Concrete Slump Loss


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Title: Concrete Slump Loss

Author(s): Robert W. Previte

Publication: Journal Proceedings

Volume: 74

Issue: 8

Appears on pages(s): 361-367

Keywords: admixtures;hot weather construction;hydration;mixing;plasticity;ready-mixed concrete;retardants;retempering;setting (hardening);slump tests;water;water-cement ratio;water-reducing agents;workablity.

Date: 8/1/1977

The effects of intial slump, temperature, and water-reducing admixtures on the loss of concrete plasticity with time were determined. Intial slump was varied by changing water-cement ratio. Slump was measured at 30 min intervals until a total mix and agitation time of 2 hr was reached. Concrete temperatures of 21 C (70 F) and 29 C (85 F) were studied. The intial slump values ranged from 8.9 cm (3.5 in.) to 19.1 cm (7.5 in.). The added water required to retemper concrete after 2 hr to a slump value of 8.9 to 10.2 cm (3.5 to 4 in.) was determined. The results of this study indicate for both reference and admixtured concrete that slump loss is proportional to the intial slump level: the higher the initial slump, the higher the slump loss. Concrete containing conventional water-reducing admixtures allows a significant reduction in the total water required after retempering. The extent of this slump loss is proportional to the concrete age. For instance, the difference in slump loss between admixtured and reference concrete is generally less than 1.3 cm (0.5 in.) at 30 min. In contrast, at 120 min., the difference is on the average less than 0.6 cm (0.25 in.). It is not desireable to compensate for expected slump loss in reference concrete by using a higher initial slump value since the retempered concrete will require a higher water-cement ratio.