A Study of Chert and Shale Gravel in Concrete

Abstract

CERTAIN CHERT AND SHALE GRAVELS HAVE LONG BEEN RECOGNIZED AS HARMFUL WHEN INCLUDED IN PORTLAND CEMENT CONCRETE EXPOSED TO FREEZING AND THAWING. MANY ORGANIZATIONS HAVE SPECIFICATIONS LIMITING PERCENTAGES OF THESE MATERIALS IN CONCRETE AGGREGATES, BUT FEW OF THESE SPECIFICATIONS DISTINGUISH BETWEEN TYPES OF CHERT AND SHALE FROM DIFFERENT GEOGRAPHICAL AREAS NOR DO THEY ALWAYS TAKE INTO ACCOUNT THE BASIC PHYSICAL PROPERTIES OF THESE MATERIALS. IN THIS STUDY, PORE CHARACTERISTICS, MINERALOGY, TEXTURE, AND STRUCTURE WERE DETERMINED FOR CHERTS AND SHALES FROM NINE INDIANA GLACIAL GRAVEL DEPOSITS BY MEANS OF MICROSCOPIC PETROGRAPHY, X-RAY DIFFRACTION, AND THE COMMON SPECIFIC GRAVITY AND ABSORPTION TECHNIQUES. BLENDS OF 2, 4, 6, AND 10 PERCENT OF CHERT OR SHALE FROM EACH SOURCE WERE MADE WITH A STANDARD DURABLE CRUSHED LIMESTONE COARSE AGGREGATE, AND THESE BLENDS WERE USED IN 3- X 4- X 16-IN. AIR-ENTRAINED CONCRETE BEAMS SUBJECTED TO UP TO 300 CYCLES OF FREEZING AND THAWING. A MEASURE OF THE AMOUNT OF DEEP-SEATED DETERIORATION OF THE BEAMS WAS PROVIDED BY DURABILITY FACTORS CALCULATED FROM THE RESULTS OF NON-DESTRUCTIVE SONIC TESTING OF THE BEAMS AT INTERVALS DURING FREEZE-THAW TESTING. SEVERITY OF SURFACE DETERIORATION WAS ALSO EVALUATED. THE INFLUENCE OF THE BASIC PROPERTIES OF THE CHERT AND SHALE GRAVELS ON THE RESULTS OF THE FREEZE-THAW TESTS WAS THEN DETERMINED. ON THE BASIS OF THE RESULTS OF THESE TESTS, THE EXISTING SPECIFICATIONS ON CHERTS AND SHALES WERE STUDIED TO DETERMINE WHETHER THE SPECIFICATIONS REALISTICALLY CATEGORIZE THESE MATERIALS. DESPITE SIGNIFICANT DIFFERENCES IN THEIR MINERALOGIES, NO DIFFERENCE WAS NOTED IN THE FREEZE-THAW DURABILITIES OF THE VARIOUS CHERT SAMPLES. FOR ALL THE CHERTS, SIGNIFICANT DEEP- SEATED AND SURFACE DETERIORATION OCCURRED ONLY IN BEAMS CONTAINING 6 TO 10 PERCENT OF MATERIAL WITH A BULK SPECIFIC GRAVITY (SATURATED SURFACE-DRY BASIS) OF LESS THAN 2.45. ALTHOUGH THE BASIC PROPERTIES OF THE SHALES VARIED EVEN MORE WIDELY THAN THOSE OF THE CHERTS, NONE OF THE SHALES CAUSED DEEP-SEATED FAILURE OF THE CONCRETE. HOWEVER, THE MOST POROUS SHALES CAUSED POPOUT DAMAGE, WHICH WAS ESPECIALLY SEVERE AT THE 6 AND 10 PERCENT LEVELS. /AUTHOR/