Defective Concrete Removal Techniques
The removal of deteriorated, damaged, porous, or defective concrete is a crucial step in any repair work. However, determining the extent of removal can be challenging as there is no clear boundary for the change in properties of concrete along its depth. One guideline for 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 recommended to remove material only up to the plane of reinforcement and have a joint between new and old materials at the same location. Even in cases where the decision is made not to fully encase the reinforcement bars, it is better to expose about three-fourths of the bar diameter and fully expose the corner bars.
The use of blasting for damaged concrete removal should be carefully planned as it can cause problems in the surrounding concrete. Impact tools may also result in small-scale cracking on the surface of the remaining concrete. Debris removal is typically followed by secondary methods such as chipping, sand blasting (impacting sand with high air pressure), or high-pressure water jetting to clean the surface. In cases where precise removal of damaged concrete in small areas is needed, saw cutting may be used, but the resulting surfaces must be treated with thin layers of materials to improve the feather edge surface.
When exposing narrow but deep areas for repair along cracks, undercutting the cavity is recommended to lock the repair material, as shown in Figure 1 for two alternate methods. For large areas, the edges of the area are sharply cut back perpendicular to the face of the existing concrete without any undercut. For large cavities to be filled, the top surface is preferably sloped towards the interior to facilitate easy placement and compaction of the repair material.
Fig. 1. Shaping Exposure Hole for Repair
Concrete removal techniques should prioritize effectiveness, safety, and cost-efficiency, while minimizing damage to the remaining concrete. Some methods may take longer but are environmentally friendly, while others allow for work to be done without removing the structure from service. It’s important to note that a single removal technique may not be suitable for all parts of a structure, and the properties of the concrete being removed should be considered when determining the method and cost of removal, which should be communicated to contractors during the bidding process. After removal, the remaining concrete surface should undergo visual inspection, sounding, microscopic examination, bond testing, and other tests to assess near-surface and subsurface conditions. It’s crucial to accurately estimate the quantities of damaged or deteriorated concrete to be removed at the outset of the project.
There are several broad categories of concrete removal methods, including blasting, cutting, impacting, milling, pre-splitting, and abrading. Blasting involves using explosives to break up concrete surfaces or structures, typically used in large-scale demolition projects. Cutting methods involve using saws or other cutting tools to slice through concrete, often used for precision removal in smaller areas. Impacting methods use tools such as jackhammers or chisels to physically break apart the concrete, commonly used in smaller-scale demolition or renovation projects. Milling techniques involve using milling machines to grind away concrete surfaces, often used for surface preparation or leveling. Pre-splitting is a technique where cracks are strategically created in the concrete to facilitate its removal, commonly used in controlled demolition scenarios. Abrading methods involve using abrasive materials or tools to wear away the concrete, typically used for surface removal or smoothing. These various methods provide options for different scenarios and project requirements in the process of removing concrete.
1. Blasting methods
The use of blasting methods involves the utilization of materials that produce rapidly expanding gas, which is contained within a series of bore holes. This technique is employed to produce a controlled fracture of concrete. Explosive blasting is regarded as the most cost-effective and efficient way of dislodging large quantities of damaged concrete. To initiate this process, a specific quantity of explosive is placed within the bore holes, which are then detonated either simultaneously or in a predetermined sequence.
Cushion blasting is a type of controlled blasting that involves drilling a line of bore holes with a diameter equal to or less than 75 mm, parallel to the removal face. These bore holes are loaded with light charges of explosive that are cushioned by wet sand before detonation. This technique produces a relatively clean surface with minimal overbreak.
It is crucial to note that all phases of the blasting operations must be performed by qualified personnel with extensive experience and ability in this field. Additionally, it is essential to obtain proper permission from government departments in advance before initiating any blasting operations.
2. Cutting methods
Various methods are available for cutting damaged concrete, and the selection of the appropriate method depends on how the cut pieces will be handled and transported. One method involves using a high-pressure water jet without abrasives. This involves using a small water jet at pressures ranging from 69 to 310 MPa or higher to cut the surface of the concrete.
Another method is to use saws, which can be diamond or carbide saws. These saws come in a range of sizes, from very small hand-held saws to very large saws that can cut depths of up to 1.3 meters.
Diamond wire cutting is another method, which involves wrapping a continuous wire with modules impregnated with diamonds around the concrete mass to be cut. This wire is then connected to a motor to form a revolving loop. The size of the concrete structure that can be cut is determined by the limits of the power source.
Mechanical shearing is another method that involves using hydraulically powered jaws to cut concrete and reinforcing steel. Care must be taken to avoid cutting into other members, and the cuts must be started from free edges or from holes made by hand-held breakers. This method is suitable for making cuts through slabs, decks, and other thin concrete members.
Stitch drilling is a method in which 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.
Finally, thermal cutting can be achieved using devices such as the powder torch, thermal lance, and powder lance. These methods use intense heat generated by the reaction between oxygen and powdered metals to melt a part of the concrete. These methods are best suited for cutting reinforced concrete, but they are generally slow, and progress depends on the rate at which the resulting slag can flow out of the slot.
3. Impacting methods
A technique used to fracture and spall concrete involves the repeated striking of a concrete surface with a high-energy tool or a large mass. However, this method may lead to micro-cracking in the surrounding concrete, especially if partial depth removal is necessary.
To carry out this process, several types of equipment can be employed. Firstly, hand-held breakers are used, which are operated by hand and are typically smaller and more portable. Secondly, boom-mounted breakers can be utilized, which are mounted on a boom and are generally larger and more powerful. Finally, scabblers are another type of equipment used in this method, which feature a series of needles that impact the concrete surface to create the desired effect.
4. Milling methods
Concrete milling techniques are utilized to eliminate a defined quantity of concrete from large horizontal or vertical surfaces. The removal depth typically ranges from 3 mm to roughly 100 mm, resulting in a solid surface that is free of micro-cracks. To cut concrete surfaces, scarifiers utilize rotary action and cutter bits. They can remove both deteriorated and sound concrete, including concrete containing form ties and wire mesh, freshly blasted surfaces with loose concrete, and concrete that has been weakened by an expansive agent. Scarifiers are available in a variety of sizes.
5. Hydro-demolition
High-pressure water jetting is a commonly used technique to remove concrete while preserving the steel reinforcement for potential reuse and minimizing damage to the remaining concrete. The main advantage of this method is its high efficiency in breaking down concrete into smaller pieces resembling sand and gravel.
This technique involves using a high-pressure water stream to disintegrate the concrete, making it easier to remove and exposing the underlying steel reinforcement. By breaking down the concrete into smaller particles, the process minimizes the risk of damage to the remaining concrete structures, reducing the need for costly repairs.
Additionally, high-pressure water jetting is an environmentally friendly method of concrete removal as it does not involve the use of chemicals or other harmful substances. The resulting debris can also be easily collected and disposed of, making it a safer and more sustainable alternative to other concrete removal methods.
Overall, high-pressure water jetting is a reliable and effective method for concrete removal that can help reduce costs and environmental impact while preserving the integrity of steel reinforcement and minimizing damage to the remaining concrete structures.
6. Pre-splitting methods
Pre-splitting methods are commonly used for removing concrete from mass structures. These methods involve inducing a crack plane in the concrete using different techniques such as hydraulic splitters, water pressure pulses, or expansive chemicals. The direction and extent of the crack planes depend on the spacing, pattern, and depth of the bore holes drilled along a line.
One of the most popular methods of pre-splitting is the hydraulic splitter. This method involves using a wedging device in pre-drilled boreholes to split the concrete. It is mainly used for removing large volumes of material from mass concrete structures. However, an additional method is required to clean the reinforcing steel after the concrete is split.
The water-pulse splitter is another pre-splitting method. Boreholes are drilled and filled with water before a small explosive charge is detonated in one or more holes. The resulting high-pressure pulse through the water transfers the impact to the concrete structure, causing it to crack. However, this method cannot be used if the concrete is severely cracked or deteriorated as it cannot hold water in the drill holes.
Finally, the expansive product agents method involves placing cementitious expansive agents in boreholes in a specific pattern within the concrete structure. The agents contain aluminum powder, and when placed in the concrete, cause it to split into pieces for removal. This method is less violent and causes minimal disturbance to the adjacent concrete.
7. Abrading blasting
Abrasive blasting is a technique commonly used to remove surface contaminants and prepare concrete surfaces for various applications. It involves propelling an abrasive medium at high velocity against the surface to abrade it. Sandblasting, shot-blasting, and high-pressure water blasting are the most commonly used methods for this purpose.
Sandblasting is the most frequently used technique for cleaning concrete and reinforcing steel. It employs common sands, silica sands, or metallic sands as the primary abrading tool. There are three methods of implementing this method: dry sandblasting, wet sandblasting, and high-pressure wet sandblasting.
In dry sandblasting, sand particles are bombarded at the concrete surface using high-pressure air in an open atmosphere. The sand particles used in this method are typically angular and range in size from passing a 2.12 to a 4.75 mm sieve. 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. Compressed air at a minimum pressure of 860 kPa is used in this method. However, the economical depth up to which sandblasting is effective is about 6 mm from the concrete surface.
Wet sandblasting, on the other hand, is the same as dry sandblasting, but the free particles bouncing back from the surface are collected in a circle of water to reduce air pollution. High-pressure wet sandblasting involves projecting sand at the concrete surface or reinforcing steel with the help of a stream of water at high pressures ranging from 10 to 20 MPa. However, this method is not as effective as dry sandblasting.
8. Shot-blasting
Shot-blasting is a method used to remove concrete from a surface, similar to sandblasting. However, instead of using sand, metal pieces are projected at high velocity onto the concrete surface. As the shot hits the surface, it erodes the concrete, which is then collected by a vacuum chamber within the machine.
Compared to other methods, shot-blasting is highly efficient and environmentally friendly. To clean a surface, a small-sized shot is used, and the machine is set to maximum travel speed. This allows for a surface cleaning operation to be done quickly and effectively. In fact, up to 6 mm of material can be removed in a single pass, and up to 20 mm thickness can easily be removed with this method.
9. High-pressure water blasting (with abrasives)
High-pressure water blasting with abrasives is a method that employs a stream of water at a high pressure ranging from 10 to 35 MPa, combined with an abrasive substance such as sand or aluminum oxide. This equipment is utilized for removing dirt, grease, and other small particles, ultimately revealing the fine aggregate.
Before disposal into a waste or storm water system, the abrasive is extracted from the water, ensuring that the water is free of any abrasive materials. By utilizing this method, high-pressure water blasting with abrasives can efficiently and effectively clean surfaces without leaving any debris behind.