Concrete Slabs: Exploring the Types & Construction 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. 

Concrete Slabs ©prodyogi
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, their types, and their construction details.

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  = 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.

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 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. 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 to Construct a Concrete Slab on the Ground?

A slab-on-ground structure requires a properly prepared ground sub-base. A stable and sturdy ground base support is necessary to keep the concrete slab level and haul for a longer period without settlement. Here is a quick guide on pouring a concrete slab on the ground.

1. Determine the Depth of the Slab

Determine the depth of the slab required and decide whether the slab is to be laid in line or above ground level. Check the drainage direction and site rules to prepare the site without any water flooding issues due to improper sloping.

2. Start Excavating the Slab Depth

Prepare the location of the ground, in the dimension of the slab (length and breadth). If you need the slab in-line with the ground, dig the slab area to the required depth of the slab. Professional expertise here can help you plan properly for digging. Dig uniformly that the bottom area is the same as the top area of the slab pit. Use dirt or soil to fill any divots or holes present. 

3. Dampen the Earth and Compact the Subgrade

Dampen the soil by spraying it with water so that it holds together. Do it in intervals so that instead of becoming a pool, the loose earth holds together. Tamp it, compact it, and level the subgrade surface, until your footprints on it are almost invisible. 

4. Lay the Base

The best base for a concrete slab is a mix of coarse and fine aggregate to form a compactable base that prevents settlement and promotes drainage.

5. Compact the Base

Use a hand or mechanical tamper to compact the gravel base. Usually, the base layer is placed in layers of 2 inches (not more than that) and compacted, following the layer. The thickness of the base layer for a concrete slab can range between 4 to 6 inches. A base of 6 inches is used for concrete driveways subjected to heavy-duty loads. Above the base, vapor barriers or a damp-proofing layer is provided based on the requirement of the site. It depends on how high is the water-table level of the site. 

Concrete Slab on Ground With Vapor Barrier
Concrete Slab on the Ground With Vapor Barrier
© 6DProjects

6. Prepare the Formwork for Concrete Slab

The formwork is prepared based on the dimension of the slab. Wood, foam boards, etc can be used for formwork. Place stakes in intervals and secure them to provide extra reinforcement for the formwork.

Stakes Support for Formwork
Stakes Support for Formwork

Apply release agent inside the formwork for easy removal from the concrete slab. Silicone rubber or cooking oil can be used for this purpose.

7. Place Reinforcement on Formwork

Reinforcement bars are not necessary for concrete slabs with thicknesses less than 4 inches. The required rebars are placed in grids. Bring together the rebars using rebar ties. Chairs can be used to keep the rebars above the height of the ground.  As per ACI 318-2019, 20.5.1.3 Specified Concrete Cover Requirements, a cover of 3 inches needs to be provided for concrete slabs directly in contact with the ground, by means of cover blocks. For slabs of small thicknesses, wire mesh can be placed as reinforcement instead of steel reinforcement bars.

8. Pour Fresh Concrete on Formwork

The concrete of required strength and properties is poured either by ready mix concrete source or as a DIY project. But with the increase in the area and depth of the slab, it is recommended to enlist professional help. The concrete slab is compacted, leveled, and finished. The slab is left for curing to gain strength and for use.

Curing Period of Concrete Slabs

American Concrete Institute (ACI) Committee recommends a minimum curing period corresponding to concrete attaining 70 percent of the specified compressive strength. The often specified seven-day curing commonly corresponds to approximately 70 percent of the specified compressive strengths. It is ideal to provide a 28 days curing period for concrete slabs with large thicknesses and for major strength gain.

What is the Cost of a Concrete Slab?

The cost of constructing a concrete slab is dependent on the area of the slab, the thickness of the slab, the extent of ground preparation, the type of concrete used, the cost of reinforcement, and labor costs.
As a rough estimate, the cost of a standard 4-inch-thick concrete slab for a typical residential driveway or patio can range from $4 to $8 per square foot. For a more durable and decorative finish, a stamped or stained concrete slab may cost anywhere from $8 to $18 per square foot.

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