In 1957 and 1958, Armand Gustaferro managed a plant that produced prestressed concrete beams and girders for 50 bridges on the Illinois Tollway. At that time, the maximum design strength given in ACI 318 was 3750 psi. Gustaferro, however, needed a compressive strength of 4000 psi at strand release and 5000 psi at 28 days. The tollway engineers were concerned that high-strength, air-entrained concrete could not be economically achieved on a daily basis. So they specified non-air-entrained concrete.
Would that happen today? If air contents really need to be in the 4% to 7% range, Gustaferro's non-air-entrained concrete should have already been replaced. But it hasn't. Even today, the non-air-entrained beams and girders his plant made are still performing well.
In 1982, a visual inspection of 20 Illinois Tollway bridges by Gustaferro, Hillier, and Janney indicated that, in general, the durability of the bridge girders had been excellent. The girders had been essentially maintenance free, and only minimal freeze/thaw deterioration or rusting of reinforcement occurred.
The authors report that the prestressed concrete piles that served as the bridge piers also were in generally good condition and required little or no maintenance. After 25 years, however, some piers suffered freeze/thaw deterioration in the zone where passing vehicles had splashed deicing-salt solutions onto the piers. This would seem to confirm current ACI 318 requirements for air entrainment in areas in contact with deicing salts.
At the Portland Cement Association, researchers made precast panels in their lab and tested them in a simulated outdoor environment. They also tested high-strength concretes using a more standard freeze/thaw test in water. Both test results indicated good performance for non-air-entrained concrete.
A.W. Isberner investigated the resistance to freezing and thawing of precast panels with facing mixes of white quartz aggregates and white portland cement, both with and without an air-entraining agent. None of the panels showed any sign of deterioration of the facing concrete after 125 cycles of freezing and thawing.
PCA funded studies in 1960 and 1978. These tests showed that even non-air-entrained concretes were very frost-resistant when air-dried before freezing and thawing in water.
The authors wrote, "... The results are of considerable practical significance, particularly to the precast, prestressed industry, which sometimes experiences difficulty in obtaining the high strengths usually specified while providing the required air contents specified for durability."
ACI 318-99, "Building Code Requirements for Structural Concrete," requires concrete exposed to freezing and thawing or deicing chemicals to be air-entrained. The minimum permissible ACI 318 total air contents for moderate exposures to freezing and thawing are low.
The 1983 revision of ACI 318 was the first to specify total air content required for frost resistance, based on a classification of severe or moderate exposure.
- Moderate exposure: Service in a climate where freezing is expected but where the concrete will not be continually exposed to moisture or free water for long periods prior to freezing and will not be exposed to deicing agents or other aggressive chemicals. Examples include exterior beams, columns, walls, girders, or slabs that are not in contact with wet soil and are located such that they will not receive direct applications of deicing salts.
- Severe exposure: Concrete that is exposed to deicing chemicals or other aggressive agents or where the concrete may become highly saturated by continued contact with moisture or free water prior to freezing. Examples include pavements, bridge decks, curbs, gutters, sidewalks, canal linings, or exterior water tanks or sumps.
ACI 211 also indicates that if a member is not continually wet and will not be exposed to deicing salts, lower air content values such as those for a moderate exposure are appropriate even though the concrete is exposed to freezing and thawing.
We agree with the ACI 211 definition that places vertical concrete in the moderate-exposure category unless it's in direct contact with wet soil or deicing salts. However, the abbreviated definition of moderate exposure in ACI 318 doesn't make it clear that if there is no exposure to wet soils or deicing salts, moderate exposure is the correct category.
Unfortunately, both ACI 301-96 and 301-99 have the same language for specifying the total air content for concrete. Section 188.8.131.52 of ACI 301-99 states "Air content: Unless otherwise specified, concrete shall be air-entrained. Unless otherwise specified, air content at the point of delivery shall conform to the requirements for severe exposure."
ACI 301 contains the definitions for moderate and severe exposures in the Optional Requirements Checklist at the end of the document, not part of the specification.
Obviously, it would be preferable to include more detailed air-content information in the specification that would allow interior concrete to be non-air-entrained and exterior vertical concrete to have air contents that fit the moderate-exposure category.
Is air free? ACI 211 states that "... each added percent of air lowers the maximum strength obtainable with a given combination of materials." As the air content increases, therefore, producers add more cement to offset the strength reduction.
For higher-strength concretes, the strength loss due to unnecessarily high air-content requirements increases, as does the cost of increasing the cement content to offset that loss.
When the air content for vertical concrete is lower than specified, the engineer or architect may provide an extended warranty, apply sealers or coatings to prevent saturation of concrete by water, or reduce payment to cover potential future maintenance or repair.
In the worst case, removal and replacement of the concrete may sometimes be justified when the air-content requirements don't meet those for moderate exposure since the concrete may not perform satisfactorily. However, if the engineer has specified air contents for severe exposure when a moderate exposure is appropriate, a warranty or pay reduction may not be necessary or reasonable. The field and laboratory data we've cited show that vertical concrete with a low air content can perform satisfactorily in a freeze-thaw environment.
Article includes a walk through Chicago's Grant Park with Leo Schlosberg of Cary Concrete Products to inspect the condition of 70-year-old, non-air-entrained precast concrete.