Any of you that have lugged a precast concrete countertop from your shop to an installation site know that concrete is heavy. It’s just the nature of the beast. Precast concrete countertops are typically made 1.5″ thick and weigh 18 pounds per square foot.
But, is it a problem? Should you be creating lightweight concrete? Let’s take a few minutes to learn more about lightweight concrete so you can decide for yourself.
Size versus Weight
First of all, the easiest way to reduce the weight of your concrete slabs is simply to make them smaller. There are 3 ways to do this:
– Make more slabs.
If you used 4 slabs to create a 16 foot long kitchen countertop, each slab would be only 4 feet long and therefore weigh a lot less than a single 16 foot long slab. However, most clients want to minimize the number of seams, so this is usually not practical.
– Make thinner (precast) slabs.
Many concrete countertop makers in the past did not understand reinforcing, and they made their slabs too thick, 2 inches or more, to compensate for their lack of confidence in their concrete. There is no need to make precast concrete more than 1.5″ thick, if you understand how to reinforce it properly. If the client wants a thicker look, you can achieve this with dropped edges.
– Use GFRC.
Glass Fiber Reinforced Concrete can go as thin as 3/4″ for the same kitchen countertop slabs that would need to be 1.5″ thick for precast. This instantly cuts the weight in half.
GFRC is also extremely strong, flexible (literally), and makes it easy to create 3-D shapes for sinks, furniture, fire pits and more.
This is by far the easiest and most obvious way of making lightweight concrete countertops, and it is the technique of choice for the vast majority of concrete countertop professionals.
If, despite the fact that GFRC will instantly cut the weight of your concrete creations in half, you still want to make the concrete itself lighter weight (so that the same volume of concrete actually weighs less) then read on.
What is Lightweight Concrete?
Lightweight concrete is made by replacing some (or all) of the normal weight aggregate (crushed limestone, granite, quartz, etc.) with a lightweight aggregate (expanded clay, shale or slate) to reduce the overall weight of the piece. Often the coarse fraction is replaced with lightweight aggregate and normal weight sand is used. Expanded clay, shale or slate are popular aggregate choices. These are created by heating the parent material to a high temperature causing the stone to “puff”, creating a substance often called foamed rock.
Lightweight vs. Conventional Concrete – What’s the Weight Difference?
- Lightweight Concrete – approx.. 115 pounds per cubic foot.
- Normal Weight Concrete – 145 pounds per cubic foot.
One square foot of 1.5 inch thick normal concrete weighs about 18 pounds. The same segment created from lightweight concrete weighs approximately 14.5 pounds. For comparison a square foot of 1.5 inch thick granite is 17.5 pounds.
Choosing Your Lightweight Aggregate
The compressive strength, elastic modulus, splitting tensile strengths and other properties of lightweight concrete are significantly affected by the structural and physical properties of the lightweight aggregate used. The aggregate itself must possess desirable properties such as adequate compressive strength, porosity, appearance, abrasion resistance and good bonding with the cement paste. For this reason you must carefully choose your aggregate if you’re working with lightweight concrete.
These don’t have the properties needed for structural concrete. They are better suited for concrete used as insulation or as lightweight filler.
- Expanded clay
- Expanded shale
- Expanded slate
Be aware that lightweight aggregate doesn’t polish well due to the porosity and internal voids. You can’t polish air. Even when polished with a 3000 grit diamond pad, the aggregate will remain dull.
Water and Lightweight Aggregate
The porosity of lightweight aggregate creates some challenges when creating a mix, especially when dosing the water. The increased porosity causes the aggregate to absorb a great deal of water, sometimes for days or even weeks. In general it is recommended that lightweight aggregate be presoaked to achieve a condition known as surface saturated dry (SSD) condition. This ensures that the aggregate will not absorb the mix water.
Extra care and attention must be paid when working with air dry lightweight aggregate, or a pre-blended lightweight concrete mix that can only have air dry ingredients in it (otherwise it would prematurely set due to the moisture inside the aggregate). The dry aggregate will readily absorb some of the mix water, requiring continued doses of extra water. It is at this point that it is extremely important that whatever extra water is added be dosed with great care, and that all batches of concrete have identical amounts of water added to it.
Concrete that has different amounts of mix water, and therefore different water to cement ratios, will have different structural, shrinkage and aesthetic characteristics. Concrete that loses mix water to thirsty aggregate during the critical phase when the concrete is setting can exhibit plastic shrinkage, surface map crazing, color variation, mottling and other undesirable and avoidable problems. Undisciplined and uncontrolled additions of unknown amounts of water will significantly affect the performance, durability and appearance of the finished concrete.
Converting a Mix to Lightweight
For concrete countertops, most mixes can be “converted” into lightweight mixes by replacing some or all of the normal weight aggregate with lightweight aggregate. While the surface texture and aggregate shape may have an affect on the workability (rougher and more angular particles make a mix that has lower workability than smooth, rounder particles, all else being equal). Most lightweight aggregates weigh about ½ to 2/3rds the weight of normal aggregate, so on average one pound of gravel can be replaced with a bit more than ½ pound of lightweight aggregate. The volume of aggregate stays the same, but the weight is reduced.
Even though the “conversion” seems simple, the inclusion of lightweight aggregates in a concrete mix will affect its properties and its workability. With appropriate lightweight aggregates, the compressive strength may not be affected, but the workability and the appearance more than likely will. Because the lightweight aggregate readily absorbs water, it is very important to calculate and keep track of all of the mix water added.
Is Lightweight Concrete Necessary for Concrete Countertops?
For most kitchen and bathroom cabinets, little or no modifications are necessary to bear the weight of 1.5” thick normal weight concrete. Lightweight concrete would not convey any significant advantage over normal weight concrete for anything but the largest slabs.
In addition, factors other than slab weight often dictate the maximum slab size and shape. Factors like site access, stairs, corners and general countertop and cabinet configurations, all impact the safe transport, handling and installation of very large slabs. The largest practical slabs may not actually be very heavy and therefore not need lightweight concrete.
The mix design for lightweight concrete is tricky and can lead to problems. The simplest solution for truly lightweight concrete countertops is to use GFRC at 3/4″ thick.