INTERNAL MICROCRACKING, MORTAR-AGGREGATE BOND AND THE STRESS-STRAIN CURVE OF CONCRETE

Abstract

INTERNAL CRACKING OF CONCRETE IS OBSERVED DIRECTLY WITH A MICROSCOPE AND WITH A NEWLY DEVELOPED X-RAY TECHNIQUE. CRACKS ARE OBSERVED BOTH AT THE INTERFACE BETWEEN COARSE AGGREGATE AND MORTAR, AND DIRECTLY THROUGH THE MORTAR. TO INVESTIGATE THE SIGNIFICANE OF THESE TWO TYPES OF CRACK, AND OF THE PRESENCE OF INTERFACIAL CRACKS BEFORE ANY LOADING, TENSILE AND SHEAR BOND STRENGTHS OF THE MORTAR-AGGREGATE INTERFACES ARE MEASURED AND COMPARED WITH STRENGTHS FOR MORTAR AND PASTE ALONE. THEORETICAL MODELS RELATING BOND STRENGTHS TO MICROCRACKING WERE DEVELOPED AND TESTED BRIEFLY. STRESSES FROM VOLUME CHANGES ON SETTING AND HARDENING ARE CONSIDERED. BOND BETWEEN PASTE OR MORTAR AND AGGREGATE IS FOUND TO BE MUCH WEAKER THAN ANY OF THE CONSTITUENTS ALONE. CONCRETE SPECIMENS ARE UNIAXIALLY LOADED, BOTH CONCENTRICALLY AND ECCENTRICALLY, IN COMPRESSION TO VARIOUS STRAINS UP TO FAILURE. INTERNAL MICROCRACKING, DISTINGUISHED AS TO BOND OR MORTAR CRACKING, IS RELATED TO AMOUNT OF STRAIN, AND A PHYSICAL, SEMI-QUANTITATIVE PICTURE OF THE PROGRESS OF CRACKING UP TO FAILURE IS DEVELOPED. THE STRAIN GRADIENT IN ECCENTRICALLY LOADED SPECIMENS SEEMS TO RETARD NOTICEABLY THE AMOUNT OF CRACKING TRHOUGH MORTAR. PROGRESSIVE DEVELOPMENT OF MICROCRACKING IS APPARENTLY DIRECTLY RELATED TO THE SHAPE OF THE STRESS-STRAIN CURVE IN SHORT-TERM COMPRESSION. THE HETEROGENEITY INDUCED BY AGGREGATES SEEMS TO BE RESPONSIBLE FOR BOND CRACKING AND THEREFORE FOR MOST OF THE SHORT-TERM INELASTIC BEHAVIOR OF CONCRETE. /A/RRL/