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Background. Proprietary concrete repair materials—primarily pre-bagged mixes
to which you add a fixed amount of water—are becoming more popular for repair of
structural concrete for a couple of reasons. First, most of these products are
produced using fairly strict quality control measures. This means that
performance of the finished product is consistent and predictable. Second, an
increasing number of products are available for a wide range of applications.
Selection of Materials. Consider several material properties when selecting or
reviewing a particular repair product. These include strength, permeability,
adhesion (bond), corrosion resistance, and application method. Base selection of
a repair material on its intended purpose. Repairs that will carry loads, such
as bearing repairs, need a higher strength. Repairs for protection of
reinforcing steel, such as restoring concrete cover, don’t necessarily need high
strength, but they should have adequate bond and lower permeability. The key to
selecting an appropriate repair material is to understand its purpose in the
repair.
Many products are capable of obtaining very high strengths, some at a very early
age. Strength is often the best selling point for various products. In general,
the higher the early strength gain, the lower the service life of the repair.
This is because the ingredients used to obtain high early strength (lots of Type
III cement, accelerators, water-reducing agents, etc.) can result in repair
concrete with a higher potential for shrinkage cracking and more brittle than
the concrete it is repairing. Use a material with strength appropriate for the
type of repair.
Some products produce very dense, low permeability patches suitable for repairs
in marine environments or where exposure to salt is high. Other products contain
corrosion inhibiting admixtures that can reduce corrosion of steel in the new
concrete. Without additional mitigation measures, these products often
accelerate corrosion in the surrounding original concrete (referred to as
"patch-accelerated corrosion" or "halo effect"). Adhesion is also an important
property to ensure that the repair material will adhere to the original
concrete. Keep in mind that adhesion is most affected by the quality of the
original concrete. No repair product, no matter how well made, will adhere to an
unsound substrate.
Application method is another key factor to consider. Repair products can be
applied by several methods: forming and pouring, pressure grouting, troweling,
or pneumatic methods (shotcrete). The 2004 specification for Concrete Structure
Repair (Item 429) refers to a new DMS-4655, "Rapid-Hardening Cementing Materials
for Concrete Repair." This document, which will be online, will list
pre-reviewed repair products by application: vertical or overhead, horizontal
applications such as decks, and pneumatic methods. The list will not be
comprehensive but can be used as a guide to review products not on the list.
Material Application. Most if not all proprietary repair products have specific
mixing and placing requirements. These are usually printed on the bag but can
come as a separate sheet of instructions. Follow these instructions exactly as
stated. Even slight deviations can result in poor performance of the repair. The
instructions will also identify limitations on the use of the product. TxDOT
encourages major suppliers of repair products to provide their instructions in
English and Spanish.
Surface preparation is the most important step in the repair process. Failure to
remove all unsound concrete will result in a repair that will not perform as
desired. The American Concrete Institute (ACI) and the International Concrete
Repair Institute (ICRI) both have excellent references for evaluating the
quality of the prepared substrate.
Need More Information? Contact the Bridge Division’s Construction and
Maintenance Branch at 512-416-2232 for information on repair products and their
application. Engineers in this branch are experienced in concrete repairs and
familiar with most of the repair products on the market.
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