New concrete is commonly poured over old concrete, which may have been damaged for any reason, for repairing purpose. However, this approach is not suitable option unless the existing concrete is in good shape structurally.

Concrete surface suffers from wearing and damages over time disregard of its strength. Placement of new concrete over old concrete shall be conducted according to a pre-planned procedure in order to achieve desired final product. For instance, clean old concrete, fix forms, install steel bars or mesh if necessary, pour concrete,compact and finish concrete, and finally cure the newly placed concrete.

How to Pour New Concrete Over Old Concrete Surface?

1. Clean Old Concrete Surface

Proper cleaning and preparation work are key to success when layering new concrete over old. Initially, sweep gravel, sand,dirt, and debris off the concrete.

After that, If the surface is flaking or spalled, scrape it with a spade to dislodge as much loose concrete as possible,then sweep the surface clean. Moreover, use a stiff-bristled broom to knock as much debris out of cracks as possible.

Finally, pressurized water may be used to clean remaining debris. It is possible to add a liquid detergent or degreaser to the water to ensure the removal of all stubborn spots.

Clean Old Concrete Surface
Fig. 1: Clean Old Concrete Surface

2. Fix Formworks

Pouring over old concrete need similar basic formworks that employed with fresh pours. Install the form, usually wood form, around the perimeter of old concrete surface.

The form shall extend above existing concrete surface based on the designated thickness of the new concrete. Make sure that the height of the formwork is even. It is required to force stakes into the ground every 0.9m and at every joint in the forms.

Moreover, specify control joint locations onto the outside of the forms, directly above existing control joints. Coat the inside edges of the forms with oil to make sure easy form stripping.

Finally, compute quantity of concrete required for the work under consideration after form fixing process ended, and then order concrete accordingly.

Fixing Forms
Fig. 2: Fixing Forms

3. Reinforcement Placement

The placement of reinforcement is a must especially when cracks of old concrete are active. Either reinforcement bars installed in a grid pattern ,as shown in Fig.3, or welded wire mesh are spread over concrete surface.

Reinforcement are held at their position by using rebar chairs. Finally, it is also possible to consider adding fibers to the concrete to control plastic shrinkage cracking.

4. Saturate Existing Concrete Surface with Water

Saturate the existing concrete surface completely prior to the placement of concrete. This prevents absorbing moisture of new concrete by old concrete. if moisture of fresh concrete is absorbed, then it would dry and the bond between old and new concrete would be compromised.

5. Pour New Concrete

After preparation work, reinforcement placement, and formwork installation are finalized, then concrete placement operation would begin using suitable means such as pumping. Fresh concrete is spread over the area and pressed down to pack the mixture into the forms.  It shall be granted that type of concrete mix ordered is best suited for the project.

How to pour new concrete over old concrete surface
Fig. 3: Pour New Concrete Over Old Concrete Surface

6. Finish Poured Concrete Surface

Smooth concrete surface with trowel and wood float can be used to float the concrete. In hot weather condition, the concrete can dry out very quickly, so finishing work shall be performed quickly after pouring is finished.

Finishing operation shall be conducted at suitable time otherwise cracks might initiate. Control joints can be cut at specified intervals after 4 in hot weather and 12 hours in cold.

Finishing Concrete Surface
Fig. 4: Finishing Concrete Surface

7. Curing New Concrete

There are several curing mechanisms which are selected and used based on weather condition and availability of materials and equipment for given curing regime. It is recommended to cure concrete for minimum of one week and use plastic sheet to keep concrete wet.