This procedure shall apply to the repair Of all defective concrete in large quantities for walls, using high strength, free flowing, cementitious micro concrete of size 40 – 300 mm thickness.
The areas to be repaired are to be as shown on the drawings or as indicated by the Client/Consultant. The areas are to be marked out in the works and agreed with the Client/Consultant before proceeding. The areas may be adjusted by the Client/Consultant as work proceeds according to the conditions.
The agreed areas shall be broken out to remove all contaminated, loose, cracked or friable concrete. Due account shall be taken of any propping or other instructions given by the Client/Consultant regarding sequence of removal and repair.
The equipment and methods used to break out the concrete shall be such that no reinforcing steel or other embedded items such as conduits or lifting sockets are loosened or damaged unless so directed by the Client/Consultant.
Breaking out shall continue along the reinforcement until sound concrete is reached or as directed by the Client/Consultant. Special care shall be exercised to ensure that any reinforcement exposed is not cut or damaged.
The prepared void shall be profiled so that the entrapment of air during the repair process using fluid micro-concrete is avoided. The minimum depth of repair shall be 40 mm throughout. The perimeter of the area to be repaired shall first be saw cut to a depth of 10 mm using a suitable tool such as an angle grinder. Feather edges will not be accepted.
This preparation shall be such as to leave a sound exposed concrete surface free from dust, debris, loose particles and any deleterious matter.
All exposed reinforcement shall be cleaned of cement deposits and any corrosion products by wire brushing blasting or other approved means to achieve a surface finish acceptable by the Engineer.
Reinforcement damaged during the removal of concrete or the preparation process shall, if required by the Client/Consultant, be repaired or replaced.
It is essential that the cleaning process is complete. To ensure this, a final wash down of the concrete substrate and reinforcement shall be carried out to ensure the removal of all residual contamination.
After cleaning the reinforcement shall be kept free from contamination and concrete replaced within 3 days.
Adequate formwork shall be provided in accordance with the relevant codes of practice. This shall be securely fixed to withstand the hydraulic pressures of the fluid micro-concrete repair material without distortion or movement during placement.
The formwork shall be watertight at all joints between panels and between the formwork and the existing concrete surface so as to prevent grout leakage.
The formwork is to be constructed from appropriate materials agreed with the Supervising Officer to achieve the required finish.
Formwork surfaces that are to be in contact with the repair micro- concrete shall be treated with a suitable mould release agent such as Reebol. This shall be used in accordance with the manufacturer’s recommendations.
Sufficient hydrostatic head or pumping pressure shall be maintained to ensure that the void is filled completely and no air remains entrapped. Where necessary, provision shall be made for controllable bleed points to prevent air entrapment and enable the extent of flow of the repair material to be assessed.
Properties of repair materials
The cementitious free flowing micro concrete shall be Renderoc LAXtra and shall:
- be shrinkage-compensated in both liquid and cured states,
- contain no metallic expansion system.
- be pre-packed and factory quality controlled to ISO 9001 and ISO 14001 accreditation.
- have a maximum equivalent sodium oxide content of 3 kg/m³ (UK Department of Transport Specification for Highway Works 1986, Clause 1704.6),
- be a free flowing cementitious material that has a coefficient of thermal expansion fully compatible with the host concrete and which complies with the following requirements:
Placing of Concrete
The micro-concrete shall be mixed and placed in accordance with the manufacturer’s recommendations, particularly with regard to water ent, mixing equipment and placing time.
The placing of the micro-concrete shall be, as far as possible, continuous. The mixing operation shall be timed so that there is minimal interruption in the material flow. If however, placing is interrupted, the operation shall recommence as soon as possible while the repair material retains its flow characteristics.
The formwork shall not be removed until the repaired area with micro- concrete has achieved a compressive strength of at least 10 N/mm² or as directed by the Supervising Officer.
Immediately after the removal of the formwork the repair area shall be cured in accordance with good concrete practice by the use of high efficiency spray applied curing membranes such as Concure 90 or Concure WB or other approved methods. Details of the methods of curing shall be submitted for approval where required.