Lightweight Concrete Floor Systems – Thickness, Uses

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Lightweight concrete floor provides more efficient strength-to-weight ratio in concrete floor systems when compared to other conventional concrete floor systems. The reduced quantity of concrete and steel reinforcement offsets the marginally higher cost of lightweight concrete floor systems.

These floors are constructed for different purposes but the reduction of dead loads on the structure is the main motivation behind using this system. There are various lightweight concrete floor systems which can be selected based on the requirements of the structure- The prevalent ones are Solid lightweight concrete floor and Composite lightweight concrete floor.

The thickness determination of lightweight concrete floor is crucial to meet the requirements and specifications of the structure. The minimum thickness of lightweight concrete floor system can be computed based on the Specifications of American Concrete Institute (ACI 318-14).

Types of lightweight concrete

1. Lightweight Aggregate Concrete

The production of lightweight aggregate concrete is similar to conventional concrete, but parts or all quantity of coarse aggregate are replaced by lightweight aggregate.

2. Foam Lightweight Concrete

It has a good mechanical strength and low thermal conductivity. Foam concrete contains confined air pockets thus reducing its weight substantially which offers great reduction in floor cost. It is mainly designed for making filling and levelling layers of floors in industrial buildings and other civic structures 

3. Polystyrene Lightweight Concrete

Polystyrene lightweight concrete's density, compression strength, and thermal conductivity are similar to that of foam concrete. It is designed for efficient construction of gradient and thermal insulating layers of flat roofs.

Types of Lightweight Concrete Floors Systems

1. Solid Lightweight Floors

This type of lightweight floor system is constructed by pouring lightweight aggregate concrete into the fixed forms of the concrete slab. It is used in the construction of multistory buildings. The thickness of solid lightweight concrete floor system is determined according to the specifications of ACI 318-14, as discussed in the following sections. 

2. Composite Lightweight Concrete Floors

According to EN 1994-1-1:2005, composite slab is composed of profiled steel decking with an in situ reinforced concrete topping. Not only does the deck acts as a permanent formwork to the concrete, but also provides adequate shear bond with the concrete.

Lightweight concrete can also be applied as a topping slab over structural concrete decks. the shape of profiled steel decking may vary as shown in Fig.3 and Fig.4.

3. Other Lightweight Floor Systems

This includes Lightweight Wood Concrete Slab, Modular Lightweight Concrete Floor System, and Precast Concrete Lightweight Concrete Floor. Some of these floor systems are modifications of the aforementioned lightweight systems.

Minimum Thickness of Lightweight Floors

For structures that will not be damaged by significant deflections,the minimum thickness of a slab can be obtained by Table 1. This table is used for normal weight concrete, but it can be factored (lightweight concrete factor) and used for lightweight concrete.

The method for calculation of lightweight Concrete factor is as follows :

lightweight concrete factor = = 1.65-.005*wc
lightweight concrete minimum thickness = * minimum thickness of table 1.

where:

wc= wt. of lightweight concrete (ranges between 14.13KN/m³ and 18.85 KN/m³)

Table 1 Minimum thickness for normal weight concrete floor system

Types of construction	Minimum thickness
Solid one-way slab
Simply supported beam	L/20
one end continuous	L/24
both end continuous	L/28
Cantilever	L/10
Ribbed one-way slab
Simply supported beam	L/16
one end continuous	L/18.5
both end continuous	L/21
Cantilever	L/8
L is the clear span of the slab

Economy Lightweight Concrete Floors

The cost of lightweight concrete floor is greater than conventional concrete by less than 1 percent. However,this increased cost is offset by number of factors which are discussed below:

1. Dead weight reduction leads to the decline of total load on foundation and hence smaller foundation would be needed.
2. Similarly, supporting members such as beams would also have smaller sizes which decline the total cost significantly.
3. Reduced loads lead to smaller inertial seismic forces.
4. Constructing precast and prestress floors, longer spans may be built and hence fewer number of columns are required
5. ACI 216.1 reported that fire resistance of lightweight concrete is greater than that of normal concrete. Consequently, the thickness of slabs may be reduced, resulting in significantly less concrete volumes.

Uses of Lightweight Concrete Floors

1. Steel frame office
2. Commercial buildings
3. Industrial buildings and warehouses
4. Leisure buildings
5. Hospitals
6. Schools
7. Cinemas
8. Individual houses
9. Residential buildings
10. Refurbishment projects