While concrete is a fantastic building material that’s used extensively in the construction industry, its properties change over time and will eventually need to be repaired. Cracking is probably the most widespread problem, but there are a vast range of other more complex issues depending on the type of structure and the length of time since the structure was built. One of the most effective and economical techniques of concrete repair is through using shotcrete, which is essentially concrete that is pneumatically sprayed at high velocity through a hose onto a receiving surface.
Shotcrete has many applications in the construction industry, however today we’ll be focusing on using shotcrete as a technique to repair concrete structures. There are many types of concrete structures that require repairing, however the advantages of the shotcrete process are best suited to repairs that require forming, such as vertical and overhead repairs. Shotcrete can be pneumatically sprayed onto prepared substrates, finished, and cured with no additional steps required.
What type of structures?
Shotcrete can be used to repair most types of concrete structures, however due to the properties of the shotcrete process, the technique is most economical on vertical surfaces such as columns, walls, and pier caps, and horizontal surfaces such as ceilings, beam bottoms, slab soffits, and deck overhangs. Generally, these types of structures need to be repaired when there are issues with structural integrity or when an additional layer of concrete is needed to cover reinforcement. Whenever forming requirements are difficult or excessively expensive, shotcrete is typically the best approach.
One of the most important requirements for successful shotcrete repair is within surface preparation. There are a specific set of measures which must be followed:
- Determine the areas of delamination or deficiency by observing the concrete
- Any unsound concrete can be removed either by a chipping hammer or an ultra-high-pressure water blaster. It’s crucial that no substrate concrete or reinforcing bar is unnecessary damaged. Once this has been done, remove any loose or fractured material.
- Any damaged reinforcement must be replaced or supplemented. New reinforcing steel should never be placed behind or in front of existing reinforcing steel.
- Remove any contaminants on the surface and use an abrasive blast or water blast on the exposed reinforcing steel.
- Finally, spray the surface with water and allow the area to dry to a saturated surface-dry (SSD) state
Once the surface has been adequately prepared and dried, the shotcrete can then be applied to the surface. Apply the shotcrete by filling the corners first and move the nozzle continuously to cover the reinforcing steel. It’s crucial that the shotcrete application is consistent and there is no build-up in any one area. Use a blow pipe to remove any overspray and rebound that builds up in corners and reinforcing steel.
Additional layers of shotcrete are almost always required, and the preparation of the immediately preceding layer is vital to achieve a successful bond. If needed, the thickness of each layer can be increased with the use of the appropriate additives. Once the last layer of shotcrete has reached just passed the desired plane, allow it to stiffen until the shotcrete can be worked without diminishing the bond. At this point, a trowel can be used to trim the material so it’s flush with the surface area. It’s crucial that the material is not overworked to a point where delamination’s or spalls are created. While excessive finishing is discouraged, proper curing procedures will enhance the physical properties of the shotcrete and reduce shrinkage cracking.
Checking the repair
Since shotcrete can be closely observed during placement, inspectors and applicators can watch the corners being filled and reinforcing steel being covered. Honeycombing generally isn’t a problem if proper care is taken to prevent voids and when overspray and rebound is removed as it accumulates. After the shotcrete has properly cured, the bond can generally be tested with a hammer, however if more information is needed, refer to the ‘Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength’. The most successful repairs typically have a tensile strength greater than 150 psi.
Using shotcrete to repair an existing concrete structure is an excellent technique to ensure the concrete surface remains intact and weather-resistant. The success of shotcrete repairs lies heavily within the experience of the shotcrete provider, so if you’re interested in using shotcrete for your next project, get in touch with the experts at Evolution Shotcrete by phoning 07 5561 8885.