Patch repair

Of all the repair techniques presented within this model, Patch Repair is the most common.  It is the application of concrete patches to damaged or deteriorated concrete.

 

Where concrete is damaged due to corrosion, it is normal practice to excavate behind the bar to replace all chloride contaminated or carbonated concrete with new, uncontaminated, alkaline mortar, this is a “Full Depth Repair”.


 

Superficial repairs, removing only corrosion damaged concrete, are an option where a technique such as cathodic protection, realkalisation or electrochemical chloride extraction will provide protection to the steel without removing the cause of corrosion.

 

Refer to Section 4.4 and Section 5 of the guidance document  to provide more guidance on the selection of patch repair for concrete.

 

This page presents a general description on applying Patch Repair.  Incorporated within the page is also a tool to calculate the total cost of applying Patch Repair.

 

There are three main techniques for applying patch repair:

·         Hand applied.

·         Recast.

·         Sprayed concrete

Activate this tool, Patch Repair  to calculate the cost of applying a Patch Repair technique.

The two main inputs expected are the 'Cost of Technique'  and 'Area to be Patched'
  

Multiplication of the two main inputs generates a total cost for the specified technique.  This cost is then added to the 'Fixed Cost' to produce a total cost for applying the technique from conception to finish.

 

The first and last two inputs which are the'Name of the Technique', 'Maintenance Cost' and 'Life of Technique' respectively is essential if the user requires the output generated by this tool for future reference.

 

The generated output is presented in a report format which contains the specified technique as well as other details relating to the calculation .

 

The output produced within the Total Cost text area can then be copied and pasted within a report or a document for future reference.

 

The patch repair process consists of the following key stages:

·         Removal of damaged concrete.

·         Substrate and steel preparation.

·         Application of the repair mortar.

The removal of the damaged concrete and substrate and steel should be carried out in accordance with current best practice.  

The application of the repair will be through one of the following processes.

Hand applied mortar

Carbonation on a building facade usually requires relatively small, isolated repairs and so lends itself to hand-applied patch repairs.

The mortar or concrete should be mixed according to the manufacturer’s instructions. The mortar is then worked around and behind the reinforcement by hand.  The thickness of each layer built up and the application procedure can vary greatly depending upon the material used and the orientation of the surface being repaired.  

A typical procedure is to apply layers of 25-50 mm thick on vertical work and 20-30 mm thick for overhead areas.   Care should be taken when applying additional layers to ensure that the previous mortar has gained sufficient strength, but has not set. If the following layer is delayed then the surface can be scoured and dampened with water before the next layer is applied, or a bonding bridge applied. The repair can be finished with a trowel using the surrounding concrete as a guide.

Recasting with concrete or mortar

This technique is usually most suitable for large-volume repairs or where large areas of dense reinforcement are present.  Access for vibration is often a problem and so flowable grouts and self-compacting proprietary micro-concretes have been developed to minimise the vibration required.

Where bonding agents are used the shutters and pouring sequence have to be carefully designed so that they can be rapidly positioned before the bonding agent dries. The concrete or mortar has to be carefully placed to avoid the entrapment of air.

Pumping is usually employed although conventional ‘letter box’ type shutters can be used on smaller pours. When pumping, the delivery hose should be at a low position in the pour to allow the air to be displaced and should be steadily raised as the pour height increases. However, in overhead pours this is more difficult. Additional concrete may need to be broken out and a second pipe used in the highest part of the broken out area to allow the entrapped air to escape.

Sprayed concrete

Sprayed concrete has been successfully used in many applications, including: bridge soffits, beams, parapets and abutments; steel and reinforced concrete framed buildings; cathodic protection overlays; cooling towers; industrial chimneys; tunnels; water-retaining structures; jetties, sea walls and other marine structures. It is often chosen to repair fire-damaged structures as it can be applied quickly and economically to the large areas typically involved.

Sprayed mortar or micro-concrete can be applied via two methods:

·         Dry process.

·         Wet process.

In the dry process, the dry constituents (i.e. sand, aggregate, cement, additives) are batched together on-site, or pre-bagged, before being conveyed by compressed air down the delivery hose to the nozzle, where pressurised water is introduced and the mix projected into place.

In the wet process, the constituents are batched and mixed together before being fed (wet) into the delivery equipment or pump. The mix is then conveyed under pressure to the nozzle, where compressed air is injected to project the mix into place. 

Further information on patch repair can be obtained from the Guidance Document