Maintaining the value of engineering structures with concrete restoration
While concrete cosmetics is primarily concerned with the appearance of exposed concrete surfaces, concrete restoration is also concerned with maintaining or restoring the structural and static properties of a concrete structure. Due to its good properties, concrete is used in areas subject to high static loads, such as for engineering buildings. There, it is all the more important to detect and repair damage at an early stage. In addition to the concrete itself, the reinforcement must also be taken into account during concrete repair. Only intact reinforcement can fully and safely absorb the tensile forces acting on a load-bearing concrete member.
Planning basics of concrete repair
A distinction is made between age-related refurbishment and the repair of new components that have defects due to improper manufacture or mechanical damage. Especially for older concrete structures, a thorough analysis of the actual condition must be performed. The following planning tasks are defined in the Repair Guideline Protection and Repair of Concrete Components of the DAfStb (German Committee for Reinforced Concrete):
- Determination of the actual state
- Development of a concrete repair concept and a repair plan
- Determination of the measures for quality assurance
- Preparation of a renovation plan with inspection intervals and maintenance measures after the renovation has been completed
In addition to the DAfStb guideline, concrete repair has also been regulated by DIN-EN 1504 since January 2009. The series of standards “Products and systems for the protection and repair of concrete structures” is divided into ten main standards.
In order to determine what damage is present in the concrete structure and what methods can be used to repair it, various examinations of the structures themselves must be carried out in advance. As a rule, analyses on the structure are combined with laboratory tests on building material samples. Among others, the following analysis methods are used, each of which can be applied non-destructively or at least non-destructively:
- Visual inspection and documentation through photos
- Localization of reinforcement layers by magnetic inductive measuring systems
- Measurement of concrete compressive strength
- Evaluation of the state of corrosion of the reinforcement
- Measurement of carbonation
In the laboratory, concrete samples of the building components continue to be examined, which are taken as drill cores using the wet drilling method. This makes it possible to assess the concrete structure and determine the bulk density of the concrete. The chemical composition of the concrete and any harmful constituents can also be determined in a wide variety of laboratory analyses.
Regardless of the chosen method of concrete repair, the substrate must be prepared. All loose constituents on the concrete surface are removed with sandblasting equipment, wire brush or other mechanical cleaning equipment until the concrete no longer sands or grits. Subsequently, the redevelopment measures are carried out:
Concrete repair as corrosion protection
If air or moisture gets to the reinforcing steel, it starts to rust, corrosion begins. If cracks appear in the concrete that penetrate to the reinforcement, it must be protected against further rusting by direct coating or by electrochemical, so-called cathodic, corrosion protection. Subsequently, the existing crack is closed. This can be done in different ways.
Close cracks in the concrete
Two-component liquid resins can be used to repair cracked concrete. These are pressed into the interior of the concrete under pressure. Depending on whether it is a waterproofing measure or the restoration of stability, elastic or frictional sealing resins are used.
Large spalling, chipping and voids are sealed with so-called concrete substitute as repair mortar. Polymer-modified or purely cementitious mortars in various grain sizes are used. Cementitious concrete mortars containing plastic are applied by hand, while mineral mortars are applied by machine as sprayed mortars. A bonding bridge ensures that concrete and mortar bond securely.
Strengthening and surface protection of the component can also be seen as a concrete rehabilitation measure. While various strengthening measures such as cross-section enlargements or additional reinforcements improve the stability, surface protection levels out the concrete surface and ensures higher resistance of the concrete through hydrophobization or oleophobization.