App>endix A 



EFFECT OF DRILLING CORES ON 

 COMPRESSIVE STRENGTH 



It is generally known that the com- 

 pressive strength of core specimens 

 obtained from a structure is usually less 

 than that of standard laboratory cast 

 cylinders. This observation is the result 

 of many influencing parameters, one of 

 which is the effect of drilling the core 

 specimens. Other parameters are: curing 

 environment, size of core specimen, and 

 direction of coring compared to casting 

 (to name a few). The specific topic of 

 drilling effect had not been directly 

 addressed in past studies. However, 

 investigations by Bloem (Ref 11) and 

 Campbell and Tobin (Ref 12) gave data 

 suitable for estimating the drilling 

 effect. 



Both investigations used concrete 

 slabs that had provisions for making and 

 curing cast cylinders as part of the slab. 

 At the time of test, the cast cylinders 

 were removed from the slab, and then 

 core specimens of the same size as the 

 cast cylinders were drilled from the slab. 



The Bloem study used plastic inserts 

 that were set in the slab to make the 



cast cylinders; these were subsequently 

 "pushed-out." Both the cast and cored 

 cylinders were soaked for 40 hours prior 

 to testing to insure similar moisture 

 contents. The Campbell and Tobin study 

 used concrete having two cement con- 

 tents, 5.33 and 7.33 sacks per cu yd. Cast 

 cylinders were made in metallic molds 

 and then placed in holes in the slab. The 

 cast and cored cylinders were soaked 

 prior to testing. 



The uniaxial compressive strength 

 data of select test runs are given in 

 Table A-1. The observed effect of 

 drilling is to reduce the strength of the 

 cast cylinders by a factor of 0.93. For 

 this report, the inverse, 1.07, is the 

 factor to increase the strength of cored 

 specimens so that they become 

 equivalent to cast cylinders. 



This factor of 1.07 was in close 

 agreement with that of Murphy (Ref 13) 

 who mentioned a factor of 1.06 but did 

 not present test results or the 

 background source. 



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