International Concrete Abstracts Portal

This paper describes the repair of a high-strength silica fume concrete structure using a high-strength repair material that also contains silica fume. The repair represented the largest single application of this material and the largest single use of low-pressure spraying of the repair material. Information is provided on the repair procedures, proficiency of the nozzlemen, acceptance criteria applied to this type of operation. Because of the lack of actual in-situ bond and compressive strength data in the literature for high strength concrete repair materials, preliminary trials were conducted for material acceptance and to establish acceptance criteria for the actual repair. Approximately 1,300 m3 of the repair material was used to repair 24,000 m2 of concrete surface damaged during a slipform operation. The damaged concrete had compressive strengths in the range of 78 to 82 MPa. The repair material had a target compressive strength of 80 MPa and an in-situ bond strength requirement (minimum) of 1.5 MPa. Using low-pressure spraying techniques because of confined working areas, the repairs were successfully completed over a 24-week period. Compressive strengths of cores from sprayed production test panels averaged 85 MPa at 28-days. The in-situ bond strength of the repairs did not appear to increase with age and averaged 1.87 MPa for all ages evaluated.

The unacceptably high failure rate for concrete repairs is a major Synopsis: problem in the repair industry. To achieve durable repairs, it is necessary to consider the factors affecting the design and selection of repair systems as parts of a composite system. Compatibility between repair material and existing substrate is one of the most critical components in the repair system. This paper summarizes research initiated by the Corps of Engineers to develop performance criteria for cement-based repair materials. Results of laboratory and field performance tests were correlated to provide a basis for development of performance criteria for the selection and specification of dimensionally compatible cement-based repair materials. Proposed performance criteria include a minimum value for tensile strength and maximum values for modulus of elasticity, drying shrinkage, and coefficient of thermal expansion. Also, resistance to cracking in restrained shrinkage tests is a requirement. A data sheet protocol is proposed for cement-based repair materials that would provide reliable, standardized information on pertinent material characteristics. Also, current efforts to develop a comprehensive analytical model to predict cracking resistance of repair materials are briefly described.

This paper presents the concrete structure repairing method, using concrete with expanded polystyrene granules (EPS). Reinforced concrete slabs are cast with depression of the top surface up to 70 mm. For the top surface leveling, with minimum selfweight load addition, a new flooring system is developed: over the sound isolation layer, EPS concrete leveling layer, 60-130 mm thick, (p = 450-500 kg/m3, Bk = 1-2 MPa) is cast. As a top finishing surface, high performance concrete layer, 15 mm thick (Pk = 60 MPa) is cast. Flooring system satisfied all the requirements, with total weight of 55-90 kg/m2.

The cracking behaviour of concrete beams having longitudinal tension reinforcement and various combinations of volume and aspect ratio of hooked end steel fibers was investigated experimentally. Eight full-scale beams have been tested. The section dimensions (200 x 350 mm2), span length (3250 mm), concrete compressive strength and longitudinal tension reinforcement were kept constant for all beams. The beams were tested at a cube compressive strength of about 40 MPa. The mechanical properties of the steel fiber reinforced concrete under tension were determined according to the Belgian standard NBN B15-238. The addition of steel fibers decreases both the crack spacing and the crack width. A greater reduction of the crack width, crack spacing respectively, can be noticed if steel fibers with a higher aspect ratio are used. Besides the experimental program also a theoretical study has been executed : a modification of the RILEM TC162-TDF-model to predict crack width is proposed.

Experimental studies are reported on upgrading the load carrying capacity of reinforced columns by jacketing with carbon fiber reinforced plastic (CFRP) flexible wraps. Several square columns with medium high strength concrete (cube strength = 64 MPa) were tested under concentric compression. The studied variables include: different upgrading configurations (continuous wrapping all over the height, discontinuous straps, straps concentrated at end zones); volume of wraps; and pre-loading prior to wrapping to simulate in situ strengthening process in practice. Effective wrapping provided lateral confinement to enhance the concrete compressive strength and the load capacity of the columns, in addition to the improvement of the ductility. With higher levels of confinement the axial capacity was further enhanced. Besides, effective retrofitting of preloaded columns restored and even increased their load carrying capacity over that of the original columns. It seems better to use effective full height wraps for enhancement of both strength and ductility and to help restrain against buckling of longitudinal bars, however the effect of confining the columns’ ends cannot be overlooked. The confined concrete strength from tests was compared with those estimated by certain empirical models and the comparisons were favorable in many cases. Other conclusions are also drawn.