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| Mobil Oil Corporation and its partners completed construction of the large offshore Hibernia concrete offshore platform in November 1996. That platform is currently in-service on the Grand Banks, offshore Newfoundland, 315 km south-southeast of St. John's, Newfoundland and approximately at the same latitude as the Treat Island Exposure Station. Four large concrete blocks (600 by 900 by 900 mm) were cast from the production concrete used for the splash zone of the outer perimeter of the structure and were installed at Treat Island in October 1996.While in-service, the Hibernia platform concrete in the splash-zone will be subjected to continual wetting and drying, freezing and thawing during the winter season, and abrasion from floating debris, particularly ice in the late spring and summer as it moves down from the Arctic Circle. The location of the structure and its exposure to the difficult wave action of the North Atlantic, make it extremely difficult to do proper evaluations of the concrete in the structure while in-service.The Treat Island blocks are to provide information on the general durability of the concrete with the passage of time.The blocks are accompanied by several 150 by 300 mm cylinders of the same concrete. The concrete is air-entrained (4.6 percent) and has a hardened density of approximately 2,170 kg/m3. | This reduced density was achieved by using a blend of normal weight coarse aggregate (crushed granite) and structural lightweight aggregate (expanded slate) at a ratio of 45 percent by volume lightweight aggregate. This type of concrete is commonly referred to as Modified Normal Density Concrete (MNDC). The sand is a crushed material resulting as a by-product of the coarse aggregate production. The cement is a blended cement consisting of ordinary Portland cement (Type 10) and silica fume (8 to 9 percent by weight). The total binder content was 470 kg/m3. The water/binder ratio was 0.32 by weight.All concrete contained high-range water reducing admixtures (HRWRA) and normal watereducing admixtures (WRA) in addition to the air-entraining r admixture (AEA). One-year cylinder compressive strengths averaged 76 MPa. In addition to the large blocks and their companion cylinders, 32 additional cylinders (150 by 300 mm) from the mixture development program for the project were placed at Treat Island in July 1997. Variables for these cylinders include all lightweight coarse aggregate, changing the HRWRA from a sulphonated naphthalene based product to a melamine based product, using another Canadian cement that inter-ground the silica fume with the clinker rather than blending it in after grinding, and the effect of both pumping and pumping plus consolidation on the MNDC. |