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: Construction Methods, Specifications, and Resistance to Chloride Ion Penetration
................................................................................................................ ii ACKNOWLEDGEMENTS ...................................................................................... iv LIST OF TABLES .....................................................................................................xv LIST OF FIGURES ...................................................................................................xx CHAPTER 1: INTRODUCTION.............................................................................1 1.1 GENERAL...................................................................................................1 1.2 FACTORS AFFECTING BRIDGE DECK DURABILITY .......................2 1.2.1 Corrosion in Concrete ......................................................................3 1.3 SIGNIFICANCE OF BRIDGE DECK CRACKING..................................4 1.3.1 Time to Corrosion Initiation ............................................................4 1.3.2 Early Age Cracking..........................................................................6 1.3.3 Crack Location.................................................................................6 1.4 TYPES AND CAUSES OF CRACKING ...................................................7 1.4.1 Plastic Shrinkage Cracking ..............................................................8 1.4.2 Settlement Cracking.......................................................................10 1.4.3 Thermal Cracking ..........................................................................10 1.4.4 Drying Shrinkage Cracking ...........................................................11 1.4.5 Orientation of Cracks.....................................................................12 1.5 FACTORS AFFECTING CRACKING.....................................................14 1.5.1 Design ............................................................................................14 1.5.2 Concrete Material Properties .........................................................15 1.5.3 Construction Practices ...................................................................16 1.6 PERMEABILITY ......................................................................................16
Ocean thermal energy conversion: The promise of a clean future
Due to the world's heavy dependence on fossil fuels for electricity, pollution and global warming is on the rise. However, numerous countries are still relying on diesel generators as their main source of energy. There is a lack of practical alternative energy source that can meet the global energy demand without posing any threat to the natural environment. Ocean Thermal Energy Conversion (OTEC) is a concept that has the potential to address this growing issue. It is basically a mechanism that exploits the temperature difference between warm surface seawater and cold deep ocean water, to produce electricity. Although OTEC has a low energy density, the thermal energy in the ocean is vastly abundant. OTEC development has been dormant for a long time since it was first proposed in 1881. However, it has now regained recognition worldwide as a realistic solution to our world energy issue. Instead of having a great potential for power generation, it also carries the ability to produce high value products from the large volume of Deep Sea Water (DSW) that can be released as byproducts of OTEC plant operation. These products can hold a very profitable place in industries such as pharmaceutical, food, cosmetics and mineral water production. Further research on ocean current and DSW properties can someday lead to the commercial used of OTEC. This paper is a review of the basic concept, present status and future prospects of OTEC around the world.