Defective Concrete Removal Techniques
Any repair work must remove deteriorated, damaged, porous, or defective concrete, but the degree of this removal is rarely known during the planning stage. It takes a long time to decide how much broken concrete should be removed and how much ageing for existing concrete should be allowed to continue. Because the change in concrete qualities occurs gradually down the depth of the concrete and there is no hard and fast border line, this decision becomes extremely challenging. In medium and high strength concretes, one recommendation for the extent of removal is to keep removing material until aggregate particles are fractured rather than merely removed from the cement matrix.
Removing material only up to the plane of reinforcement and having a joint between new and old materials at the same position is not a good technique. Even if the bars are not fully encased, it is preferable to expose around three-quarters of the diameter of the bars and the corner bars completely.
Blasting damaged concrete can cause difficulties in the surrounding concrete, thus it must be used with caution. Impact tools may cause small-scale cracking on the surface of the concrete that has been left in place. After removing the debris using various basic methods, a secondary approach such as chipping or sand blasting (impacting sand with high air pressure) is frequently used. Alternatively, high-pressure water jetting can be used to clean the surface. Saw cutting can be used to remove defective concrete in tiny places with greater precision, but the resulting surfaces must be coated with thin layers of materials to improve the feather edge surface.
When exposing narrow but deep areas for crack repair, it’s best to undercut the cavity to lock the repair material in place; see Fig. 1 for two alternate methods. The borders of big portions are sharply cut back perpendicular to the face of the existing concrete without any undercut. When filling big cavities, the top surface should slant toward the interior to allow for simple placement and compaction of the repair material.
Shaping Exposure Hole for Repair
The concrete removal procedures employed should be practical, safe, and cost-effective, with minimal harm to the remaining concrete. Some eradication techniques take significantly longer than others yet are less harmful to the environment. Some solutions allow a portion of the work to be completed without the structure being taken out of service. For all parts of a structure, a single removal procedure may not be the best option. The qualities of the concrete to be removed give critical information for evaluating the technique and cost of concrete removal, and this data must be made available to contractors for bidding purposes.
Visual inspection and sounding at the surface, microscopic examination or bond testing for near-surface damage, and cores, pulse velocity tests, and pulse echo tests for subterranean condition of surviving concrete are performed after the removal. All defective or deteriorating concrete must be removed, and quantities must be determined as precisely as possible from the beginning.
Blasting, cutting, impacting, milling, pre-splitting, and abrading are the basic types of concrete removal processes. The following are the specifics of the various methods:
1. Blasting methods
To achieve controlled concrete fracture, blasting procedures use ingredients that produce quickly expanding gas confined within a sequence of bore holes. Explosive blasting is often regarded as the most cost-effective and efficient method of removing huge amounts of damaged concrete. A predetermined amount of explosive is placed in drilling bore holes and detonated simultaneously or in a predetermined sequence.
Cushion blasting is a controlled blasting technique in which a line of bore holes with a diameter of less than or equal to 75 mm is drilled parallel to the removal face, each hole is loaded with light explosive charges, the charges are cushioned by wet sand, and the explosives are marked. This approach generates a surface that is relatively clean and has less overbreak. All phases of the blasting operations must be carried out by qualified professionals with demonstrated expertise and ability in this field, and clearance from the appropriate government departments must be obtained ahead of time.
2. Cutting methods
The following methods can be used to cut damaged concrete, and the method chosen is based on how the cut pieces will be handled and transported.
a) High-pressure water jet (without abrasives): To cut the concrete surface, a high-pressure tiny water jet with pressures of 69 to 310 MPa and above is utilised.
b) Saws: Diamond or carbide saws come in a variety of sizes, from small hand-held saws to massive saws capable of cutting depths of up to 1.3 metres.
c) Diamond wire cutting: To generate a revolving loop, a continuous wire with diamond-impregnated modules is wrapped around the concrete mass to be cut and attached to a motor. The size of the concrete structure that can be cut is determined by the power source’s limitations.
d) Mechanical shearing: For cutting slabs, decks, and other thin concrete members, hydraulically powered jaws cut concrete and reinforcing steel. Cuts must be made from free edges or holes formed by hand-held breakers, with caution exercised not to cut into other members.
e) Stitch drilling: This method involves drilling overlapping bore holes along the removal perimeter to chop out specified concrete pieces. This method is very beneficial for cutting out cuts in concrete members where only one face may be accessed and the cut depth is larger.
(f) Thermal cutting, which uses tremendous heat generated by the reaction between oxygen and powdered metals to melt a section of concrete, can be done with the powder torch, thermal lance, and powder lance. These processes are usually slow, and the rate at which the generated slag may flow out of the slot determines the rate of progress. These are the best tools for cutting reinforced concrete.
3. Impacting methods
Repeated striking of the concrete surface with a high-energy tool or a big mass is used in these procedures to fracture and spall the concrete. If partial depth removal is required, this approach may induce microcracking in the adjacent concrete. This approach makes use of the following tools:
a) Breakers that can be held in one’s hand
b) Breakers mounted on a boom
4. Milling methods
Milling techniques are used to remove a certain amount of concrete from broad portions of horizontal or vertical surfaces, with removal depths ranging from 3 mm to 100 mm. These techniques usually result in a smooth, micro-crack-free surface.
Scarifier: A scarifier is a rotary-action concrete cutting instrument with cutter bits that cut concrete surfaces. This equipment can remove degraded and sound concrete, including form ties and wire mesh, as well as loose concrete from newly blasted surfaces and concrete that has been split and weakened by an expanding agent. Scarifiers come in a broad variety of sizes.
When it is necessary to maintain and clean the steel reinforcement for reuse while minimising damage to the concrete that remains in place, high-pressure water jetting is employed as the principal method of concrete removal. This process is highly efficient at disintegrating concrete and converting it to sand and gravel-sized fragments.
6. Methods of pre-splitting
Hydraulic splitters, water pressure pulses, or expansive chemicals injected in bore holes drilled along a line to produce a crack plane for concrete removal are used in pre-splitting procedures. The pattern, spacing, and depth of the bore holes determine the direction and extent of the crack planes that propagate.
a) Hydraulic splitter: This approach is used to remove enormous volumes of material from mass concrete constructions, and the apparatus comprises of a wedging mechanism that is used to split concrete in predrilled boreholes. To clean the reinforcing steel, a secondary method of separating and managing the concrete is required.
b) Water-pulse splitter: This approach involves drilling and filling boreholes with water, then detonating a device holding a very small explosive charge in one or more holes. This creates a high-pressure pulse in the water, which transfers impact to the structure and causes concrete cracking. If the concrete is substantially broken or damaged, this procedure will not work since it will not be able to contain water.
c) Expansive product agents: Concrete can be divided into pieces for removal by inserting cementitious expansive agents (for example, products containing aluminium powder) in boreholes arranged in a predetermined pattern within the concrete structure. This procedure is less aggressive and provides the least amount of disruption to the surrounding concrete.
7. Blasting with abrasive
Abrading blasting abrades the concrete surface by forcing an abrasive material at a high velocity against it. Abrasive blasting is commonly used to remove impurities from the surface and as a final surface preparation. Sandblasting, shotblasting, and high-pressure water blasting are all common processes.