International Concrete Abstracts Portal

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.

Many Corps of Engineers rubble-mound breakwaters and jetties have become permeable to sand transport and wave transmission, resulting in increased dredging costs, risks and delays to navigation, and damage to moored vessels by excessive wave activity. Some Corps coastal districts have applied grouting techniques for sealing these structures by using cementitious and chemical grouts for creating a vertical barrier through a series of vertical holes drilled along the centerline of the structure. To ascertain the effective useful life of such grouts, durability time-dependent tests were conducted by U. S. Army Engineer Waterways Experiment Station (WES) to determine how the materials would endure under near-actual field conditions. A cementitious mixture previously used at Buhne Point, California (Buhne Point Mixture), and a new mixture design (WES Mixture) developed by the WES Structures Laboratory (SL) were evaluated. Specimens were exposed at three weathering stations (including Treat Island, Maine) for the eight-year period 1987 to 1995. Nondestructive tests (ultrasonic pulse velocity and transverse flexural frequency) were conducted periodically during the eight-year evaluation period. Long-term durability exposure field tests revealed spalling of the Buhne Point Mixture due to freezing and thawing. However, nondestructive tests indicated the integrity of all specimens was maintained, as there appeared to be minimal changes in the properties of these cementitious grouts. Grout laced within the core of rock structures may not actually be exposed to the extreme conditions on the weathering platform at Treat Island. Either the WES Mixture or the Buhne Point Mixture may be used as grouting materials to rehabilitate existing Corps rubble-mound breakwaters and jetties by filling voids to prevent passage of excessive wave energy and sediment through such structures.

Presents results of a study on the effect of several types of coatings and surface treatmets on the corrosion activity on specimens containing U-shaped epoxy-coated reinforcing bars cast in concrete slabs. These reinforced concrete slabs were subjected to severe exposure conditions and evaluated after four years. To obtain results in a short time period, a high water-cement ratio of 0.6 and concrete cover of only 20 mm were used. Four different types of epoxy coatings were investigated, with some bars being coated before bending and some coated after bending. The reinforcing steel had been surface treated by the supplier in seven different ways before coating, with some of the bars receiving a primer coating before application of the epoxy coating. Also, the surfaces of some bars were contaminated with salt prior to coating. Some reinforcing steel bars were intentionally damaged with the damaged area being seven 6 mm x 6 mm square spots evenly distributed over the surface of each bar. The exposure conditions were a laboratory test chamber simulating a marine environment and a natural marine environment site at Treat Island, Maine. Assessment of corrosion activity was carried out using linear polarization resistance and open circuit potential techniques. A visual survey was done as well. The results indicated that undamaged epoxy-coated reinforcing steel bars performed very well, with no corrosion occurring after four years of severe exposure conditions with low strength concrete (w/c=0.6) and only 20 mm of cover. The test results indicate that, after four years of exposure, most of the various surface treatments or types of coating were equally effective in terms of long-term corrosion protection of the reinforcing steel. However, the exception was a salt contaminated surface, in which a corrosion rate of up to 50 percent of that for uncoated reinforcing steel was observed.

The concrete lining the outfall canal of Munmorah Power Station, built by the Electricity Commission of New South Wales (now operating as Pacific Power) in the mid-1960s, has been subjected to flowing seawater for 30 years. Two types of concrete, a portland cement concrete and a fly ash concrete, were used for the construction of the canal. This presented an ideal opportunity for a comparison to be made of the performance of the two binders in concretes which were subjected to the same aggressive environment. Limited information was available on the concretes from trial mixture records. A recent field investigation revealed similar chloride ingress into the two concretes in the tidal zone. This was so despite the fact that the fly ash concrete had a lower binder content than the portland cement concrete. As such, a lower strength grade and, hence, a fly ash concrete with higher water permeability can perform as well as a portland cement concrete. In the dry area above the high tide mark, the carbonation depth of the fly ash concrete was greater than the portland cement concrete. No corrosion was found in any reinforcing steel, as there was sufficient cover in both concretes to prevent the chloride ions or carbonation front reaching the steel. The effectiveness of a number of investigative techniques was evaluated during the investigation. It was found that the apparent chloride diffusion coefficients, determined from short-term immersion, the water permeability coefficients, and copper to copper sulfate half-cell potential measurements were poor indicators of the real long- term performance.

A series of hollow, cylindrical mortar specimens of 0.44 water-cement ratio were prepared without reinforcement and exposed to flowing natural seawater for up to 256 days. A direct electrical current of 2, 10, or 50 mA was impressed between the specimen core and exterior, using a pair of titanium or mixed metal oxide electrodes and a power supply. Embedded strain gage output and measurement of cylinder length and outside diameter as a function of exposure duration indicated that specific volume of the mortar increased with time with net expansion and expansion rate being greater for specimens exposed to current compared to baseline (no current) specimens. The cause of the enhanced expansion of the mortar in the presence of a direct current is discussed in this paper; the results are evaluated within the context of practical situations involving seawater exposure of cementitious materials.