Lightweight Concrete – Is It Really Necessary for Countertops?

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. 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 do not understand reinforcing, and they make 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.

This article is not about any of these techniques, it is about making the concrete itself lighter weight, so that the same volume of concrete actually weighs less.

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.  

Don’t use:

  • Perlite
  • Vermiculite
  • Styrofoam
  • Air

These don’t have the properties needed for structural concrete. They are better suited for concrete used as insulation or as lightweight filler.

Do use:

    • 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.

expanded shale in concrete

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.

Restaurant gets a kick out of a custom concrete bench and table top

Check out this one of a kind project by Concrete Countertop Institute student Thomas Lancaster of Lancaster Concrete Designs in Alpine, TX. It makes me want to kick up my heels and go line dancing, but I think that’s the point. 

Boot and table set

This project was actually inspired by Thomas’ wife. She had the idea one day while meditating and told Thomas they needed to make it happen. After a lot of planning and over a month of templating and designing, this project was ready for concrete. It was to be installed in a local restaurant, the Saddle Club, with a Western theme.

PreCast Tops

The tops are made from precast concrete.

The tops of the bench and table are made from precast concrete. The base of the boot is constructed using GFRC. It features two distinct pieces held together with magnets for easy moving. Since the entire project is made in pieces, future versions can easily be shipped out to ordering customers.

Creating the boot base was the most difficult part of the project. Here it is, almost complete.

Creating the boot base was the most difficult part of the project. Here it is, almost complete.

The boot top is made using precast concrete and sits on the base over rubber dowels. The idea was just a concept when this project started, but since the boot bench has been in place, it’s certainly been tested. Just last weekend the club had a long music festival with thousands of guests, and the boot held up beautifully.

The table base is an imported wine barrel with a precast concrete top. It features a decorative rope edging and steel inlays. The turquoise color is stunning.

Extras Matter

The versatility of concrete makes it easy to incorporate unique accents and special features. Adding these custom additions can make a piece more usable and more appreciated. In this special boot bench and table set, Thomas incorporated several special features for a truly unique project.

One-of-a-Kind Features:

  • Bottle Opener on Side of the Boot- this piece is in a club/restaurant and the bottle opener certainly comes in handy.
  • Decorative Horse Shoes in Table Top- Thomas used pony shoes as a decorative accent in the table top to complement the restaurant’s Western theme.
  • Multi-Part Construction- With over 40 square feet of concrete, the finished piece weighs about 400 pounds, a lot for any one person to lift. To make transport easier, the boot is constructed in multiple pieces and can be easily moved.
  • Company Branding- Anyone wanting to know who created this work of art won’t have to look far; Thomas included his name at the base of the boot for all to see.
The bottle opener and company name are just two of the custom features added to this project.

The bottle opener and company name are just two of the custom features added to this project.

Beautiful work Thomas! We’re sure the Saddle Club patrons get a kick out of this unique piece. I know I’d certainly love to enjoy a good meal on a bench like that.

Thomas attended The Ultimate Concrete Countertop Training in March 2011. View more of Thomas’s work at www.lancasterconcretedesigns.com or www.facebook.com/LancasterConcreteDesigns.

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