By Gert Schmidt
Surface preparation is a critical component for successful concrete repairs. If prepared correctly, a strong and lasting bond between the repair material and host substrate is formed. If the host substrate has not been adequately prepared, concrete repairs will fail. This is irrespective of how proficient the repair system is.
When applied, the repair material should become an integral component of existing concrete. In this way, the original monolithic strength of the concrete is restored. The composite system formed will facilitate uniform transfer of stresses in the system.
There is no point in trying to achieve good adhesion to a weak friable substrate as the concrete repair will fail imminently. Even concrete repairs performed on a sound host substrate will not perform.
Other factors that contribute to good adhesion is the chemical bond of the repair materials and exposure conditions.
Delineate concrete repair zones
The first step in preparing substrates is to delineate the perimeter of concrete repair zones. This is conventionally undertaken with a disc-type mechanical grinder. The saw cut is right angled and between 10mm and 15mm deep to avoid feather edging. It is never deeper than the reinforced concrete cover. The edges are also roughened slightly with a needle gun or hacking. If undertaken correctly, these saw cut edges help to contain the repair material.
A common mistake made by many inexperienced concrete repair contractors is to try and outline the shape of the distressed concrete. This creates tortuous edges which often causes shrinkage stresses. Shrinkage, tensile stress-induced deformation across concrete triggers cracking. This creates a pathway for aggressive agents, such as chloride-ions and sulphate-ions, into the concrete. Therefore, the long-term durability and serviceability of the structure has been compromised, undermining the overall objective of concrete repairs. Good repair work prolongs the lifecycle of structures as opposed to merely addressing deterioration. Refer to https://www.sciencedirect.com/topics/engineering/shrinkage-stress.
Chipping tools are used in smaller areas where it is impossible to undertake saw cuts. Seasoned concrete repair contractors will always ensure that the edges of the repair area are cut perpendicular to the substrate.
Concrete repair areas
The deteriorated centre of the concrete repair areas is then removed. Many surfaces require roughening. Aggregates also need to be exposed and damaged, delaminated and loose concrete removed.
Defective concrete is broken back to a sound base and dense concrete surface. This is after shoring has been installed.
The removal of concrete progresses from the centre towards the edges. Concrete near the edges is then broken out while taking care to avoid damaging the sound concrete at the interface.
A skilled and experienced concrete repair contractor will know which tools to deploy when removing damaged concrete. The most common methods include hand-held percussive equipment; pneumatic breakers; chipping hammers; and scabblers. High pressure water jetting; water demolition; and sand or shot blasting are also regularly used to remove damaged concrete.
Next step in concrete repair
The next step in the concrete repair process entails cleaning the sound concrete that remains after the damaged portions have been removed. Bear in mind that the impact methods deployed can result in micro-cracking and bruising. These, in turn, form weak planes that act as “bond breakers”.
This is usually done using air blasting or water jetting at pressures of between 15MPa or lower and 35MPa or less.
Worryingly, many contractors do not do this because they want to save time or costs. Yet, surface cleanliness is a critical step in preparation for concrete repairs. Any dirt, debris or loose particles can affect bond strength of the repair materials. Cleaning the surface ensures that the repair materials directly contact the host concrete. Roughening the surface also enhances anchorage of the repair material.
Effective concrete repair
Another critical factor for effective concrete repair is to prepare rebar surfaces. This is to remove the rust layer which is mechanically and poorly bonded to the surface.
All weak; spalled; severely cracked; damaged; and easily removable concrete is also removed from the corroded reinforcement steel. Once the entire circumference of the corroded reinforcement is exposed, it is cleaned. This is to remove loose scale; corrosion deposits; and other contaminants. Generally, a clearance of between 20mm and 25mm is required around the rebar to thoroughly clean it. It also ensures proper encasement and bond of repair materials. Damaged steel reinforcement must also be exposed to the intact section. Thereafter, concrete covering must be removed to expose at least 50mm of the non-corroded steel section.
The products that have caused deterioration of concrete reinforcement, such as chloride ions, must also be removed. Refer to https://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=282.
Rebar that has lost more than 25% of its cross-sectional area due to corrosion should be spliced. This is done by butt-welding bars with backing plates. Proficient concrete-repair contractors will also lap them with supplemental reinforcement or use coupler mechanical joints to make the connection. The steel is then treated with an unbroken coat of anti-corrosion zinc-rich epoxy primer to protect it. There are many other ways to protect rebar from corrosive forces. Hindle Mason Projects deploys them widely.
Concrete repair bonding materials
It is also important to understand the role of concrete repair bonding materials. A bonding agent can never supplement poor surface preparation. Many concrete repairs have failed because contractors have incorrectly assumed that merely treating surfaces with bonding agents will be sufficient. This is a common misperception. In other instances, concrete repair contractors have done this deliberately to save in time and costs. This enables them to undercut their competitors when pricing for projects.
The professional engineers with whom Hindle Mason Projects works only specify bonding agents to enhance bond at the repair interface. They also know that we have experience working with these products. If incorrectly applied, bonding agents create an additional layer that causes a weak plane. Thus, extra care needs to be taken to ensure that these systems do not cure prior to the application of the repair mortar. If this occurs, they become bond breakers as opposed to a way of enhancing bonds. Sprayed repair mortars, in particular, do not require bonding agents. This is considering that the shotcrete process provides excellent bonding characteristics by itself.
Concrete repairs with cementitious mortars
Before concrete repairs are undertaken with cementitious mortars, surfaces need to be wet. This is to achieve a saturated surface dry (SSD) condition after cleaning. An SSD condition means that the substrate is damp and saturated. However, there is no free water at the surface that will impair the bond at the interface due to shrinkage. This will reduce repair material and bond strength.
Achieving an SSD condition prevents the host concrete from absorbing the repair mortar moisture which is required for hydration.
Pre-soaking is especially important if the concrete is dry. This should be done thoroughly to slow down water movement rate from the repair material to the host concrete.
A substrate in SSD condition is damp and saturated but does not contain any free water on the surface. Free water at the surface must be avoided because it can impair the bond at the interface due to shrinkage. As a result, material and bond strength will be negatively impacted.
Concrete repair is always safe
Irrespective of its various phases, concrete repair is always safe. Health and safety risk before and during the surface preparation, therefore, needs to be assessed.
This includes understanding the impact of the of removal of damaged concrete on structural integrity. Only then can spalled concrete or damaged reinforcement be removed. Hindle Mason Projects temporarily braces structural elements before starting operations. This is done by trained and experienced professionals whose operations are also supervised by our safety officers.
When concrete is removed, dust and debris must also be contained. Moreover, the repair areas should be examined to ensure that there are no embedded electric conduits, sockets or utility connection lines.
Tensile pull-off adhesion tests of the existing concrete will also be undertaken as part of the condition evaluation.
Strength tests inform concrete repairs
Certainly, these strength tests also inform concrete repairs.
Significant deviations of the prepared surface pull-off strength from the existing concrete tensile strength also inform concrete repairs. In this way, engineers can establish and specify the realistic adhesion strength requirements for the on-site repair condition.
To prequalify concrete repairs, it is important to evaluate surface preparation quality and eventually bond durability. This is undertaken by conducting a direct pull-off test on a representative sample area for the cured in-situ repair material. Thus, the tensile repair material tensile bond strength and that of the existing host concrete can be ascertained.
Surface preparation must be validated periodically by benchmarking engineers’ specifications and values obtained during reference sample prequalification. Generally, the mean adhesion or pull-off strength to concrete at seven days should not be less than 0,75MPa. No individual result should be less than 0,65MPa for substrate mode of tensile failure within existing concrete substrates. Bond values for shotcrete and form-and-pour repairs typically exceed 0,75MPa and, in most cases, are more than 1,0 MPa.
Leading concrete repair specialist
Hindle Mason Projects is a leading concrete repair and corrosion/erosion protection specialist.
As such, we are approved applicators of:
- SprayLock Concrete Protection [https://www.concreteprotection.com/]
- StonCor Africa [https://www.stoncor.co.za/]
- ABE Saint Gobain [https://www.abe.co.za/waterproofing-product-selector/]
- Sika [https://zaf.sika.com/?gad_source=1&gclid=EAIaIQobChMI0fr46-a3hgMV1ZlQBh3AcQOREAAYASAAEgJBK_D_BwE]
- Mapei [https://www.mapei.com/za/en/home-page]
- Bostik [https://www.bostik.com/global/en/]
- Penetron Africa [https://penetron.africa/]
Gert Schmidt is a Director of Hindle Mason Projects