International Concrete Abstracts Portal

Concrete repair materials applied in thin layers often fail under severe weathering conditions and high loading due to sensitivity in the bonding area to water, alkalinity, and mechanical strain. High-strength concrete, with its dense cement matrix, makes it even more difficult to connect repair materials to the old concrete. More than 15 years of experience in development and use of different systems for repair of high-strength concrete has shown that cementitious mortars with modification by high amounts of superplasticizers perform best. Practical aspects of application are shown on a large project carried out on a high-strength concrete floor in an airplane hangar. Cementitious repair systems are suitable for any kind of high-strength concrete repair where adequate surface preparation and the application of a special cementitious bridging agent is provided, but have to be adopted to the individual job site conditions. Shrinkage compensation techniques and sophisticated curing methods have to be used to achieve improved results with respect to drying shrinkage cracking. The durability of high-strength floor repairs with new technologies, used on a large scale in Europe, has proved to be reliable even under severe service conditions.

Results of studies on the application of a high-strength concrete, with compressive strength of 42 to 60 MPa, to a high-rise reinforced concrete residence are presented. First, experiments were performed in accordance with the construction procedure, applying full-scale test structure modeling on part of the actual building. As a result, workable high-strength concrete was achieved by using a high-range water-reducing agent at the plant where concrete is being manufactured, and by adding a superplasticizer and placing the concrete carefully on site. In addition, for the quality control method of a ready-mixed concrete, water-cement ratio measurement before placement was useful. It is desirable to control the structure strength of high-strength concrete by not only using a test specimen cured in water on site, but also by taking out core specimens. Secondly, requirements for a construction method were set, by reference to the test results, and construction of the actual building was undertaken. Results of all the tests satisfied the requirements necessary to demonstrate the stable manufacturing control of ready-mixed concrete.

A National Project lasting five years has been promoted by the Ministry of Construction of Japan since 1988 to develop super high-rise reinforced concrete buildings in seismic zones. The strength of concrete and reinforcing steel bars ranges from 30 to 120 MPa (4.3 to 17.4 ksi) and from 400 to 1200 MPa (58 to 174 ksi), respectively. The following is investigated in the Project: 1) production, quality control, and placement of high-strength concrete; 2) production of high-strength steel bars; 3) mechanical properties of high-strength concrete and steel bars; 4) behavior of members and subassemblages; and 5) structural design methodology.

A 5-year National Research Project on advanced concrete buildings with high-strength and high-quality materials has been in progress in Japan since 1988. A High-Strength Concrete Committee was organized to establish guidelines to be used in applying the high-strength concrete of 30 to 120 MPa to reinforced concrete buildings; it has started to investigate the following items: development of cements, aggregates, chemical admixtures, mineral admixtures of high-strength concrete and establishing of the quality standards of these materials and the design method of mix proportion; establishing the evaluation method for properties of fresh concrete required in construction; establishing of evaluation methods for compressive strength and other properties of hardened concrete; and establishing of the quality control procedure and evaluation method for concrete strength in structures. Paper describes the problems of production, transportation, and placement when high-strength concrete is applied to reinforced concrete buildings standing in seismic zones and urban areas such as Tokyo. The results obtained from the preliminary studies and experiments by the high-strength concrete committee will also be briefly described.

Two test methods were used to investigate the frost resistance of high-strength concrete with and without air-entraining agents: a volume deterioration method (ASTM C 666) and a salt-scaling method (SwedishStandard SS137244) similar to ASTM C 672. In addition, low-temperature calorimetry was used to measure ice formation in concretes after a drying/resaturation treatment. For concretes with 0 and 10 percent silica fume contents and water-binder ratios from 0.40 to 0.25, the calorimetry results showed only very minor ice formation down to 20 C. The cement used was a high-strength type (Norwegian P30 4A). This result contrasts an earlier calorimeter result with ordinary portland cement, and indicates that the P30 4A cement produces a more finely divided capillary pore structure. The salt-scaling tests showed that the high-strength concrete with water-to-binder ratios less than about 0.37 exhibits acceptable resistance to salt-scaling, even without air entrainment. The ASTM C 666 test results showed relatively severe damage to concretes with water-to-binder ratios down to 0.28. No air-entrained concrete was tested with ASTM C 666. This result is in apparent conflict with the calorimetry results and suggests that the damage may be related not to ice formation but to thermal fatigue effects caused by differences that are too large between the thermal expansion coefficients of aggregates and binders.