Pouring new concrete over old concrete is a common practice when repairing damaged surfaces. However, this method is only effective if the existing concrete is structurally sound. Without proper preparation and careful execution, the bond between the old and new layers can fail, resulting in premature cracking or delamination. To ensure a durable and high-quality repair, it’s essential to follow a systematic procedure covering surface preparation, formwork installation, reinforcement placement, concrete pouring, finishing, and curing.
1. Clean the Old Concrete Surface
Proper cleaning of the old concrete is the foundation for success when overlaying new concrete. Start by sweeping away gravel, sand, dirt, and any loose debris. If the surface has flaking or spalling areas, scrape them off using a spade or similar tool to remove as much loose material as possible. After scraping, sweep the surface thoroughly.
Next, use a stiff-bristled broom to clear out debris from cracks and crevices. For stubborn dirt or grease spots, wash the surface with pressurized water. Adding a liquid detergent or degreaser to the water can help remove contaminants that might otherwise interfere with bonding. Ensure the surface is completely clean and dry before proceeding to the next step.
2. Fix Formworks
Formworks serve as molds to contain the new concrete and shape the repair area. Install wood forms around the perimeter of the existing concrete surface. The forms should extend above the old surface by the thickness of the new concrete layer you plan to pour. Make sure the form height is level throughout the perimeter.
Secure the forms by driving stakes into the ground approximately every 0.9 meters and at every joint in the form. Mark control joint locations on the outside of the forms, aligning them with existing control joints in the old concrete. Applying oil to the inside edges of the forms ensures easier removal after curing. Finally, calculate the volume of concrete required and place your order accordingly.
3. Reinforcement Placement
If the old concrete has active cracks or if added strength is needed, reinforcement is necessary. Place reinforcement bars in a grid pattern or lay welded wire mesh over the prepared surface. Rebar chairs can be used to hold the reinforcement at the proper height within the new concrete layer.
Additionally, consider incorporating fibers into the concrete mix to help control plastic shrinkage cracking. Proper reinforcement improves the structural integrity and longevity of the repair.
4. Saturate the Existing Concrete Surface with Water
Before pouring new concrete, saturate the old concrete surface thoroughly. This step prevents the old concrete from absorbing moisture from the fresh concrete, which can weaken the bond between the two layers. The old surface should be damp but free of standing water when pouring begins.
5. Pour New Concrete
Once preparation, reinforcement, and formwork are complete, begin placing the new concrete. Use suitable equipment such as concrete pumps or wheelbarrows to distribute the mix evenly over the surface. Spread and compact the concrete to ensure it fills all voids and forms a dense, uniform layer.
Make sure the concrete mix you ordered is appropriate for the project’s requirements, considering factors like strength, workability, and setting time.
6. Finish the Poured Concrete Surface
After pouring, finish the surface using a trowel and wood float to achieve a smooth, even texture. In hot weather, the concrete can dry rapidly, so finish the surface quickly to prevent early cracking. Timing is crucial — premature finishing or delays can cause surface defects.
Cut control joints at specified intervals after approximately 4 hours in hot conditions or 12 hours in cooler weather. These joints help control cracking as the concrete cures.
7. Cure the New Concrete
Proper curing is essential for concrete strength and durability. Various curing methods can be used depending on weather conditions and material availability. A minimum curing period of one week is recommended to maintain moisture in the concrete.
Cover the surface with plastic sheets to retain moisture and protect against evaporation. Regularly inspect the curing process to ensure the concrete remains wet and cures evenly.
Conclusion
Pouring new concrete over old concrete requires careful attention to each step to achieve a long-lasting repair. From thorough cleaning and proper formwork installation to reinforcement placement, timely pouring, finishing, and curing, each phase plays a critical role in ensuring the new concrete bonds well and performs as expected. Following this structured procedure helps avoid common problems such as poor adhesion and cracking, resulting in a durable, high-quality concrete surface.