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The acceptability of unbonded neoprene caps has been established for over 20 years, and both bonded (ASTM C617) and unbonded (ASTM C1231) caps are currently used in commercial testing and in research. In this study, compressive strength results from one hundred 6 x 12 in. (152 x 304 mm) cylinders cast from a single batch of ready mixed concrete, tested with both types of caps, indicated that while either cap type produced approximately equivalent strength, there was a significant difference in the standard deviations of the two populations. Further, strength obtained with unbonded caps was less sensitive to variations in cylinder eccentricity during testing. Response to eccentricity was most pronounced at 1/4 in. (6 mm), with neoprene-capped specimens showing increased strength and sulfur-capped specimens showing decreased strength. Predominant failure modes differed with both cap type and eccentricity. Observed differences in variability would impact the probability of meeting building code requirements for acceptance based on compressive strength.
Keywords: acceptance criteria; bonded caps; compression strength; cylinder testing; neoprene caps; quality control; sulfur caps; test methods; unbonded caps; variability
INTRODUCTION
The primary purpose of this study was to investigate the influence of bonded versus unbonded caps and eccentricity of load on the variability of the compressive strength and failure modes of concrete cylinders. One hundred cylinders were cast from a single batch of ready mixed concrete, thus distinguishing this study from its many predecessors in which smaller numbers of specimens represented a large number of concrete mixtures of varying strengths. Part I of this paper explores the impact of cap type and eccentricity on the mean and variance of compressive strength for concrete with an average 28-day strength of 6000 psi (41.4 MPa). Part II analyzes the effect of these same factors and cylinder end conditions on failure mode.
RESEARCH SIGNIFICANCE
The industry has come to expect no significant difference in average compressive strength for bonded or unbonded caps. However, this study demonstrated a statistically significant and reduced variability in compression strength as a result of using unbonded neoprene caps. This difference could impact the acceptability of concrete strength test results when evaluated in accordance with the requirements of ACI 318 or ACI 301.
BACKGROUND FOR PART I
The use of unbonded cylinder caps in accordance with...