Monday, March 24, 2014

Sausalito Belvedere Swimming Pool Design Shotcrete Strengths

ACI 318-11 requires that the shotcrete walls on pools be increased in strength and thickness.
For clarification, "Shotcrete" is defined by the ACI as either the wet or dry (incorrectly referred to as "gunite") process of pneumatically placing concrete.

ACI - The American Concrete Institute

ACI is the definitive expert on concrete.  So much so, that they publish a myriad of standards that relate to concrete and it's use.  The ACI standards are actually adopted by and incorporated into the International Building Code (the building code adopted across the entire United States).

Because ACI Standard 318-11, "Building Code Requirements for Structural Concrete" has been directly incorporated into the International Building Code by the California Building Code.  This effectively makes it the law of the land.

ACI 318-11 Changes the Requirements

ACI 318-11 redefined the parameters of the required coverage of concrete required over the reinforcing steel.  Prior versions of ACI 318 were ambiguous as to what was actually defined as sulfate or corrosive conditions.  It establishes the additional shotcrete protection required when exposed to defined levels of sulfates, water containing chlorides (salts) and permeability.

ACI 318-11, Table 4.2.1 indicates that shotcrete swimming pools are required to meet the criteria for Permeability - P1 (condition defined as: "in contact with water where low permeability is required") AND Corrosion Resistance - C2 (a condition defined as: "concrete exposed to moisture and an external source of chlorides from deicing chemicals, salt, brackish water, seawater, or spray from these sources").  It does not define the PPM level of salt required to create an "external exposure to chlorides."

Those resulting P1 and C2 classifications require specific mix design ratios (ACI 318-11, Table 4.3.1).

ACI 318-11, Table 4.3.1, P1 classification sets a maximum water to cement ratio (w/cm2) of 0.50 and a MINIMUM 28 day compressive strength (f'c) of 4,000 PSI.  While the C2 classification establishes a maximum water to cement ratio (w/cm2) of 0.40 and a MINIMUM 28 day compressive strength (f'c) of 5,000 PSI.

Therefore, any swimming pool that is going to be treated with sodium compounds is REQUIRED to have a maximum water to cement ratio (w/cm2) of 0.40 and a designed compressive strength (f'c) of 5,000 PSI.  Sodium compounds utilized in swimming pool sanitization include sodium chloride compounds and solutions (chlorine), sodium bromide (bromine) or sodium (salt-electrolysis chlorination).  Furthermore, the sodium (salt) levels in pools treated with sodium chloride compounds (chlorine) actually becomes more concentrated over time as the chlorides are consumed.

The C2 classification (Table 4.3.1) further specifies that the requirements of ACI 318-11, 7.7.6 "Corrosive Environments" be met.  Section 7.7.6 states that the coverage over reinforcing steel shall not be less than 2 inches for walls and slabs exposed to such corrosive environments.

Structural Engineers & Building Departments often ignore the standards

Many structural engineers and building departments do not abide by the requirements of ACI 318-11.  They either attempt to justify design criteria less than the standards, are not versed on the most up to date requirements or simply refuse to comply with the codes.

When there is an issue of structural integrity on a pool, the most recently adopted version of the IBC, CBC or ACI standards will apply in court, whether or not they were followed by the engineer, building department or enforced by the building inspector.

Do you want to be left hanging by a structural engineer who will have to defend his decision to design a structure that was below the requirements of ACI 318-11 and the building codes?

Do you want to take the risk that he will be able to convince a jury of lay people that a weaker and substandard structure was acceptable?

I thought not!

ACI's position is to design to a standard of:
A.  A minimum compressive strength (f'c) of 5,000 PSI
B.  A maximum water to cement ratio (w/cm2) of 0.40
C.  A minimum concrete coverage of 3 inches between the earth and steel
D.  A minimum of 2 inches of coverage over the steel (water side).

Therefore, the minimum wall thickness is 3 inches, 2 times the bar diameters (assuming a single curtain of steel bars that cross each other in a grid pattern), plus 2 inches of coverage.  For #4 (1/2 inch) bars this would be 3" + 1" + 2" = 6" thick wall.

Contact the author, Paolo Benedetti of Aquatic Technology Pool and Spa at: or 408-776-8220. Visit his website at: All Contents © Aquatic Technology Pool & Spa, 2013. All rights reserved.

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To contact the author, Paolo Benedetti of Aquatic Technology pool and Spa, email him at: or call 408-776-8220.
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