Friday, March 15, 2013

Swimming Pool - Minimum Concrete Strengths & Special Inspections

4,000 PSI Shotcrete - there ought to be a law...
aah, but there is and it is rarely adhered to or understood.

Code Requirements 

Variations of the International Building Code (IBC) have been adopted in every state of the US, except Minnesota.  The IBC has many sub-books, the most popular being the International: Residential Code (IRC), Mechanical Code, Plumbing Code, Electrical Code, and yes, Swimming Pool Code (ISPC).

No one code addresses everything one needs to comply with in order to build anything, whether it is a wood framed house, steel barn, concrete bridge or swimming pool.  Instead, one needs to comply with the various portions that apply to their project that are found in the different areas of various codes.

Concrete & Shotcrete Codes

The IRC contains sub-chapters on concrete construction and shotcrete.  Shotcrete (wet or dry mix methods of pneumatically applying concrete) is concrete construction.  The code recognizes this, so the shotcrete chapter specifies that the chapter on concrete shall also apply to shotcrete.

The concrete chapter specifies that the MINIMUM strength for the construction of any concrete structure is 3,000 PSI.  But, the chapter also directly references and incorporates ACI 318-96 (1996), which specifies that shotcrete shall be 4,000 PSI.

Further complicating matters, the chapter on shotcrete makes direct reference to and incorporates ACI 506R-90 (1990), which also specifies that shotcrete shall be 4,000 PSI.

So, merely by reference and inclusion in the IRC/IBC, the American Concrete Institute (ACI) has set the minimum building code standard for shotcrete construction at 4,000 PSI. 

Special Inspections & Testing

The IRC sub-chapter on shotcrete also requires special inspections of the following (not a totally inclusive list):
reinforcing steel
shotcrete process and placement
initial curing process
mix design
testing and sampling
28 day compression testing (laboratory)  

A special inspection, is not the regular city building official's inspection.  Rather it is a qualified inspector from an independent observation and testing firm.  The special inspector (sometimes called a deputy inspector) is required to stand and observe the entire shotcrete placement process.

They are also required to monitor the mix designs and obtain test panels for later compression testing.  This does not mean that the shotcrete company shoots material into a test cylinder.  Instead, they must build panels of a prescribed size and actually shoot the material just as if they were creating the structure.

Structural Engineers  

In an effort to "save" contractors the expense of special inspections and worse yet, the oversight of an independent inspector, some structural engineering firms specify only 2,500 PSI shotcrete.

The code requires that concrete specified over 2,500 PSI be subject to all of the above special inspections.  But, because the code specifically states that shotcrete shall be a minimum of 3,000 PSI, this maneuver is no longer valid.  Special inspections and testing can no longer be danced around - they are the law.  While not every state has included the specific shotcrete chapter in their adopted versions of the IRC, it becomes included through the inclusion of the ACI standards.

So, now if your structural engineer specifies only 3,000 PSI, they can still be in violation of the building code.  Because, through the reference of the ACI standards, the IBC/IRC (and the even more restrictive California Building Codes) now make 4,000 PSI shotcrete and special inspections THE STANDARD.


Paolo Benedetti 
Aquatic Artist, Construction Consultant & Expert Witness 
"Creating water as art."™ 
Aquatic Technology Pool and Spa 
©www.aquatictechnology.com

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