Structure Repair with Advance Cast in Place Concrete Types
Cast-In-Place Fiber-Reinforced Concrete
When metallic or polymeric fibers (made up of steel, glass or synthetic and natural plastics) are added in conventional concrete to get greater resistance to plastic shrinkage, drying shrinkage, and cracking, the resulting material is called fiber-reinforced concrete. Fiber-reinforced concrete is placed for repairs using conventional and shotcrete methods.
Although the fibers are not generally used as the main reinforcement, these can be used to provide sufficient reinforcement in thin overlays where the thickness is not enough for placement of steel reinforcing bars. The function of this fiber reinforcement is not to resist the applied load but is to increase durability, reduce cracking of repair material due to plastic shrinkage, increase in shock / vibration strength, and increase in blast resistance.
The addition of fibers can reduce the slump and can make the placing and compaction of concrete more difficult. Rust stains may also be produced at the surface of steel fiber-reinforced concrete due to corrosion of fibers.
Cast-In-Place Low-Slump Dense Concrete (LSDC)
Low-slump dense concrete (LSDC) is a special type of conventional concrete having a moderate-to-high cement factor, a w/c less than 0.40, and having slumps of 50 mm or less, which gains strength quickly, has high density and possesses reduced permeability. LSDC provides increased resistance to chloride-ion penetration when tested according to ASTM C 1202. LSDC is used as an overlay / wearing course in a repair to obtain abrasion-resistant and protective layer with a durable concrete surface.
LSDC requires the use of a high-range water-reducing admixture (HRWRA), high compacting efforts and at least 7 days of continuous moist curing to obtain adequate hydration. Galvanic corrosion, drying-shrinkage cracks, chloride-ion intrusion, etc., are some of the problems associated with LSDC. Its use must be made if these environmental effects are minimal.
Cast-In-Place Silica-Fume Concrete
Silica-fume concrete is a normal Portland-cement concrete that is modified with a silica fume and is used as an alternative to latex modified concrete (LMC) or low-slump concrete. Silica fume is a by-product in the manufacture of silicon and ferrosilicon alloys. It is an efficient pozzolanic material and is normally added to concrete in quantities of 5 to 15% by mass of cement to obtain compressive strengths of 85 to 105 MPa. Adding silica fume and a HRWRA (high range water reducing admixture) to a concrete mix significantly increases compressive strength, decreases permeability, increases density, increases chemical resistance of concrete, acts as cement replacement material, improves performance in a wide range of applications and increases abrasion-erosion resistance.
The high strength of silica-fume concrete and the resulting abrasion-erosion resistance provide an economical solution to abrasion-erosion problems in repair of hydraulic structures, particularly where available aggregate may result in unacceptable concrete in this respect. Silica-fume concrete is also used extensively in overlays on parking structures and bridge decks to reduce the intrusion of chloride ions into the concrete.
Problems with silica fume concrete can be more drying and plastic shrinkage crackling, significantly greater cost, difficult to get steel trowel finish due to no bleed water, early required start of wet curing, minimum wet curing for 7 days and maintaining curing temperatures above 4 °C. The addition of silica fume to the mixture makes it sticky, and thus more difficult to finish.