Revised formula for predicting the long-term deflection multiplier of normal and high strength concrete

M H Muhaisin,H K Ammash,A R Jawdhari

doi:10.1590/s1983-41952019000600007

Abstract

Abstract High strength concrete (HSC) has found many applications in civil engineering structures such as in high-rise buildings, and bridges. The mechanical properties of HSC are sometimes different than of normal strength concrete (NSC). In particular, HSC possess lower creep strains compared to NSC. As a result, members constructed using HSC have been found to deflect less under sustained long-term loads. However, formulas used by current codes of practice such as ACI (318) code and Australian standard (AS-3600) for predicting the long-term deflections don’t account for effects of HSC. This study aims to present a theoretical formula to calculate the long-term deflections for reinforced concrete beams made from NSC and HSC, taking into account the influence of HSC. The formula was derived from curve fitting analysis of long-term deflections obtained from several experimental tests available in literature. The presented equation considers the effects of several factors, such as compressive strength of concrete, and reinforcement at compressive zone, found in the experiments to have a significant impact on long-term deflections. The results of the equation were compared with experimental results of other researchers, and a good agreement was obtained. Following a parametric study, the long-term deflections were found to decrease to about 50% when increasing the concrete’s compressive strength from 20 to 100 MPa. The compressive steel reinforcement was found less effective in the case of HSC.

Highlights

High-strength concrete (HSC) has been used in countless applications, especially for column and shear wall construction in high-rise buildings, and in bridges such as
This study aims to present a theoretical formula to calculate the long-term deflections for reinforced concrete beams made from normal strength concrete (NSC) and High strength concrete (HSC), taking into account the influence of HSC
This study investigated the long-term deflections of RC members with focus on the effects of high strength concrete

Summary

Introduction

High-strength concrete (HSC) has been used in countless applications, especially for column and shear wall construction in high-rise buildings, and in bridges such as (prestressed girder bridges, box girder bridges, and cable-stayed bridges). The decrease in member’s dimensions may create serviceability problems of excessive deflection due to the reduced stiffness and/ or expected increase in slenderness ratio. It is, of paramount importance while optimizing the strength and weight requirements, to take appropriate measures to control deflections of HSC members under service loads. Serviceability requirement is more pronounced for HSC than NSC, given that HSC is typically used to provide a longer span and smaller cross-section (e.g. slender columns in high-rise buildings or long span girders in bridges) in which serviceability might control the design rather than strength [1,2]. Some research has found that HSC exhibits higher shrinkage than NSC made of similar materials

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