Pre-treatment of concrete
The main goal of pre-treatment of concrete surfaces is to remove the remnants of older coatings, dust, algae, striking agents, surface pollution, as well as the removal of damaged, carbonated or aggressive media contaminated. The concrete base must be load-bearing so repair substances can be applied, and the grains of small-grained or coarse gravel can be seen to be exposed and uncovered. Cleaning of reinforcement from corrosion products may also be a part of this technological procedure.
Concrete pre-treatment involves removing large layers, fine roughening the surface, including opening the pores in the cement sealant system and removing corrosion products from reinforcement; and in the final phase, perfecting the removal of all dust from the concrete and reinforcement surfaces.
The removal of damaged surface layers must be carried out in such a manner that the condition and quality of the steel reinforcement are not compromised and that the concrete on the structural core is not unnecessarily endangered. Moreover, concrete surface layers should be removed in a manner that ensures compliance with the relevant health and safety standards while also keeping the safety of workers and the environment in mind.
Usually, the thickness of the removed concrete layer is determined by structural analysis. When removing the surface concrete layers, the structure’s static suitability must not be compromised. This aspect is particularly important for thin-walled structures. After the pretreatment of the substrate (concrete and reinforcement), it is necessary to check the surface pretreatment (visual inspection, acoustic tracking, tensile tests, etc.). It is also necessary to document the occurrence of cracks found in the area.
Mechanical impacting is traditionally used for rough concrete removal. The most widespread and widely used method is the demolition hammering technique, consisting of repeated canine (tooth) strikes that create cracks in concrete surfaces and break them into fragments. Compressed air (pneumatically), petrol engines, electric current, and hydraulic current can all be used to drive hammers. Their size is very variable, from small electro-pneumatic hammers weighing several kilograms to huge hammers placed on mobile chassis.
It is effective on horizontal, vertical, and suspended surfaces, but it has the disadvantage that cracks are also generated in healthy, unremoved concrete and it is rather nonselective.
It is therefore imperative to proceed with great care when hammering, especially near the released reinforcement, to ensure that it does not interfere with the surrounding healthy, unremoved concrete. The removal of incoherent layers by mechanical blows must be carried out at the smallest possible angle, i.e. these damaged layers must be separated by peeling them off. From a health and safety point of view, this method is quite problematic, burdening the operator with noise, dust and vibrations, especially when using hand tools.
The methods used for fine concrete removal include:
Cutting and blasting using high-pressure jet technology are among the most used methods for preparing concrete surfaces. With this technology, a jet or a beam of water jets with a thickness of 0.1-1 mm is used to cut and blast. Depending on the desired effect (cleaning and roughening of the surface up to 60 MPa, cutting concrete at higher pressures), these beams are generated by pressures between 20 and 150 MPa.
A highly abrasive effect is obtained through the direct impulse of compressed water and cavitation phenomena. The principal advantages of the use of high-pressure water jet technology are as follows:
• During normal temperatures, there are practically no dynamic effects on the structure, it is dust-free, and it does not spark.
• It does not generate microcracks.
• Differentiated withdrawal of materials (only materials of unsatisfactory quality are removed; regulation is achieved by continuous pressure selection). The depth of withdrawal can be adjusted from 0 mm (washing) to 80 mm; with abrasive cuts, up to 1,000 mm possible.
• Exposing the reinforcing inserts without damaging them or releasing them from the concrete.
• Vibration-free technology.
• It does not burden the environment with dust and fumes.
• Absence of thermal or mechanical stress.
• There is no risk of explosion.
• After treating the structure’s surface with this method, no further surface cleaning operations are required. All debris and dirt are carried away by the water jets used for the initial concrete removal process.
• This technology can be used to clean the concrete surface and remove adhering particles that could act as separations when applying additional repair layers.
• The field of application of high-pressure water jet technology is very wide, from surface cleaning, and repair surface layers of cement screeds to local removal of concrete structures.
