Mechanical properties of steel changes when the steel is exposed to high tem- perature of fire. Figure 3.31 shows yield stress and tensile strength at room
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Fig. 3.31. Yield stress and tensile strength after exposed t o high t e m p e r a t u r e .
temperature of USD685A, USD685B and USD980 for axial reinforcement, USD785 for lateral reinforcement, and SD345 for comparison, after exposed to high temperature ranging from 400 to 800 degree Celsius. It is seen from the figure that both yield point and tensile strength of axial bar USD685 and lateral bar USD785 starts dropping at heating of 700 degree Celsius, and the
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Temperature in degree Celcius (a) Yield stress at high temperature 1400
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Fig. 3.32. Yield stress and tensile strength at high temperature.
reduction rate is slightly greater than SD345, and that yield point and tensile strength of USD980 which was made by off-line heat treatment starts dropping at heating of 600 degree Celsius. Up to 500 degree Celsius, which is the highest temperature expected in case of fire, there is no mechanical property change of high strength re-bars similarly to currently used SD345 steel.
Figure 3.32 shows yield stress and tensile strength tested while being heated in the electric furnace up to the designated temperature. There is a general trend of larger reduction of yield stress and tensile strength for higher strength steel, but minimum remaining tensile strength of about 200 MPa can be insured at 600 degree Celsius for any grades of steel including SD345.
3.2.6.2. Corrosion Resistance
When different metals touch in the corrosive environment, the metal on elec- trically base side tends to corrode due to difference of ionization. Furthermore, high strength re-bars contain many special chemical elements compared to ordinary steel. Considering the possibility of mixed use of high strength and ordinary strength bars, corrosion tests were conducted of steel in contact as well as isolated in the solution of sodium chloride and calcium hydroxide to simulate the environment in fresh concrete. Tests consisted of three items. The first was isolated immersion test, to observe and measure rust appearance, corrosion loss of mass, and corrosion hole depth, during 30 days of immersion at 25 degree Celsius. The second was measurement of electrochemical natural potential, to determine inertness-break voltage by measuring natural potential and anode polarization (re-bar in corrosion side). The third was measurement of coupling current between different grade steel, tested as shown in Fig. 3.33.
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Fig. 3.33. Measurement of electric current between different grade steel.
After equilibrium in 30 days of immersion, coupling current was measured and corrosion area ratio and corrosion speed were calculated. SD345, USD685 and USD980 bars were used as specimens.
It was found that corrosion resistance of isolated specimens of high strength steel was similar to currently available ordinary strength steel. Corrosion due to different metal touch tended to occur on the lower strength steel while higher strength steel is corrosion-proofed. However the speed of corrosion in the pH 12 environment was in the range of 0.001 to 0.019 mm per year, so it was very small and was on the same order as the corrosion of isolated bodies.
3.2.7. Splice
Current reinforced concrete construction employs various splicing methods of re-bars, such as lap splice, gas butt welding, arc welding, and mechanical splices. As high strength bars of USD685 are manufactured by addition of strengthening chemical elements and controlled hot rolling or heat treatment, gas butt welding or arc welding are not suitable. Metal crystalline structure is apt to change in the heat-affected zone of these splices, which results in re- duced strength. Hence mechanical splices are more desirable for high strength re-bars. Among them, the most advantageous would be the use of deformed bars with screw type surface deformation, spliced with screw coupler with grouting. This kind of coupling does not require special skill of technicians, and is relatively easy to keep good quality control, hence is most practically feasible.
Screw coupler splices are currently available for screw type deformed bars up to SD490 steel. They use steel couplers with female screw on the internal face conforming to the screw shaped surface deformation of bars. After the coupler is installed to connect bar ends, grout material is injected through the hole at the center of the coupler. Both organic grout of epoxy resin and inorganic grout of cementitious material are available.
Applicability to USD685 high strength re-bars was investigated of this type of screw coupler splices. Both epoxy grout splices and inorganic grout splices were applied to D19, D22, D25, D32, D35, D38 and D41 bars, and three kinds of tension tests, specified in the bar splice performance acceptance criteria (1982) of the Building Center of Japan, were carried out.
(1) One-way loading test. As shown in Fig. 3.34, specimen is first loaded up to 0.95 times the specified yield stress and unloaded to 0.02 times the yield. Secant moduli at 0.70 times, and at 0.95 times, the yield stress are
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Fig. 3.34. One-way loading test.
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Fig. 3.35. Cyclic test in the elastic range.
measured. Offset strain at 0.02 times the yield, corresponding to bar slip, is also measured. Then it is loaded all the way up to failure to determine maximum strength.
(2) Cyclic test in the elastic range. As shown in Fig. 3.35, load is reversed 20 times between 0.95 times the yield in tension and 0.50 times the yield in compression, and then increased in tension to the point of failure. Stiffness in the first and twentieth cycles is measured and the ratio is calculated. Slippage in 20 cycles is also determined as shown.
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Fig. 3.36. Cyclic test in the plastic range.
(3) Cyclic test in the plastic range. As shown in Fig. 3.36, load is reversed four times between twice the yield strain in tension and 0.5 times the yield stress in compression, and then increased in tension to the point of failure.
Slippage is determined from the fourth loop as shown in the figure.
Criteria of acceptance for these tests of high strength re-bars have not been established, so the criteria for ordinary re-bars were simply extrapolated to match the strength of USD685. It was shown that all splice specimens broke in base metal, and that the splices possessed the capacity corresponding to Class A splices specified by the Ministry of Construction.
Lapped splices are not likely to be used in the New RC buildings, be- cause bars in New RC buildings are mostly large diameter bars, and either prefabricated cages or precast members would be used in practice. Hence no investigation was conducted in the New RC project into the performance of lapped splices.
Splices were also tested in the structural tests. Figure 3.37 shows a specimen of cantilever beam having splices of axial bars at the critical section.
The purpose of this test was to see whether splices induce strain concentra- tion at the critical section. High strength re-bars have relatively high yield ratio, which may lead to strain concentration at the section of first yield with- out allowing the plastic hinge zone to expand, particularly of flexural members with low steel percentage. This trend may be exaggerated by the presence of bar splices. Hence structural tests of specimens as in Fig. 3.37 were conducted using steel with yield ratio of 90 percent and 75 percent, with or without bar splices.
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Fig. 3.37. Test specimen for beams using re-bars with different yield ratio.
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Fig. 3.38. Load-deflection curves of beams using re-bars with different yield ratio.
Figure 3.38 shows load deflection relations of two specimens, (a) yield ratio of 90 percent without splices, and (b) yield ratio of 75 percent with splices.
Marks indicate points corresponding to particular bar strain. It can be seen that higher yield ratio results in smaller deflection at a given bar strain, meaning that strain is more concentrated in case of higher yield ratio. Although not shown in the figure, trend of the specimen of 90 percent yield ratio with splices was quite similar to Fig. 3.38(a), hence the presence of splices did not accelerate the strain concentration even in case of yield ratio of 90 percent. However, as shown in Fig. 3.38(a), bars broke in tension in the reversal of loading at an amplitude of 5 percent in terms of deflection angle. Bars also broke in the same specimen with splices. This test results form the basis of yield ratio limitation in the mechanical property specification of high strength reinforcing bars.