The replacement concrete technique is utilized for repairing defective concrete when the affected area is substantial. This method involves the removal of faulty concrete from the structural element, followed by the preparation of the area for repairs. Finally, replacement concrete is employed to complete the repair process.
This method of concrete repair is generally used when the damaged area exceeds one square foot and has a depth greater than six inches. Additionally, it is applied when the affected area is sizable and continuous. The replacement concrete method is a viable solution for remedying large-scale defects or cracks in concrete, allowing the structural integrity of the element to be restored.
Replacement concrete repairs are also used for:
When it comes to repairing concrete structures, holes that extend entirely through the concrete sections can be a common issue. Such holes can be problematic, especially when they exceed a certain size or depth. For instance, holes that are deeper than 4 inches and larger than 1 square foot may require concrete replacement, unless epoxy-bonded concrete replacement is allowed.
In the case of holes that go through reinforced concrete, the repair work becomes more complicated. Holes greater than half a square foot that extend beyond the reinforcement require careful attention. One solution is to use replacement concrete, which involves bonding new concrete to the repair areas without the use of a bonding agent or Portland cement grout.
By using replacement concrete, the defective concrete is removed and replaced with high-quality concrete that closely matches the surrounding material. This approach ensures that the repair is compatible in terms of thermal expansion and other physical and chemical properties with the old concrete. Consequently, replacement concrete is often the preferred method for repairing concrete structures, as it offers a reliable and long-lasting solution.
Preparation of Surface for Replacement Concrete:
To achieve satisfactory results with the replacement concrete method, certain preparation steps must be followed. Firstly, the reinforcement in the reinforced concrete should be exposed by a minimum of 25mm around it to use replacement concrete. Additionally, the depth of concrete removal for replacement concrete should be a minimum of 25mm and must extend until all the defective concrete has been removed.
In the case of a vertical member, the top of the hole should be cut in a 1:3 upward slope from the back toward the face from which the concrete will be placed. This slope is crucial to allow for proper vibration of the concrete without leaving air pockets at the top of the repair. If concrete is removed from one side of the structural member to the other, the replacement concrete should be filled completely in the hole.
It’s important to cut damaged concrete in a way that doesn’t cause any harm to the existing concrete or result in spalling of the concrete. For repairs on surfaces that are exposed to destructive water action and other exposed surfaces, the outlines of areas to be repaired should be saw cut to a depth of 1-1/2 inches before removing the defective concrete.
In order to bond new concrete to old concrete surfaces, the latter must be clean, rough, and in a saturated surface dry condition. Any extraneous material on the joint that results from form construction must be removed before placement of the replacement concrete.
Materials for Replacement Concrete:
When repairing concrete, it is important to use a water-cement ratio that is the same as what was used in similar new or existing structures. However, this ratio should not exceed 0.47 by weight. Using aggregate that has a large maximum size and low slump can help minimize the amount of water needed, which in turn reduces shrinkage. Entrained air should also be added to the concrete in the amount of 3 to 5 percent.
To further minimize shrinkage, the concrete should be kept as cool as possible during placement, preferably at around 20 °C or lower. Materials should be stored in shaded areas during warm weather, and ice may need to be used in the mixing water. When batching materials, it is important to do so by weight to ensure uniformity, especially since batches for this type of work are typically small.
The slump of the concrete should be kept to a minimum, ideally around 75mm, when replacing concrete. Structural concrete placements should begin with an over-sanded mix containing a maximum aggregate size of 20mm, a water-cement ratio of 0.47 by weight, 6 percent total air by volume of concrete, and a maximum slump of 100mm. This mix should be placed several inches deep on the joint at the bottom of the placement. It is not recommended to use a mortar layer on construction joints.
Application of Replacement Concrete:
The effectiveness of a repair job is not solely dependent on using low-slump concrete, but also on the thoroughness of the vibration during and after the concrete is poured. There is no risk of over-vibrating, and immersion-type vibrators should be used when possible. After filling the hole completely, pressure should be applied to the fill and the form should be vibrated. This process should be repeated every 30 minutes until the concrete hardens and stops responding to vibration. Pressure can be applied through wedging or tightening bolts extending through the pressure cap. Only materials that have been proven to produce durable concrete should be used for repairs on open-top forms, such as those used for reconstructing walls, piers, parapets, and curbs. Top surfaces should be sloped to allow for efficient drainage. Forms used for concrete replacement repairs can usually be removed the day after the concrete is poured, unless doing so would damage the green concrete, in which case stripping should be postponed for an additional day or two. Some replacement concrete may not require forms.
Replacing damaged or deteriorated paving or canal lining slabs, where the entire depth of the slab needs to be replaced, requires the same procedures that are used for original construction in order to achieve optimal results.
Curing and Protection of Replacement Concrete:
Proper curing is crucial for replacement concrete to avoid failure. Even after the forms are removed or the concrete has hardened, it is essential to provide adequate curing. Insufficient curing can lead to complete failure of the repaired concrete. Due to the small volume of most repairs, it is common for old concrete to absorb moisture from new material. Therefore, water curing is highly recommended, especially during the first 24 hours.
A soil-soaker hose can be laid beneath a plastic membrane covering the repair area to provide effective water curing. When a curing compound is used, the best approach is to start with an initial water-curing period of at least 24 hours, followed by a uniform coat of the compound while the surface is still damp. The optimal water-curing duration is seven days. After this, a coat of curing compound can be applied to ensure the best curing combination.