Synopsis This paper deals with the bond strength of steel and concrete at high temperatures because, although bond strength is of major importance in the determination of the behaviour of concrete structures in fire, there is no detailed information available on this problem. For this reason a test apparatus was developed which enabled pull-out experiments to be carried out on cylindrical concrete specimens with short embedment lengths. During the tests the slip at the non-loaded end of the concrete specimen and the pull-out force were reassured. In the temperature range 20 to 800°C the pull-out system enabled two different types of experiment to be carried out: force–slip measurements at thermal steady state and at non-steady state conditions. In these experiments normal-weight concrete made with Portland cement and siliceous aggregate and three different types of reinforcing steel (ribbed steel bars, plain round bars and deformed prestressing bars) were used. Differences between the results for the different types were noted, especially between the ribbed steel and plain round steel bars.
The spalling of high strength concrete under f i e attack is discussed in this paper. Two series of f i e tests on reinforced concrete elements were carried out in order to investigate the spalling behavior and effect of spalling on the fire endurance of reinforced concrete elements. The results of the f i e tests on 30 specimens can be summarized as follows: a) the spalling is significantly affected by the water-cement ratio of the concrete, the permeability of the concrete, the amount of steel reinforcement, the f i e intensity and the age of the concrete at f i e exposure, b) the addition of polypropylene fiber into concrete is a good measure for the prevention of spalling and can improve the f i e endurance of reinforced concrete elements, c) the addition of polypropylene fiber into concrete at a ratio of Ikg polypropylene fiber per lm3 concrete reduces the depth of spalling by about 18mrn.
Though, concrete in general is a non-combustible building material, modern High Performance Concrete (HPC) is very susceptible to violent explosive spalling during a fire attack. This requires protective measures for fire safety design of concrete structures. The current most worthwhile method to prevent explosive spalling is the addition of monofilament Polypropylene fibres (PP-fibres). However, since it has become common knowledge that PP-fibres are suitable for fire safety design, a variety of theories concerning the mode of action of PP-fibres have been suggested. The present article summarizes the most important hypothesis and presents an innovative method for the analysis of micro structural processes in heated specimens. The results show that due to the thermal decomposition of PP-fibres capillary channels are created. Simultaneously, a netlike micro crack formation occurs, which connects these capillary channels. This enables the relief of internal stresses (mechanical effect) and the formation of a permeable transport system for the escaping water vapour (permeation effect).
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