There have been many pull-out tests of single and double layered deformed bars to investigate bond strength in R/C members2), 4)-10). In these tests, the development lengths of the bars were identical as shown in Fig. 1 (a). These tests indicated that the bond strengths in the second layer weakened as the stresses in the first layer became large. On the other hand, some of longitudinal bars at the end sections of an R/C beam are cut off in the span if those are no longer needed against required loads. In the previous experimental studies of R/C beams11), 12), it was indicated that maximum bond stresses in cut-off bars in the second layer were larger than those in bars placed through the span, as shown in Fig. 2. In order to evaluate the bond capacities of multi-layered reinforcing bars, a new method was proposed13). This method is for evaluating the capacity of all the tension bars, and can be applied to tension bars including cut-off bars. However, the method may underestimate a bond capacity of a singly reinforced beam with cut-off bars, and tentative assumption was adopted for the method because of limited experimental studies.
In this study, pull-out tests of reinforcing bars of different development lengths, as shown in Fig. 1 (b), were carried out. The specimens were intended for the condition in the beam including cut-off bars. Test variables were the number of layers, the number of bars in the second layer, development length, ld, of bars in the first layer, shear reinforcement ratio, and concrete strength. Three bars were placed in the first layer in all the 28 specimens, and each test variable was as follows: single or double layers; two or three bars in the second layer; short ld of 500mm or long ld of 688mm; shear reinforcement ratios of 0.2%, 0.4% and 0.6%; specified concrete strengths of 21, 24, 36, and 54 N/mm2. In this investigation, bond capacities in the range of 500mm that was the same as short ld, was focused on. As a result of the monotonic pull-out tests, the following conclusions can be drawn.
(1) When single layered reinforcement is not sparse arrangement, regardless of a mixture of short and long ld, the bond strength of the short ld bar can be evaluated by the calculation for side splitting failure with safe margin.
(2) Regarding the double layered specimens, the bond strength of the bars in second layer can be evaluated by Tsuihiji formula9) by substituting bond force ratio for load ratio between first and second layers.
(3) In the specimen with different development length bars, total bond force in the range of 500mm reaches its peak before or at the same time as the peak of the bond stress of short ld bar. Because the total bond force balances on truss action in an R/C beam, it is reasonable in design to evaluate the total bond capacity in terms of checking resistance of the member. However, an additional examining must be needed for a local failure of a cut-off bar.
(4) The total bond capacity of double layered bars was larger than that of single layered bars. Effects of increasing shear reinforcement in single and double layered specimens were almost the same. However, the total bond capacity of tension bars of different ld was smaller than that of identical ld. Especially, the specimens which the mid bar in first layer was shorter than bars around it, showed lower capacities than the specimens with identical ld bars by about 80%.
