Lightweight Concrete Mixes

By Jeffrey Girard, P.E., President, The Concrete Countertop Institute

Copyright 2006 The Concrete Countertop Institute, LLC. All Rights Reserved. May be reprinted only with the express written permission of The Concrete Countertop Institute, LLC.

Definitions of Lightweight Concrete

Lightweight concrete mixes are commonly used in the construction industry where weight savings is an important factor. One of the most common uses for lightweight concrete is with floor, roof or bridge decks; others include pavement systems, masonry blocks and offshore oil structures. Lightweight concrete is made by replacing some or all of the normal weight aggregate with lightweight aggregate. Often the coarse fraction is replaced with lightweight aggregate and the fines are normal weight sand.

Structural lightweight aggregate concrete is defined as concrete which:

• is made with lightweight aggregates conforming to ASTM C 330,
• has a compressive strength in excess of 2500 psi at 28 days when tested in accordance with methods stated in ASTM C 330,
• and has an air dry density of no more than 115 pounds per cubic foot as determined by ASTM C 567.

High performance lightweight concretes are typically made using expanded clay, shale or slate. These lightweight aggregates weigh less than normal weight aggregates (crushed limestone, granite, quartz, etc.) due to the porous cellular structure of the individual aggregate particles. The cellular, or “foamed”, structure is created at temperatures of about 2000 degrees F or higher. At these high temperatures the parent material “puffs” and expands to form foamed rock.

Properties of Lightweight Aggregates

Color: Lightweight aggregates are typically dark gray, brown, reddish brown, rust-colored or even orange.

Polishing: Because they have a large amount of internal voids, the aggregate does not polish well. Lightweight aggregate polished with a 3000 grit diamond pad will still remain dull because of the open nature of the aggregate. Air does not polish.

Strength: The compressive strength, elastic modulus, splitting tensile strengths and other properties 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, non-structural lightweight aggregate such as perlite, vermiculite, Styrofoam and air are not considered appropriate for structural concrete, but rather find uses in concrete meant for insulation or as a lightweight filler.

* Note: It is also possible to make concrete lighter weight by using cenospheres. See this article for information about cenospheres.

Consequences of Absorptive Aggregates

Lightweight aggregates have a cellular structure and are therefore more porous than ordinary crushed stone. As such they absorb and hold more moisture than ordinary stone. Because of this greater porosity, extra care must be exercised when designing the concrete mix and when dosing the mix water. Because of the greater porosity, lightweight aggregates continue to absorb water over time. In fact, they absorb water for hours, days and even weeks after first being wetted.

When lightweight concrete is selected for an important structural application, good standards of practice dictate that the lightweight aggregates should first be pre-soaked in order to achieve a surface-saturated dry (SSD) condition. The SSD condition ensures that the lightweight aggregate will not absorb some of the mix water, which would rob the fresh cement paste of a carefully calculated dose of water chosen to achieve a specific strength with a certain degree of workability. Varying the cement paste water content has a profound effect on its stiffness, color and shrinkage characteristics.

A side benefit of absorptive aggregates is that the internal moisture acts as a reservoir, assisting in internally curing the concrete. However this benefit only becomes realized if the lightweight aggregate was at or very near an SSD condition before it was mixed into the concrete. Thus for important applications lightweight concrete is almost always made from batched ingredients.

The Importance of 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 looses 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 of 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 Really Necessary for Countertops?

Normal weight concrete has a cast density that is around 145 pounds per cubic foot (pcf). Lightweight concrete has a cast density of about 115 pcf. A square foot of 1.5” thick concrete weighs about 18 pounds for normal weight concrete and just under 14.5 pounds for lightweight concrete (a weight savings of less than 4 pounds per square foot). For comparison, 3cm (about 1.25” thick) granite weighs about 17.5 lbs per square foot.

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. However, 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 affect 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.

References

1. Harmon, K. S. “Physical Characteristics of Rotary Kiln Expanded Slate Lightweight Aggregate”. Carolina Stalite Company, Salisbury, NC. www.stalite.com
2. “Guide for Structural Lightweight Aggregate Concrete”. ACI 213R-87, American Concrete Institute, Detroit, Michigan. 1987.
3. “Advantages of Structural Lightweight Aggregate Concrete”. Expanded Clay, Shale and Slate Institute, www.escsi.org
4. “Standard Practice for Selecting Proportions for Structural Lightweight Concrete”. ACI 211.2-98, American Concrete Institute, Detroit, Michigan. 1998.