Removal of deteriorated, damaged, porous or defective concrete is a must for any repair work but the extent of this removal can not usually be determined in the planning stage. The decision about how much damaged concrete is to be removed and what level of aging for existing concrete is allowed to remain in place is very tedious. This decision becomes particularly difficult because the change in properties of concrete occurs gradually along the depth of concrete and there is no hard and fast border line. One guideline for the extent of removal in medium and high strength concretes is to continue removing material until aggregate particles are being broken rather than simply removed from the cement matrix. It is not a good practice to remove material only up to the plane of reinforcement and to have a joint between new and old materials right at the same location. Even in those cases where it is decided not to encase the bars fully, it is better to expose about three-fourth of the bars diameter and to expose the corner bars fully.
Blasting of damaged concrete can produce problems in the surrounding concrete and hence its use must be carefully planned. Use of impact tools may also produce small-scale cracking to the surface of the concrete left in place. The debris removal by some primary means is usually followed by using a secondary method such as chipping, sand blasting (impacting sand with high air pressure) or high pressure water jetting to clean the surface. For more precise removal of damaged concrete in small areas, saw cutting may be used but the surfaces obtained must be treated with thin layers of materials to improve the feather edge surface.
When exposing narrow but deep areas for repair along cracks, the cavity is better to be undercut to lock the repair material, see Fig.1 for two alternate methods. For large areas, the edges of the area are cut back sharply perpendicular to the face of the existing concrete without any undercut. For large cavities to be filled, the top surface is preferably made slopping towards the interior for easy placement and compaction of the repair material.
Fig. 1. Shaping Exposure Hole for Repair
The concrete removal techniques used should be effective, safe and economical, and should produce minimum damage to the concrete left in place. Some removal techniques take much longer time than the others but are environment friendly. Some techniques permit a portion of the work to be accomplished without removing the structure from service. A single removal technique may not be the optimum solution for all portions of a given structure. The properties of the concrete to be removed provide important information required determining the method and cost of concrete removal and this information must be made available to contractors for bidding.
After the removal, the concrete surface is checked by visual inspection and by sounding at the surface, microscopic examination or bond testing for near-surface damage and by taking cores, pulse velocity tests and pulse echo tests for subsurface condition of remaining concrete. All damaged or deteriorated concrete is to be removed and the quantities must be estimated as accurately as possible in the start.
The broad categories of concrete removal methods are blasting, cutting, impacting, milling, pre-splitting, and abrading. The details of various methods are given below:
1. Blasting methods
Blasting methods use materials producing rapidly expanding gas confined within a series of bore holes to produce controlled fracture of the concrete. Explosive blasting is considered to be the most cost effective and quick way of dislodging large quantities of damaged concrete. A designed quantity of explosive is placed in drilling bore holes that is simultaneously or in predefined sequence detonated.
One technique of controlled blasting is cushion blasting, in which a line of less than or equal to 75 mm diameter bore holes are drilled parallel to the removal face, each hole is loaded with light charges of explosive, the charges are cushioned by wet sand, and the explosives denoted. This method produces a relatively clean surface with less overbreak. All phases of the blasting operations must be performed by qualified personnel having proven experience and ability in this field and proper permission from the government departments must be taken in advance.
2. Cutting methods
Following methods can be used to cut the damaged concrete and the selection of the method basically depends on handling and transportation of the cut pieces.
a) High-pressure water jet (without abrasives): A high pressure small water jet is used producing pressures of 69 to 310 MPa and above to cut the concrete surface.
b) Saw: Diamond or carbide saws are available in sizes ranging from very small hand-held saws to very large saws capable of cutting depths of up to 1.3 m.
c) Diamond wire cutting: A continuous wire having modules impregnated with diamonds is wrapped around the concrete mass to be cut and is connected to a motor to form a revolving loop. The limits of the power source determine the size of the concrete structure that can be cut.
d) Mechanical shearing: The mechanical shearing is achieved by hydraulically powered jaws to cut concrete and reinforcing steel for making cuts through slabs, decks, and other thin concrete members. The cuts must be started from free edges or from holes made by hand-held breakers and care must be taken to avoid cutting into other members.
e) Stitch drilling: In this method, overlapping bore holes are drilled along the removal perimeter to cut out desired sections of concrete. This method is especially useful for making cutouts through concrete members where access to only one face is possible and the depth of cut is greater.
f)Thermal cutting: The powder torch, thermal lance, and powder lance can be used for thermal cutting that use intense heat generated by the reaction between oxygen and powdered metals to melt a part of concrete. These methods are usually slow and the progress depends on the rate at which the resulting slag can flow out of the slot. These devices are best to cut reinforced concrete.
3. Impacting methods
In these methods, repeated striking of concrete surface is done with a high energy tool or a large mass to fracture and spall the concrete. This method may cause micro-cracking in the adjoining concrete particularly if partial depth removal is required. Following equipments are used for this method:
a) Hand-held breakers
b) Boom-mounted breakers
4. Milling methods
Milling methods are used to remove a specified amount of concrete from large areas of horizontal or vertical surfaces; having removal depth ranging from 3 mm to approximately 100 mm. These methods usually produce a sound surface free of micro-cracks.
Scarifier: A scarifier is a concrete cutting tool that employs the rotary action and its cutter bits cuts concrete surfaces. This equipment can remove deteriorated and sound concrete in which some of concrete contains form ties and wire mesh, loose concrete from freshly blasted surfaces and concrete that is cracked and weakened by an expansive agent. Scarifiers are available in a wide range of sizes.
High-pressure water jetting is used as a primary means for removal of concrete when it is desired to preserve and clean the steel reinforcement for reuse and to minimize damage to the concrete remaining in place. This method has a high efficiency and disintegrates concrete changing it back to sand and gravel-sized pieces.
6. Pre-splitting methods
Pre-splitting methods use hydraulic splitters, water pressure pulses, or expansive chemicals placed in bore holes drilled along a line to induce a crack plane for the removal of concrete. The direction and extent of the crack planes that propagate depend on pattern, spacing, and depth of the bore holes.
a) Hydraulic splitter: This method is used as a primary means for removal of large volumes of material from mass concrete structures and the apparatus consists of a wedging device that is used in predrilled boreholes to split concrete. Secondary means of separating and handling the concrete is required to clean the reinforcing steel.
b) Water-pulse splitter: In this method boreholes are drilled and are filled with water and a device containing a very small explosive charge is detonated in one or more holes. This creates a high-pressure pulse through the water transferring impact to the structure and causing the cracking of the concrete. This method does not work if the concrete is severely cracked or deteriorated as it can not hold water in the drill holes.
c) Expansive product agents: By placing cementitious expansive agents (for example, products containing aluminum powder) in boreholes located in a decided pattern within the concrete structure, the concrete can be split in to pieces for removal. This method is less violent and causes minimum disturbance to the adjacent concrete.
7. Abrading blasting
Abrading blasting removes concrete by propelling an abrasive medium at high velocity against the concrete surface to abrade it. Abrasive blasting is typically used to remove surface contaminants and as a final surface preparation. Commonly used methods include sandblasting, shot-blasting, and high-pressure water blasting.
Sandblasting: This method is the most commonly used technique to clean concrete and reinforcing steel, where common sands, silica sands, or metallic sands is used as the primary abrading tool. There are following three methods of implementing this method.
a) Dry sandblasting: The concrete surface is bombarded with sand with the help of high-pressure air in the open atmosphere. The sand particles are usually angular and may range in size from passing a 2.12 to a 4.75 mm sieve, larger size particles are used for rougher required surface condition. Compressed air at a minimum pressure of 860 kPa is used in this method. Finer sands are used for removing loose materials and grease from the concrete and reinforcing steel, while coarser sands are commonly used to expose fine and coarse aggregates in the concrete or tightly bonded corrosion products from reinforcing steel. The economical depth up to which sand blasting is effective is about 6 mm from the concrete surface.
b) Wet sandblasting: This method is same as dry sandblasting but the free particles bouncing back from the surface are collected in a circle of water to reduce air pollution.
c) High-pressure wet sandblasting: Sand is projected at the concrete surface or the reinforcing steel with the help of stream of water at high pressures ranging from 10 to 20 MPa. This method is not as effective as dry sandblasting.
This method is also similar to sandblasting but here metal pieces are projected at the concrete surface at a high velocity. The shot erodes the concrete from the surface and the removed material is collected by a vacuum chamber in the machine. The shot-blasting process is highly efficient and environment friendly method. A surface cleaning operation is done by using a small-sized shot and setting the machine for maximum travel speed. Removal of as much as 6 mm in a single pass is possible and up to 20 mm thickness can easily be removed.
9. High-pressure water blasting (with abrasives)
High-pressure water blasting with abrasives uses a stream of water at high pressure of 10 to 35 MPa with an abrasive, such as sand, aluminum oxide. This equipment can remove dirt, grease or other small particles exposing the fine aggregate. The abrasive is removed from the water before it is disposed into a storm or waste water system.