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.

Fatigue properties of high-strength concrete in compression were studied. Two types of normal-density concrete and one type of lightweight aggregate concrete have been tested. The numbers indicate the planned mean strength in MPa of 100 x 100 x 100 mm cubes. The influence of different moisture conditions was studied in an introductory investigation. Three different sizes of cylinder were tested for each of the three curing and testing conditions: in air, sealed, and in water. The tests showed that the fatigue properties of both the air and water conditions were scale-dependent, while the sealed condition was hardly influenced by the sizes of the specimens. The main investigation dealt with the influence of the variation in stress levels on the fatigue life. Test conditions with constant maximum stress levels showed significantly longer lives when the stress range was reduced. If the load levels were defined relative to the static strength, there was no obvious difference between the fatigue properties of the concrete qualities included in these tests. An additional investigation was performed on ND95 cylinders exposed to different combinations of cyclic load levels. It was found that initial cycling at lower load levels was beneficial for the fatigue life at the higher load levels. Based on the results of the experimental work, a design proposal for fatigue of concrete in compression was established.

Six reinforced concrete beams and four slabs with different reinforcement ratios were tested to failure. The behavior of specimens manufactured with normal strength concrete (fc = 36 MPa) and high-strength concrete (fc = 83 MPa) was compared with respect to cracking and deflections. It was found that crack widths and crack spacings were fairly comparable for both concrete types in the region of stabilized cracking. Deflections decreased by using high-strength concrete due to the increased modulus of elasticity and cracking moment. However, for the beams, this gain diminishes at higher load levels.

Presents the results of an investigation undertaken at the Technical University of Denmark to determine the parameters that affect the ultimate load capacity of a concrete structure subjected to concentrated loads originating from reinforcement bars bent 90 deg. The following parameters have been found to have a decisive influence on the ultimate load capacity of the concrete bar: bar diameter, internal height of the specimen, side concrete cover, and concrete compressive strength. The results show that the relative load-carrying capacity of the concrete åc / fc decreases for increasing concrete compressive strength. However, the use of high-strength concrete (HSC) still results in an increase in the absolute load-carrying capacity of the concrete whencompared to normal strength concrete (NSC).