International Concrete Abstracts Portal

Recently, there has been a great demand for high-quality concrete and concrete structures with high performance. In this context, silica fume is one of the most remarkable mineral admixtures that can give concrete high performance, such as high workability, strength, and durability. However, it is unclear as to the types of form silica fume takes in concrete, mortar, and cement paste. Some researchers point out that silica fume may be in high agglomeration. Therefore, it is very important to disperse silica fume in concrete effectively to get high-performance concrete. Consequently, this paper deals with the effect of physical treatment (ultrasonic homogenizer) and chemical treatment (superplasticizer) of silica fume on the properties of mortar. In this study, different silica fumes were used, one Japanese and five imported. The investigated properties of mortar were workability (flow values), compressive strength, and total pore volume. The study resulted in the following conclusions: 1) Silica fumes in the Japanese market were highly agglomerated in the natural state. This agglomeration of silica fume can be broken up by using some treatment methods, such as ultrasonic homogenizer and superplasticizer. 2) Physical treatment (ultrasonic homogenizer) before mixing mortar was useful to improve compressive strength and to decrease total pore volume of mortar containing silica fume. The use of superplasticizer could result in highly workable mortar. 3) The effectiveness of ultrasonic homogenizer treatment and that of superplasticizer treatment are different.

The latest developments in concrete platform concepts for deep water and floating structures have indicated the need for further development in the field of practical concrete technology. Paper presents some of the most significant factors in this challenge such as increased compressive strength, improved workability, and stability of fresh concrete, use of high-strength lightweight aggregate concrete, measures to improve the concrete E-modulus, and utilization of variable concrete density to optimize the platform design. This has been achieved through further development of the constituent materials, refinements of the mix design, and advancements in production methods, as well as the use of high-quality lightweight aggregates.

The troll GBS platform is the world's largest concrete offshore concrete platform. The platform is designed for an operational lifetime of 70 years and will be installed in the North Sea during 1995. To improve the buoyancy of the platform during tow-out to the field, a concrete mixture with reduced density has been developed, providing a characteristic 28-day cube compressive strength of at least 75 MPa and an in situ density of 2250 kg/m 3. The weight reduction has been obtained by partly replacing the natural coarse aggregates by high-quality lightweight aggregates. The concrete is denoted as modified normal density (MND) concrete. The modification was expected to reduce both compressive strength, Young's E-modulus, and material ductility to some extent. A comprehensive testing program comprising laboratory tests and full-scale tests has been performed to investigate and to document all relevant concrete properties related to mechanical, durability, and constructibility performance of the concrete. A secondary purpose of the investigations has been to evaluate the possibility of retaining the mechanical properties of the original normal density concrete by replacing the remaining coarse granite aggregate with a more rigid quartz-diorite aggregate. The laboratory investigations included the determination of the following concrete properties: fresh concrete properties, compressive strength development, compressive strength at sustained load, compressive E-modulus, tensile strength and E-modulus, stress-strain in compression, fatigue, fracture energy and characteristic length, shrinkage, creep, water intrusion, and alkali-silica reactivity.

The Hibernia offshore concrete platform is being constructed in Newfoundland, Canada, and will be used in hydrocarbon production on the Grand Banks off the east coast of Canada. The 111-m tall concrete structure will contain approximately 165,000 m 3 of high-strength concrete. Construction of the concrete platform through 1993 consisted of a 108-m-diameter base slab that rested on a series of precast and cast-in-place concrete skirts. Specified 28-day compressive strengths (cylinder) for the skirts and base slab were 49 and 69 MPa, respectively. Actual average compressive strengths achieved were73.8 and 81.7 MPa, respectively. The remaining construction will be completed by 1996. The use of two different concrete production systems and their results are described.

When a damaged concrete slab is repaired, new concrete will generally be placed in a downward direction. Form panels must be set up first under the damaged concrete slab, and then concrete must be placed between the form panels and the damaged slab. However, compaction by vibrator is needed in most concrete (slump: 8 to 21 cm), and therefore concrete placement under a damaged slab that needs repair is impossible realistically. A repair method using high-performance concrete without vibration or compaction was developed. The method was applied to the repair of concrete slabs that had been damaged by heat. It was confirmed that this repair method was easier and that repair was possible while the slab was in use. In addition, the costs and construction period were reduced greatly by this repair method.