What is Lightweight Concrete?
Lightweight concrete mixture is made with a lightweight coarse aggregate and sometimes a portion or entire fine aggregates may be lightweight instead of normal aggregates. Structural lightweight concrete has an in-place density (unit weight) on the order of 90 to 115 lb / ft³ (1440 to 1840 kg/m³).
Normal weight concrete a density in the range of 140 to 150 lb/ft³ (2240 to 2400 kg/m³). For structural applications the concrete strength should be greater than 2500 psi (17.0 MPa).
Lightweight aggregates used in structural lightweight concrete are typically expanded shale, clay or slate materials that have been fired in a rotary kiln to develop a porous structure. Other products such as air-cooled blast furnace slag are also used.
There are other classes of non-structural LWC with lower density made with other aggregate materials and higher air voids in the cement paste matrix, such as in cellular concrete.
Classification of Lightweight Concrete
It is convenient to classify the various types of lightweight concrete by their method of production. These are:
- By using porous lightweight aggregate of low apparent specific gravity, i.e. lower than 2.6. This type of concrete is known as lightweight aggregate concrete.
- By introducing large voids within the concrete or mortar mass; these voids should be clearly distinguished from the extremely fine voids produced by air entrainment. This types of concrete is variously knows as aerated, cellular, foamed or gas concrete.
- By omitting the fine aggregate from the mix so that a large number of interstitial voids is present; normal weight coarse aggregate is generally used. This concrete as no-fines concrete.
LWC can also be classified according to the purpose for which it is to be used: it can distinguish between structural lightweight concrete (ASTM C 330-82a), concrete used in masonry units (ASTM C 331-81), and insulating concrete (ASTM C 332-83).
This classification of structural lightweight concrete is based on a minimum strength: according to ASTM C 330-82a, the 28-day cylinder compressive strength should not be less than 17 MPa (2500 psi).
The density (unit weight) of such concrete (determined in the dry state) should not exceed 1840 kg/m³ (115 lb/ft³), and is usually between 1400 and 1800 kg/m³ (85 and 110 lb/ft³). On the other hand, masonry concrete generally has a density between 500 and 800 kg/m³ (30 and 50 lb/ft³) and a strength between 7 and 14 MPa (1000 and 2000 psi).
Types of Lightweight Concrete
1. Lightweight Aggregate Concrete
In the early 1950s, the use of lightweight concrete blocks was accepted in the UK for load bearing inner leaf of cavity walls. Soon thereafter the development and production of new types of artificial LWA (Lightweight aggregate) made it possible to introduce LWC of high strength, suitable for structural work.
These advances encouraged the structural use of LWA concrete, particularly where the need to reduce weight in a structure was in a structure was an important consideration for design or for economy.
Listed below are several types of lightweight aggregates suitable for structural reinforced concrete:-
- Pumice – is used for reinforced concrete roof slab, mainly for industrial roofs in Germany.
- Foamed Slag – was the first lightweight aggregate suitable for reinforced concrete that was produced in large quantity in the UK.
- Expanded Clays and Shales – capable of achieving sufficiently high strength for prestressed concrete. Well established under the trade names of Aglite and Leca (UK), Haydite, Rocklite, Gravelite and Aglite (USA).
- Sintered Pulverised – fuel ash aggregate – is being used in the UK for a variety of structural purposes and is being marketed under the trade name Lytag
2. Aerated Concrete
Aerated concrete has the lowest density, thermal conductivity and strength. Like timber it can be sawn, screwed and nailed, but there are non-combustible. For works in-situ the usual methods of aeration are by mixing in stabilized foam or by whipping air in with the aid of an air entraining agent.
The precast products are usually made by the addition of about 0.2 percent aluminums powder to the mix which reacts with alkaline substances in the binder forming hydrogen bubbles.
Air-cured aerated concrete is used where little strength is required e.g. roof screeds and pipe lagging. Full strength development depends upon the reaction of lime with the siliceous aggregates, and for the equal densities the strength of high pressure steam cured concrete is about twice that of air-cured concrete, and shrinkage is only one third or less.
Aerated concrete is a lightweight, cellular material consisting of cement and/or lime and sand or other silicious material. It is made by either a physical or a chemical process during which either air or gas is introduced into a slurry, which generally contains no coarse material.
Aerated concrete used as a structural material is usually high-pressure steam-cured. It is thus factory-made and available to the user in precast units only, for floors, walls and roofs. Blocks for laying in mortar or glue are manufactured without any reinforcement.
Larger units are reinforced with steel bars to resist damage through transport, handling and superimposed loads. Autoclaved aerated concrete, which was originally developed in Sweden in 1929, is now manufactured all over the world.
3. No Fines Concrete
The term no-fines concrete generally means concrete composed of cement and a coarse (9-19mm) aggregate only (at least 95 percent should pass the 20mm BS sieve, not more than 10 percent should pass the 10mm BS sieve and nothing should pass the 5mm BS sieve), and the product so formed has many uniformly distributed voids throughout its mass.
No-fines concrete is mainly used for load bearing, cast in situ external and internal wall, non load bearing wall and under floor filling for solid ground floors (CP III: 1970, BSI). No-fines concrete was introduced into the UK in 1923, when 50 houses were built in Edinburgh, followed a few years later by 800 in Liverpool, Manchester and London.
This description is applied to concrete which contain only a single size 10mm to 20mm coarse aggregate (either a dense aggregate or a light weight aggregate such as sintered PFA). The density is about two-third or three quarters that of dense concrete made with the same aggregates.
No-fines concrete is almost always cast in situ mainly as load bearing and non load bearing walls including in filling walls, in framed structures, but sometimes as filling below solids ground floors and for roof screeds.
No-fines concrete is thus an agglomeration of coarse aggregate particles, each surrounded by a coating of cement paste up to about 1·3 mm (0·05 in.) thick. There exist, therefore, large pores within the body of the concrete which are responsible for its low strength, but their large size means that no capillary movement of water can take place.
Although the strength of no-fines concrete is considerably lower than that of normal-weight concrete, this strength, coupled with the lower dead load of the structure, is sufficient in buildings up to about 20 storeys high and in many other applications.
Types of Lightweight Concrete Based on Density and Strength
LWC can be classified as :-
- Low density concrete
- Moderate strength concrete
- Structural concrete
1. Low Density Concrete
These are employing chiefly for insulation purposes. With low unit weight, seldom exceeding 800 kg/m³, heat insulation value are high. Compressive strength are low, regarding from about 0.69 to 6.89 N/mm2.
2. Moderate Density Concrete
The use of these concrete requires a fair degree of compressive strength, and thus they fall about midway between the structural and low density concrete. These are sometimes designed as ‘fill’ concrete. Compressive strength are approximately 6.89 to 17.24 N/mm² and insulation values are intermediate.
3. Structural Concrete
Concrete with full structural efficiency contain aggregates which fall on the other end of the scale and which are generally made with expanded shale, clay, slates, slag, and fly-ash. Minimum compressive strength is 17.24 N/mm².
Most structural LWC are capable of producing concrete with compressive strength in excess of 34.47 N/mm².
Since the unit weight of structural LWC are considerably greater than those of low density concrete, insulation efficiency is lower. However, thermal insulation values for structural LWC are substantially better than NWC.
Uses of Lightweight Concrete
- Screeds and thickening for general purposes especially when such screeds or thickening and weight to floors roofs and other structural members.
- Screeds and walls where timber has to be attached by nailing.
- Casting structural steel to protect its against fire and corrosion or as a covering for architectural purposes.
- Heat insulation on roofs.
- Insulating water pipes.
- Construction of partition walls and panel walls in frame structures.
- Fixing bricks to receive nails from joinery, principally in domestic or domestic type construction.
- General insulation of walls.
- Surface rendered for external walls of small houses.
- It is also being used for reinforced concrete.
Advantages of Lightweight Concrete
- Reduced dead load of wet concrete allows longer span to be poured un-propped. This save both labor and circle time for each floor.
- Reduction of dead load, faster building rates and lower haulage and handling costs. The eight of the building in term of the loads transmitted by the foundations is an important factor in design, particular for the case of tall buildings.
- The use of LWC has sometimes made its possible to proceed with the design which otherwise would have been abandoned because of excessive weight. In frame structures, considerable savings in cost can be brought about by using LWC for the construction floors, partition and external cladding.
- Most building materials such as clay bricks the haulage load is limited not by volume but by weight. With suitable design containers much larger volumes of LWC can haul economically.
- A less obvious but nonetheless important characteristics of LWC is its relatively low thermal conductivity, a property which improves with decreasing density in recent years, with the increasing cost and scarcity of energy sources, more attention has been given the formerly to the need for reducing fuel consumption while maintaining, and indeed improving, comfort conditions buildings. The point is illustrated by fact that a 125 mm thick solid wall of aerated concrete will give thermal insulation about four times greater than that of a 230 mm clay brick wall.
Durability of Lightweight Concrete
Durability is defined ass the ability of a material to withstand the effect of its environment. In a building material as chemical attack, physical stress, and mechanical assault:-
Chemical attack is as aggregate ground-water particularly sulfate, polluted air, and spillage of reactive liquids LWC has no special resistant to these agencies: indeed, it is generally move porous than the ordinary Portland cement. It is not recommended for use below damp-course. A chemical aspects of durability is the stability of the material itself, particularly at the presence of moisture.
Physical stresses to which LWC is exposed are principally frost action and shrinkage and temperature stresses. Stressing may be due to the drying shrinkage of the concrete or to differential thermal movements between dissimilar materials or to other phenomena of a similar nature. Drying shrinkage commonly causes cracking of LWC if suitable precautions are not taken.
Mechanical damage can result from abrasion or impact excessive loading of flexural members. The lightest grades of LWC are relatively soft so that they subject to some abrasion were they not for other reasons protected by rendering.