Durability of New-to-Old Concrete Bondings

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Title: Durability of New-to-Old Concrete Bondings

Author(s): Francois Saucier and Michel Pigeon

Publication: Special Publication

Volume: 128

Issue:

Appears on pages(s): 689-706

Keywords: adhesives; bond (concrete to concrete); concrete durability; repairs; shear tests; silica fume; tension tests; wetting; Materials Research

Date: 11/1/1991

Abstract:
Concrete repairs always include bonding surfaces between the new concrete and the repaired structure, but the factors influencing the durability of new-to-old concrete bondings are still unclear. To study this question, many types of basic new-to-old concrete bondings were fabricated and submitted to three different aging conditions. At the time of writing, the results from 376 tests (of six composite specimens each) were available. All these results are not reported in the paper, but the main conclusions are all illustrated by typical results. The principal parameters that were studied are the composition of the bonding agent and the state of saturation of the base concrete when the new concrete is cast. Cement slurries with and without silica fume with water-cement ratios (w/c) of 0.30 and 0.60 were used as bonding agents, plus two latex-based commercial bonding agents. Test specimens were obtained by casting fresh concrete on the sawn surface of a base concrete air-dried for at least 28 days. After 7 days of water-curing, the composite specimens were submitted to freezing and thawing cycles (FTC), wetting and drying cycles (WDC), or were simply allowed to dry in air. The durability of the bondings was evaluated by shear or tensile tests of the specimens at various ages, up to 620 days in certain cases. From the results, it clearly appears that bondings fabricated with a bonding agent with a w/c of 0.60, although considered acceptable in certain standards, were the least durable. Another important finding is that almost all bondings tested resisted adequately to daily FTC for many months. WDC were harmful only for some of the bondings made with the 0.60 w/c slurry. Almost all bondings, however, were found to be damaged by simple drying, underlying the key role of differential shrinkage in the debonding process. The addition of silica fume in the cement slurries used as bonding agents and the wetting of the base concrete before casting the new concrete did not appear to significantly influence the durability of the bonding. Only a few sets of specimens were submitted to a combination of aging treatments, but they revealed durability problems not identified by exposure to a single aging treatment.