Spalling concrete

Repair of spalled concrete by use as an adhesive and protective coating... 

The aim of the visual survey is to give a first indication of what is wrong and how extensive the damage is. If concrete is spalling off then that can be used as a measure of extent of damage. In some cases weighing the amount of spalled concrete with time can be a direct measure of the deterioration rate. In most cases loose concrete should be removed to avoid safety problems. 

Cracked and spalled concrete - this can distort the current path or give low readings due to poor contact between steel and electrode. 

Before patching carbonated concrete, the cracked and spalled concrete must be removed from around the rebar or as far as the carbonation front goes which ever is the lesser. The cementitious patch material is chosen to ensure that the steel is back in a high pH, alkaline environment. This will encourage the reformation of the passive layer to stop further corrosion. 

Although not strictly used as prime structural adhesives, polymer latices or dispersions have been included here because of their increasing use as aids to bonding in the patch repair of spalled concrete. They usually take the form of a polymer-cement slurry which is applied to the moistened and prepared concrete surface. In general, the repair mortar must then be applied before a film... 

Replacement of damaged and spalled concrete with high-strength repair mortar... 

Once the source and cause of the deterioration have been identified, corrective action can be taken. There are several accepted procedures and methods for repairing cracks in concrete structures, spalled concrete floors, and pavements. Table 5.65 outhnes several types of materials for such repairs. The materials described also can be used for repair of masonry members. 

A spalled concrete surface is the beginning of continued disintegration of the concrete. If the cost of replacement is at all comparable with the cost of repair, replacement is recommended. Replacement is also recommended when changing the level of the final surface is impractical. Otherwise, the concrete can be resurfaced with new concrete, epoxy topping, latex mortar, or iron topping. 

Figure 14.10 General view (a) and detail (b) of spalling concrete coating on water pipe. (Courtesy of Mark Lewis, East Bay Municipal Utility District)...

The corrosion of buildings and concrete structures is a major area of concern to engineers and builders. Repairs to concrete structures are essential to maintain their integrity. The design of the structure should have sufficient accessibility for repairs as shown in Fig. 8.47. During repair, spalled concrete is taken out ensuring that the salts have been sufficiently removed and the steel is cleaned. The concrete is replaced by a suitable mortar or concrete with proprietary additives. It has been observed in recent years that epoxy coated reinforcement provides an excellent protection against reinforced concrete corrosion. 

Two basic approaches to concrete repair have been followed. The first repair methodology utiUzes concrete or other cementitious materials alone. Essentially, these procedures involve the removal of loose, spalled concrete, followed by further systematic removal of the concrete surrounding the corroded rebar. Finally, the rebar and concrete surfaces are cleaned and primed before the new repair concrete is applied. The repair procedures thus create three different material zones that interact with the reinforcing steel (1) the old chloride-con-taminated/carbonated concrete, (2) the new concrete, and (3) the interface between the old and new concrete. The interface may represent a zone of weakness with respect to further ingress of corrosive species. 

Unfortunately, some concrete structures do corrode. When this happens, remedial action can include repairing the cracked and spalled concrete, coating the surface to prevent further entry of corrosive chemicals into the structure, and cathodic protection, an electrical means of corrosion control. 

The application of a CFRP lining system to a pipeline is a bond critical application, so the surface of the pipeline must be prepared sufficiently to ensure proper adhesion. Concrete surfaces are profiled using abrasive or water blasting to a minimum International Concrete Repair Institute (ICRI) concrete surface preparation CSP 3, as shown in Figure 1.3, whereas a metallic surface will be prepared using abrasive blasting to a near white metal surface profile. Surface contaminants (laitance, carbonated and weak concrete, cracked and spalled concrete) are removed and the substrate is repaired as necessary. The work requires the pipe surface to be thoroughly cleaned, dried, and dehumidified. 

Building durability, by preventing decay of wood-based materials, corrosion of metals, and spalling of masonry and concrete caused by freeze-thaw cycles... 

When mature concrete is contaminated by chloride, e.g. by contact with deicing salts, the cement chemistry is more complex, and less chloride is taken up by the cement hydrate minerals and a larger proportion is free in the pore solutions and can therefore pose a greater hazard. When embedded steel corrodes, the production of a more voluminous corrosion product pushes the concrete from the steel with resultant cracking and spalling of the concrete. 

An interesting new use of galvanised steel is for reinforcement of concrete. This reduces the risk of spalling and staining or can enable the depth of the concrete cover to be reduced leading to slimmer structures of lower cost. 

Brick Construction Brick-lined construction can be used for many severely corrosive conditions under which high alloys would fail. Brick linings can be installed over metal, concrete, and fiberglass structures. Acid-resistant bricks are made from carbon, red shale, or acid-resistant refractory materials. Red-shale brick is not used above 175°C (350°F) because of spalling. Acid-resistant refractories can be used up to 870°C (1600°F). See Table 25-10. 

Spalling Debris or fragments generated from the impacted face of a concrete or masonry wall. 

The more common type of spalling failure of concrete occurs when (and where) the transmitted compressive wave reflects from the free surface back face of the slab as a tensile wave, and the head of the reflected tensile wave and tail of the transmitted compressive wave combine to produce net tensile stress exceeding the dynamic tensile strength of the concrete. This process is shown schematically in Figure 21 for the simplified case of a plane, triangular compressive... 

The normal stress must be zero at the free surface, so a tension wave of a similar profile but opposite sign must start propagating in from the rear surfaces when the compressive front reaches this surface. The actual stress state shortly thereafter is shown in state 2 in Figure 21. When the tensile stress exceeds the tensile strength of the material, spall occurs on a plane parallel to the free surface. The normal stress then drops to zero again, and the process continues. In brittle materials weak in tension (such as concrete), it is possible for multiple spalls to occur before the reflected tensile waves decay enough to fall below the tensile strength. 

Design for Spalling. Breeching, and Fragment Penetration. Test results have been used to empirically derive relationships for the reinforced concrete thickness required to prevent spalling, breeching, or fragment penetration. 

If spalling is a hazard it can be eliminated with spall plates or by using the minimum concrete thickness, t (in). 

Extensive. The fiimace door was blown off, the roof was destroyed, and the back wall buckled. Some 2(KX) kg of aluminum spilled on the concrete floor with significant spalling. Personnel injuries resulted primarily from aluminum splash, not shock waves... 

Simple calculations of concrete structural fire resistance are not readily available. Above 300°C (572°F), the strength of concrete is reduced by 25% and should be discounted structurally (Spouge, 1999). Experiments on concrete slabs with hydrocarbon fires indicate that after 2 hours the outside 130 mm exceeds 300°C (572°F). Normally this concrete will spall away. 

If the concrete member is designed to have 4-hour fire resistance in a cel-lulosic fire test, as recommended in design guidance by the Institute of Structural Engineers and the Concrete Society, then 3-hour fire resistance would be expected in hydrocarbon pool fires, provided that spalled material remained in place (Spouge, 1999). 

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