Journal of Materials in Civil Engineering

The electrical resistivity and mechanical properties of carbon fiber-reinforced cement mortar (CFRCM) were investigated. Both cylindrical and prism specimens were used in this investigation to determine the effects of size and shape of specimen on the physical and mechanical properties of the CFRCM systems. Carbon fiber loading was varied up to 6% (w/w), and water-to-cement ratios (w/c) of 0.5 and 1 were used. The specific electrical resistivity of the plain mortar with w/c ratio of 1 $(>\mathrm{66,000}\mathrm{Ohm}\mathrm{m})$ was reduced to $\mathrm{3,750}$ and $0.23\mathrm{Ohm}\mathrm{m}$ upon increasing the carbon fiber content to 1 and 6% (w/w), respectively. The percolation theory was used to represent the variation of the electrical specific resistivity with fiber content in the cement mortar. Increasing the w/c ratio decreased the strength and toughness of the CFRCM composites. Increasing the fiber content increased the peak strain and toughness, but decreased the Young’s modulus and electrical resistivity of cement mortar composites. Pulse velocity tests showed that the specimen shape also affected the dynamic Young’s modulus, dynamic shear modulus, and the dynamic Poisson’s ratio. The compression strength of the CFRCM was influenced by the w/c ratio and the fiber content. A model was used to predict the compressive stress-strain behavior of different CFRCM systems, and relations between static and dynamic properties have been developed. Empirical relations were developed to relate the specific electrical resistivity to unit weight, Young’s modulus, and pulse velocity.

This article reviews the results of two studies examining the effects of exposure to elevated temperatures on the dynamic elastic (Young’s) modulus $\left(E_d\right)$ and air permeability index (API) of concrete. The relationship between $E_d$ and API in fire damaged concrete, and the use of $E_d$ and API to determine both the extent of damage due to exposure to fire and the presence and extent of damage gradients are reviewed. Cores were removed from a precast, prestressed concrete beam containing lightweight aggregate and from a reinforced concrete structural wall containing conventional mineral aggregates. Both of these structures were exposed to intense fires and the laboratory investigation of cores was part of a forensic engineering evaluation to determine the extent of damage to the structure and the necessary repairs prior to returning to service. The $E_d$ and API were determined using $25\mathrm{mm}$ thick disks sawn from the cores. The findings of the study suggest that $E_d$ and API are linearly related for a given set of raw materials until damage is significant and permeability increases nonlinearly when significant damage is sustained for both types of concrete that were evaluated. The findings also suggest that permeability of lightweight aggregate concrete increases more rapidly than conventional mineral aggregate concrete per unit change in Young’s modulus, under exposure to fire. Analyzing $E_d$ and API of concrete disks at $25\mathrm{mm}$ $\left(1\mathrm{in.}\right)$ depths provided insight into damage gradients and was found to be an effective tool in the assessment of damage gradients. API was found to be particularly sensitive to fire damage.

This paper deals with the study of the influence of a mixture composition on the properties of foam concrete at the fresh state. The stability of the base mix in terms of the water–solids ratio for getting a stable foam concrete mix of design density was determined for different mixture parameters and their influence on the water–solids ratio requirement was studied. The consistency of the foam concrete mix was measured in terms of the spread percent using a flow cone test and flow time using a Marsh cone test. The fresh state properties of foam concrete, viz; (1) stability of foam concrete and (2) consistency of the foam concrete, were very much affected by the water content in the base mix and the amount of foam added along with the other solid ingredients in the mix. Regression equations for predicting the spread flow value of the foam concrete based on the experimental results were reported. This will help in arriving at the water content of a foam concrete mix for a given consistency requirement. An appropriate value of consistency for obtaining a stable and workable foam concrete was also proposed.

Temperature fluctuations in asphalt pavements significantly affect pavement stability and the selection of asphalt grading. Ability to accurately predict asphalt pavement temperatures at different depths and horizontal locations based on environmental conditions will greatly help pavement engineers, not only in the selection of the asphalt grade to be used in various pavement lifts, but also in the accurate assessment of thermal stresses in and between various asphalt lifts. This is especially critical when the asphalt pavement is exposed to extreme freeze and thaw conditions. In this paper, a simulation model is proposed that uses a transient, two-dimensional finite difference approach to calculate the temperature distribution in asphalt pavements in response to hourly thermal environmental conditions and to calculate associated thermal stresses. Previous attempts to determine thermal stresses in asphalt sections range from correlations of observed cracking to ambient environmental conditions, to detailed numerical solutions of stress equations. The difficulty lies in adequately describing rheological properties of asphalt as a function of temperature. This paper uses the stiffness index to estimate the thermal stress distribution in asphalt pavements, and preliminary thermal stress maps during seasonal and diurnal freeze/thaw cycles are presented. Simulation results show that the greatest thermal stresses are observed in cases where pavement lifts have dissimilar thermal properties.

This paper presents a proposal for an expert system for inspection and diagnosis of epoxy resin on industrial floor coatings (PIRE). The system proposed intends to contribute to normalize the inspections and related reports and make the in situ inspector activities more objective and standardized. A classification system for epoxy resin floor defects and their most probable causes are proposed. With these data, anomaly files were prepared, in which the designation, a brief description, possible causes and consequences, the main aspects to be checked, inspection parameters, and a classification of the anomaly according to its importance/urgency, are presented. In parallel, the floor coatings’ diagnosis and repair techniques were also identified and classified. With these data, correlation matrices between anomalies and causes, anomalies between themselves, anomalies and diagnostic techniques, and finally, anomalies and repair floor techniques, were built. The validation of the classification system was made via a number of cases of standardized inspections. The paper presents indispensable data for industrial floor designers, builders, and users in terms of maximizing the service life of epoxy resin coatings.

The feasibility of using cement kiln dusts (CKDs) as potential stabilizing agents for Na-montmorillonite clay was investigated using a suite of four CKDs with different chemical and physical characteristics. The Atterberg limits, pH values, unconfined compressive strengths (UCSs), stiffnesses (stress-strain behavior), and stability of the CKDs-modified Na-montmorillonite clay were measured as functions of CKDs content and curing periods. These properties were compared with those of the untreated clay to determine the extent of enhancement, which was used as a measure of effectiveness of CKDs as stabilizers. All CKDs considerably decreased the plasticity index, thereby improving the workability of the clay, while they also considerably increased the initial pH value of clay, providing a favorable environment for further chemical (pozzolanic) reaction. The addition of CKDs and subsequent compaction substantially increased the UCS and the stiffness of the clay, thus improving its structural properties. The extent of improvement of the clay characteristics was found to be a function of the chemical composition of the particular CKD, specifically its free lime content. It was also found that the length of curing period after compaction had a major role in the stabilization process.

Creep compliance (or relaxation modulus), which is a fundamental property that determines the strain (or stress) development in flexible pavements or damage evolution in asphalt mixtures, can be determined from either a creep compliance test using static loading or a complex modulus test using cyclic loading. Since the nature of each test is different, creep compliance determined from the complex modulus test was significantly different compared with that determined from the creep compliance test. From this rigorous experimental and analytical study, it was concluded that the creep compliance or complex modulus test alone is not capable of providing complete information over the typical time or frequency range used in single-temperature tests. In general, the complex modulus test provides accurate creep compliance at short loading time, while the creep compliance test provides accurate creep compliance at longer loading time. An approach to determine accurate creep compliance from the combination of creep and complex modulus tests at a single temperature was developed. The method provided accurate creep compliance values that corresponded to short-term and long-term experimental data.

The influence of fiber inclusion on the abrasion resistance of concrete specimens of various mix compositions was measured by means of a rolling-wheel apparatus. Microhardness, mercury intrusion porosimetry, and petrographic examinations were used to investigate the microstructural features of the cement matrix of the exposed surfaces. The results indicate that variations in the water–cement ratio for both plain and fiber-reinforced concrete mixes influence the microhardness and the pore structure of the surface matrix. The petrographic examination revealed that variations in the water–cement ratio for both plain and fiber-reinforced concrete influenced the formation and dimensions of the locally abundant microcracks observed below the parts of the surfaces subjected to abrasion testing. For the systems investigated, this was attributed to the phenomenon of rolling contact fatigue.

In this study, 68 reinforced concrete columns were tested under uniaxial compression after being jacketed externally with carbon fiber-reinforced polymer (CFRP) sheets. Forty specimens were cast using low strength concrete and inadequate internal transverse reinforcement, while 28 specimens were cast with medium strength concrete and adequate internal transverse reinforcement. Thickness of the CFRP jacket, cross-section shape, concrete strength, amount of internal transverse reinforcement, corner radius, existence of predamage, loading type (monotonic or cyclic), and the bonding pattern (orientation, spacing, anchorage details, additional corner supports) of CFRP sheets were the main test parameters of this extensive experimental work. Test results showed that external confinement of columns with CFRP sheets resulted in an increase in ultimate strength and ductility. While the strength enhancement was more pronounced for specimens with circular cross section, specimens with square and rectangular cross sections exhibited larger ultimate axial deformations without a substantial loss in strength. The efficiency of retrofitting was much more pronounced in the case of relatively lower strength concrete. The proposed model, together with two other available models, were used for predicting the strength and corresponding axial deformations of more than 300 specimens tested by other researchers, as well as more than 100 specimens tested by the writers during this study and before. It was shown that the predicted results by the proposed model were in reasonable agreement with this extensive database of experimental studies.

Although many attempts have been made in previous research to identify the various parameters governing the rheology of cement paste, there has been little progress in evaluating the combined effects of these parameters. In this paper, a new parameter, the excess water to solid surface area ratio, is proposed to evaluate the combined effects of water content, packing density, and solid surface area on the rheological properties of cement paste. For the purpose of determining the value of this new parameter, a new wet packing method has been developed to measure the packing density of cementitious materials so that the voids content of the cementitious materials and the amount of excess water in the cement paste can be quantified. A number of cement paste samples containing different proportions of cement, pulverized fuel ash and condensed silica fume, and different water contents have been tested and their rheological properties, as measured by a rheometer, correlated to the new parameter. The correlation revealed that the excess water to solid surface area ratio, which serves as an indicative measure of the average water film thickness, is the single most important factor governing the rheology of cement paste.