International Concrete Abstracts Portal

Little research is reported in the literature on the effect of pozzolans such as silica fume on structural behavior of reinforced concrete, namely on bond and anchorage characteristics of reinforcing bars in concrete. The objectives of the study were to investigate the effect of silica fume on bond and anchorage characteristics of reinforcing bars in high performance concrete, to study the validity of the upper limit of 70 MPa (10,000 psi) imposed by the ACI Building Code 318-95 on the concrete compressive strength for determination of development length, and to evaluate the reliability of the empirical equation of Orangun, Jirsa, and Breen in estimating the bond strength of deformed bars embedded in high-strength concrete. Sixteen beam specimens were tested. Each beam was designed to include two bars in tension, spliced at the center of the span. The splice length was selected so that bars would fail in bond, splitting the concrete cover in the splice region, before reaching the yield point. The beams were loaded in positive bending with the splice in a constant moment region. The variables used were the percentage replacement by weight of cement by silica fume, casting position, and the superplasticizer dosage. Test results indicated that replacement of 5 to 20 percent of the cement by an equal weight of silica fume resulted in an average 10 percent reduction in bond strength regardless of casting position or the superplasticizer dosage used.

A survey of the rules in various national codes for detailing of concrete structures reveals wide variations in the bond strength reduction specified for fusion-bonded epoxy-coated reinforcing bars. Investigation of the background to code rules reveals that the differing test methods and bar rib patterns used in research reports consulted by the code drafters have led to the adoption of differing requirements. The paper reports two sets of tests carried out to investigate the effect of a fusion-bonded epoxy coating on bond. Fundamental tests of friction characteristics show that coating reduces friction between steel and concrete by around 40 to 50 percent when stress normal to the interface is low, but that the difference in behavior reduces as normal stress increases. Results of friction tests are related to results of pullout-type bond tests on bars with short embedment length. Bond tests demonstrate that the bond reduction due to a fusion-bonded epoxy coating depends on the bar-concrete slip at which the comparison is made, the reduction being greatest at a small slip, and on the inclination of the face of the bar ribs. It is suggested that the difference in development length requirements could be reduced if restrictions were placed on rib geometry of bars selected for coating. However, it is concluded that it would not be feasible to raise performance of coated bars to that of currently acceptable uncoated bars by specifying a minimum rib face angle for coated bars.