Is Lightweight Concrete Necessary for Countertops?
Question:
Because concrete countertops often must be installed on existing cabinetry, can the weight of the concrete be a problem? Are there advantages to using lightweight concrete?
Answer:
Lightweight concrete mixes are commonly used in the construction industry where weight savings is an important factor. Typical uses include floor, roof, and bridge decks. For most concrete countertops, however, lightweight concrete would not convey any significant advantage over normal-weight concrete for anything but the largest slabs. Factors such as site access, stairs, corners, and general countertop and cabinet configurations often prohibit the use of very large slabs. So in general, the largest practical slabs for countertops are not actually very heavy and therefore do not need lightweight concrete.
To illustrate, normal-weight concrete has a cast density of about 145 pounds per cubic foot (pcf) while lightweight concrete has a cast density of about 115 pcf. A square foot of 1.5-inch-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, a 1.25-inch-thick granite countertop weighs about 17.5 pounds per square foot. For most kitchen and bathroom cabinets, therefore, little or no modifications are necessary to bear the weight of 1.5-inch-thick normal-weight concrete.
If you do run into a situation requiring a lighter-weight slab, most countertop mixes can be “converted” into lightweight mixes by replacing some or all of the normal-weight aggregate with lightweight aggregate. Most lightweight aggregate weighs about one-half to two-thirds the weight of normal aggregate so, on average, 1 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 aggregate in a concrete mix will affect its appearance, performance properties, and workability. Lightweight aggregates are typically dark gray, brown, reddish brown, rust-colored, or even orange (see photo). They weigh less than normal-weight aggregates (crushed limestone, granite, quartz, etc.) due to the porous cellular structure of the individual aggregate particles. 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 remain dull because of the open nature of the aggregate (air does not polish).
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. For countertops, it’s important to use aggregates that possess desirable properties such as adequate compressive strength, porosity, appearance, abrasion resistance, and good bonding with the cement paste. For this reason, nonstructural lightweight aggregates such as perlite, vermiculite, and Styrofoam are not considered appropriate for structural concrete, but rather find uses in concrete meant for insulation or as lightweight filler.
Because lightweight aggregates have a cellular structure and are more porous than ordinary crushed stone, they absorb and hold more moisture. Because of this greater porosity, extra care must be exercised when designing the concrete mix and when dosing the mix water. Lightweight aggregates continue to absorb water over time—for hours, days, and even weeks after first being wetted. Concrete that looses mix water to thirsty aggregate during the critical phase when the concrete is setting can exhibit plastic shrinkage, surface crazing, color variation, mottling, and other undesirable and avoidable problems. Therefore, it is very important to calculate and keep track of all of the mix water added. Undisciplined and uncontrolled additions of water will significantly affect the performance, durability, and appearance of the finished concrete.
For more information about the properties and performance of lightweight concrete, check out the following references:
Dark gray and orange lightweight aggregate in gray cement.
Because concrete countertops often must be installed on existing cabinetry, can the weight of the concrete be a problem? Are there advantages to using lightweight concrete?
Answer:
Lightweight concrete mixes are commonly used in the construction industry where weight savings is an important factor. Typical uses include floor, roof, and bridge decks. For most concrete countertops, however, lightweight concrete would not convey any significant advantage over normal-weight concrete for anything but the largest slabs. Factors such as site access, stairs, corners, and general countertop and cabinet configurations often prohibit the use of very large slabs. So in general, the largest practical slabs for countertops are not actually very heavy and therefore do not need lightweight concrete.
To illustrate, normal-weight concrete has a cast density of about 145 pounds per cubic foot (pcf) while lightweight concrete has a cast density of about 115 pcf. A square foot of 1.5-inch-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, a 1.25-inch-thick granite countertop weighs about 17.5 pounds per square foot. For most kitchen and bathroom cabinets, therefore, little or no modifications are necessary to bear the weight of 1.5-inch-thick normal-weight concrete.
If you do run into a situation requiring a lighter-weight slab, most countertop mixes can be “converted” into lightweight mixes by replacing some or all of the normal-weight aggregate with lightweight aggregate. Most lightweight aggregate weighs about one-half to two-thirds the weight of normal aggregate so, on average, 1 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 aggregate in a concrete mix will affect its appearance, performance properties, and workability. Lightweight aggregates are typically dark gray, brown, reddish brown, rust-colored, or even orange (see photo). They weigh less than normal-weight aggregates (crushed limestone, granite, quartz, etc.) due to the porous cellular structure of the individual aggregate particles. 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 remain dull because of the open nature of the aggregate (air does not polish).
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. For countertops, it’s important to use aggregates that possess desirable properties such as adequate compressive strength, porosity, appearance, abrasion resistance, and good bonding with the cement paste. For this reason, nonstructural lightweight aggregates such as perlite, vermiculite, and Styrofoam are not considered appropriate for structural concrete, but rather find uses in concrete meant for insulation or as lightweight filler.
Because lightweight aggregates have a cellular structure and are more porous than ordinary crushed stone, they absorb and hold more moisture. Because of this greater porosity, extra care must be exercised when designing the concrete mix and when dosing the mix water. Lightweight aggregates continue to absorb water over time—for hours, days, and even weeks after first being wetted. Concrete that looses mix water to thirsty aggregate during the critical phase when the concrete is setting can exhibit plastic shrinkage, surface crazing, color variation, mottling, and other undesirable and avoidable problems. Therefore, it is very important to calculate and keep track of all of the mix water added. Undisciplined and uncontrolled additions of water will significantly affect the performance, durability, and appearance of the finished concrete.
For more information about the properties and performance of lightweight concrete, check out the following references:
- “Physical Characteristics of Rotary Kiln Expanded Slate Lightweight Aggregate,” Carolina Stalite Company (http://www.stalite.com/).
- “Guide for Structural Lightweight Aggregate Concrete,” ACI 213R-87, American Concrete Institute (http://www.concrete.org/general/home.asp).
- “Advantages of Structural Lightweight Aggregate Concrete,” Expanded Clay, Shale and Slate Institute, (http://www.escsi.org/).
- “Standard Practice for Selecting Proportions for Structural Lightweight Concrete,” ACI 211.2-98, American Concrete Institute (http://www.concrete.org/general/home.asp).
Dark gray and orange lightweight aggregate in gray cement.
posted by Jeff Girard
at
2:09 PM
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