International Concrete Abstracts Portal

The moisture condition of the substrate surface prior to repair influences the plastic shrinkage and resultant physical properties of the repair material after repair. Absorption of moisture by the substrate may cause not only plastic shrinkage of the repair material adjacent to the interface, but also expansion at the surface to be repaired due to uptake of moisture and may lead to the premature debonding of the repair material. In this investigation, substrate prisms of four different thicknesses (30, 48, 66 and 1OOmm) under three different moisture condition (oven dry, natural dry and saturated surface dry) were employed to investigate the absorption and expansion of the substrate when it is immersed in water or comes into contact with the freshly cast repair material. Based on the test results obtained, a suitable pre-wetting time before repair work is begun is proposed.

Ferrocement, a thin structural composite material, exhibits better crack resistance, higher tensile strength-to-weight ratio, ductility and impact resistance than conventional reinforced concrete. These properties have been exploited through the use of ferrocement for structural upgrading and the rehabilitation of concrete structures. This paper highlights over 20 years of research and innovative applications of ferrocement by a team of researchers at the Centre of Construction Materials and Technology at the National University of Singapore. A number of successful commercial precast applications of ferrocement for use in the upgrading of building fixtures and services have resulted from the research carried out. Salient features of the design, construction and performance of ferrocement structural elements used in upgrading and rehabilitation are discussed briefly in this paper. Experimental studies into the durability, strength and impact properties are also discussed. Once viewed as a labour intensive construction material, the production of ferrocement precast elements through mechanization have rendered the material competitive and economically viable for use in construction, upgrading and structural rehabilitation in land scarce Singapore with its high labour cost.

Protective coatings are often used to waterproof their substrate, to increase the durability of concrete, or simply to improve the aesthetics of a structure. However, there are side effects. For example, coatings that shed water would inhibit the curing effect that could be derived from subsequent wetting due to outdoor exposure. A protective coating could also increase the carbonation of its substrate since it provides a favourable dry environment for carbonation to proceed. This paper presents long-term results to illustrate these two ‘side effects’ on the use of protective coatings.

Durability limitations of steel reinforced concrete are well documented. Corrosive environments (e.g. the presence of chlorides), carbonation of concrete structures, poor workmanship and other factors can cause corrosion of the reinforcing steel. Normally, reinforcing steel embedded in concrete is protected because the concrete cover acts as a barrier and the high pH value of the pore fluid assures a passive state. Both the presence of chloride ions at concentrations above a given threshold level and carbonation can put reinforcing steel into an active state and result in corrosion rates that markedly decrease the expected service live of reinforced concrete structures. Aqueous surface-applied corrosion-inhibiting impregnations are featuring the ability to reduce the corrosion rate of corroded steel reinforcement embedded in hardened concrete due to their corrosion-inhibiting action and in the case of a carbonated concrete structures by realkalization. Additionally the corrosion rate is further reduced due to the water repellent action of organosilicone compounds. The careful selection of hydroxyalkylamino compounds as well as of inorganic-or organic acid compounds allows the formulation of corrosion-inhibiting impregnations with high buffer capacity. When applied on the surface of a concrete structure, said corrosion-inhibiting compositions are capable of penetrating into the concrete, and raising the pH value of the pore fluid in the vicinity of the reinforcing steel to a level, where the corrosion rate is markedly reduced. Laboratory tests and results from field applications are reported.

Roller-compacted concrete (RCC), because of its superior properties, Synopsis: low cost, rapid and relatively simple construction methods, and proved performance, has been used extensively in recent years in rehabilitation projects at existing concrete and earth and rockfill embankment dams and related hydraulic structures. These applications include increasing the existing spillway capacities, construction of new service and emergency spillways, overtopping protection, and seismic strengthening. This paper presents case histories of selected applications of RCC in rehabilitation of dams and related hydraulic structures. These case histories are presented to show the range of previous applications and to illustrate typical design and construction practices in repair and rehabilitation of dams with RCC. For each of the case histories presented in this paper, an attempt is made to discuss (a) the description of the project, (b) the cause and extent of the deficiency that necessitated rehabilitation, (c) design details, (d) RCC mixture proportions, (e) description of materials, equipment, and placement procedures, (f) cost, and (g) RCC performance to date.