Q: We are working on a project where the owner is trying to earn LEED credits. So we are trying to convince the structural engineer to use lightweight block. How can we get him to use lightweight block as an alternative?
A: With the possible exception of very high compressive strength requirements, lightweight concrete masonry units (CMUs) can be specified any time. A common misconception is that lightweight units are not structural. Lightweight CMUs meeting the requirements of ASTM C90 have all the structural properties of normal weight load-bearing units.
Higher compressive strengths that allow higher design strength of masonry (f'm) are common with both lightweight and normal weight units. Lightweight units offer improved fire resistance and thermal performance compared to normal weight units of the same configuration, so they often are specified for fire-rated walls. In addition, lightweight units can lower wall costs by increasing mason productivity and reducing labor costs, which make up the largest portion of the installed wall cost.
Q: I also have reviewed the National Concrete Masonry Association website. It appears that lightweight units perform better than normal weight units from a fire and thermal standpoint, as well as increased productivity. However, it seems from an acoustic standpoint, the normal weight units perform better then the lightweight units. Is this true?
A: There are two different measures of acoustic properties of walls—STC and NRC. STC (Sound Transmission Class) measures how well a wall prevents sound from passing through the wall itself. In general, the heavier the wall, the higher the STC will be. So, normal weight CMU walls have a somewhat higher STC than lightweight CMU walls. However, both normal weight and lightweight CMU walls have STCs of 45 or higher (minimum 8-inch wall), far superior to light-frame construction.
NRC (Noise Reduction Coefficient) measures how well a wall absorbs sound, reducing the reflection of noise within a room. Generally, lightweight CMU walls have twice the NRC of normal weight CMU walls. This principle applies to both unpainted and painted walls.
Q: How does lightweight concrete contribute to sustainability?
A: Lightweight concrete (LWC) solutions, including structural lightweight concrete, concrete masonry, and prestressed/precast concrete, improve energy performance, reduce the volume of materials resulting from lower dead loads, reduce transportation requirements (fewer truck loads mean less fuel is used and less pollution is generated by delivery), improve service life, and lower life-cycle costs.
Lightweight fine aggregate also can contribute to recycled content, but sustainability goes far beyond the use of recycled materials.
Q: Why or how does lightweight concrete lower a project's costs?
A: Because LWC has greater fire resistance than normal weight concrete, required fire ratings of floor slabs can be achieved with thinner slabs. This reduction in thickness reduces the dead load by 20% to 25%. In addition to the 20% density reduction, the thinner, lighter slabs can be supported by smaller beams, columns, and foundations. This further reduces the volume of materials.
The reduced floor-to-floor height also reduces the quantities of utilities and exterior cladding. In the case of pre-stressed/precast concrete structures, the number of truckloads required to deliver the structural members to the jobsite can substantially reduce costs. For concrete masonry, labor savings can substantially reduce the installed wall cost.
The answers were provided by Jeff Speck, vice president of sales and marketing for Big River Industries, a producer of expanded clay lightweight aggregate. Visit www.bigriverind.com.