Conventional cement concrete is a heavy building material. For structures such as multi-story buildings, it is desirable to reduce the dead loads. Lightweight Concrete is generally more suitable for such types of construction works.
Lightweight Concrete is particularly suitable for use where low density, good thermal insulation, or fire protection are required but not all of the available aggregate are equally suitable for any particular application.
Light Weight Concrete Produced By:
It is best produced by entraining air in the cement concrete and can be obtained by any one of the following methods:
1. Concrete making with coarse aggregate and cement only:
Sometimes this type of concrete referred to as no-fines concrete. Suitable aggregates are natural aggregate, blast furnaces slag, clinker, foamed slag, etc. In this concrete fine aggregate are not added, therefore the voids will be created and the concrete produced will be lightweight.
2. Concrete mix by replacing coarse aggregate with porous or cellular aggregate:
The concrete produced is known as cellular concrete which is further classified as follows:
Based on Method of Manufacturing:
It is classified as foam concrete and gas concrete
Based on Types of Building Materials:
Classified as gas and form concrete (Portland cement), gas and foam concrete (lime and sand), gas slag and foam slag concrete (lime and finely divided blast furnace slag or fly ash.
Types of Light Weight Concrete:
1) Foam Concrete:
Foam concrete is made by mixing a cement paste or mortar with stabilized foam. After the curing of concrete and concrete get harden the foam cell in concrete foams the cellular structure.
The foam is generally formed by stirring a mixture of resin soap and animal glue. The most preferable foaming agent is alumino sulpho naphthene compounds and hydrolyzed slaughter blood. Foam concrete is more suitable for heat insulation purposes.
2) Heat Insulating Foam Concrete:
It is cast into blocks measuring 100 × 50 × 50 cm and over, which, once hardened, can be sawn into slabs of 100 × 50 × 50 to 100 × 50 × 12 cm.
Heat insulating foam concrete has a strength of up to 2.5 N/mm2 and a coefficient of heat conductivity of 0.10–0.20 kCal/m.h.°C. This kind of foam concrete is used as heat insulating material for reinforced concrete floors, partitions, etc.
3) Structural and Heat Insulating Foam Concrete:
It has a strength of 2.5 – 7.5 N/mm2 and a coefficient of heat conductivity of 0.20–0.40kCal/m.h.°C and is used for exterior walls.
4) Structural Foam Concrete:
It is used to make reinforced floor components, the items being reinforced by two wire meshes from wires of 3–5 mm thick. Structural foam concrete has strength up to 15 N/mm2 and a coefficient of heat conductivity of 0.40–0.60 kCal/m.h°C. Heat insulating foam concrete is widely used in three-layer exterior walls of heated buildings.
5) Gas Concrete:
The gas concrete is generally manufactured by expanding the binding material paste, which may or may not include aggregates. It is also known as aerated concrete. The mix is expanded by gas-forming substances, but care should be taken to synchronize the end of gas formation with the beginning of mix setting. The setting time of cement may be regulated with the aid of accelerators (such as dihydrate gypsum) or retarders (such as industrial sugar, or molasses, introduced in amounts from 0.1 to 2.5 kg/m3).
The approximate component proportion of gas concrete ingredients is as follows:
90% Portland cement, 9.75% powdered lime, 0.25% aluminum powder (for water to cement ratio of 0.55–0.65). In this, about 65% of sand is ground in a wet state. The basic considerations in choosing the proportion of lightweight concrete are economy consistent with placability and adequate strength, and attainment of specified bulk density with the lowest consumption of cement.
Lime for preparing gas concrete should be of the top grade, quick-slaking and low-magnesium variety. In sand intended for gas concrete, the content of clay impurities should not exceed 1.5% by weight, since the impurities lower the strength and slow down gas evacuation and concrete expansion.
The gas-forming agent used is finely ground aluminum powder. The evolving hydrogen produced during the chemical reaction between hydrate calcium oxide and aluminum according to the equation expands the cement paste which retains its porous structure as it hardens.
2Al + 3Ca(OH)2 + 6 H2O = 3 CaO.Al2O3.6H2O + 3H2
Items from gas concrete are manufactured in the manner described below. A mixture of ground sand and water is fed to the stirrer and mixed with cement, aluminum powder, water, and un-ground sand, after which the mix is cast into moulds.
After 4–5 h of hardening, gas concrete is cut into slabs and loaded into autoclaves where the items finally harden at a temperature of 175°C and at a pressure of 8 atm. Autoclaving enhances the strength of gas concrete and, in addition, substantially reduces the consumption of cement which can thus be fully or partially replaced by lime.
Gas concrete is similar to foam concrete in properties and is used for the same purpose. However, it is simpler to manufacture, and items form it has more stable qualities than from
Foam Concrete: in particular, this applies to their bulk densities. These are the chief advantages of gas concrete over foam concrete. Among the main shortcomings of cellular concrete is a high tendency to deformation, shrinkage, etc.
Characteristics Lightweight Concrete:
The density of LWC varies from 300–1200 kg/m2
Due to low density and the characteristic texture of porous aggregate especially in the crushed state, the workability of concrete needs special attention.
In general, placing compacting, and finishing lightweight aggregate concrete requires relatively less effort; therefore, even 50 to 75 mm slum may be sufficient to obtain workability of the type that is shown by 100 to 125 mm slump of normal-weight concrete.
3) Unit Weight:
Unit weight and strength are the two properties generally sought from lightweight concrete. In concrete, it is generally preferred to have the highest possible strength/ unit weight ratio with the lowest cost of concrete. The air-dried unit weight of concrete is limited to a maximum of 18.40 kN/m3.
The use of normal sand to control the properties of hardened concrete tends to increase the unit weight, although this tendency is partially offset from the balancing effect of entrained air, which is invariably prescribed for improving the workability.
Most structural lightweight concrete weighs between 16.00 to 17.60 kN/m3; however, job specifications in special cases may allow higher than 18.40kN/m3.
This concrete has the design strengths of 20 to 35 MPa, 28-day compressive strengths are common, although using a high cement content and good quality lightweight aggregate of small size has made it possible, in some precast and pre-stressing plants, to produce 40 to 48 MPa concrete.
Lightweight aggregate is developed with microporosity have been developed to produce 70 to 75 MPa lightweight concrete which generally weighs 18.40 to 20.00 kN/m3.
Thermal Insulation: is about 3–4 times more than that of bricks and about 10 times than that of concrete.
5) Fire Resistance
The Fire resistance of light weight concrete is excellent.
6) Sound Insulation:
The Sound insulation of light weight concrete is generally poor compare to normal concrete.
Aerated concrete is slightly alkaline. Due to its porosity and low alkalinity the reinforcement may be subjected to corrosion and as such, require special treatments.
The lightweight concrete elements can be easily sawn, drilled, or nailed which makes for easy construction and repairs.
Due to high strength and lightweight to mass ratio, the cellular products are quite economical.
Advantages of Light Weight Concrete:
- The basic economy of LWC can be demonstrated by the savings achieved in the associated reinforcement requirements.
- LWC has superior resistance of shear elements to earthquake loading since seismic forces are largely a direct function of deal weight of a structure, are also one among the other advantages of LWC.
- Due to lower handling transportation, the construction cost, the lightweight concrete is ideally suited for the production of precast concrete elements and prefabricated elements.
Applications Light Weight Concrete:
- Lightweight Concrete or low-density concrete is used for precast floor and roofing units.
- As load bearing walls using cellular concrete blocks.
- As insulation cladding to exterior walls of structures.
You May Also Like:
- Vacuum Concrete – Procedure, Pros, & Cons
- Pervious Concrete – Its Application & Advantages
- What is Green Concrete? Its Materials & Application