A design consideration for durability of high-performance concrete
Introduction
The castatrophic earthquake that took place in Taiwan on 21 September 1999 caused serious causality and attracted much attention to the quality of construction in terms of its strength, ductility and durability. Poor quality has often been a shortcoming of traditional concrete. It poses danger to the safety of the users' life and property. Comparatively speaking, greater emphasis is usually paid to the strength and workability of the concrete mixture while durability has never been given much attention in the mix design. Consequently, two kinds of problems arise. One is the over-stiffness of the mixture which lacks in workability. To remedy such a problem, water is added, leading to further deterioration in quality. The other is the serious drying shrinkage cracking as a result of excessive amount of cement used. In the past, ACI 318-89 used to take account of the water to cement ratio (W/C) as a safety criterion. As a result, deterioration, corrosion, bleeding, efflorescence or cracks were commonly found in many buildings within a few years after construction. These problems are signs of aging caused by the neglect of durability in the concrete design.
The mixture design of high-performance concrete (HPC) relies heavily on the appropriate amount of paste used. In addition, chemical admixture and pozzolanic materials as well as the low water content and low amount of cement paste were used to achieve high durability [1]. ACI 318-95 [11] has been recently revised, specifying that structural concrete should have high durability, as presented by the water to binder ratio (W/B) [2]. A long-term monitoring device should be made available for large structures so as to establish durability data and to accumulate further experience in following up on concrete quality which can serve as feedback for future design. In this study, the corrosion behavior of singly reinforced HPC beams under repeated loading in marine environment was examined. The results, thus obtained, may shed light on the design of highway structure concrete in humid coastal environments.
Section snippets
HPC mix proportioning
HPC designed according to the densified mixture design algorithm (DMDA) has to be safe, durable, workable, economic and ecological. However, this eugenic mixture logic is different from that of the traditional ACI mixture design [3]. The following is a comparison between the two mixture designs with respect to considerations of workability, safety, durability, economy and ecology.
Experimental plan
The durability of beams made according to the HPC mixture proportion algorithm and those fabricated following the ACI 318-89 [11] are compared with respect to the following: (1) optimum packing density of materials; (2) application of pozzolanic materials; (3) boundary activator.
Medium and long-term durability index
As the W/C ratio can only control early-age strength of concrete [3], the major influence on long-term strength development lies in the W/B ratio because it takes into account the pozzolanic effect. Among the existing mixture specifications, durability is guaranteed by controlling the air entrained, the W/B ratio and the content of chloride content without considering the negative effect of physical and chemical interactions [1]. Meanwhile, low W/B ratio may reduce the amount of cement required
Conclusions and suggestions
- 1.
HPC contains pozzolanic materials such as fly ash, slag for lower water permeability, smaller voids and higher impedance resistivity coefficient, and thus improving durability.
- 2.
The addition of steel fiber could prevent steels from corrosion. However, after concrete cracks, for any mix design of concrete, the steel corrosion would accelerate. However, HPC group has better corrosion resistance.
- 3.
The ACI provisions with respect to cracking width of reinforced concrete structures cannot handle the
Acknowledgements
The authors thank the National Science Council of the Republic of China for the financial support that made this publication possible. We are grateful to Professor J.C. Chen and T.S. Liao,Y.F. Chan, J.S. Guo for their pioneering work and assistance on this subject to help finish this research.
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