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MC Magazine Archive

Lightweight Aggregates

Lighten your load with low-density aggregates.

By Dean A. Frank, P.E.
Dean Frank is Director of Technical Services at NPCA.

A fundamental advantage of precast concrete is that it is produced at a factory in a quality-controlled environment. Since every piece must be shipped to another location, precasters are often required to limit the weight of each shipment due to highway weight restrictions. These restrictions limit the number of units that can be placed on each shipment, even if more space is available on the truck or trailer.

One alternative that allows you to include more products with each shipment is to use concrete that weighs less. Since aggregates make up a significant percentage of the weight of concrete (60 percent to 70 percent), it makes sense to focus on them. Besides, although there is such a thing as lightweight cement, it is not widely available. And who has ever heard of lightweight water?

How light is lightweight?
Lightweight aggregate is more accurately referred to as low-density aggregate, since it is usually compared with normal-density aggregate. However, the generally accepted industry term used is lightweight aggregate.

There are differing definitions for concretes that can be produced with lightweight aggregates. Low-density concrete generally is produced with perlite or vermiculite aggregates, rarely exceeds 50 lbs/ft3 (800 kg/m3) and has very low compressive strengths in the range of 100 to 1,000 psi (0.7 to 6.7 MPa). Structural lightweight concretes are typically produced with expanded shales, clays, slates and slag. They can also be made with pumice or scoria, which are naturally occurring volcanic aggregates. By definition, structural lightweight concretes have a minimum compressive strength of 2,500 psi (17.2 Mpa) and an air-dried unit weight of 90 to 115 lbs/ft3 (1,440 to 1,850 kg/m3). Moderate strength concretes fall somewhere in between low-density and structural lightweight concrete. For comparison, normal-weight concretes have a typical dry unit weight of 145 to 150 lbs/ft3 (2,300 to 2,400 kg/m3).

By adjusting the proportions of both lightweight and normal-weight aggregate, the concrete density can be varied, depending on the project requirements. This type of lightweight concrete is often referred to as specified-density concrete.

It is important to note that lightweight concrete can also be produced with special production methods in combination with or without lightweight aggregates.

Advantages of using lightweight aggregates
The primary advantage of using lightweight aggregates to precasters is the reduction of product weight. Reduction in weight can lead to improved economy of structural components because there will be less dead load for the structure to support. Also, as mentioned previously, this may significantly affect the way in which products can be shipped. Table 1 shows a comparison of the shipping costs for two projects. The comparison shows that the savings in transportation costs well outweigh the additional expense of the lightweight aggregate. These savings were realized because additional product could be loaded on each truckload and, consequently, fewer shipments were needed. While these examples show that saving money is possible, each precast concrete producer must analyze variables that would ultimately affect the overall production and shipping costs at his/her own plant.

Table 1

Transportation Savings/Cost Analysis of Two Example Projects Using Lightweight Concrete
  Example Project 1 Example Project 2
Number of Loads Required
- Normal Weight
- Lightweight
Reduction in Truck Loads

431
- 287
144

87
- 66
21
Transportation Savings
- Shipping Cost Per Truck
- Load Reduction in Truck Loads
Transportation Savings
$1,100
x 144
$158,400
$1,339
x 21
$28,119
Savings/Cost Analysis
- Transportation Savings
- Premium Cost for Lightweight Concrete
Increase in Gross Margin
$158,400
- $17,245
$141,155
$28,119
- $3,799
$24,320

Courtesy of ESCSI/Big River Industries Inc.

Another reason to consider using lightweight aggregates is that sometimes the dead load of a product is near or above the capacity of the crane being used at a plant or at a jobsite. With lightweight aggregates, it may be possible to reduce the weight of the product so that special cranes would not be necessary, or to produce larger sections than would be possible with normal-weight concrete. Also, a reduction in crane movements may be realized since longer reaches are possible with lighter loads.

Lightweight aggregates can also provide unique and potentially useful properties to concrete besides reduced weight.
  • Lightweight concrete is thermally efficient. With warmer walls, there is less risk of condensation. In fact, it may be possible to reduce or possibly eliminate insulation layers in sandwich panels.
  • Lightweight concrete is fire-resistant. Because lightweight aggregates have already been pre-fired, they are stable and do not decompose in high temperatures. This is ideal for building components or refractory products.
  • Lightweight concrete absorbs energy well. With the addition of fiber reinforcement and even foaming agents, an extremely lightweight concrete could be produced and used for such products as highway impact attenuators or sacrificial blast-resistant barriers.

With reduced weight, lightweight concrete will have a correspondingly reduced hydrostatic pressure on formwork. This is useful when casting large, custom products. Also, water that has been absorbed into the porous structure of lightweight aggregates is said to provide additional water for internal curing.

Special considerations when using lightweight aggregates
When producing concrete using lightweight aggregates, there are several things you should be aware of in addition to the extra cost involved. Fluctuations in the gradation, specific gravity, absorption and moisture content are of particular concern. This means that additional quality-control testing is usually necessary to quickly catch problems associated with these variations.

Quality Control. To assure uniform quality, manufacturers should ensure that gradations and dry, loose unit weight of the lightweight aggregates are consistent. Reports including this data can usually be obtained from the aggregate supplier. If this is not the case, plant personnel should do the tests themselves. Variation in either the aggregate gradations or the dry, loose unit weight generally requires adjustments to the mix proportions in order to produce uniform concrete. In addition, both unit weight and slump testing of the fresh concrete should be performed frequently in order to verify consistency of the mix constituents and the concrete itself. Slumps should be as low as possible while remaining sufficiently easy to place, consolidate and finish.

Air content testing cannot be performed by the pressure method with air meters typically used for normal concrete (i.e., ASTM C231 Type-B meters). Instead, air content testing must be performed according to ASTM C173, “Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method.” This test uses a volumetric air meter, often referred to as a Roll-a-Meter.

Finally, you should ensure that aggregate used for structural lightweight concrete is graded and conforms to ASTM C330, “Specification for Lightweight Aggregate for Structural Concrete.” This specification covers lightweight aggregates intended for use in structural concrete in which the primary objective is reducing the density while maintaining the compressive strength of the concrete.

Concrete Strength. Lightweight concretes generally have what is called a “strength ceiling.” This is the maximum compressive or tensile strength a certain mix can obtain despite improvements to the cementitious materials. This limiting strength is dependent on the strength of the lightweight coarse aggregate and/or the quality of the contact zone and bond between the aggregate and the surrounding cement paste. It is possible to achieve a slight increase in strength by reducing the maximum size of coarse aggregates. Given that these limits exist, it should be noted that structural lightweight concrete strengths compare favorably with that of normal-weight concretes – in the 3,000 to 5,000 psi-range.

Moisture Contents. Highly absorptive lightweight aggregate should be wetted at least 24 hours prior to use, allowing time for the porous aggregate to become fully saturated. Wetting aggregate may be a logistical challenge depending on the weather, with freezing a possibility in cold weather and moisture loss in the hot weather. However, one benefit of wetting is that it helps keep the aggregate particles from segregating during handling.

It is not recommended that dry lightweight aggregate be directly batched and mixed because the aggregate particles can continue to absorb water from the mix. This can cause the mix to segregate or stiffen before it can be placed.

Because of the high variability of aggregate moisture contents, water-cement ratios are generally not specified for lightweight concretes. Calculation of water-cement ratios is hampered by the uncertainty of the total amount of water contained in the aggregates.

Proportioning. Many lightweight aggregate suppliers offer recommended mix designs that can be used as starting points for trial batching. Additionally, ACI 211.2, “Standard Practice for Selecting Proportions for Structural Lightweight Concrete,” provides procedures for designing lightweight concrete mixes.

Production Considerations. It may be necessary to extend mix times for lightweight concrete compared with conventional concrete to ensure that all of the mix constituents are properly mixed. Greater variations in workability should be expected, compared with conventional concrete with the same slump. Along those same lines, the amount of air-entraining admixture necessary to produce a constant amount of air content could also vary widely. Consult your admixture supplier for more information.

Depending on the porosity and the degree of the aggregate angularity, the concrete could be more difficult to place and finish. In some cases it is possible for the aggregate and the other mix constituents to separate, allowing the lightweight aggregate particles to float toward the concrete surface. Overworking the concrete can also cause the mix to segregate. This situation can be remedied by adjusting the aggregate gradation to reduce the size of the larger aggregates, adding natural sand or other filler materials. Although this is the opposite problem encountered with self-consolidating concrete, where the goal is to keep aggregates suspended in the mix, it is likely that similar rheological mix enhancements can help stabilize the mix.

As with other conventional concrete mixes, ease of placement can be enhanced by including air entrainment if not already a part of the mix design. Air entrainment also reduces bleeding and segregation and improves durability.

Finally, lightweight concretes may have an increased tendency to experience drying shrinkage and creep (strain increase over long periods of time with a constant load). Steam curing effectively reduces the likelihood of both drying shrinkage and creep.

Lighten up - It's your decision
The decision to use lightweight aggregates in order to economize transportation costs is one that requires you to take into account many variables. Although not every project is a good candidate for the use of lightweight aggregate, some projects definitely are. In addition, some of the more unique properties that lightweight aggregates offer could enhance specialized products, whether they are fire-resistant, blast-resistant or other insulating products. It’s good to keep your options open and be ready to take advantage of opportunities that allow you to boost your plant’s income. Ultimately, only you can decide if your products should lose some weight. As with our own diet, if you wish to lose weight, you must be patient and should expect some trial-and-error in finding the right combination of what works best.

References
ACI 213R, “Guide for Structural Lightweight Aggregate Concrete,” American Concrete Institute.

ACI 211.2, “Standard Practice for Selecting Proportions for Structural Lightweight Concrete,” American Concrete Institute.

ASTM C330, “Standard Specification for Lightweight Aggregates for Structural Concrete,” ASTM International.

ASTM C567, “Standard Test Method for Determining Density of Structural Lightweight Concrete,” ASTM International.

Concrete Manual. A Water Resources Technical Publication. Ninth Edition, Bureau of Reclamation, Denver, CO.

For more information
Expanded Shale, Clay & Slate Institute (ESCSI), 2225 Murray-Holladay Road, Suite 102, Salt Lake City, UT 84117, 801-272-7070, Fax: 801-272-3377, e-mail: info@escsi.org, www.escsi.org.

National Slag Association (NSA), 25 Stevens Avenue, Bldg. A, West Lawn, PA 19609, 610-670-0701, Fax: 610-670-0702, e-mail: useslag@aol.com, www.nationalslagassoc.org.

 
 
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