Lightweight Aggregate as Internal Curing Agent to Limit Concrete Shrinkage
Drying shrinkage is a principal cause of cracking in concrete structures. The cracking can be accommodated or modified so that the performance of the overall structure is not affected. This is not true for bridge decks where drying shrinkage cracking combines with plastic shrinkage, settlement, and thermal cracking to lower the durability of the deck by allowing water and deicing chemicals to penetrate the concrete and increase the susceptibility of the reinforcing steel to corrosion. Work is underway at the University of Kansas to develop concrete mixtures that will minimize concrete shrinkage to minimize cracking in bridge decks. It has been well established that the lower restraint provided by lightweight aggregates (LWAs) can lead to increased drying shrinkage compared with mixtures containing normal aggregates, and if not adequately prewettted, the unsatured pores lead to greater water demand during construction and result in inadequate water for hydration. Due to the absorption and desorption properties of the aggregate, vacuum-saturated LWAs can readily provide an internal source of water to increase hydration and lessen the drying effects on a concrete mixture. The effectiveness of prewetted, vacuum saturated (PVS) light-weight aggregates as an internal curing agent to reduce concrete shrinkage is evaluated for curing periods of 7 and 14 days in this paper.
Characterization of superabsorbent poly(sodium-acrylate acrylamide) hydrogels and influence of chemical structure on internally cured mortar
Internal curing of mortar through superabsorbent polymer hydrogels is explored as a solution to self-desiccation. Four different hydrogels of poly(sodium-acrylate acrylamide) are synthesized and the impact of chemical composition on mortar is assessed with relative humidity and autogenous shrinkage testing. The hydrogels are characterized with swelling tests in different salt solutions and compression tests. Chemical composition affected both swelling kinetics and gel network size. Mortar containing these hydrogels had increased relative humidity and markedly reduced autogenous shrinkage. Additionally, the chemical structure of the hydrogels was found to significantly impact the mortar’s shrinkage. Hydrogels that quickly released most of their absorbed fluid were able to better reduce autogenous shrinkage compared to hydrogels that retained fluid for longer periods (>4 h), although this performance was highly sensitive to total water content. The release of absorbed water in hydrogels is most likely a function of both Laplace pressure of emptying voids and chemically-linked osmotic pressure developing from an ion concentration gradient between the hydrogels and cement pore solution. If the osmotic pressure is strong enough, the hydrogels can disperse most of the absorbed water before the depercolation of capillary porosity occurs, allowing the water to permeate the bulk of the mortar microstructure and most effectively reduce self-desiccation and autogenous shrinkage.
Ground Fault Protection for Electric Utility Generating Station Medium Voltage Auxiliary Power Systems
Most generating station designs have medium voltage auxiliary power systems with low resistance grounded neutrals.1 In order to provide even higher reliability, the auxiliary power distribution system configurations supply increasingly redundant process equipment.