Obtaining swimming pool concrete cores

Obtaining concrete cores from a swimming pool, spa or watershape for the purpose of testing the in-situ (in place) strength requires expert witness supervision and adherence to sampling and testing standards.

Where to core

The locations on where to obtain the samples should be chosen to obtain a representative sample of the placed concrete.  Areas where inexperienced shotcrete operators would allow the accumulation of rebound and trimmings should be tested.  Walls and vertical surfaces should be tested as well, to ensure proper shotcrete placement and correct steel encapsulation.

Core size - Diameter & Length

The testing standards require that the ideal length to diameter (l/d) ratio should be 2, and in no instance less than 1.  Therefore, if the shell is 12 inches thick, the core diameter should be 6 inches.  An 8 inch thick shell would require a core diameter of 4 inches.

Equipment

The coring machine should be mounted to the surface or of such mass, that it does not move or allow the diamond core bit to wobble.  Wobbling can impart stress into the core sample.

The core bit must be water cooled during the coring operation.  This may saturate the outer surface of the core sample.  However, in a swimming pool that has been in place for a period of time, the shotcrete may already be fully saturated.

Reinforcement Encapsulation

A few cores should be taken through the steel reinforcement, in order to verify the complete encapsulation of the reinforcement.  Hollow pockets from shaddowing and honeycombs are a common occurrences in inferior shotcrete.  

If improper contact lap splices were used when installing the reinforcement steel, then particular attention should be paid to these areas.  The large mass of bars frequently causes honeycombs behind these splices.

Cores containing reinforcement should not be relied upon to evaluate the in-situ strength of the concrete.

Test Results

ACI 318-08, Section 5.6.3.4, specifies two criteria that must be met for accepting the strength of in place concrete (fc' = specified strength of the concrete):
1) the average of any three consecutive strength tests equals or exceeds 85% of fc';
AND
2) No individual strength test shall fall below 75% of fc'.

In other words, the average of THREE CONSECUTIVE tests must meet or exceed 85% of fc' AND the strength of every sample must be at least 75% of fc'.  

These lower acceptable margins are due to the inconsistent and unreliable development of concrete's strength in-situ.  In contrast, poured cylinders that are prepared as samples of the delivered concrete, are water cured under controlled conditions, and therefore expected to develop higher strengths (ACI 318-08, Section 5.6.3.3).



Contact the author, Paolo Benedetti of Aquatic Technology Pool and Spa at: info@aquatictechnology.com or 408-776-8220. 

Visit his website at: www.aquatictechnology.com. 
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