What is Lightweight Concrete (LWAC) ? Properties & Features

Lightweight concrete is concrete made from lightweight aggregates that have a having an oven-dry density of not less than 800kg/m3 and not more than 2000kg/m3.

[Ref: From Handbook of Sustainable Concrete and Industrial Waste Management, 2022.]

The LWA can be either natural or artificial or manufactured. The natural aggregates include volcanic cinder, dolomite, or pumice. The artificial aggregates can be blast furnace slag, sintered fly ash, or bloated clay.

Properties of Lightweight Aggregate Concrete

Commonly the LWA has a lower density, is highly durable, has low shrinkage, and low thermal conductivity. The strength of the LWA particles decreases with decreasing density. 

The following are the properties of LWAC

• Particle Size, Shape, and Surface Texture 
• Specific Weight, Density, and Bulk Density 
• Water Absorption Capacity 
• Unit Weight 
• Compressive Strength 

Particle Size, Shape, and Surface Texture

The shape of LWA is rounded, angular and irregular in shape. The shape of LWA aggregates will vary based on the manufacturing process. Surface texture may be smooth with a small number of pores or irregular with a large rock-marked surface. The surface texture and the shape are very important in the mix design procedure, as they will control the degree of moisture absorption and will affect the workability of fresh concrete.
The granularity of LWA is related to the quality of the aggregate. The size will vary from 5mm to 20mm.

Specific Weight, density, and Bulk Density

The specific weight of the LWA is equivalent to the weight of the unit volume of the porous material (including the pores in the particle). The density of LWA is lower than that of normal weight aggregate.

For coarse LWA density varies from 600kg/m3 to 1600kg/m3. For fine aggregates, the density will change from 1300 kg/m3 to 1850 kg/m3.

The bulk density of LWA is based on the weight of 7 to 15mm grains. The bulk density of the LWA can vary from 1/3rd to 2/3rd times that of normal-weight aggregates. The order of bulk density of LWA is 700kg/m3 to 800 kg/m3.

Water Absorption Capacity

The water absorption capacity of the coarse LWA is in the range of 5% to 30% while for fine LWA it is 5% to 15%. Pumice LWA has a high water absorption capacity and the maximum capacity is 50%.

Unit Weight

For Coarse LWA unit weight is 1000 kg/m3 to 1700 kg/m3 and for fine LWA it is 1300 to 1600 kg/m3. For pumice stones, the unit weight varies from 1200 to 1600 kg/m3.

Compressive Strength

The compressive strength of coarse LWA is 10 to 60MPa. For Fine LWA it is in the range of 20 to 50MPa. This is the raw material, expanded clay. For pumice-based LWAC, the compressive strength varies from 5 to 15MPa.

Production of Lightweight Concrete (LWAC)

The LWAC can be manufactured by proper and efficient mixing, transporting, and compacting of the binder material(cement), water, fine aggregate, and lightweight aggregates. The production or the different stages of manufacturing LWAC is the same as that of natural or ordinary concrete.

• Effective batching 
• Proper mixing 
• Correct transporting 
• Sufficient compaction after the proper placing 
• Efficient curing is the important step that will influence the maximum or the best qualities of LWAC. 

Factors Affecting the Strength of Lightweight Concrete

The strength and durability of the Lightweight Concrete are influenced by the following parameters:

• Grade and Granularity of the Aggregates
• Grade and the proportion of the cement
• Mixing water and consistency
• Method of mixing
• Compaction
• Curing

Grade and Granularity of the Aggregates

For increased strength, density, and improved weather resistance, the fine grains of lightweight aggregate are substituted with sand. This will, in turn, increase the cement requirement.

Grade and the proportion of the cement

Lower cement content of 150 to 300kg/m3 might result in the lowering of the density and a higher strength is gained for higher cement content of a range of 350 to 450kg/m3 for properly compacted concrete. Rich concrete if not compacted may result in the same strength as that of well-compacted weak concrete.

Mixing Water and Consistency

The concrete must have a good consistency which can be checked using the slump. The plastic consistency can be induced by the addition of 2 to 4% of the vinsol resin (by weight of cement), an entraining agent without any loss in the strength of the cement.

Method of Mixing

Gravity-type mixers are adequate for the concretes having a low content of fine aggregates (10 to 15%). Positive-type mixers are most effective if the proportion of the fine aggregate is 30 to 50%. They lead to a strong increase of 30 to 100% as compared to the gravity-type mixers.

Compaction

Strength and density are closely related to the degree of compaction. The strength varied with the vibrating frequency and amplitude as well as with the type of vibrator used. The compaction efficiency can be improved by applying pressure during vibration.

Curing

Fresh concrete is extremely sensitive to intensive sunshine and wind. Hence, they must be protected by using damp sheets and covers. Also, methods of spraying have to be carried out. Cool-weather concreting will slow down the hardening procedures. The steam-cured LWAC concrete must not be exposed to temperatures that are lesser than 15 degrees Celsius.

Applications of Lightweight Concrete

1. Used in Oil exploration Concrete Structures
2. Bridge Deck Construction
3. Floating Bridge Construction
4. Post-tensioned box girder Concrete Bridges
5. Offshore Construction

Read More On: Fiber Reinforced Concrete (FRC)