Understanding Different Types of Lightweight Concrete
The use of lightweight concrete dates back to as early as the eighteenth century, and as advances in building and construction technology have increased, so has the use of lightweight concrete as the benefits of lighter dead load concrete have become apparent. In the U.S., lightweight concrete became more common around the 1930s and continues to provide advantages to the building and creative industries as different types of lightweight concretes are developed for use. Those advantages include not only weight considerations, but also insulation value, sound reduction, and workability.
While the description of lightweight concrete is fairly simple – lightweight concrete simply weighs less due to lower density aggregates than standard concrete, and can range from 35-100 pounds per cubic foot – it quickly becomes apparent that with both advances in technology and new materials being tested, lightweight concrete is not all created equal.
Where Lightweight Concretes Differ
The real difference, and it can be a significant one, is in the aggregates used to form the concrete mixture. These affect not only the finished weight, but also often will determine the process or the products necessary for a successful installation.
Low-density natural aggregate concrete
Most low-density aggregates are volcanic in origin and include pumice, tuff, scoria, and cinders. Diatomite is also something used as the aggregate component of lightweight concretes. The advantage to these types of aggregate is that they often do not require processing apart from crushing or screening.
Pumice is the most commonly used, and is actually a glass that forms when frothy volcanic eruptions cool quickly into rock. It is sometimes heat-treated for additional strength, as it can have higher absorption rates if it is not structurally strong in its original form. Other natural materials like perlite or vermiculite are also used, although they generally expand, quickly heating the material.
Cinders as a by-product of coal or coke combustion are also sometimes used but are often limited because of the chemical presence of sulfur compounds that can skew the pH and performance of the concrete.
Processed or synthetic aggregate concrete
For some lightweight concrete formulations, either processed by-products or synthetic material is used to form the aggregate.
- Expanded shale or clay – prepared shale or clay materials are heated, which expand the materials as the gasses within expand. Other materials with higher fusion points are sometimes added as coatings to avoid having the material stick together during mixing or storage.
- Expanded slag – when treated with steam or water, blast-furnace slag can also produce an acceptable aggregate product for lightweight concretes.
- Synthetics aggregates – the range of synthetic aggregates being tested and produced covers a variety of manufactured products that range from recovered fly ash or oil sands to recycled plastics, papers or glass to products like Styrofoam. Obviously, weight gains vary widely from product to product but the potential of “green” uses for by-product and recycled materials has become a focus for this type of aggregate.
Cellular or aerated concrete
This type of lightweight concrete is the result not just of product, but also of process. It is produced by introducing tiny air pockets into the concrete mixture. This can be done through a chemical reaction produced by using hydrogen peroxide or an aluminum powder in the batch mix that generates gas within the concrete. As the concrete is poured, the chemical reaction actually expands the concrete, which is then cured with high-pressured steam to “set” the micro air pockets. The other method uses a pre-mixed foam that is stirred into the slurry to create tiny air voids in the final concrete.
High performance concrete
High performance cellular concrete involves “enhancing” the concrete to allow for longer placement time requirements, or specific properties of density, volume or performance in a severe or specialized environment. These generally involve not only the use of low-density aggregates but also additional admixtures to obtain the desired qualities for the finished concrete.
Because of the numerous possible combinations of aggregates, admixtures, processes and end products, “lightweight concrete” as a general category term is a broad umbrella that must be spec’d out on a job-by-job basis, and fully understood for the final density, compressive strength and production/installation requirements of each specific blend.
Where Lightweight Concretes Are the Same
For all their various compositions, all lightweight concretes have the same need for accurate moisture testing during the drying stages or for subsequent testing if moisture intrusion is suspected. With so many variables that may affect the final drying schedule of a lightweight concrete slab, only accurate moisture testing can provide the green light for finish or flooring installation.
For lightweight concretes, the only concrete moisture test allowed by the ASTM is relative humidity (RH) testing. Any surface-based testing, including calcium chloride (CaCl) testing, plastic sheet testing, or hood testing, has proven to be highly problematic in measuring the moisture content of lightweight concretes. In fact, CaCl testing has been specifically disallowed for lightweight concrete applications.
RH testing, like with Wagner Meters’ award-winning Rapid RH®, provides an accurate indication of internal moisture conditions that will impact the long-term performance of the concrete by placing a sensor at 40% of the slab’s depth1 – the distance scientifically tested and proven to give an indication of the final moisture content level if the slab were sealed at that point. Moisture does not evenly distribute through the slab during the drying process, but once the slab is sealed, any moisture remaining will eventually equilibrate through the slab. This is the moisture level that will remain in contact with installed flooring or applied finishes over time.
Only RH testing allows an accurate indication of the internal moisture conditions in a lightweight concrete installation so that building schedules and product decisions or adjustments can be made in an informed setting.
Advances in lightweight concrete technology continue to expand the possibilities of long-lasting, resource-responsible manufacturing and building in a variety of industries. Only RH testing can help those advances best stand the test of time.
1 40% of the slab’s depth is the correct depth for the test hole if the slab is drying from one side; the correct test hole depth for slabs drying from two sides is 20%.
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