🕑 Reading time: 1 minute
What is Lightweight Concrete?
Lightweight concrete is a specialized concrete mixture made with lightweight coarse aggregates, and sometimes lightweight fine aggregates, instead of traditional aggregates. This type of concrete possesses an in-place density (unit weight) typically ranging from 90 to 115 lb/ft³ (1440 to 1840 kg/m³), in contrast to the density of normal weight concrete, which falls within the range of 140 to 150 lb/ft³ (2240 to 2400 kg/m³). For structural applications, lightweight concrete must exhibit a compressive strength greater than 2500 psi (17.0 MPa). The lightweight aggregates used in structural lightweight concrete are usually materials such as expanded shale, clay, or slate, which undergo a firing process in a rotary kiln to develop a porous structure. Additionally, products like air-cooled blast furnace slag are also utilized.
There are other classes of non-structural lightweight concrete (LWC) with lower density made with different aggregate materials and higher air voids in the cement paste matrix, as seen in cellular concrete.
Classification of Lightweight Concrete
Various types of lightweight concrete can be classified based on their method of production:
- Lightweight Aggregate Concrete: Utilizes porous lightweight aggregates with a low apparent specific gravity, typically lower than 2.6.
- Aerated Concrete: Incorporates large voids within the concrete or mortar mass, achieved through methods such as aeration with stabilized foam or air entrainment.
- No-Fines Concrete: Opts to omit fine aggregate from the mix, resulting in a large number of interstitial voids. Normal weight coarse aggregate is generally used in this concrete type.
Lightweight concrete can also be classified according to its intended use, distinguishing 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 requirement, as per ASTM standards.
Types of Lightweight Concrete
1. Lightweight Aggregate Concrete
- In the early 1950s, the use of lightweight concrete blocks gained acceptance in the UK for load-bearing inner leaf of cavity walls. New types of artificial lightweight aggregates (LWA) paved the way for introducing lightweight concrete (LWC) suitable for structural work. Some types of lightweight aggregates suitable for structural reinforced concrete include:
- Pumice: Used for reinforced concrete roof slabs, mainly for industrial roofs in Germany.
- Foamed Slag: The first lightweight aggregate suitable for reinforced concrete produced in large quantities in the UK.
- Expanded Clays and Shales: Capable of achieving sufficiently high strength for prestressed concrete, established under various trade names.
2. Aerated Concrete
Characterized by its lowest density, thermal conductivity, and strength, aerated concrete can be sawn, screwed, and nailed like timber. It is usually high-pressure steam-cured and available as precast units for floors, walls, and roofs.
3. No Fines Concrete
Composed of cement and coarse aggregate only, with uniformly distributed voids throughout its mass. It finds applications in load-bearing walls, internal wall casting, non-load-bearing walls, and under-floor filling.
Types of Lightweight Concrete Based on Density and Strength
Lightweight concrete can be classified into:
- Low Density Concrete: Primarily used for insulation purposes with low unit weight, high heat insulation value, and relatively low compressive strength.
- Moderate Density Concrete: Positioned between low density and structural concrete, offering a balance of strength and insulation.
- Structural Concrete: Featuring aggregates suitable for structural efficiency, with minimum compressive strength requirements.
Most structural LWC is capable of producing concrete with compressive strength in excess of 34.47 N/mm². While the unit weight of structural LWC is considerably greater than that of low-density concrete, its thermal insulation values are substantially better than those of normal-weight concrete.
Uses and Advantages of Lightweight Concrete
Lightweight concrete finds diverse applications, including screeds, walls, insulation, and structural elements. Its advantages include reduced dead load, faster building rates, lower transportation costs, and enhanced thermal insulation properties, contributing to energy efficiency in buildings.
Durability of Lightweight Concrete
Durability is defined as the ability of a material to withstand the effects of its environment. Lightweight concrete faces challenges such as chemical attack, physical stress, and mechanical damage, which require proper precautions and protective measures to ensure long-term performance in various environmental conditions.