Preparation is key to a long lasting repair
Like with anything in life, preparation is crucial to the success of what we do. If we don’t prepare and plan we can expect the endeavour to fail. This is no different when we are dealing with concrete repairs, or the application of coatings.
BS EN1504 states; 'The preparation of the substrate of concrete and reinforcement shall be suitable for the required condition of the substrate and the structural status of the structure, so that the products and systems can be properly applied, and shall be carried out in such a way as to produce protection or repair which is in accordance with this and other parts of this standard.'
Cleaning and preparation on a repair contract can be broken down into 2 parts; preparation of the concrete substrate (this could be for the application of a protective coating, fairing coat, or mortar/repair concrete) and preparation of the reinforcing. Both are equally important in ensuring the success of the repairs, and/or coatings.
All repair and rehabilitation projects require some form of cleaning and preparation. There are 4 parts to this activity;
- Cleaning of the concrete surface (to be able to identify defects which were hidden by moss, algae, or coatings),
- Forming a perpendicular edge around the patch to prevent feather edging,
- Cleaning the reinforcing, and
- Breaking out the concrete behind the steel and inside the patch up to the perpendicular edge.
The cleaning of the concrete surface is normally carried out by high pressure cleaning at between 2000 and 3000 psi. Water jetting removes all the contaminants and roughens the surface via the impact of jets of high pressure. Varying pressures are used to obtain different surface profiles depending on the material to be applied to the concrete surface.
The forming of the perpendicular edge can be achieved by either saw cutting around the perimeter of the patch, or if using hydro demolition, by precision cutting around the patch.
Cleaning of the reinforcing to SA2½ is usually done by grit blasting. This has the added advantage of producing a micro profile on the surface of the patch which assists in bonding the new repair material onto the parent concrete. Bear in mind though that it is not normaly required to clean the reinforcing to SA2½.
Lastly, the breaking out of any spalled, delaminating, cracked, or sub-standard concrete to behind the steel and inside the patch up to the perpendicular edge can be done using either hydro demolition, and/or mechanical means (electric or pneumatic).
Hydro demolition, which uses water at pressures of between 16,000 and 20,000 psi, is generally much easier on the environment, both from an acoustic and dust point of view. However, it does produce a fair amount of wastewater with a high PH which needs to be collected, filtered and treated before it can be discharged into normal storm water drains. Hydro demolition also provides a vibration-free solution, resulting in no damage or cracking to adjacent surfaces. At the same time it cleans the reinforcing to an acceptable level and leaves an excellent micro profile on the inside of the patch for the new repair material to bond onto. From a H&S perspective, hydro demolition has a low reaction force and almost zero HAV’s risk.
Mechanical breaking out is still a widely accepted means of preparing the concrete patches for repair, and is generally used where the patches are small, there is not a large quantity of patches and water would cause a problem. HAV has been a problem in the past and is still an issue with mechanical breaking out that needs to be considered and dealt with. However, the equipment used nowadays has come a long way and the vibration magnitudes of the new breakers are so low that when coupled with exposure time produce relatively low exposure values.
CRL, with over 65 experience in concrete repairs are well qualified and experienced to carry out all cleaning and preparation works on the projects it undertakes. We have carried out studies, and are involved in various groups investigating noise, silica and vibration. This helps us understand the problems associated with these activities and deal with/risk assess the likely exposures before a project begins.