Recent research confirms that producers can blend lightweight and normal-weight aggregate in their block mix designs and achieve required fire ratings. Offering customers lightweight concrete masonry units is becoming less of a value-added marketing strategy and more of a necessity for producers.

Two lightweight aggregate suppliers are testing to prove that blended units meet fire-resistance requirements, enabling masonry to earn a bigger piece of the construction market.

The new International Building Code has adopted ACI 216.1/TMS 0216.1, "Standard Method for Determining Fire Resistance of Concrete and Masonry Construction Assemblies." The standard provides for blending aggregate as long as the unit meets the required fire rating in actual testing or if it is cast with a minimum equivalent thickness for a blended unit, calculable using weighted averages found in Table 3.1 of the standard.

With the fire-resistance hurdle cleared, Northfield Block, a leading Chicago-area block producer, is launching a gradual, wholesale switch from normal- and medium-weight units to blended lightweight ones using Witelite by Tarmac's pumice aggregate. Northfield Block sales representatives are handing out a two-page flyer titled, "Protecting the Future of Masonry: Lightweight Masonry Units." The flyer lists increased productivity, lower workers' compensation costs, shorter construction time, safer scaffolding, and easier cutting with its lightweight units. For architects, the flyer emphasizes lightweight block's thermal resistance, strength, durability, fire ratings, and contribution to lower construction costs.

According to Jeff Hunt, sales manager, "When we came up with a mix design where we could take 10 pounds out of an 8-inch block, still have the durability to meet ASTM C 90, and make it cost-effective for the basic contractor, that became very exciting to us. Our 12-inch block is 33 pounds-that's less than an 8-inch block that's currently in the marketplace."

This article includes the formula for calculating equivalent thickness for blended block and two tables showing UL fire testing of blended concrete block.