REPAIR OF CONCRETE STRUCTURES BOOK DOWNLOAD Before any repair work is put in hand, the cause of the damage must be identified as clearly as possible.
REPAIR OF CONCRETE STRUCTURES BOOK DOWNLOAD
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General considerations
Before any repair work is put in hand, the cause of the damage must be identified as clearly as possible. This principle may seem self-evident but it is surprising how often it is disregarded, with the result that further repairs have to be carried out within a short time.
Sometimes the cause is obvious, for example, in many cases of accidental damage, but careful investigation is required more often than not.
The next step must be to consider the objective of the repair, which will generally be to restore or enhance one or more of the following: Durability Structural strength Function Appearance.
Of these four requirements, restoration of durability is by far the most common in repair work. One must also consider whether the repair is to be permanent or temporary.
Only after deciding on the most likely cause of damage, whether it is likely to recur, and the purpose of the work, should the method of repair be chosen.
- The basic choice of repair systems is between those based on Portland cement and those based on synthetic resins. In reinforced concrete, they protect the reinforcement from corrosion in different ways.
- Cement-based materials provide an alkaline environment for the steel (pH of the order of 12) and, in these conditions, a passivating film forms on the surface of the steel.
- Corrosion will occur if the alkalinity of the concrete surrounding the steel is reduced by carbonation—i.e. penetration of carbon dioxide from the atmosphere—or if aggressive ions such as chlorides are present
- Consequently, the provision of an adequate thickness of the dense concrete cover is important. Resin-based materials do not generally provide an alkaline environment: they normally rely for their protective effect on providing cover that will exclude oxygen and moisture, without which corrosion will not take place.
- It is usually desirable that the mechanical properties of the repair material should resemble as closely as possible those of the structure being repaired. This means that, as a general rule, careful consideration should be given to the use of cement-based repairs.
- They can be made relatively inconspicuous although it is very difficult to hide them all together without using an overall coating. They can provide fire resistance, while resins soften at relatively low temperatures. Cement is cheaper than resins but this is seldom the deciding factor because labour usually accounts for a large proportion of repair costs.
- Most building operatives are more familiar with the use of cement-based material than with resins. For some applications, however, resins are more suitable. Their properties can be adjusted within fairly wide margins by suitable formulation so that they can, to some extent, be tailored to fit the job in hand.
- This is particularly valuable when working time is limited and rapid curing is required. Sometimes the thickness of cover to reinforcement is less than it should be and it may not be possible to increase the dimensions of the member or to move the reinforcement.
- In these cases resin mortars may provide less permeable cover then cement mortar although the permeability of the latter can be reduced by incorporating polymer admixtures.
- Sometimes feather edges cannot be avoided and, although polymer admixtures may make it possible to use cement mortar patches, resin mortars are often more suitable.
- Some compounds are not suitable for use in confined spaces and good ventilation is always desirable (6, 7). Heat is evolved while resin compounds cure and, in general, the faster the cure the greater the rate of heat evolution.
- This may not be important in the case of a small patch from which heat can escape easily, but there may be a considerable temperature rise in a patch with a small ratio of surface area to volume, and this may give rise to unacceptably high thermal stresses as the material cools.
- For a similar reason too great a volume of material should not be mixed in one batch. The rate of reaction is both exothermic and temperature-dependent
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