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.
Low-Cracking, High-Performance Concrete Bridge Decks
Cracks in concrete bridge decks provide easy access for water and deicing chemicals that shorten the life of the deck, and field surveys show that the problem has become progressively more severe since at least the 1980s. A two-phase, 10-year Pooled Fund study to minimize cracking in bridge decks is now under way. Twenty bridge decks have been constructed in the program to date. Comparison with conventional decks shows that the techniques embodied in low-cracking, high-performance concrete (LC-HPC) bridge deck specifications have been highly successful in reducing cracking in bridge decks. The results also show that high-slump high-strength concretes result in greater cracking in bridge decks than low-slump, moderate-strength concretes and that concrete temperature control and early application of curing counteract the negative effects of casting concrete under high-temperature conditions. Early owner and contractor buy-in is needed for successful LC-HPC bridge deck construction, and top performance requires the adherence to all aspects of the specifications.
Mechanical behaviour and durability of high volume fly ash cementitious composites
The purpose of this research is to separate different morphological particles of ASTM class F fly ash, and study their effect on mechanical behaviour and durability of high volume cementitious mixtures. In this research wet separation of raw fly ash is carried out, which resulted in three layers of different morphological particles. The first layer of particles float, comprise of about 1-5% of fly ash, is identified as cenospheres or hollow spheres. The second layer of particles is measured to be 55-60% of raw fly ash and consisting of porous spherical and rounded particles rich in Si and Al. The third layer particles is measured to be about 35-40% of raw fly ash. High volume fly ash cementitious composites containing second or third layer particles are tested under compression and bending, highlighting a higher strength and ductility in comparison to cementitious ones containing raw fly ash particles. KEYWORDS. Cementitious matrix; Raw fly ash; Second layer particles; Third layer particles.