Changes in Properties of Portland Cement and Their Effects on Concrete Durability

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Title: Changes in Properties of Portland Cement and Their Effects on Concrete Durability

Author(s): G. J. Osborne and J. L. Hardcastle

Publication: Special Publication

Volume: 170

Issue:

Appears on pages(s): 1411-1432

Keywords: Carbonation; chlorides; compressive strength; corrosion; durability; permeability; physical properties; portland cement; seawater.

Date: 7/1/1997

Abstract:
paper summarises results of studies carried out at the Building Research Establishment on the performance and longer term durability at 5 years of two concretes, which contained different normal portland cements, designed to give equal 28-day compressive strength by adjusting the cement contents and water/cement ratios. The two cements were chosen to provide examples of normal Portland cements with the widest difference in tricalcium silicate (C3S) content. It was necessary to obtain the low C3S cement from Israel to satisfy this requirement. The cements originating from the UK and Israel, had C3S contents of 67 and 33.5%, and tricalcium aluminate (C3A) contents of 8.4 and 12.3%, respectively. The concretes assessed were of similar mixture proportions, although an extra 25 kg of the low C3S cement and a lower water/cement ratio were required compared with the high C3S cement, to achieve equal 28-day strength concretes of 40 (-+ 2) MN/m2. Several types of concrete specimens were prepared using two curing regimes (wet and dry-curing), before carrying out a range of long-term tests. These included: compressive strength, seawater attack rating, carbonation, oxygen permeability, chloride ingress, and corrosion of rebar. This study showed that by designing concretes to give equal 28-day strengths, but using high-early strength cements, concrete performance should be quite satisfactory in most indoor and outdoor environments. However, concrete durability may be compromised, even with good curing, if the cement content is not sufficient or the w/c is too high for certain severe exposure conditions, such as in the marine tidal zone. In such cases the reduced cement content and higher w/c could result in discernible loss in long term strength development and reduced durability. These data are of direct relevance to the UK concrete industry practice and support the approach adopted in the current British Standards and Codes of Practice of specifying concrete in terms of minimum cement content and maximum W/C, as well as by minimum strength grade, rather than by 28-day strength attainment.