Sprayed Concrete Technology

Sprayed Concrete Technology

In Québec, liquid air-entraining admixtures are commonly used to improve the
durability of dry-mix shotcrete repairs exposed to severe winter conditions. The
liquid air-entraining admixture is mixed with the water that is added at the nozzle
during the shooting operation, particularly in order to improve the resistance of
shotcrete to scaling due to freezing in the presence of deicer salts. This procedure
requires special equipment and may cause problems in the field. This paper

presents the results of a study undertaken to verify the efficiency of powder air-
entraining admixtures to entrain an adequate air void system in dry-mix

shotcrete. Eleven mixtures were shot, using three powder and one liquid air-
entraining admixtures. The test program included compressive strength, deicer

salt scaling resistance, spacing factor and the volume of permeable voids tests.
The results obtained show that powder air-entraining admixtures can improve
both the quality of the air void system (i.e. lower the spacing factor), as well as
the resistance to scaling due to freezing and thawing in presence of deicer salts.
Keywords: compressive strength, dry-mix shotcrete, powder air-entraining
admixture, scaling, spacing factor.

Two conditions must be fulfilled in order to obtain durable repairs: the repair
material must be durable and the bond between the new material and the old
concrete must also be durable. To improve the durability of the shotcrete to
freezing and thawing cycles in the presence of deicer salts, the following
technique has been developed: a liquid air-entraining admixture is mixed with
the water added at the nozzle. This has been observed to enhance very
significantly the durability of the repair material. To obtain a durable bond,
proper surface preparation and construction practices are required [1]. Usually,
shotcrete develops good bonding characteristics with the old concrete [2].


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