【正文】 results to an increase of the stress applied to the bars. By considering Eqs. (3) and (4), the values of the applied stress on the reinforcing bars can be calculated as (8) with σ0 being the applied stress for the uncorroded material. For the case under consideration, it refers to a bar with d0 = 8 mm. An example of the increase on applied stress as a result of the reduction of the load carrying crosssection with increasing duration of the salt spray exposure is shown in Fig. 8. The values taken for σ0 for the two curves in Fig. 8 were 280 and 320 MPa, respectively. The synergistic effect of the observed decrease on the effective strength values of the material and the increase of applied stresses due to the crosssection reduction may reduce appreciably the safety factors involved in the calculations of a reinforced concrete structure. Note that the safety factor normally used when designing reinforced concrete structures is . Furthermore, it should be noted that the reduction of the crosssection of a reinforcing bar also reduces the moment of inertia of the steel bar. (22K) Fig. 8. Applied stress increase as a function of the duration of corrosion exposure. The effects of increasing corrosion damage on the tensile ductility of the investigated steel bars are shown in Fig. 9 and Fig. 10. Both elongation to fracture, Fig. 9, and energy density, Fig. 10, decrease appreciably with increasing duration of the salt spray exposure. The value of elongation to fracture meets the requirement fu 12%, as requested by the standards in [1], for exposures to salt spray of up to 35 days. As discussed above, the corrosion damage referring to 35 days laboratory salt spray exposure is not unrealistic for corroded reinforcing steels of older buildings at coastal sites. Fig. 9 and Fig. 10 have been approximately fitted using the Weibull function. The Weibull constants C1 to C4 are given in Table 2. (19K) Fig. 9. Effect of the duration of corrosion exposure on elongation to fracture. (16K) Fig. 10. Effect of the duration of corrosion exposure on energy density. The standards do not require for the evaluation of the energy density W of the reinforcing steel. Energy density is a material property which characterizes the damage tolerance potential of a material and may be used to evaluate the material fracture under both, static and fatigue loading conditions [26]. Note that energy density may be directly related to the plain strain fracture toughness value, KIC, . [27], which evaluates the fracture of a cracked member under plain strain loading conditions. The observed appreciable reduction on tensile ductility may represent a serious problem for the safety of constructions in seismically active areas. As during the seismic erection, the reinforcement is often subjected to stress events at the region of low cycle fatigue, the need for a sufficient storage capacity of the material is imperative. 4. Conclusions ? The exposure of the steel bars S500s tempcore to salt spray environment results to an appreciable mass loss which increases with increasing duration of exposure. Durations of laboratory salt spray exposures of 40 days or longer are realistic for simulating natural corrosion damage obtained at members of old buildings at coastal sites. ? The effect of salt spray exposure on the strength properties of the steel S500s is moderate. Yet, with regard to the observed appreciable mass loss, the increase on the effective engineering stress is essential such as to spend the reserves on strength which are required in the standards trough safety factors. ? The effect of salt spray exposure on the tensile ductility of the material is appreciable. For salt spray exposures longer than 35 days, elongation to fracture drops to values lying below the fu = 12% limit which is required in the standards. ? Present day standards for calculating strength of reinforced concrete members do not account for the appreciable property degradation of the reinforcing steel bars due to the gradually accumulating corrosion damage. Although, a revision of the standards such as to account for the above corrosion effects on the material properties seems to be required, further extensive investigation is needed to conclude on proper remendations for such a revision. References [1] Hellenic Regulation for the Technology of Steel in Reinforced Concrete。C. The temperature in the zone of the reinforcement material exposed inside the salt spray chamber was maintained at 35 176。 S, %。 P, %。C, the solution was in the pH range from to . The pH measurements were made at 25 176。8 tempcore steel Test series Test series description Corrosion exposure prior to tensile test Number of tests conducted 1 Tensile tests on noncorroded control specimens None 4 2 Tensile tests on corroded specimens Salt spray corrosion for 10 days 3 3 Tensile tests on corroded specimens Salt spray corrosion for 20 days 3 4 Tensile tests on corroded specimens Salt spray corrosion for 30 days 3 5 Tensile tests on corroded specimens Salt spray corrosion for 40 days 3 6 Tensile tests on corroded specimens Salt spray corrosion for 60 days 3 7 Tensile tests on corroded specimens Salt spray corrosion for 90 days 3 The performed tensile tests aim to provide information on: 1. the gradual deterioration of the mechanical properties of the S500s tempcore steel reinforcement during salt spray corrosion。 1989. [3] . Wallbank, The performance of concrete in bridges, HMSO, London (1989). [4] . Manning, Design life of concrete highway structures – The North American scene, Design Life Struct (1992), pp. 144–153. [5] . Broomfield, Corrosion of steel in concrete, E amp。 1999. p. 497–505. [20] DIN 4881, Reinforcing steel grades, properties, marking。 P, %。8Tempcore 鋼 檢驗系列 系列測試說明 前張力接觸腐蝕試驗 一些測試