by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
Concrete In Australia : March 2013
38 Concrete in Australia Vol 39 No 1 FEATURE: ANCHORING & PRECAST effective concrete area around the bar improves confinement and increases bond strength); and the nature of the applied loading (dynamic and cyclic loads reduce the ultimate bond stress, but this effect is yet to be adequately quantified). Within the development length of a deformed bar, the deformations bear on the surrounding concrete and the bearing forces F are inclined at an angle β to the bar axis as shown in Figure 1a 5. e perpendicular component of the bearing forces exerts a radial force on the surrounding concrete. Tepfers 6, 7 described the concrete in the vicinity around the bar as acting like a thick walled pipe as shown in Figure 1b and the radial forces exerted by the bar cause tensile stresses that may lead to splitting cracks radiating from the bar if the tensile strength of the concrete is exceeded. Bond failure is often initiated by the splitting cracks within the development length of an anchored bar (Figures 1c and 1d) or within the lap-length at a lapped tension splice (Figure 1e). Transverse reinforcement across the splitting planes (Atr in Figures 1c, 1d and 1e) delays the propagation of splitting cracks and improves bond strength. Compressive pressure transverse to the plane of splitting delays the onset of cracking in the anchorage region thereby improving bond strength. For a reinforcing bar of diameter db, the ultimate bond force over the development length (πdbLsy.t fb) must not be less than the maximum bar force (fsy Ast = fsy π db2/4). erefore, Lsy.t ≥0.25dbfsy/fb (1) Expressions for the development length in most codes of practice are similar to Equation (1) with the average ultimate bond stress fb directly related to the tensile strength of concrete and modified by coefficients of varying form and complexity to account for the factors affecting bond strength. At a lapped splice, the two parallel bars are developing stress in close proximity with the bond stresses on each bar developing in opposite directions. e anchorage of each bar is adversely affected by the presence of the other bar. As a consequence, the minimum lapped splice length Lsy.t.lap is usually specified in codes of practice as being somewhat longer than the development length, depending on the percentage of reinforcing bars being spliced at the same location and the ratio of area of steel provided to area required. When 50% or more of the reinforcement is spliced at the location, AS3600-2009 requires that Lsy.t.lap = 1.25 Lsy.t.lap. In ACI318.08 (2), the multiple is 1.3 and, in Eurocode 2 3, the multiple is as high as 1.5. Existing approaches for calculating the development length or lapped splice length of reinforcement are statistically based and their applicability outside the range of the test data is uncertain. In addition, there is little test data to indicate the effect of repeated or cyclic loading on the degradation of bond at bar anchorages or bar splices and the effects of such loading has not been adequately considered. 3.0 THE EXPERIMENTAL PROGRAM 3.1 Test specimens and loading regimes At the time of writing, a total of 24 specimens had been fabricated and tested on two different specimen types. Test Series 1 consisted of eighteen development length specimens (as shown in Figure 2) and Test Series 2 included six lapped splice specimens (as shown in Figure 3). Each specimen was cast and moist cured for a period of 14 days prior to the commencement of drying, with loading commencing at ages between 28 days and 40 days. e compressive strength and elastic modulus of concrete at age 28 days was measured on standard 150 mm diameter concrete cylinders and the flexural tensile strength was measured on 100 mm x 100 mm x 400 mm concrete prisms. Each specimen was subjected to one of two loading regimes, either static loading or cyclic loading. e static loading involved monotonically increasing the applied load on the specimen at a slow rate until failure occurs in the specimen, either by bond failure or yielding of the reinforcement. e cyclic loading involved repeatedly loading and unloading the specimen from 10% to about 50% of its static strength. Each cyclic loading specimen was subjected to at least 40,000 cycles of loading at a rate of 1.0 hertz (unless premature failure occurred) and then was loaded to failure, in order to assess the effects of repeated loading on the bond strength. e response of specimens subjected to a third loading regime (sustained loading) is also being investigated. is involves initially loading the specimen Figure 1. Splitting failures around developing bars 8. (a) Forces exerted on concrete by a deformed bar in tension (b) Tensile stresses in concrete (c) Horizontal splitting (d) Cover splitting (e) Splitting at a lapped splice adb cb cs Atr Splitting cracks Atr Tensile stresses F F F T