International Concrete Abstracts Portal

SP-158 The Building Code Requirements for Masonry Structures will provide solutions to the most complex masonry construction questions. Replacing all existing masonry code, these important new ACI/ ASCE documents have been especially prepared to facilitate adoption, by reference, in a general building code. They have been adopted by B.O.C.A. and SBCC (model code groups). The code contains: a standard construction specification as part of the design standard; provisions for analytical and empirical design; provisions for seismic design; and provisions for design of brick and block composite and cavity walls. Other topics covered by the new code include: permits and drawings; quality assurance; materials; placing embedded items; strength and serviceability; flexural and axial loads; shear; walls; columns; pilasters; beams and lintels. The quality, inspection, testing and placement of materials used in construction are covered by reference to specifications and the appropriate ASTM Standards.

The tumulus (an earthen mound) disposal concept can provide a major means for the disposal of low-level radioactive waste (LLRW) provided the concrete structure of the tumulus disposal units is designed and fabricated for the long- term durability. The concrete used in an experimental disposal facility, Tumulus II, was designed to have an excellent resistance to frost attack and a very low permeability to chloride ions. The present study reports numerous research results, including those from an accelerated alkali-aggregate reactivity test (Accelerated Concrete Core Method), which showed that a local coarse aggregate was potentially reactive to alkali. The reactivity to alkali was substantially reduced by incorporating 30 percent fly ash (Class F) by weight of cement. Additional studies were performed on field concrete samples incorporating nine percent silica fume by weight of cement which showed effective reduction in alkali-aggregate reactivity. Expansion mechanisms of the local coarse aggregate and reference alkali- carbonate reactive Pittsburgh aggregate in concrete were studied by digesting the powdered aggregates under the accelerated test condition (1.0 N NaOH solution and 80 C) and monitoring clay mineral phases in the aggregates with x-ray diffraction (XRD) analysis at various digestion ages. The results showed that transformation of non-expansible clay phases (vermiculites/smectites) could occur in a highly alkaline environment which is typical of many concrete pore solutions. The expansible clays thus formed are, at least in part, responsible for the expansion of concrete cores containing the local coarse aggregate and Pittsburgh aggregate, as observed by the accelerated alkali-aggregate reactivity tests.

Several engineered disposal technologies involving concrete structures have been proposed for low-level radioactive waste disposal. The long-term performance and behavior of reinforced concrete structures in disposal units have been examined. Under most conditions, the reinforcing steel and concrete work well together to withstand the natural forces. Under certain conditions, however, the reinforcement and concrete may be subject to environmental attack which may cause degradation of the reinforced concrete. Water infiltration through the structure may increase as a result of cracking and increasing permeability and thereby increase the potential for contaminant release. The model for estimating time to onset of reinforcing steel corrosion due to presence of chloride is presented. Requirements for design of reinforced concrete structures for low-level radioactive waste disposal facilities are suggested.

Interest in the use of reinforced concrete structures in LLW disposal facilities has preceded the development of a comprehensive understanding of the long-term performance of these disposal technologies. With this in mind, Rogers and Associates Engineering Corporation has developed a new assessment computer model, restrict, that adopts a more complete, mechanistic approach to modeling concrete degradation, groundwater infiltration, leaching, and radiological risks.

The cement/silicate method of solidifying wastes was investigated. Emphasis was placed on the interaction between aqueous sodium silicate and portland cement hydration reactions. A definition of the role which the alkali- silicate plays in increasing the ability of cement hydration reactions to immobilize waste ions was the principal objective. Characterization relied upon calorimetry, X-ray diffraction, microstructural examination by scanning electron microscopy, and monitoring strength development of the waste forms. Increasing additions of sodium silicate to cement pastes accelerate hydration reactions, specifically the hydration of C 3 A and C 3 S, and decrease the presence of portlandite. Effects on compressive strengths of cement pastes were varied; at a water-cement ratio of 0.83, strengths increased with moderate sodium silicate additions, while at higher water-cement ratios, sodium silicate additions decreased strengths.