How to Pour Concrete Over-Existing Concrete Slab?

🕑 Reading time: 1 minute

As everything in the world, concrete also has a lifespan and it wears out over time. Imperfections forms when the concrete hardens or sinks into the ground. Pouring fresh concrete over the existing concrete is a common way to level out old slabs and patch damage.

Steps Involved in Pouring Concrete over Existing Concrete Slab

Step 1: Cleaning Old Concrete Surface

Sweeping dirt’s debris over the existing concrete slab will allow bonding to take place. This includes removing sand, dead leaves, and gravel. One of the best tools that can be clean all the debris out of crack is a stiff-bristle broom.

• 

• 

This cleanliness needs to be done thoroughly, it’s recommended to use pressure washer so that the hidden debris. Another way to achieve great cleanliness is by using liquid detergent or degreaser when washing the area, to remove stubborn spots.

We have cleaners meant for concrete use, one needs to scrub spot that proves difficult with it. The last step when cleaning is to make the surface wet.

Step 2: Setting Up Slab Perimeter

Measure the perimeter of the area to pour the concrete over the existing slab. Depending upon the requirement of the preferred use of the slab and the loads coming upon the slab. The thickness of the slab is designed. The measurements acquired can then be used to estimate the amount of concrete required.

• 

• 

The braces are installed around the area to be filled with concrete usually made of wood.  It must be ensured that the braces are evenly spread which is can be done by running a bubble or string. For the thick concrete slab, a wire-mesh is needed on the braces to provide strength.

Step 3: Pouring a Primer Coat

The mixture of water and cement is made in slurry and spread over the existing slab to form a bonding coat between the old and new concrete to be laid. The standard mixture is 1:7 of water to cement.

• 

• 

Step 4: Pouring of Concrete

Fine sand or crushed stone is preferred for small thickness concrete and coarse aggregate is used for concrete of higher thickness. The mixture of aggregate and cement is mixed and poured it all into the existing concrete. Hand trowel or a paver is used to spread the concrete evenly by pressing it hard.

• 

• 

• 

A bonding adhesive is used in the concrete to ensure that the new concrete stick existing concrete successfully. To protect the new-laid concrete from wearing out soon, a protective layer is applied by using curing compound and sprayed it all over the surface which usually takes seven days to cure.

Disadvantages of pouring new concrete over existing concrete.

1. Raises the Level

Pouring a new layer of concrete over an existing walkway, patio or porch will raise the level by several inches. If the slab runs up to a door, the raised surface may not provide the necessary clearance for the door to open.

For a walkway, raising the level by several inches can throw off its alignment with the driveway, steps or another structure. Adding a slight ramp from the driveway to the raised surface helps eliminate a trip hazard and keep debris from collecting in the corner.

2. Fewer Years of Service

A well-poured concrete slab with a deep, sturdy foundation can last for 30 to 40 years. Pouring concrete over old concrete instead of directly over a new gravel foundation limits your ability to maximize the slab's lifespan.

The condition of the existing slab is the primary factor that determines how long the new concrete will last. If the foundation beneath the slab is not sound, the new concrete could sink or develop deep potholes.

3. Bonding

If new concrete binds to the existing slab, cracks are inevitable. Keeping the new concrete from binding to the existing slab helps prevent new damage from forming and any damage in the old slab from spreading to the new one.

It also allows both slabs the flexibility to shrink and expand when temperatures fluctuate. One straightforward method to prevent the slabs from bonding is to lay plastic, sand or other durable material between the two layers.

4. Maintenance

While pavers, stones and other types of paving materials offer a more decorative look, concrete is valued for its strength and low maintenance. A slab poured over an existing slab is more vulnerable to frost heave damage and cracks.

Patch damages as soon as they develop to prevent them from spreading, and seal the new layer with a deep penetrating sealer to prevent water damage. If you notice long, deep cracks, have a professional evaluate the damage to determine if there's a structural problem.

Read More:
How Thick Should a Concrete Slab be?
Garage Concrete Floor Slab – Construction, Thickness and Cost
Concrete Topping Slab -Types, Thickness and Uses