Title: Flexural Microcracking in Unreinforced Concrete Beams

Author(s): Paul W. Abels and Chao Hsiung Hu

Publication: Journal Proceedings

Volume: 68

Issue: 10

Appears on pages(s): 779-786

Keywords: beams (supports); flexural strength; microcracking; photoelasticity; plain concrete; stress-strain relationships; tensile strength

DOI: 10.14359/15239

Date: 10/1/1971

Abstract:
There is a similarity between the development of flexural microcracking in reinforced and pre-stressed concrete beams and also in plain concrete beams when the load-deflection curve deviates from that of a straight line. Compressive and ten¬sile strain measurements were made on unrein¬forced concrete beams at Duke University in 1967 to obtain the stress-strain relation; moreover, photoelastic studies were made to record visual microcracking of such unreinforced beams. Three different types of beams were available: they were made for auxiliary tests on composite pre-stressed concrete beams. Altogether, 24 beams of a 4 x 4 in. cross section and a 3-ft span, tested with the loads at the one-third points, were available and also four beams with a 3.5 x 8 in. and 3.5 x 10 in. cross section, respectively, tested at 7 ft 6 in. span by two central point loads 2 ft apart. At testing of the first quarter of the beams, the strains were measured by mechanical strain gages, positioned at the side faces of the beams, but the strain readings were insufficiently exact. Consequently, on the remaining three-quarters of the specimens, electrical resistance strain gages were attached to the compressive and tensile faces, respectively. The diagrams obtained from these strain measurements can be used to study the shape of stress-strain curves on a statistical basis. In addition, the relation between flexural micro-cracking and modulus of rupture was studied using photostress techniques.