November 29, 2006

SCMs

Supplementary materials’ impact on durability examined

Peter Mitham

Correspondent

When a highway overpass in Laval, Quebec, collapsed at the end of September, some speculated that the use of supplementary cementing materials such as fly ash may have been to blame.

“In the industry, it’s not entirely unheard of for builders to rely a little too heavily on extra helpings of economical fly ash,” reported the Toronto Star on October 8, little more than a week after the overpass collapse that killed five people. “Add too much of it, however, and concrete can be weakened considerably.”

During a November 23 workshop in Vancouver sponsored by the Cement Association of Canada and the Ecosmart Foundation, engineer Phil Seabrook of Levelton Consultants Ltd. allayed some of the recent concern surrounding SCMs with a more nuanced perspective.

Seabrook said the Toronto Star article and other media reports may have thrown the durability of concrete made with SCMs such as fly ash and silica fume into question of late, but he pointed out that SCMs have long been known to pose problems when insufficient time is allowed for the curing and maturation of the concrete.

“If you’re going to use SCM, please cure (the concrete) or you could face durability problems,” he said.

Still, Seabrook said concrete that incorporates high volume fly ash or silica fume is a durable product if Canadian Standards Association guidelines are followed.

“Up to about 25 per cent (fly ash content), there is no problem,” he said. Seabrook also addressed reactive aggregates, which have driven the use of supplementary cementing materials in several areas of B.C.

A recent CSA study found that 45 per cent of aggregates tested in B.C. failed the CSA’s prism test, which tests a prism of concrete over the course of a year for expansion.

SCMs can mitigate the effects of reactive aggregates on concrete, in part because expansion requires water and concrete made with SCMs requires less water.

Seabrook said conventional concrete may require 145 to 150 litres of water per cubic metre, while concrete made with SCMs requires as little as 130 to 135 litres per cubic metre.

The latest CSA guidelines indicate that once fly ash content exceeds 30 per cent of a concrete’s composition, the mixture’s water content must be boosted. A curing plan to ensure the concrete strengthens appropriately is also required.

Seabrook emphasized that users should take into account various other factors to ensure concrete cures properly, however. Proper curing is not just a question of time or moisture content, but also climate and current drying conditions.

Phil Seabrook

Levelton Consultants Ltd.

He showed slides of a concrete at the site of a prestigious building in Vancouver where the finishers moved in on a pour that took place in cool conditions. The finishers were too quick, and the concrete delaminated.

Repairing the damage was expensive, given that the concrete encased an in-floor heating system.

“It’s just unfortunate that somebody didn’t think things through,” Seabrook said.

Taking CSA guidelines and making appropriate adjustments for the lower moisture content of the concrete, lower bleed and longer set time, Seabrook said most builders should find SCM concrete to be more durable than conventional concrete.

“But there can be durability problems if you don’t accommodate the differences (between the two),” he said.

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