Concrete Topping Slab -Types, Thickness and Uses

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Concrete topping slab is an overlay designed to provide a dense, abrasion-resistant, and a finished floor surface for multiple purposes such as providing a wearing course to support traffic loads in parking facilities and bus terminals, providing a level surface for interior floors, providing special base for electrical and mechanical equipment, and to resurface worn or damaged floors.

There are two major types of concrete topping slab namely bonded and unbonded toppings. Both are suitable for heavy duty and light duty concrete slab. The former generally includes residential, office, institutional, and light commercial uses. The latter, used typically in industrial buildings, is subjected to heavy moving and static loads, abrasion, and sometimes aggressive chemical attack.

In some cases, concrete slabs are exposed to extremely hot or cold temperatures. Thickness of toppings is specified based on the expected loads and the variations in temperature a slab may have to go through.

Types of Concrete Topping Slab

1. Unbonded Topping Slab

In this system, the topping slab is not bonded to the underlying concrete slab. The thickness of unbonded topping may influence final floor elevation and may interfere with other service requirements such as doorway clearances.  

If cracks in the base slab are repaired, they may eventually reflect through a partially bonded overlay. Reflective cracking can be reduced by using thicker overlays.

1.1 Motivation for Application

It is used for contaminated slab for instance with oil, or for slabs with which bonding is not possible. Unbonded topping is also used when it is preferable to not bond the topping to the base course, so that the two courses can move independently, or so that the top courses can be more easily replaced at a later point of time.

1.2 Materials

For light-duty slab, concrete topping material is similar to the slab concrete. However, hard-steel troweling which commonly has a higher compressive strength than the base course is used as topping for heavy duty slab. Plastic sheeting, roofing felt, or a bond-breaking compound is used to prevent bond to the base slab.

1.3 Joints in Unbonded Topping Slab

Joint spacing in the topping should be coordinated with joint spacing in the base slab. Additional joints should be considered if the topping slab thickness mandates a closer spacing than the base slab to limit uncontrolled cracking.

1.4 Minimum Thickness

1. For unbonded topping, the minimum thickness is 75 mm.
2. Unbonded construction with a minimum thickness of 100mm is recommended if there are cracks in the base slab or if good bond is only partially attainable or totally lacking.
3. For applications with heavy traffic concentrated load of 35.6 to 58.7 kN, a minimum unbonded topping slab thickness of 127 mm provides a better serviceable life.

2. Bonded Topping Slab

In bonded system, the topping is adhered to the underlying concrete slab. It is used for both light duty slab in commercial applications and heavy-duty slabs in industrial applications subjected to heavy traffic loads and impact.

The surface of the base slab should have a rough, open pore finish and be free of any substances that would interfere with the bond of the topping to the base slab.

The topping can be placed before hardening base concrete slab, or after the base slab has hardened.

2.1 Materials

The topping for light duty slab is a concrete mixture similar to that concrete slab. The topping for a heavy-duty slab requires a multiple pass, hard-steel-trowel finish, and it usually has a higher strength than the base slab.

A bonded topping can also make use of an embedded hard aggregate or a premixed mineral aggregate or metallic hardener for addition to the surface.

2.2 Joints Bonded Topping Slab

Joint spacing in the topping should be coordinated with construction and contraction joint spacing in the base slab. Saw-cut contraction joints should penetrate into the base slab a minimum of 25 mm.

If the topping is placed on a base slab before the joints are cut, joints in the topping should extend into the base slab and depth should be appropriate for the total thickness of the combined slab.

If the topping is installed on a previously placed slab where joints have activated, additional joints in the topping are unnecessary as shrinkage relief cannot occur between the slab joints in the bonded topping.

2.3 Minimum Thickness

Bonded topping minimum thickness is 19 mm .

Topping Slab Strength and Specifications

For bonded and unbonded concrete topping slab, strength of concrete for concrete topping slab is governed by function of the slab; 30MPa for light duty, 40 MPa for medium duty, and 50 MPa for heavy duty.

The concrete topping used should have a maximum slump of 75 mm. If low w/c is employed, then a water-reducing admixture or high-range water reducing admixture should be used to increase the slump.

Steel Reinforcements

For both bonded and unbonded concrete topping slabs, reinforcement, such as deformed bars, welded wire fabric, bar mats, or fibers, may be placed in the topping to reduce the width of shrinkage cracks.

Topping Slab Construction

1. Prepare concrete slab surface if it is an existing surface. This stage is not needed if the topping is placed just after the construction of slab base.
2. Fix forms.
3. Place insulation in the case of unbonded topping such as plastic sheet.
4. Apply adhesive agent to concrete surface in case of bonded topping. This process sometimes becomes optional since roughness and pores of concrete surface may be adequate to generate enough bond strength between slab and the topping.  
5. Install reinforcements
6. Pour concrete
7. Finish concrete surface using suitable means based on the type of topping and materials used.
8. Apply suitable curing methods.