Akhter B. Hossain

Quantifying Early-Age Stress Development and Cracking in Low Water-to-Cement Concrete: Restrained-Ring Test with Acoustic Emission

Early-age cracking has been found to occur in some concrete bridge decks, slabs, and pavements when the volumetric changes associated with drying, hydration, and temperature reduction are prevented. While free-shrinkage tests can quantify length change, they may not always be sufficient for detecting materials that are prone to cracking, since the potential for cracking is influenced by complex interactions of strength gain, stiffness development, creep, shrinkage, the degree of restraint, and toughness. The simplicity of the ring test enables it to be used as a comparative test to screen potential mixture designs. From the use of this test, AASHTO developed a provisional standard ring test that establishes specimen geometry; however, the provisional standard does not provide an approach for quantifying stress development or indicating how close a specimen may be to failure. Described is a simple stress solution for quantifying the results of the ring test. Issues related to ring and free-shrinkage specimen geometries are discussed to improve the fundamental understanding of the information provided by the ring test. Also described is how elastic stress and actual stress can be compared to measure the stress relaxation in a material. To better illustrate the microcracking and visible-crack development process, acoustic-emission testing was performed. These experiments indicated that specimens with a higher level of restraint exhibited more microcracking as a part of the stress relaxation process.

Properties of Concrete Containing Vitreous Calcium Aluminosilicate Pozzolan

A laboratory study was done on the influence of vitreous calcium aluminosilicate (VCAS), a new pozzolanic mineral admixture, on the properties of fresh and hardened portland cement concrete. A relative comparison was done of the performance of concrete containing VCAS with that of concrete containing silica fume (SF), one of the widely used mineral admixtures. The addition of VCAS pozzolan increased the slump (improved consistency) of the mixtures, while the addition of SF decreased it. The VCAS pozzolan was found not to have significant influence on the plastic shrinkage cracking potential of the mixtures while SF increased it. The compressive strength development of both the VCAS and SF mixtures was found to be higher than that of the control mixture. The experimental results showed that both VCAS and SF increased the free shrinkage of concrete. The rapid chloride permeability (RCP) values for mixtures with VCAS and SF specimens were significantly lower than those of the control concrete. At equal replacements of cement with either VCAS or SF, the reduction in RCP values was higher for the SF modified mixtures. Incorporation of either VCAS or SF also reduced the sorptivity and moisture diffusion coefficient of the concrete mixtures. This study provides insight into the behavior of concretes containing two high-performance mineral admixtures.

Comparative Study of Rural Stop-Controlled and Beacon-Controlled Intersections

The characteristics of traffic flow at rural, low-volume intersections controlled by stop signs and by intersection control beacons plus stop signs were examined. The measures of effectiveness included vehicular speeds, stop-sign violations, service delay, gap acceptance, and accidents. In addition, the effects of sight distance, traffic volume, and day and night conditions were also considered. It was found that intersection control beacons generally reduced vehicular speeds in the major directions, particularly at intersections with inadequate sight distance. The intersection control beacons had, in general, little or no effect on accepted or rejected gaps and on service delays. A large proportion of drivers (40 to 90 percent) violated stop-sign laws by not completely stopping at the intersections, which indicates that intersection control beacons were not effective in reducing stop-sign violations. Intersection control beacons did not appear to be effective in reducing accidents.

Comparative analysis of injury severity resulting from pedestrian-motor vehicle and bicycle-motor vehicle crashes on roadways in Alabama

A comparison is provided between the influence of a variety of variables and the injury severities resulting from pedestrian-motor vehicle and bicycle–motor vehicle crashes in Alabama with the incorporation of the effects of randomness across the observations. Given the occurrence of a crash, random parameter logit models of injury severity with possible outcomes of fatal, major, minor, and possible or no injury were estimated. The estimated models identified a variety of statistically significant factors influencing pedestrian and bicyclist injury severities. According to these models, some variables were found to be significant in only one model (pedestrian or bicycle) but not in the other. For example, variables such as roadway crossing, crashes on Saturdays, and crashes on private roadways were found significant only in the pedestrian model. By contrast, variables such as the use of retroreflective clothing, crashes during the evening peak hour, and crashes in summer were found significant only in the bicyclist model. In addition, some variables—such as older pedestrians and bicyclists, dark roadway, and low-speed roadway—were found significant in both models. Also, estimation findings showed that three parameters in the pedestrian model (between midnight and 7:00 a.m., clear weather, and at intersections) and two parameters in the bicycle model (level grade on tangent and clear weather) could be modeled as random parameters; these findings indicated their varying influences on injury severity. From the results obtained, the effects of different variables on pedestrian and bicyclist injury severities and their possible explanations are discussed.

Properties of Concrete Incorporating Ultrafine Fly Ash and Silica Fume

A laboratory study on the influence of the combination of ultrafine fly ash (UFFA) and silica fume (SF) on the properties of fresh and hardened concrete is described. Also compared are the performance of concrete incorporating UFFA and SF (ternary blend of cement), concrete incorporating UFFA or SF (binary blend of cement), and control portland cement concrete. The test results show that the incorporation of SF or UFFA in concrete resulted in higher strength and improved durability (resistance to chloride penetration). These benefits were found to be more pronounced in the SF concrete. However, the SF concrete demonstrated several limitations such as low slump and high early-age shrinkage. These limitations were not observed in the UFFA concrete; addition of UFFA increased the slump and decreased the early-age shrinkage. To minimize the shortcomings of SF without losing its strength and durability benefits, a concrete mixture incorporating both SF and UFFA was prepared. The test results show that the incorporation of both SF and UFFA produces a concrete mixture that demonstrates high early-age strength and improved durability similar to those properties in SF concrete. In addition, unlike SF concrete, the new concrete mixture demonstrates a higher level of slump and a lower level of free shrinkage.

Comprehensive Injury Severity Analysis of SUV and Pickup Truck Rollover Crashes

The research described in this paper explored the factors contributing to injury severity resulting from rollover accidents of light trucks—SUVs and pickups in particular—in Alabama. Given the occurrence of a rollover crash, random parameter logit models of injury severity (with possible outcomes of fatal, major, minor, and possible or no injury) were estimated. The estimated models identified a variety of statistically significant factors influencing the injury severities resulting from SUV and pickup rollover crashes. According to these models, some variables were significant in one model (SUV or pickup) but not the other. For example, variables such as roadway downgrade, female drivers, and daylight were significant only in the SUV model. Variables such as driver fatigue, driver or occupant wearing a seat belt, and freeway were significant only in the pickup model. In addition, some variables (such as not wearing a seat belt, two-lane roadway, horizontal curve, and old driver) were significant in both models. Estimation findings showed that two parameters (horizontal curve and at intersections) in the SUV model and two parameters (horizontal curve and dry roadway surface) in the pickup model could be modeled as random parameters, indicating their varying influence on the injury severity related to unobserved effects. The results obtained are used in this paper for a discussion of the effects of variables on pedestrian and bicyclist injury severities and their possible explanations.