Specifying lightweight concrete block units versus normal-weight concrete block units comes with its share of benefits.A close look at the payoffs such as sustainability, performance and cost effectiveness offers more than meets the eye at a glance. So, we asked Jeff Speck, vice president of sales & marketing at Big River Industries (www.bigriverind.com , to give us his take on the topic and answer a few related common questions. His experience stems from the fact Big River Industries manufactures expanded clay lightweight aggregate used in multiple applications, including concrete masonry units, structural lightweight concrete for buildings and highway bridge decks, and internal curing of low w/cm concrete.
Concrete Products: When should lightweight concrete block units be specified versus normal weight concrete block?
Jeff Speck: 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 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 are often specified for fire-rated walls. In addition, lightweight units can lower wall costs by increasing mason productivity and reducing labor requirements, which make up the largest portion of the installed wall cost.
CP: It appears that lightweight units perform better than normal-weight units from a fire and thermal standpoint as well as increased productivity. Is this also true from an acoustical standpoint, and do you have insights about this factor?
JS: There are two different measures of acoustic properties of walls—STC and NRC. STC (Sound Transmission Class) is a measure of how well a wall prevents sound from passing through the wall itself. In general, the heavier the wall, the higher the STC; 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 (min. 8-in. wall), far superior to light frame construction. NRC (Noise Reduction Coefficient) is a measure of how well a wall absorbs sound, reducing the reflection of noise within a room. In general, lightweight CMU walls have twice the NRC of normal-weight CMU walls. This principle applies to both unpainted and painted walls.
CP: How is lightweight concrete used to improve sustainability?
JS: Lightweight concrete (LWC) solutions, including structural lightweight concrete, concrete masonry and prestressed/precast concrete offer improved energy performance, reduced volume of materials resulting from lower dead loads, reduced transportation requirements (fewer truck loads mean less fuel is used and less pollution is generated by delivery), improved service life, and lower life-cycle costs. Lightweight fine aggregate can also contribute to recycled content, but sustainability goes far beyond the use of recycled materials.
CP: Why or how does lightweight concrete contribute to lower costs?
JS: Because LWC has greater fire resistance than normal-weight concrete (NWC), required fire ratings of floor slabs can be achieved with thinner slabs. This reduction in thickness reduces the dead load by 20–25 percent; in addition to the 20 percent density reduction, the thinner, lighter slabs can be supported by smaller beams, columns and foundations, further reducing the volume of materials. The reduced floor-to-floor height also lowers the quantities of utilities and exterior cladding. In the case of prestressed/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.