STRUCTURAL CONCRETE REPAIR

Concrete structures inevitably have a design life per the relevant Australian Standards; as such, maintenance and concrete repair will always be required. 

There are, however, sources that will expedite the degradation of the concrete, which should always be considered both pre and post-construction.

At Slabtec, we provide solutions for all types of concrete repair. 

Whether the remediation is the result of chlorides, carbonation, damage or defective concrete, we are able to put forward a solution that will ensure the durability and strength of the section is fully reinstated.

The surface preparation, approach and materials used are critically important to the result. 

Corbel Repair of Concrete

The use of corbels to transfer loads across movement joints, both temporary and permanent, is very common in the Australian construction industry. Unfortunately, corbels are typically one of the most poorly constructed (and detailed) elements on many building projects.

These deficiencies, in both the design office and on-site, can cause structural deterioration of the corbel, which often manifests itself visually as cracking and concrete spalling. However, it can also seriously impede the corbel’s ability to carry the design load safely.

Slabtec’s Engineering background allows us to provide concrete repair solutions that address the visible effects of the deficiencies, the cracking and spalling and, importantly, the root causes of the problem in the first instance.

In our experience, the majority of distressed corbels are the result of one or more of the following:

POOR CONSTRUCTION PRACTICES

• Incorrectly scheduled/bent reinforcement
• Incorrectly placed reinforcement, particularly the main U bars position in the corbel itself
• Inadequate vibration of concrete
• Inadequate fixing of the slip joint prior to the pour
• Inadequate use of compressible material to allow for any “closing” movement at relevant positions along joints, if any
• Trying to construct the corbel upside down
• Trying to pour the (upper) supported side of the corbel first

PT Anchorage End Pocket Repairs

The concrete patching of a post-tensioning anchorage primarily performs two basic functions:

• Fire rating of the anchorage
• Durability of the anchorage

The most common problems we see in day to day construction of PT end pockets relate to the cover of the cut strand and the quality of the dry pack material.

Using conventional abrasive disc cutters, getting much better than an average of 20mm to 25mm cover to cut strands in a typical void former recess is difficult. This is why the quality of the dry-pack concrete repair material, as well as the application of the material, is so important.

The quality of the tendon grout is also critical in encapsulating the internal anchorage void during the tendon grouting process.

Unfortunately, you will often see PT pockets with very low levels of cover to the strand, to the point where you can almost make out the individual strands visually. Over the years, there has also been very limited quality control on the products being applied as the dry pack material; in many cases, it was purely a prebagged sand/cement where the cementitious content could typically be very low.

Before specifying the concrete repair remedial solution, it is important to thoroughly inspect the anchorage zone to determine the scope of the work required. Still, it is very important to remember that the anchorage zone is a very highly stressed area, even in the grouted condition, meaning a thorough understanding of PT anchorage systems is necessary before undertaking any work in the vicinity of the anchorage.

The inspection should include the following as a minimum:

• Corrosion level on the strand and anchorage components
• The level of cover to the strand ends
• Evidence of PT grout around the wedges and in between the wires of the strand
• Minimum cover levels to the top of the slab and the soffit, particularly around façade fixings/rebates
• Minimum spacing requirements between anchorages
• Visible cracking in the anchorage zone, if any
• Visible reinforcement, if any
• Any post-pour fixings or other alterations

Concrete repair procedures are determined on a case-by-case basis following the above inspection and reflecting the environmental conditions in which the anchorage is located.

Subject to the above, as a minimum, Slabtec typically adopts an R4 class high-build concrete repair mortar as a part of an overall repair system, including a bonding bridge with an anti-corrosion primer, the high-build mortar itself and a levelling/finishing mortar where required.

In areas subject to severe durability conditions, further considerations may be necessary.

Concrete Cancer Repair Services

Concrete cancer is the terminology most commonly used to describe the visible deterioration of a concrete member due to ongoing reinforcement corrosion.

Good quality fresh concrete is highly alkaline, and this alkalinity allows a passivating layer to form around the reinforcement, which prevents the process of corrosion from occurring.

For the electrochemical process of reinforcement corrosion to occur, the alkalinity in the concrete protecting the reinforcement must be reduced, which can happen due to a number of reasons, typically the:

• The ingress of chlorides into the concrete, or
• The gradual loss of alkalinity in the concrete through carbonation

As defined by Australian Standards, concrete cancer in a structure within its normal design life of 40 to 60 years is typically the result of poor design, detailing, concrete repair or construction practices.

Poor quality concrete mixes, insufficient levels of cover to reinforcement, aggressive toppings such as magnesite, cracked members and poorly drained / waterproofed structures are all factors that can cause concrete cancer.

The visible signs of concrete cancer in the early stages are often cracking and rust staining leaching from the cracks. As the process progresses and the corrosion becomes more significant, spalling can occur due to the expansion of corrosion products. Concrete spalling, as well as being unsightly and hazardous in its own right, results in the loss of concrete cross-section and the corrosion process, in general, reduces the quantity of effective reinforcement in the member, which can seriously reduce the member’s capacity.

The earlier the concrete cancer is detected in a structure; the more cost-effective the remediation/concrete repair will be. 

In the early stages, with more localised areas of concrete cancer, the solution will likely result in removing and replacing the affected concrete zones with engineered mortars, including reinforcement if required.

More significant cases can be addressed through combinations of the approaches above and protective coatings and membranes.