Q.: We have been building concrete arch culverts with shotcrete. They're generally protected by earth cover, but we're wondering about durability at the water line. How well will they resist freezing and thawing damage? Should we use air-entrained shotcrete?
A.: The American Concrete Institute's "Recommended Practice for Shotcreting" (ACI 506-66: revised 1983) cautions that it is not feasible to entrain air in dry-mix shotcrete. However, air-entraining admixtures may be used in many wet-mix applications. To entrain a given percent of air in the shotcrete as placed, ACI 506 recommends about twice that quantity in the original mix to allow for loss of air in gunning. According to ACI 506, data on the effect of entrained air in shotcrete are limited, but air entrainment would be expected to increase resistance to freezing and thawing damage and to make some mixes more workable. We received the results of some high-strength shotcrete tests from Rusty Morgan of Hardy BBT Limited, Burnaby, British Columbia. One objective of the tests was to study freeze-thaw durability of both wet and dry mixes, and the results are interesting although the number of specimens was limited.Morgan tested three samples from each of six mixes under study:
- Plain control
- With 13.1% silica fume (microsilica)
- With silica fume and steel fibers
- Plain control
- With 12.6% silica fume
- With silica fume, steel fibers, and accelerator
All the wet mixes also contained an air-entraining agent. The dry mix was designed with a water-cement ratio of 0.29 and the wet mix, 0.35. Their 28-day compressive strengths were in the 9000-12,000-psi range.
|Shotcrete Air Void and Durabiltiy Results
Study by Morgan and others
|Mix||Hardened air content %||Air void spacing factor, inch||Durability factor, %|
After 300 freeze-thaw cycles, the study reports that excellent durability factors above 96% were achieved with all of the mixes tested, whether containing an air-entraining agent or not. Spacing factors for the air voids in all the mixes were significantly higher than the 0.008 inch (0.2 mm) generally recommended for air-entrained concrete. Tests were conducted in accordance with ASTM C 666, "Test for Resistance of Concrete to Rapid Freezing and Thawing," Procedure A, with one exception. Longer periods of moist curing of the specimens were used to permit most of the pozzolanic reaction with the silica fume to take place before the freeze-thaw cycling. Measurements of the air void system were determined using ASTM C 457-82 "Microscopical Determination of Air-Void Content and Parameters of the Air-Void System in Hardened Concrete," modified point count method.
Compressive strengths of all the shotcrete mixes studied were higher after undergoing the freeze-thaw tests; the increase ranged from 10% to 65%. Overall, these results suggest high durability of high-strength shotcretes, regardless of air-entrainment. A complete report of these tests has been submitted to a technical society for publication, and Concrete Construction will advise readers when it is available.
It would still seem prudent to use an air-entraining agent for lower strength shotcretes where exposure to freezing and thawing is expected. According to Schrader and Kaden,* "The high-quality paste and mortar achievable with shotcrete having low water-cement ratios, low porosity, and relatively high-strength results in a higher initial resistance to damage, but it will still deteriorate progressively and extensively when saturated and frozen if it has strengths less than about 10,000 psi, and if it does not contain proper entrained air bubbles."
However, they mention the possible effectiveness of latex modifiers in shotcrete that is not air entrained. "Because properly formulated and applied latex modified shotcrete has extremely low porosity and permeability, and because of the apparent reduction in interconnecting capillaries, it is virtually impossible to saturate it sufficiently to cause freeze-thaw damage. It is virtually watertight and may become vaporproof.
Reference: Schrader, E., and Kaden, R., "Durability of Shotcrete," pages 1071-1101, Concrete Durability, ACI Special Publication SP-100, American Concrete Institute, Box 19150, Detroit, Michigan 48219, 1987.