included in this study was determined to be 3 days. Compared to this, the minimum lengths of curing for the plain portland cement concrete mix and the fly ash concrete mix were 3.75 days and 6.5 days, respectively.