Concrete Slabs- Types & Design Features

A concrete slab is a horizontal structural element with length comparatively larger than its width usually of uniform thickness constructed to support structures like foundations, or building components. They are mostly observed as roofs or floors in building structures.

Concrete slabs are flexural members that are primarily designed to resist bending, shear, and deflection under the action of loads. The structural behavior of a concrete slab is dependent on its thickness, reinforcement details, support conditions, and type of loading. 

A Cross-Section of a Simple Concrete Slab Constructed On Ground
©prodyogi

Concrete slabs are the most sturdy and compact slab type used for building structures. In this article, we will discuss the structural features of concrete slabs and their types.

Different Types of Concrete Slabs

Concrete slabs can be classified based on:

1. Flexural behavior
2. Structural features 
3. Construction Method

Classification of Concrete Slabs
Based on Flexural Behaviour	Based on Structural Features	Based on the Construction Method
One-Way Slab	Flat Slab	Post-Tensioned Slabs
Two-Way Slab	Ribbed Slab	Precast Slabs
	Waffle Slab

1. One-way Slab

A one-way slab is designed to span in one direction only and is supported by beams or walls on opposite sides. If the length-to-breadth ratio (l/b) is greater than or equal to 2 then it is considered a one-way slab.

The structural behavior of one-way slabs is that they bend in a single direction in order to support all loads coming over them. The main reinforcement bars in the slab are oriented in a direction perpendicular to the direction of the span. 

One-way slabs are most suitable for spans ranging between 3- 6 m. 

Read More On: One-Way Slabs

2. Two-way Slab

A two-way slab is designed to span in both directions and is supported by columns or walls on all four sides. If the length-to-breadth ratio (l/b) is less than 2 then it is considered a two-way slab.

These structures bend in two directions to support the loads, hence the main reinforcement bars in the slab are oriented in both directions. 

Read More On: Two-Way Slabs

3. Flat Slab

A flat slab is a reinforced concrete slab that does not have beams, and instead uses column capitals or drop panels to increase its load capacity. Flat slabs can be designed as either one-way or two-way slabs, depending on the direction in which they are supported. 

2. Ribbed Slab

A ribbed slab is similar to a flat slab, but it has a series of ribs or beams underneath the surface to increase its strength and load-bearing capacity.

3. Waffle Slab

 A waffle slab is a type of ribbed slab that has a grid-like pattern of beams and joists, creating a series of square or rectangular "waffles" or voids in the slab.

4. Post-tensioned Slab

A post-tensioned slab is a type of reinforced concrete slab that uses high-strength steel cables or tendons to increase its strength and durability.

5. Precast Slab

A precast slab is a type of reinforced concrete slab that is cast in a factory and then transported to the construction site for installation.

All the slabs can be designed as one-way or two-way slabs based on their support conditions and arrangement in the building. 

How Thick Should a Concrete Slab be?

The thickness of concrete slabs ranges between 4 to 6 inches to comply with most building codes. The thickness of concrete slabs for residential construction is generally 4 inches. For slabs subjected to occasional heavy loads, a thickness of 5 to 6 inches is provided.

The recommendations are based on construction practices following the ACI standards. Most building codes recommend having concrete laid at a minimum of 3.5 inches with a minimum compressive strength of 3000 psi.

It must be noted that the higher the thickness, the higher the strength of the concrete slab. But engineers always design slab thickness that is economical and with serviceability conditions.

Thickness of Unsupported Concrete Slabs (Slab-On-Ground)

For instance, the thickness of unsupported concrete slabs that are directly placed on the ground (slab-on-ground) for various construction applications is listed in the table below.

Type of Concrete Slab	Thickness
Concrete Driveway Slabs (Standard Use)	4 inches
Concrete Driveway Slabs (Heavy Duty)	6 inches
Concrete Slabs for Car lift	4 inches [ Lift: 8500 lbs to 10,000 lbs] 6 inches [Lift: 11,000 to 15,000 lbs]
Patio	4 inches
Concrete Garage Slabs	6 inches
Concrete Shed	4 inches

Note: The above details are from real and successful construction practices but are not specifically listed in standard codes.

Thickness of Supported Concrete Slabs (One-Way & Two-Way Slabs)

The thickness of supported concrete slabs is determined based on the following conditions:

1. Type of slab ( One-Way Slab or Two-way slab)
2. Prestressed on non-pressed type
3. Support conditions of the slabs ( simply supported, one end continuous (three sides supported), both ends continuous, cantilever) 

A thickness is determined from the span and depth of the slab using expressions given by the standard code. This thickness is checked for deflection and if it passes, the thickness is fixed as the design depth of the slab.

Expression for Minimum Thickness of One-Way Slab (Non-Prestressed)

The data below shows the expression for determining the minimum thickness of solid non-prestressed one-way slabs as per ACI 318-14, Table 7.3.1.1. The ℓ = span length of the beam or one-way slab and h = overall thickness, height, or depth of the member, in inches. 

Note: The above expression is varied for higher fy values, or if lightweight concrete is used For more details review: Section.7.3.1.1.1 in ACI 318-4.

Expression for Minimum Thickness of Two-Way Slab (Non-Prestressed)

Similarly, the expression for determining the thickness of the slab (h) for two-way slabs is given in detail in ACI 318-4, Chapter 8. The details will be discussed in the upcoming articles.

Read More: Which Cement is Best for R.C.C Slab Construction?