The Lighter Side of Concrete
Introduction to Lightweight Concrete
In 1971, the One Shell Plaza pierced the Texas skyline, breaking the city record for tallest building. This 52-story building broke another record as the world’s tallest building made of lightweight concrete. Today, this record has been surpassed with towers reaching far more impressive heights and standards. Most notably, Renzo Piano’s The Shard is a marvelous structure made of lightweight concrete. All 95 floors of the Shard only took four years to build.
Suspended floors have been benefiting from lightweight concrete for years as it reduces the weight on the structure and presents better fire-resistant properties. Due to the fact that lightweight concrete is typically thinner than traditional concrete and utilizes different ingredients, floors can be up to 45% lighter than normal-weight floors. Even though lightweight concrete and its production can cost more than traditional concrete, contractors end up saving significant amounts of money on frame costs and benefit from other advantages.
What is Lightweight Concrete?
The distinction between lightweight concrete and other types of concrete is its composition of light weight aggregates. Specific types of lightweight aggregates make varying forms of lightweight concrete. These vary from lightweight aggregate concrete, foamed concrete or Autoclaved Aerated Concrete (AAC). Distinctly, the aggregates used in AAC are extremely fine, even smaller than a grain of sand. This composition makes AAC highly suitable for pre-casting and reshaping. AAC structures are soft enough that they can be sawn by hand and pierced by nails and screws.
Builders using lightweight concrete have the option of using a method of “internal curing”. The lightweight aggregate enables the concrete to cure inside out. This option is not afforded when using normal weight concretes because it is not as porous as lightweight concrete.
What is Internal Curing?
Curing is “the maintenance of a favorable temperature and moisture environment for repair and protection materials during some definite period following placing, casting, or finishing so that the desired properties may develop.” Internal curing is defined by ACI as “a process by which the hydration of cement continues because of the availability of internal water that is not part of the mixing water.”
The first benefit attributed to internal curing is a major selling point for AAC because the concrete’s porosity enables it to sustain adequate moisture overtime. This ultimately prevents the concrete from cracking and shrinking as the structure cures.
With AAC, the moisture inside the concrete remains high because the aggregate is prewetted prior to mixing. The small pores in AAC allow for a steady release of moisture from all layers of the structure. The ingenuity of internal curing is a time-saver in contrast to the traditional curing process. In normal-weight concrete different techniques are used to retain moisture at the surface to prevent cracking and premature drying.
Benefits of AAC
AAC is highly favourable for various reasons including:
- Increased strength due to internal curing
- Improved durability
- Decreased shrinkage cracking due great moisture retention
- Reduced weight on structural elements
- Superior fire resistance as the material composition is
- Reduced time and cost of labour as well as easy and light transportation (specific to precast)
Weighing in the options
As outlined, AAC makes for great precast structures, however there are drawbacks in using this type of concrete for cast-in-place projects. Mainly, the use of AAC for the base of a flooring system may have its disadvantages. Specifically, a slower drying time than normal weight concrete, meaning contractors might have to wait longer to proceed with flooring application.
However, the successful outcome of a flooring system made from normal concrete or lightweight concrete is determined by understanding the processes to eliminate the risks of floor failures. It is actually the long-term cost considerations that will influence buyer preferences for using lightweight concrete in projects.
According to Emily Hopps, Associate Principal at Simpson Gumpertz & Heger Inc., the initial unit cost of lightweight concrete is more expensive than normal weight, “[…] but the unit cost usually is more than offset by the overall reduction in concrete volume and steel tonnage for the structural system.” This means, using lightweight concrete allows for higher building structures since it minimizes the unit cost of adding reinforcing steel to support the weight of normal concrete.
Ultimately, one has to consider the pros and cons of using lightweight concrete over normal-weight concrete based on what type of structure they are building. If initial time and costs are not the sole determinants of that decision, the long-term benefits associated with this product are key.