S.-J. Han/D. H. Lee/S.-H. Cho/S.-B. Ka/K. S. Kim · Estimation of transfer lengths in precast pretensioned concrete members based on a modified thick-walled cylinder model
Table 4. Dimensional and material properties used for parametric study
bw (mm)
h (mm)
fpu (MPa)
C (mm)
fct (MPa)
200
200
1860
20–100 20–100
fpj (MPa)
db (mm)
0.4–0.8fpu
9.5–15.2
lengths since the confining stresses induced by the surrounding concrete become larger as the concrete cover thickness increases. When the concrete cover thickness is greater than about 40 mm, however, its effect on the magnitude of the transfer lengths becomes marginal. This is because sufficient confining stresses can be developed by the surrounding concrete with a certain cover thickness. Fig. 7b illustrates the effect of the compressive strength of concrete at release on the magnitude of the transfer length. The analysis results show that the transfer lengths decrease as the compressive strength of concrete at release increases, which is because the confining stresses developed by the uncracked concrete cylinder pc and the residual tensile stress in the cracked cylinder sq increase as the compressive strength increases. Consequently, the bond stresses between prestressing tendon and concrete increase, and so the transfer lengths decrease. For the magnitude of the initial prestress and the diameter of the prestressing tendon as shown in Figs. 7c and 7d, the transfer lengths increase as both factors become larger. This is because the prestressing force itself increases in proportion to the magnitude of the initial prestress and the diameter of the prestressing tendon. The proposed model provided reasonable trends for the transfer lengths of PSC members according to the various influential factors. Its accuracy for estimating the transfer lengths of test specimens will be described in the following section.
a) Effect of concrete cover thickness
b) Effect of concrete compressive strength at release
4.3 Verifications Fig. 8 shows a comparison of the transfer lengths of specimens reported by Oh et al. [12] and those estimated by the proposed model. The longitudinal bond stress distribution of the test specimens calculated by the proposed model is shown for reference at bottom right of each graph. Note that the initial prestress of the tendon is about 0.7 fpu for all test specimens. As shown in Figs. 8a and 8b, the transfer length of specimen M12-N-C4-1 was calculated to be smaller than that of specimen M12-N-C3-2, which is due to the larger concrete cover thickness C of specimen M12-N-C4-1 than that of specimen M12-N-C3-2. In other words, the pretensioned concrete member with a larger concrete cover thickness can provide higher confinement, and thus its transfer length is surely smaller than that of a member with small concrete cover thickness. In addition, as shown in Figs. 8b and 8c, the transfer length of specimen M12-H-C4-1 was calculated to be smaller than that of specimen M12-N-C4-1, and this tendency was also observed in the test results. This is because even though the concrete cover thickness C of both specimens was 40.0 mm, the tensile strength of the concrete ft of specimen M12-H-C4-1 was greater than that of specimen M12-N-C4-1. Therefore, specimen M12-H-C4-1 showed a relatively higher bond strength, as shown in the bond
c) Effect of magnitude of initial prestress
d) Effect of prestressing tendon diameter Fig. 7. Effect of various parameters on transfer length
Structural Concrete 17 (2016), No. 1
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