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.
Development and Construction of Low-Cracking High-Performance Concrete (LC-HPC) Bridge Decks: Free Shrinkage, Mixture Optimization, and Concrete Production
The development and evaluation of low-cracking high-performance concrete (LC-HPC) for use in bridge decks is described based on laboratory test results and experience gained during the construction of 14 bridges. This report emphasizes the material aspects of the construction process; a companion report will provide a detailed discussion of the construction, design, and environmental factors affecting the performance of LC-HPC bridge decks. The KU Mix design methodology for determining an optimized combined gradation uses the percent retained chart and the Modified Coarseness Factor Chart. The process begins by developing an ideal gradation followed by the determination of an optimum blend of user-selected aggregates. A Microsoft® Excel workbook enhanced with Visual Basic for Applications is available to perform the optimization process at www.iri.ku.edu. The second portion of the study involves evaluating the effect of paste content, water-cement (w/c) ratio, coarse aggregate type, mineral admixture type (silica fume, slag cement, and Class F fly ash each at two levels of replacement), cement type and fineness, a shrinkage reducing admixture, and the duration of curing on the free-shrinkage characteristics of concrete mixtures in the laboratory tested in accordance with ASTM C 157. The final portion of the study presents the specifications, construction experiences, and the preliminary evaluation of 14 LC-HPC bridge decks that have been built or are planned in Kansas. The techniques used to reduce cracking in these bridge decks are presented, and the field experiences for the 18 individual LC-HPC placements completed to date are presented. The results indicate that LC-HPC decks with an optimized aggregate gradation and design w/c ratios of 0.44 and 0.45 with cement contents of 317 and 320 kg/m³ (535 and 540 lb/yd³) have more than adequate workability, finishability, and pumpability, in addition to reduced cracking. A preliminary evaluation of these decks indicates that, on average, the LC-HPC decks are performing at a level approximately equal to or exceeding the best performing monolithic decks in Kansas surveyed over the past 15 years.
Do Potential People Have Moral Rights?
By a potential person I shall mean an entity which is not now a person but which is capable of developing into a person, given certain biologically and/or technologically possible conditions. This is admittedly a narrower sense than some would attach to the term ‘potential'. After all, people of the twenty-fifth century, if such there will be, are in some sense potential people now, even though the specific biological entities from which they will develop, i.e. the particular gametes or concepti, do not yet exist. For there do exist, in the reproductive capacities of people now living and in the earth's resources, conditions adequate to produce these future people eventually, provided of course that various possible catastrophes are avoided. Indeed, in some sense of ‘potential’ there have been countless billions of potential people from the beginning of time. But I am concerned not with such remote potentialities but with currently existing entities that are capable of developing into people.