For more than 70 years since its inception, autoclaved aerated lightweight concrete has enjoyed a reputation for excellent insulation properties and low density. Traditional production of aerated lightweight concrete employs portland cement, fly ash, lime and fine sands as major raw material components and involves curing products in an autoclave. Estimated costs for an autoclaved aerated lightweight concrete plant range from $25 million to $50 million.

CJS Technology Inc. of Ontario and Hamburg, N.Y.-based Advanced Materials Technologies, LLC have developed technologies (patent pending) for manufacturing ultra-lightweight fiber-reinforced cellular concrete — Cret-O-Lite — using waste materials as the major component, including high carbon fly ash and waste glass typically not suited to conventional concrete. Also added to the concrete mix is a certain amount of portland cement, fiber and proprietary chemical additives.

Furthermore, a procedure has been developed to design concrete with strengths from 1,000 to 7,000 psi and densities from 50 to 90 lb. per cu. ft. for both nonloadbearing and loadbearing construction applications. Several trial production runs have been successfully conducted in at Rehoboth, Mass., precast plant., where the manufacturing process has been developed and refined in collaboration with operator Oldcastle Precast Inc. Following is a discussion of production techniques and properties of Cret-O-Lite, plus potential applications.

Production and properties

Though industrial by-products or recycled materials such as coal fly ash, slag and/or recycled glass comprise a major component of the ultra-lightweight fiber-reinforced cellular concrete, the material can be mixed, placed and finished like conventional concrete. (Note: Figure 1 showing the concrete mixture in a conventional turbine concrete mixer, and Figure 2, which illustrates the pouring of the concrete mixture into a modular mold.) Aeration can be introduced before or after placement. Though a small portion of air voids may be lost during pouring and finishing if aeration happens prior to placement, the concrete products can be finished immediately and their size controlled exactly to meet specifications. If aeration happens in molds after placement, most air voids can be retained; however, the concrete products should not be finished until the aeration process is completed. (Figure 3 shows the cellular pore structure in hardened concrete.)

Like conventional concrete, fiber-reinforced nonautoclaved cellular lightweight concrete products can be cured either at room temperature or at elevated temperatures. If cured at room temperature, they can be demolded the second day, though several days of curing may be required before shipment. If cured at elevated temperatures, the products can be demolded and shipped out the next day.

Compressive strength

For any given product, the strength of the fiber-reinforced nonautoclaved cellular lightweight concrete is dependent upon the density of the concrete. Figure 4 illustrates the relationship between compressive strength after steam curing and dry density of concrete composed of 60 percent industrial by-products or recycled materials including low-carbon fly ash, high carbon fly ash, ground glass, and/or ground granulated blast-furnace slag. Evidence indicates that with a dry density below 90 lb. per cu. ft., compressive strength could reach over 6,000 psi just after 14 hours of low-pressure steam curing. The strength continues to increase with time when concrete samples are kept in moist conditions because the industrial by-products or recycled materials hydrate much more slowly than portland cement.

Though adding fiber has little bearing on compressive strength, it increases the impact resistance and ductility of concrete. Figure 5 shows 4-in. cubes of conventional concrete and Cret-O-Lite after compression testing. As illustrated, the conventional concrete specimen cracked along a 45-degree diagonal line and broke into pieces, while the Cret-O-Lite specimen remained in one piece.

Flexural strength

It is generally agreed that the addition of polymer fiber does not increase flexural strength of concrete. However, within the studied strength range, as shown in Figure 6, the compressive/flexural strength ratio increases in a linear fashion with compressive strength of concrete. As the compressive strength reaches 5,000 psi, the compressive/flexural strength ratio reaches approximately 10, a value typical for conventional concrete.

Thermal conductivity

The thermal conductivity of concrete depends upon its density. Figure 7 illustrates the relationship between density and thermal conductivity of concrete, as demonstrated by Valore¹. Testing of concrete containing different materials such as low carbon fly ash, high carbon fly ash, blast furnace and ground glass has indicated that the thermal conductivity/density relationship for Cret-O-Lite follows the same pattern. Thus, the use of the above-mentioned materials does not have an obvious effect on the thermal conductivity of concrete for a given density.

Handling properties

As a hydraulic cement-based product, fiber-reinforced nonautoclaved cellular lightweight concrete is water-resistant. It can also be cut, sawed and nailed like wood, as shown in Figures 8-11.

Product development

Precast concrete panels. Small panels of 2-ft. width, 4-ft. length and 2-in. to 6-in. thickness have been manufactured in the laboratory of Advanced Materials Technologies. More than a dozen panels ranging in size from 8 ft. to 28 ft. long, 2 ft. to 10 ft. wide and 3 in. to 6 in. thick have been manufactured at Oldcastle Precast's plant in Rehoboth. Both welded 4- × 4- × 4-in. wire mesh and carbon scrims were used as primary reinforcements.

Table 1 shows the properties of Cret-O-Lite mixtures used for the precast panels and modular units (discussed later). The concrete mixtures exhibited a slump of 8 in. suitable for vertical pouring. The density of the fresh mixture was 98 lb. per cu. ft. After steam curing in the precast plant, the hardened concrete had a wet density of 97 lb. per cu. ft. Specimens were dried in either an office room or an oven. After four weeks in the office room, air-dry density registered 91 lb. per cu. ft., while the oven-dry concrete measured only 78 lb. per cu. ft., approximately half that of regular concrete.

After steam curing in the plant, the concrete mixture showed a compressive strength of 3,800 psi, adequate for demolding, handling and transportation. The strength continued to increase with time, reaching 5,200 psi at seven days and 6,500 psi at 28 days. Thus, sufficient strength for most precast elements and modular units was obtained.

TABLE 1 Properties of Cret-O-Lite for Precast Concrete Panels

Fresh Concrete		Hardened Concrete
Slump (inch)	Density (lb/pcf)	Wet Density	Air-Dry Density (lb/pcf)	Oven-Dry Density (lb/pcf)	Compressive Strength (psi)
					After Steam (lb/pcf)	7 days Curing	28 days
8	98	97	91	78	3,800	5,200	6,500

Figure 12 shows a 10-ft. × 10-ft. × 3-in. concrete panel directly lifted after 13 hours of steam curing. A simple center loading test of the panels is demonstrated in Figure 13. Applying a total load of about 6,000 lb., no cracking was observed during the the test, although some deflection was measured.

Modular concrete. Cret-O-Lite has been employed in the manufacture of several different types of modular concrete. The mix used for modular concrete in this case was the same as that described in Table 1. Lifting of a 4-ft. × 4-ft. × 6-ft. (4-in.-thick) Cret-O-Lite modular unit is illustrated in Figure 14. A modular washroom with wall thickness from 2 to 4 in. has been successfully manufactured using flowable Cret-O-Lite mixtures.

TABLE 2 Cret-O-Lite Mixtures for Nonloadbearing and Loadbearing Masonry Blocks

Batch No.	Wet Density (lb/pcf)	Oven-Dry Density (lb/pcf)	Compressive Strength (psi)
			After Steam Curing	27 days in Fog Room After Steam Curing
1	70	56	1,030	1,380
2	81	65	1,400	2,060

Concrete masonry block. Various batches of concrete mix were prepared to cast masonry blocks (Figure 15). According to ASTM C90², the minimum strength requirement for loadbearing masonry units is an average of 1,900 psi for three units and 1,700 psi for a single unit. For nonloadbearing masonry units, ASTM C129³ mandates a minimum strength of 600 psi for the average of three units and 500 psi for an individual unit.

Table 2 describes two Cret-O-Lite mixtures designed for nonloadbeaing and loadbearing masonry blocks. Designed for nonloadbearing masonry units, Batch 1 had a wet density of 70 lb. per cu. ft. and a dry density of 56 lb. per cu. ft. The dry density constitutes about half that of conventional lightweight concrete and one-third that of conventional regular concrete. By contrast, the compressive strength of Batch 1 measured 1,030 psi immediately after steam curing and 1,380 psi following 27 days in the fog room after steam curing: both values far exceed the strength requirement for nonloadbearing masonry units. The weight of the nonloadbearing block is about 16 lb. compared with approximately 45 lb. for regular concrete blocks.

Batch 2 was designed for loadbearing masonry units with a wet density of 81 lb. per cu. ft. and a dry density of 65 lb. per cu. ft. Although the densities of Batch 2 were only slightly higher than those of Batch 1, the strength of Batch 2 reached 2,060 psi after 28 days of curing, which meets the strength requirement specified in ASTM C90.

Economical considerations

The cost of Cret-O-Lite materials is comparable to or higher than conventional concrete. However, the low density of Cret-O-Lite products facilitating transportation, handling and construction processes may result in significant overall savings. Additionally, the use of ultra-lightweight concrete reduces the dead weight of the building, thereby decreasing foundation requirements, which can also save money. Where the ground is soft, ultra-lightweight concrete may be the only materials option available, both technically and economically. In any case, a high volume of industrial by-products or recycled materials used in the manufacture of Cret-O-Lite products can reduce potential environmental pollution and waste disposal costs.

Conclusion

With very low density, Cret-O-Lite has sufficient strengths for both nonloadbearing and loadbearing construction applications. Although it can be designed for cutting, nailing, and screwing like wood products, it is water- and fire-resistant. Savings in transportation, handling and construction costs due to its considerably lower density may readily compensate for the initial cost of Cret-O-Lite materials.

Caijun Shi and Yanzhong Wu are affiliated with CJS Technology Inc., 2116 Upland Dr., Burlington, Ontario, Canada. Along with Monte Riefler, the former is also associated with Advanced Materials Technologies, LLC, 5225 Southwestern Blvd., Hamburg, N.Y. 14075. Harold Messenger is employed by Oldcastle Precast Inc., 41 Almeida Road, Rehoboth, Mass. 02769.

References

1. R.C. Valore, “A Study of Cellular Concrete,” NBS Technical News Bulletin, National Bureau of Standards, pp.41-43, March 1955.
2. ASTM C90-99, Specifications for Loadbearing Concrete Masonry Units, Annual Book of ASTM Standards, Vol. 04.05, ASTM, West Conshohocken, Pa., 2000.
3. ASTM C129-99, Specifications for Nonloadbearing Concrete Masonry Units, Annual Book of ASTM Standards, Vol. 04.05, ASTM, West Conshohocken, Pa., 2000.