Strain and bond stress distributions in tension lap joints in reinforced concrete

Abstract

Fifteen reinforced concrete tension specimens were tested, each reinforced with a pair of lapped rods. Longitudinal reinforcement strain distributuions were measured using electrical resistance strain gauges installed within the reinforcement, and strain concentration gauges were used to obtain very detailed information at the ends of some rods. Thirteen of the specimens were loaded incrementally to study short-term effects and the other two were used for a preliminary investigation of sustained loadings. Results show the changing nature of the strain distributions, first as transverse cracks were developed and subsequently as lap failure was approached. The test results have been compared with the theoretical work of Tepfers. Peak and mean bond stresses at the formation of each crack were calculated and compared and bond stress distributions at lap failure are discussed.

The behaviour of long tension reinforcement laps

“…Tested bar diameters were ∅ = 12 mm and ∅ = 20 mm with lap lengths ranging between 10∅ and 63∅. Judge et al (1990) found longitudinal splitting cracks to initiate at peaks in the bond stress distribution which occurred at lap ends and adjacent to transverse cracks. The reinforcement strain distribution became more linear near failure but detailed strain measurements revealed significant variations in bond stress immediately prior to failure.…”

“…Finally, strain gauges are positioned prior to gluing the bar halves together with epoxy resin. Judge et al (1990) used this technique in tests of axially loaded tension prisms reinforced with single pairs of lapped bars of either the same or mixed diameter. Tested bar diameters were ∅ = 12 mm and ∅ = 20 mm with lap lengths ranging between 10∅ and 63∅.…”

“…The reinforcement strain distribution became more linear near failure but detailed strain measurements revealed significant variations in bond stress immediately prior to failure. Judge et al (1990) compared their strain distributions with the predictions of the simplified modulus of displacement theory of Tepfers (1980) in which interaction with concrete is neglected by assuming the reinforcement percentage to be infinite. Transverse cracks are assumed to form at the ends of lapped bars.…”

“…Transverse cracks are assumed to form at the ends of lapped bars. The model is applicable to lapped bars of equal diameter and is valid prior to the development of longitudinal splitting cracks which change bond behaviour (Judge et al, 1990). According to Tepfers the reinforcement stress and bond stress along lapped bars 1 and 2 of equal diameter are given by:…”

“…and at the ends of laps. Outside this load range, the model is not applicable due to changes in bond behaviour (Judge et al, 1990). …”