Q.: ASTM C 42-99, Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete, requires measuring the length of the capped specimen to the nearest 0.1 inch (2 mm) and using this length to compute the length-to-diameter ratio prior to testing. What length do I use when I test the cores with unbonded pad caps instead of capping them?
A.: We contacted Tony Fiorato, president of Construction Testing Laboratories, Skokie, Ill., who believes that you should not use pad caps when testing cores following ASTM procedures.
Section 7.4 of ASTM C 42 states: The ends shall be sawed or ground to tolerance or capped in accordance with capping procedures for hardened concrete specimens of Practice C 617. ASTM C 617, Standard Practice for Capping Cylindrical Concrete Specimens, references as an alternative procedure ASTM C 1231-00, Standard Practice for Use of Unbonded Caps in Determination of Compressive Strength of Hardened Concrete Cylinders. However, Section 1.1 of ASTM C 1231 states: This practice covers requirements for a capping system using unbonded caps for testing concrete cylinders molded in accordance with Practice C 31/C 31M or C 192/C 192M.
Fiorato's understanding is that the current standard (ASTM C 42) doesn't permit the use of unbonded caps for testing cores because there is a lack of data on their performance over the range of l/d ratios that can be encountered with core specimens. Also, there is some concern over the fact that the edges of the core ends tend to be chipped during drilling, and this may affect results more significantly when unbonded caps are used.