Bridge Decking

EXPANDED SHALE CLAY AND SLATE INSTITUTE-SALT LAKE CITY, UTAH 84117

Freeze-Thaw Durability of Structural Lightweight Concrete

INTRODUCTION

The performance of structural lightweight concrete in a freezethaw environment has proved to be excellent in a variety of applications over many years. Two major laboratory test programs have been undertaken by members of the Expanded Shale, Clay and Slate Institute (ESCSI) to confirm this performance and to investigate the effect that different construction practices have on the freeze-thaw durability of structural lightweight concrete. The purpose of this Information Sheet is to summarize the test programs and the results obtained from them.

FREEZE-THAW EXPOSURE AT AN EARLY AGE

As was done in the ESCSI studies, it is normal practice to conduct freeze-thaw durability tests under laboratory conditions using ASTM C666 procedures, where specimens are cured 14 days before freezethaw exposure is begun. In actual field practice the first freezing cycle may occur as soon as the first night after the concrete is placed if proper winter concreting practices are not employed. Because lightweight concrete, especially that which is placed by pumping, has a relatively high moisture content, there is a natural concern about the freeze-thaw durability of this material.

SCOPE OF LABORATORY INVESTIGATIONS

Freeze-thaw durability studies of various lightweight aggregates produced by members of the ESCSI were conducted by the Civil Engineering Department of the University of Toledo in 1968 and again in 1974. The 1968 program evaluated eight different lightweight aggregates for freeze-thaw durability and studied the effect of replacing the lightweight fine aggregate with normal weight aggregate. Normal weight fine aggregate is often used in lightweight concrete. This is generally done to reduce costs, but results in a higher density concrete. Other variables in the 1968 study included various air curing periods prior to testing and the effect of the moisture content of the lightweight aggregate (prior to mixing) on the freeze-thaw durability.

Since lightweight concrete often has more than 14 days curing prior to freeze-thaw exposure, the effect of various air curing periods was evaluated. The moisture content of the aggregates prior to mixing was varied from approximately 2/3 Of the 24 hour absorption up to 100 percent of the 24 hour absorption. This was done to determine if relatively higher initial moistures resulted in a loss of freeze-thaw durability.

The 1968 study confirmed that excellent freeze-thaw durability can be achieved using lightweight aggregates in structural concrete. In the years that followed, placement of lightweight concrete by pumping became predominate. To achieve satisfactory pumping results the moisture content of the lightweight aggregate needed to be higher than these studied in the 1968 series. Thus the main objective of the 1974 series was to evaluate the freeze-thaw durability of lightweight concrete produced with lightweight aggregate that was saturated to enhance pumpability.

TESTING PROGRAM - CONCRETE MIXES

In the 1968 series the aggregate's average 24 hour absorption was 5.7%. In the 1974 study the average aggregate moisture contents were 4.8% for the 2/3 of 24 hour absorption condition, 10.4% for the maximum ambient absorption condition, and 19.1 % for the thermal or vacuum saturation condition. In both studies mixes were made with a cement content of 611 Ibs. per cubic yard of concrete, and concrete was made with a 4 inch slump and 6% air content. The fresh weight of the lightweight concrete in the 1968 study was approximately 102 Ib./cu. ft. The sand fines "semi-lightweight" concrete in both the 1968 and 1974 studies averaged approximately 116 Ib./cu. ft. Twenty-eight day strengths averaged 5460 psi in the 1974 series and 6100 psi in the 1968 series.

The 1968 study involved lightweight aggregate from eight different producers. For each of the eight aggregates tested, mixes were made with both lightweight aggregate fines and natural sand fines, a commercial blend sand from Elgin, Illinois. The lightweight aggregate in each of the mixes was pre-wetted to have either 66% or 100% of its 24 hour moisture at time of batching.

TESTING PROGRAM—TEST SPECIMENS AND CURING

For each aggregate in the 1968 program, nine concrete prisms were made for each of the four different mixes tested. All prisms were moist cured for 14 days, and then three prisms were air dried (73· F. 50% R.H.) for 14 days, three for 28 days, and three for 56 days. The 1974 series specimens were cured the same as the 1968 series, but three additional prisms were made and air cured for 98 days for each mix . Companion test cylinders for compressive and splitting tensile strength were made for each mix as well. Cylinders were moist cured 7 days, then air cured 21 days and tested .

After air curing all freeze-thaw prisms were soaked in water for 24 hours, then placed immediately into the curing chamber to begin freeze-thaw cycling. Except for the air curing periods, all procedures employed were in accordance with ASTM C666 "Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing''. Both freezing and thawing were performed in water, and all values reported are the tests conducted after 300 freeze-thaw cycles were performed.

RESULTS

CONCLUSIONS

All of the lightweight aggregates tested demonstrated good, if not excellent, freeze-thaw durability. A slight improvement in the Durability Factor was evident when normal weight sand was substituted for lightweight fines in the mixes and also when air curing periods were extended. The high degree of aggregate saturation required for pumping appeared to lower the Durability Factor, but the amount was not significant and all of the values obtained for concrete containing even the highly saturated aggregates were within the normal range. The most significant variable in the studies was the source of the rotary kiln expanded shale, clay or slate lightweight aggregate. Even the worst performing aggregate had an average Durability Factor barely under 90, which is good performance. The freeze-thaw durability test programs verified that properly designed and cured structural lightweight concrete mixes have good to excellent durability, whether they are designed for conventional or pump placement.

REFERENCES

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