An understanding of the rate and amount of heat generated during the early stages of hydration of concrete mixes is essential in the prediction of the thermal behavior of concrete structures. This paper describes the design and operation of a low-cost, computer-controlled adiabatic calorimeter used for the determination of the heat of hydration of concrete mixes using small samples of the concrete. Heat measurements are started approximately 10 min after water is added to the concrete and a continuous plot of the rate of heat generation is obtained. Furthermore, the use of the calorimeter for temperature-matched curing tests on concrete is also discussed.
The dynamic behavior of a reinforced concrete beam was studied theoretically and experimentally. A relationship between the resonance frequency of the beam and the applied bending moment was established. A frequency criterion corresponding to the crack control criterion in the ACI Building Code was developed. It was found that resonance frequencies and damping will change rapidly when cracks are initialized and growing. The resonance frequency shift could be as large as 20 to 25% of the original value, and the decay rate could change by a factor of 4. The applied bending moment not only determines the number and the size of cracks but also determines the crack opening or closing condition that affects both resonance frequency and damping. To accurately estimate the frequency and damping of a cracked-reinforced concrete beam, empirical formulas are proposed based on a small number of experiments. The technique has promise for global nondestructive evaluation of reinforced concrete structures such as highway bridges.
Difficulties experienced using ASTM Method E 582-88 for determination of minimum ignition energies of compounds more difficult to ignite than simple hydrocarbons are described along with remedies for overcoming those difficulties. Minimum ignition energies and associated quenching distances are reported for four hydrofluorocarbon compounds using a modified Method E 582-88 apparatus and procedure.
Support surface interface (footwear, flooring systems, etc), thickness, and hardness strongly influence stability in men of all ages. We hypothesize interfaces influence stability through their effect on proprioception. We tested this by means of an experiment based on a randomized, cross-over, and controlled comparison design. Footwear midsole hardness and thickness were independent variables. Dependent variables were foot position and perception of foot position, measured concurrently. Thirteen subjects were a random sample of healthy older men (mean age 72 years, sd ± 4.50). They were tested barefoot using six support surface interfaces consisting of shoes that were identical, except for midsole hardness and thickness, that spanned the respective ranges in current footwear. Measures were balance failure frequency defined as falls per 100 m of beam walking, rearfoot angle measured via an optical position measurement system, perceived maximum supination estimated by subjects via a ratio scale when walking, and foot position error, defined as rearfoot angle minus perceived maximum supination. The results demonstrated: (1) foot position awareness was positively related to stability; (2) foot position error was negatively related to support surface interface thickness; (3) foot position error was positively related to support surface interface hardness; and (4) foot position error correlated best with maximum supination. We conclude that instability induced by support surface interfaces is caused by its effect on foot position awareness. Thin hard-soled shoes provide superior stability for older men. Most currently available footwear provides poor stability because soles are too soft and thick. Since suboptimal support surface interfaces are encountered by everyone daily, they represent substantial safety hazards. Likewise, improving this situation through setting stability safety standards is a promising means of improving public safety. At the very least, the public must be informed about products imparting suboptimal stability so that they can anticipate potential problems and compensate for them without falling.
This research evaluates a new concept to measure the tensile strains at the bottom of the asphalt concrete layer in flexible pavements. The concept consists of using the Hall Effect sensor in an H-gage configuration to measure the dynamic strains in flexible pavements. The evaluation program included both laboratory and field experiments to evaluate the fundamental and operational properties of the recommended gage design. The laboratory experiment showed that the Hall Effect gage can withstand the temperature and moisture conditions that are encountered during the construction and operation stages of hot mixed asphalt concrete pavements. It also indicated that the dynamic characteristics of the gage are suitable for measuring pavement strains under moving vehicle loads. The field experiment evaluated the in-service characteristics of the Hall Effect gage under a large number of combinations of vehicle speed, axle load, and tire pressure. The field data showed that the Hall Effect gage has good survivability and repeatability and it compares favorably with other strain gages that have been used in flexible pavements.
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