Lightweight Concrete: Weighing in on the Differences
Lightweight concrete presents a number of benefits to the building industry–lighter weight on a building’s structural load, greater sound absorption, better shock absorption and flexibility, improved insulation values–when compared to standard concrete blends. However, it has also been suspect in increasing cases of moisture-related flooring failures. Knowing the advantages and disadvantages of lightweight concrete can be the foundation for making better choices at the design and installation stages.
Concrete Mix Differences
The prime difference between standard concrete mixes and lightweight concrete mixes lies in the aggregate that is used in the concrete mixture.
Standard concrete mixes typically use crushed natural stone as the coarse aggregate in the mix (along with Portland cement, water and sand). The actual weight may vary slightly according to local stone availability, but generally natural stone is relatively dense for its size, adding to the weight of the concrete mix. (This also adds to increased transportation and equipment costs.)
Lightweight concrete instead uses a variety of alternate aggregate materials that may include more porous rock like pumice, manufacturing byproducts like fly ash or slag, or clay, shale or slate that has been treated with a heat process that expands the material and creates a series of internal pores in the aggregate. In effect, these materials provide lower mass per volume in the aggregate. Lightweight concrete may also be “foamed” by mixing a cement slurry into a pre-formed foam or AAC (autoclaved aerated concrete) in a process which introduces entrained air into the mix to reduce the final concrete weight.
While the change in the aggregate for lightweight concrete does not seem to significantly impact the final concrete’s compressive strength, any type of lightweight concrete has a significant trade-off in two areas: the advantage of reduced structural weight once its dry, and the disadvantage of moisture retention that makes that drying process significantly longer.
Concrete Moisture Testing Differences
The weight advantage of lightweight concrete may provide the best benefit, but the same feature that reduces the weight–the pores in the aggregate or foam, or in the air introduced during mixing–also become additional spaces within the concrete that can trap and retain moisture.
Unless a self-desiccant or other chemical material that locks moisture into the slab is also introduced, the water from the initial mix and hydration process must eventually find its way to the surface to evaporate away. Certain admixtures like fly ash retain moisture for longer periods, and the sheer volume in the many additional pores of the aggregate will also increase the amount of water retained, as well as the weight that additional moisture would cause until it is released.
Because lightweight concrete has this increased capacity for moisture absorption, it can take two to three times longer than regular aggregate concrete to dry. This can present real challenges for the builder or contractor tasked with keeping the building schedule on track and under budget if this characteristic of lightweight concrete was not accounted for during the design and planning stage. The delays can be significant setbacks to the schedule or additional costs for dehumidification processes and equipment.
It also means that surface-based concrete moisture test methods are at an extreme disadvantage in trying to gauge the level of moisture in a lightweight concrete slab. In fact, this became evident as moisture-related flooring failures became more common as the use of lightweight concrete expanded, and based on the findings, the ASTM has specifically disallowed calcium chloride (CaCl) testing for lightweight concrete.
So how can you mitigate the disadvantages of lightweight concrete’s propensity for retaining moisture when applying a flooring or finish to the slab?
- Allow time.
As has already been pointed out, lightweight concrete takes significantly longer to dry. By understanding this at the planning stage, the schedule can be adjusted to maximize the drying time.
- Optimize ambient conditions.
There are a number of environmental factors that influence concrete drying time. Having the slab at service condition, or adjusting air flow, RH levels and temperature to maximize the drying process can optimize the movement of the moisture from the slab to the surface and away.
- Test with relative humidity (RH) testing.
Only RH testing, like the Rapid RH® 4.0, can accurately gauge the moisture content of a lightweight concrete slab. Because the Rapid RH® places sensors within the slab at the optimal depth, test results will provide an accurate picture of the moisture content within the slab and allow for informed decisions about installation schedules, adhesive or finish choices and remedial steps if necessary. The Rapid RH® provides fast, easy-to-use technology for immediate updates on the slab conditions. It also complies with ASTM F2170 to allow documented warranty information when necessary.
Lightweight concrete has distinct advantages and disadvantages in the building industry. Knowing what they are allows building and flooring professionals to make informed decisions, and includes the necessity of accurate RH testing to confirm and document the readiness of the slab. The Rapid RH® 4.0 helps you be sure that the slab is ready for the next step towards a long-lasting and beautiful floor.