International Concrete Abstracts Portal

Describes the results of marine durability studies carried out on concretes containing high-alumina cement (HAC) and ground granulated blast furnace slag (GGBFS) blended cements. Concrete cubes of 100 mm were initially cured for 28 days at 5, 20, and 38 C in water and in air at 20 C prior to their storage in the different marine environments. The specimens were exposed for up to four years in spray, tidal, and full- immersion zones at the Building Research Establishment's marine exposure site on the Thames estuary at Shoeburyness. Chloride penetration data down to depths of 36 mm were determined and evidence of frost damage sought in these non air-entrained concretes after four years of marine exposure. All HAC/GGBFS concretes performed well in terms of their low chloride ingress and excellent frost resistance, irrespective of early curing temperature or marine exposure zone. Most of the plain HAC concrete performed equally well, with the exception of the converted specimens, pre-cured at 38 C prior to storage in seawater. These concretes were frost resistant, but showed some signs of chemical attack and had high levels of chloride at 36 mm depths. The practical significance of these data is discussed.

Steel reinforced concrete specimens, with nominal strength grades of 25, 35, and 45 MPa and different fly ash levels (0 to 50 percent), were exposed to various curing treatments during the first 28 days prior to exposure in the tidal zone of a seawater exposure site. Chloride concentration profiles and reinforcing bar weight losses were measured for specimens after one, two, and four years exposure. Chloride profiles were also measured for specimens after 28 days immersion in seawater under laboratory conditions. Fly ash concretes showed substantially increased resistance to the penetration of chlorides compared with control specimens. The improved resistance of fly ash concrete to the penetration of chlorides resulted in reduced corrosion of steel bars embedded in the concrete. Threshold chloride levels for corrosion were estimated from relationships between steel weight loss and chloride content at the location of the steel. The threshold concentration decreased with increasing fly ash content. Chloride concentration profiles after 28 days of immersion in seawater showed that considerable chloride penetration occurred during this period due to sorption (capillary suction) of the seawater into the unsaturated specimens. This results in a significant error in diffusion coefficients calculated from the concentration profile using the standard solution to Fick's second law. The error may be substantial for fly ash concretes in which the penetration of chlorides due to sorption in the first few days far outweighs subsequent diffusion during continued seawater exposure. Alternative methods of solution that take account of initial sorption effects are presented.

Long-term research studies on the effectiveness of corrosion protection systems for reinforced concrete exposed to aggressive environmental conditions of the United Arab Emirates have been in progress at the exposure site adjacent to Dubai Creek since December 1991. These studies cover reinforcing bar coating systems and different products comprising pore blocking admixtures and a penetrating surface sealer. The parameters included in the studies are two water-cement ratios of 0.44 and 0.6, two types of curing regimes (laboratory conditions and actual construction practices), two concrete reinforcing bar covers (10 mm and 30 mm), and three site exposure conditions above ground, below ground, and in the tidal zone). The effectiveness is assessed through accelerated laboratory tests and site exposure tests. The tests performed at different ages include compressive strength, water absorption, water penetration, capillary rise and chloride ingress, crack appling, and measurements. Also conducted were electrochemical testing comprising half-cell potential, resistivity, linear polarization, corrosion current, and AC impedance. One-year and two-year results have been previously published; the present paper updates the findings, emphasizing the electrochemical testing results. The results to date show that the performance of the epoxy coated reinforcing bar is encouraging, while the performance of the most of the products studied is not satisfactory.

To determine the most effective end-anchorage protection for post- tensioned beams, 20 air-entrained post-tensioned concrete beams were fabricated and placed at the Treat Island severe exposure station in 1961. The beams were fabricated using four types of post-tensioning systems with 12 different types of end-anchorage protection over external and recessed anchorages. End- anchorage protection was applied to the beams using six different types of joint preparation: bush-hammering, epoxy adhesive on sandblasted concrete surface, retarding agent, sandblasted, sandblasted with primer, and no preparation. The end protections were made from three different mixtures: portland cement concrete, epoxy concrete, and sand-cement mortar. The 20 post-tensioned test beams have been inspected annually by the Corps and biennially by other interests. Eleven of the beams were autopsied at the Waterways Experiment Station (WES) in 1973-74 and 1983. This paper assembles the data obtained, evaluates the data, and summarizes the important findings related to durability. The main findings are that the flush (recessed) anchorage protection using portland cement concrete is the superior detail. The external portland cement concrete anchorage protection, properly anchored with reinforcing steel across the join with adequate concrete cover, also provides an effective protection.

The use of dry-mix shotcrete for the repair of structures in a marine environment was the subject of an investigation carried out for the Canadian Coast Guard. In addition to the control mixture, four different mixtures using two aluminate-based accelerating admixtures at two different dosages were prepared. All mixtures contained steel fibers and silica fume and were air- entrained. For each mixture, a panel of approximately 1.0 m x 1.0 m was shot on a wharf (in the St. Lawrence River north of Quebec City) to study the influence of freezing and thawing cycles in the presence of salt water, the influence of wetting and drying cycles, and the abrasion due to ice packs. Three panels were also shot with each mixture for laboratory testing purposes (mechanical strength, durability, and microstructure). The accelerating admixtures that were used were not found to have any adverse effects on any of the properties of the hardened shotcretes. The initial setting time was reduced to values below six minutes. For all five mixtures, the resistance to scaling due to freezing in the presence of deicer salts (ASTM C672) was found to be not very good. This confirms the results of previous tests which have also shown a negative influence of silica fume on the scaling resistance of dry-mix shotcrete measured in the laboratory.