Stronger Concrete for Durability in North
In 2008, the American Concrete Institute (ACI) completely revamped Chapter 4 regarding the durability of concrete. This code is used in correlation with IBC 2009 and IBC 2012. The ACI code now asks the engineer of record to designate an exposure category and class. This designation comes with a required minimum concrete strength.
F1 and F2 Exposure Class
Northern states have footings subject to cycles of freezing and thawing. Footings at sites that have a concrete pad under the footprint of the canopy will be mostly protected, having only occasional exposure to moisture. The footing concrete will be designated an F1 exposure class (moderate), which comes with 4500 psi minimum compressive strength concrete which also needs to be air-entrained.
We will encounter the F2 exposure class (severe) in northern states for sites where the canopy does not have a concrete pad beneath its footprint, and columns go directly into gravel or grass. These locations will have freezing and thawing cycles, as well as frequent exposure of moisture on the footing. Expect more air-entrainment.
Strength vs Durability Requirements
Previously, we have specified a minimum compressive strength of 3000 psi, very standard in the construction industry. For our canopies, especially in northern states, we are using a 4’ deep footing to get the bottom of the footing below the frost line. The strength of the concrete is not critical in the footing design due to having 4’ thick of concrete. What governs our footing design are stability and bearing checks.
Even though the 3000 psi strength of the footing is sufficient in carrying loads, by code we must use the more stringent of what’s required for strength vs durability. For northern states, we must go with 4500 psi strength concrete. For southern states, the concrete will be designated as F0, which comes with 2500 psi minimum compressive strength concrete, no air-entrainment. For southern states, our previous standard of 3000 psi will still be specified.
Designated Minimum Compressive Max Max. Air-Entrainment
Exp. Class Concrete Strength 28 days (psi) w/cm Aggregate (± 1.5%)
F0 3000 (strength) N/A 1.5” Not required
F1 4500 (durability) 0.4-0.5 1.5” 4.5%
F2 4500 (durability) 0.4-0.5 1.5” 5.5%
Example of general notes for our F101 footing drawing:
“Specifications are for canopy footings of normalweight concrete. Slump shall not exceed 4”. The concrete shall attain a minimum compressive strength of 2925 psi at 7 days, and 4500 psi at 28 days. The concrete canopy footing is designated as F1 exposure class. Concrete supplier shall follow requirements of ACI 318 Chapter 4. Maximum water to cementitious material ratio (w/cm) is 0.40-0.50. Concrete shall be air-entrained a minimum of 4.5% (± 1.5%) for 1.5” max aggregate size.
Owner is responsible for arranging and analyzing results of tests. If test result for 7 days is less than required, construction on the footings shall halt until passing 28-day tests are attained.
Installation of canopy on footings that have attained a minimum of 2925 psi (65%) of the total 4500 PSI designed F’c compressive strength within 7 days, is acceptable for this type of structure.”
Planning for 2016
It seems that some engineers have not caught on to the changes in the code on minimum durability requirements yet, so this was a breakdown for how it affects canopy piers. The engineers for convenience stores and carwash buildings should also review the chapter and apply it to their designs and specifications. It is a good time to make the necessary changes while we all plan for 2016 construction.