Saturday, July 5, 2014

MINUMUM Swimming Pool Strength Shotcrete Gunite PSI MPA

There are minimum specifications for the strength of swimming pool shotcrete.  

What ever you call it, it's all Shotcrete
All pneumatically applied (sprayed) concrete is called shotcrete, regardless if they are using the wet or dry mix method.  The term "gunite" is often incorrectly used to describe the dry mix method.  The American Shotcrete Association (ASA), which is a sub-organization of the American Concrete Institute (ACI) only recognize the term shotcrete.  And they only refer to the shotcrete "wet" or "dry" methods.

Specifications are the LAW
ACI 318 is incorporated and included in every state building code in the United States.  By statute, this makes the standards of ACI 318 MANDATORYSo, regardless of the concrete strengths that a structural engineer can justify with their calculations, ACI 318 dictates the minimum strengths.  This is exactly where the problems start... structural engineers think that they are doing their clients and contractors a favor by specifying a lesser strength concrete.  

Concretes that are specified above 2500 PSI require strength validation by a testing laboratory.  The testing lab will obtain samples from the job site and then test them to verify that the proper strengths are met.  2500+ PSI concretes also require that an independent special (deputy) inspector observe the entire shotcrete placement process.  Structural engineers think that they are doing everyone a favory specifying sub-standard 2500 PSI concrete to avoid these additional quality assurance measures.

ACI 318-11 Durability Standards
As can be seen in the chart below, ACI 318-11, 4.2.1 specifies durability requirements for concrete (which includes shotcrete):

Swimming pools by their very nature are always a Permeability Class P1.  They are always in contact with water where low permeability is required.

Corrosion protection for swimming pools more than likely fall into the Corrosion Classification C2, due to their exposure to both water and chlorides (this includes pools treated with chlorine, salt chlorinators or chlorine compounds).  ACI does not specify a MINIMUM level of chloride exposure, to require a C2 classification.  In any case, the C1 classification would be the absolute minimum level of corrosion protection required for swimming pools.

So swimming pools must meet the P1 and C1/C2 durability requirements.

Concrete Strengths for P1 & C1/C2 Classifications

It is clear from the following chart from ACI 318-11, 4.3.1, that swimming pool shotcrete should have a minimum strength of 4,000 PSI to satisfy Classification P1.
Classification C1 allows for the use of 2500 PSI concrete, while C2 requires the use of 5,000 PSI.

Therefore, swimming pools should be either 4,000 or 5,000 PSI, with 4,000 PSI being the ABSOLUTE MINIMUM STRENGTH.

Don't let your structural engineer do you any favors.  If there are any issues with the performance of the pool structure in the future, substandard concrete will come back to haunt everyone involved!

The Uniform Building Code, Uniform Swimming Pool and Spa Code and the California Building Codes all recognize the requirements of ACI 318.

Paolo Benedetti, SWD Aquatic Artist, Watershape Consultant, Expert Witness, International Construction Management 
Contact the author at: or 408-776-8220 "Creating water as art."™ 
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