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1、Residual bond strength between steel bars and concrete after elevated temperatures A. Ferhat Bingl Rstem Gl Civil Engineering Department, Atatrk University, Erzurum, Turkey ARTICLE Article history Received in revised form Revised 28 March 2009. Accepted 3 April 2009. Available online 6 May 2009. Key

2、words Concrete； Elevated temperatures； Pull-out； Bond strength； Cooling regimes Abstract The effects of elevated temperatures and cooling regimes on the residual (after cooling) bond strength between concrete and steel bars are investigated. For this study, ribbed steel bars of 8 mm diameter are emb

3、edded in to C20 and C35 concrete blocks with embedment lengths of 6, 10 and 12 cm. Unsealed specimens are heated to 12 different temperatures ranging between 50 and 700 C and then cooled in water or in air. Pull-out tests are carried out on the specimens, and the effects of elevated temperatures on

4、the residual bond strength are investigated by comparing the results against unheated specimens. Increases in bond strength are observed for temperatures up to 150 C； however, there is decrease for all other temperatures. The effect of the cooling regime is less pronounced for the concrete-bar bond

5、strength. Moreover, it is concluded that concrete-bar bond strength increases with the increase in compressive strength of concrete and embedment length of the bar. 1. Introduction Reinforced concrete, one of the most widely used construction materials for a variety of structures, is a composite mat

6、erial consisting of reinforcing bars in a hardened concrete matrix. Concrete is unique for its versatility and large capacity to resist compressive stresses. However, its low tensile capacity makes it imperative to incorporate another material capable of resisting and transmitting tensile stresses.

7、Steel is recognised worldwide as the most competent reinforcing material in structural concrete due to its high tensile capacity and similar rate of thermal expansion and Poissons ratio as of concrete 1. More attention has been paid to the mechanical properties of concrete at high temperature or to

8、the residual properties of concrete after exposure to high temperatures. Concrete may be exposed to elevated temperatures during a fire or when it is closer to furnaces and nuclear reactors. Its mechanical properties such as strength, modulus of elasticity and volume deformation decrease and this re

9、sults in structural quality deterioration of concrete 2, 3, 4, 5, 6, 7 and 8. Of particular importance are loss in strength and elastic modulus, cracking and spalling, ductility, and loss of bond with any steel reinforcement 3 and 9. An assessment of the degree of deterioration of the concrete struc

10、ture after exposure to high temperatures can help engineers to decide whether a structure can be repaired rather than required to be demolished 10. Deterioration in mechanical properties of concrete upon heating may be attributed to material factors and environmental factors. Material factors are pr

11、operties of aggregate, properties of cement paste and aggregatecement paste bond and their thermal incompatibility between each other. Environmental factors can be listed as heating rate, duration of exposure to maximum temperature, cooling rate, loading conditions and moisture regime 11 and 12. Sil

12、iceous aggregates containing quartz may cause distress in concrete at about 573 C since the transformation of quartz from to form is associated with a sudden expansion of 0.85% 11. The structure of concrete material can be approximately classified into micro level (less than 1 m), meso level (betwee

13、n 1 m and 1 cm), and macro level (greater than 1 cm). For concrete subjected to high temperature, with the increase in temperature, strength and Youngs modulus decrease at macro level, internal structures degenerate and micro defects develop at micro and meso levels 13. The effect of fire and high t

14、emperature on the behaviour and properties of reinforced concrete studies includes compressive strength, modulus of elasticity, shear modulus, thermal conductivity, specific heat and creep of concrete along with modulus of elasticity and coefficient of thermal expansion and tensile strength of reinforcing steel. Information on the bond between concrete and steel is limited. In studying bond strength, pull-out tests are applied and average loads are used to calculate average bond strength 14.