Effect of Retempering With Superplasticizer on Properties of Fresh and Hardened concrete mixed at higher ambient temperatures

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Title: Effect of Retempering With Superplasticizer on Properties of Fresh and Hardened concrete mixed at higher ambient temperatures

Author(s): M. A. Samarao, V. Ramakrishnan, and V. M Malhotra

Publication: Special Publication

Volume: 119

Issue:

Appears on pages(s): 273-296

Keywords: compressive strength; flexural strength; fresh concretes; hardened concretes; high temperature; hot weather construction; plasticizers; retempering; water-cement ratio; workability; Materials Research

Date: 9/1/1989

Abstract:
In hot climates, it may be necessary to retemper the concrete to maintain the required workability. The retempering is done by additional dosages of superplasticizer. However, there is a paucity of information on the effects of retempering at higher temperatures. Therefore, an extensive investigation was undertaken to determine the slump loss characteristics and effect of repeated retempering with superplasticizer on the strength and elastic properties of concretes mixed at ambient temperatures ranging from 30 to 60 C (86 to 140 F). A total of 15 concrete mixes with water-to-cement ratios 0.40, 0.50, and 0.60 were made at ambient temperatures of 30, 40, 50, 55, and 60 C (86, 104, 122, 131, and 140 F) to give constant workability. Immediately after mixing and after two retemperings at 30 min intervals, fresh concrete properties (slump, air content, concrete temperature, and unit weight) were determined. Specimens were cast after initial mixing and after two retemperings and tested for hardened concrete properties at 7 and 28 days. The additional water and cement needed to achieve the same workability at higher temperatures is very high for low w/c concrete, whereas there is only a slight increase in the water demand for concretes with w/c 0.50 and 0.60. The increase in the rate of slump loss is not significantly higher at higher temperatures. When retempering with superplasticizer, there are no detrimental effects on fresh as well as hardened concrete properties even under the unfavorable high ambient temperature of 60 C (140 F). The performance characteristics are the same for concretes mixed at higher ambient temperatures and retempered with superplasticizer.