What is Concrete Topping ?
Concrete topping is the process of overlaying (resurfacing) the hard, non-abrasive, structurally tough layer of concrete over any existing surface.
The old surface may be worn out or structurally not suitable for the special purpose; such as installing pieces of machinery and big equipment causing impacts and vibration at workshop, factories and garages, establishing bus stops and parking, for a smooth and raised level surface, or creating an impermeable surface layer.
Sometimes concrete topping is also done simultaneously after the construction of the base surface to provide a rigidly and cost-effective surface.
The concrete topping is generally classified as cost-effective and sustainable techniques of maintenance, rehabilitation and reconstruction. Their thickness may vary from 2 inches (50mm) to 10 inches (250mm).
Types of Concrete topping
On basis of the design and their structural construction or say how the old base surface is treated, they are generally categorized into two types.
1) Bonded Concrete topping
In this type of topping, the base existing surface is treated as the structural component which contributes most of all the structural strength in pavement or slab.
Thus, the overlaid concrete topping slab doesn’t require much structural strength but should be focused on its toughness and hardness in order to protect it from abrasion and corrosion. And so, these concrete toppings are relative of less thickness.
In bonded concrete topping, the concrete topping slab is required to be bonded and anchored with existing pavement having structural capacity, so that it can act as a monolithic slab.
So in such topping, the design of the bonding and anchorage between two layers, joints and curing is a more important focus. They are likely to be preferred for regular maintenance, rehabilitation and the special purpose of creating the required level surface.
2) Unbonded concrete topping
In this type of topping, the existing surface doesn’t contribute structural strength much effectively rather it treats the existing surface just only as a high-quality subbase.
Thus, the concrete topping slab should fully consider being designed in order to provide structural strength. They are relative of greater thickness. They are design or preferred for higher durability and longer life span.
As the word “Unbounded Topping”, itself says the anchorage and proper bonding is not required between two layers.
Moreover, if the existing surface is concrete capable of providing structural strength and the bonding is made between the structurally designed concrete topping, then there is the probability of concrete distress between two layers and the deformation and cracks are prone.
This effect is less in asphalt or another flexible base as they are not capable of creating differential stress between two layers.
However, on the basis of the base surface type they can also be also divided as:
- Topping over asphalt pavement
- Topping over concrete pavement
- Topping over the composite pavement
Materials Required for concrete topping
- Formwork: The formwork for creating a suitable size panel of the concrete slab are needed to be made. The formwork at the joint between two slabs can be later on cut or saw to 2/3 of the depth and filler materials are inserted.
- Reinforcement bars: The reinforcement bars are designed according to the necessity of structural functioning for the unbonded slab. But for the bonded slab, generally, wire mesh or bar mats of small diameter rods are made.
- Bond breaker membrane: They are only required for the unbonded type of concrete topping. They can be simply plastic, thin ply, or thermocouple sheet which are laid over old concrete surfaces.
- Bonding agent or slurry and shear bolts: They are only required for the bonded type of topping, for proper adhesion and anchorage with old layer to act monolithically
- Joint fillers and seal: they are generally sealed at last for sealing the joint
- Cement concrete: The normal or specialized concrete with different modification in admixture and aggregates can be used to provide a tough surface
- Concrete surface hardener: They are the liquid solution that is sprayed or painted over the topping slab after their full curing and hardening. Some commonly used hardener solutions are Sodium Silicate, Magnesium Silicofluoride, and Zinc Silicofluoride.
Specification of Topping Slab
- The minimum strength of hardened concrete should be 28 MPa(4000 psi) at 28 days.
- The maximum size of aggregate is nearly 1inch (or 2.38 mm) and 3/8 inch for the congested area and conditions.
- All Aggregates should pass from Us standard Sieve No.8 (2.38mm) and Sieve No. 16 (1.18mm)
- Mix design must include up to 10% of replacement of cement with fly ash along with plasticizers and admixture for the dense and plastic mix. So that topping is easily surfaced and toughed.
- The slab should be constructed in a number of panels.
- For Bonded topping concrete slab, any of the dimension should not exceed 4m.
And for unbonded slab, any of dimension should not exceed 2m.
- The minimum Thickness of the Bonded slab is 2inch(50mm) and can extend up to 4inch.
- The minimum Thickness of the unbonded slab is 3inch(75mm) and can extend up to 10inch or more.
- The length of any panel should be greater than 1.5 times its breadth. But for the floor slab which is needed to bonded, no such rules are considered.
Topping on Concrete Slab
Concrete Topping on a concrete slab, generally done with bonded method. However, if the old concrete base has deteriorated very badly then unbonded techniques are also used.
The general thumb rule for using boned and unbonded techniques also depends upon the thickness of topping, i.e. if less than 3inc, use bonded techniques; if greater than 3-inch use unbonded techniques.
The bonding agent is used over the old surface concrete if it has to be bonded. Before application of such bonding agent or slurry, the old concrete must be scrapped creating a rough surface and should well wash, dampened and dust-free.
Pressure washing and sandblasting is also used in many cases. Generally, fibre mesh or wire mesh is used to top over the concrete surface
Construction Process for the concrete toppin
The construction process for the concrete topping includes series of following steps.
- Inspection of existing surface
The initial construction process of concrete topping includes the inspection and analysis of the condition of existing surfaces. The slab(surface) whether sounds structurally or not is determined so that the bonded or unbonded type of topping required can be determined.
The slab whether its asphalt, concrete or any other also must be analyzed. Further, the cracks and joints present and the condition of the surface is also inspected.
- Preparation of surface
If the topping is bonded type, then the surface must be well prepared. The surface should be roughened by the grinder or any other scrapping equipment along with the removal of mortar cakes, loose particles and laitance. And the dust should be well cleaned.
Then the surface should be well washed and moistened enough before application of bonding agent or slurry.
But if the topping is unbonded type, the surface is not needed to be cleaned and roughened. Instead, the separating film or thin ply or plastic can be used to separate e old and new concrete slab.
- Separation of joints and construction of panels
The topping process can be completed in a series of panel slabs along with proper separation of joints. For this, the form works are initially placed according to the designated dimension of each panel.
Then the reinforcement is also placed according to design. The joints are then later placed with filler
- Placing and Finishing
The concrete is poured into each panel thoroughly. The placed concrete is well vibrated and tamped so that coarse aggregate doesn’t appear over the surface. They are then proceeded to finish with help of trowels.
The slabs are then left for curing after their final setting. The minimum curing duration is 10 days.
- Application of concrete hardener
After the time period of 28 days, the slabs are maintained at a well clean and dried state. Then the concrete hardener is applied over it. The rate of application and number of coats of such hardener depends upon the prescribed instruction manual of the company.
Generally, wire mesh or bar mats of very less diameter are used for bonded concrete topping as their thickness is less. And moreover, if the higher diameter reinforcement is used, there might be a high probability of cracks in the concrete due to insufficient cover.
The reinforcement for unbonded concrete topping is structurally designed after the analysis of the load it probably might encounter during its life span.
However, there is generalize reinforcement detail information which can be applied o all type of concrete topping. They are:
- A maximum number of 4 tie bars are to be used when they are in need. Tie bars are a small piece of the bar kept between two adjacent slabs.
- Epoxy tie bars are preferred to be applied over existing pavements.
- Dowel bars are used between two longitudinal panel slabs if the slab is designed for dynamic loading.
- Shear connectors are also embedded in the old concrete surface by bolting techniques and half embedded in the topping concrete slab for better bonding.
- Reinforcement should be located at 1/3rd of the slab thickness.
Granolithic Concrete Topping
This concrete topping contains special types of aggregate along with a rich concrete mix. The aggregates used are generally, Granite, basalt, Trap and Quartzite. These concrete toppings have higher hardness, good surface texture and high wearing resistance to alkali and mineral oils.
They are adopted for the heavy engineering factories, workshops, garage, warehouse where there is the possibility of heavy load, severe abrasion along with the impacts.
I hope this article on “Concrete topping slab” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha