a King Saud University, [email protected]
b King Saud University, [email protected]
c King Saud University, [email protected]
If steel manufactures usually comply with the minimum code specifications, the nominal yield strength of rebar can however be significantly exceeded in many countries, depending on the steel manufacturing processes. Such an increase in yield strength can have negative effects on the flexural behavior of beams designed as tension controlled, and reduce their ductility, an essential property in seismic resisting structures. An experimental and analytical study of the flexural behavior of reinforced concrete (RC) beams was conducted through the investigation of the Moment-Curvature relationships and the ultimate steel strains. The main variable was the level of the actual steel yield stress as compared to the nominal value. It was found that unexpectedly high values of steel yield stress reduce the beam ductility and violate the tension-control condition which was enforced in the design stage. Appropriate design corrections are proposed to account for high yield stress values in order to achieve the desired ductility of beams while maintaining the moment capacities.
Keywords: Reinforced concrete; beam flexure; ductility; moment curvature; yield strength
While much attention has been given to the effect of the variability of concrete strength and properties on the response of reinforced concrete structures, there is little, if any, information on the effects of variability of steel strength. This lack of interest may be explained by the assumed assurance that steel manufactures are always complying with minimum code specifications. However, steel mechanical properties are sometimes exceeding the minimum nominal strength values for a specific grade of steel. This happens in many countries as there are different steel producers who adopt different methods of manufacturing. The mechanical properties of steel are governed by the production technique, chemical compositions, mechanical working of rebar, and the method of heat treatment as described by ( Davis et al., 1982 ). Seismic standards have been developed with the aim of allowing steel to yield without rupture during an earthquake, in order to enhance seismic energy absorption of the structure and avoid collapse. For this purpose, steel bars should possess high strength with sufficient ductility and low variation in yield strength to experience large number of inelastic cycles of deformation with large plastic strains. In this regard, most international specifications outline and control the mechanical properties requirements for rebar to be used in seismic resistant systems as reported by ( Milbourn, 2010 ), and shown in Table 1.
Table 1 Mechanical properties requirements for seismic resistant rebars.
|Rebar Specifications||SteelGrade||Strength requirements Ratio = Actual/Nominal||Ductility requirements|
|f y nominal (MPa)||f y ratio||f u ratio||Elongation (%)|
|Chinese Standard GB 1449.2:2007||HRB400E||400||≤ 1.3||≥ 1.25||≥ 16|
|HRB500E||500||≤ 1.3||≥ 1.25||≥ 15|
|Australian/New Zealand Standard AS/NZS 4671:2001||300 E||300||≤ 1.27||≥ 1.15, ≤ 1.5||≥ 15|
|500 E||500||≤ 1.20||≥ 1.15, ≤ 1.4||≥ 10|
|British Standard BS 4449:2005||B500B||500||≤ 1.30||≥ 1.08||≥ 5|
|B500C||500||≤ 1.30||≥ 1.15, ≤ 1.35||≥ 7.5|
|American Standard ASTM A706 – ASTM A615- 09b||Grade 60||420||≤ 1.29||≥ 1.25||≥ 14 : Φ = 10-19 mm|
|≥ 12 : Φ = 22-36 mm|
|Grade 80||550||≤ 1.23||≥ 1.25||≥ 12 : Φ = 10-36 mm|
The increase in both yield and ultimate strengths of rebars will certainly improve the member’s strength but however it may also affect adversely the member’s ductility. This is particularly important for beams which are usually designed as tension controlled members, where at the ultimate state, tension steel should exceed the yield limit and reach a minimum strain of 0.005 as specified by ( ACI-318R-11, 2011 ). For such beams, steel bars will sufficiently yield before concrete reaches its compressive crushing strength allowing significant increase in deflections, and showing enough warning before failure. For such reasons, beams with a minimum tensile steel strain of 0.005 are considered to be fully ductile. An unsuspected increase in yield strength may therefore change the tension control of a beam and violate the minimum 0.005 strain condition.
( Al-Haddad, 1995 , 2006 ) studied the effect of high yield strength of Saudi rebar on the curvature ductility factor. It was concluded that the ACI-318 provisions of limiting maximum longitudinal steel ratio do not ensure sufficient ductility for conventional and seismic designs. ( Youcef and Chemrouk, 2012 ) also reported ductility reduction with higher steel ratios and yield limits. Appropriate compression steel to retrieve the desired ductility was recommended. ( Zhou et al., 2011 ) studied the effects of steel ratio and yield level on both deformability and strength of RC beams, and made appropriate recommendations in order to achieve the desired level of ductility without adversely affecting the strength by combining compression and transverse steel.
In the Middle East and Gulf Region, there are at least six producers of deformed rebars, all complying with ASTM A 615M specifications. Steel is manufactured using either tempered or quenched processes to produce bars with minimum yield strength of 420 MPa, and minimum ultimate strength 620 MPa. However, large differences in yield strength values are noticed among producers using different manufacturing processes, leading to substantially different mean-to-nominal yield strength ratios. There is consequently a substantial risk that a beam initially designed to fail in a ductile mode may fail in a brittle manner. It is therefore necessary to investigate the effect of the increase in rebar yield strength on beam ductility.
The objective of the present study is to investigate the effect of local manufactured steel on the flexural behavior of beams in terms of ductility and moment capacity. The investigation is performed experimentally and verified analytically. In addition, a parametric study is conducted to demonstrate the effect of high yield strength of rebars on the ductility and bending capacity of beams for different steel ratios and beam sections.
2 EXPERIMENTAL INVESTIGATION
2.1 Variation of Steel Strength
In the first part of the experimental investigation, random samples were taken from two different local steel producers “A” and “B” that are confirming to ASTM A 615M grade 60, but with unspecified manufacturing processes. The nominal yield and ultimate strengths are 420 MPa and 620 MPa respectively. Three specimens were tested for each bar diameter, the yield and and ultimate strengths were recorded as well as the mean values and coefficient of variation. The ratio of mean yield strength to nominal yield value (γy) and the ratio of mean ultimate strength to nominal ultimate value (γu) were computed for each rebar diameter, and the results are shown in Table 2 for both producers “A” and “B”. In fact both yield and ultimate strengths vary considerably from small to large diameters even for the same steel producer. For producer “A”, the ratio of mean yield stress to nominal value varies from 1.26 to 1.39, while the ultimate ratio ranges is from 1.16 to 1.26. For producer “B”, the range variation is 1.30 to 1.46 in yield stress and 1.06 to 1.13 in ultimate strength.
As the steel tensile strength depends on the manufacturing process, an investigation was made to compare the tensile strength of grade 60 rebars produced by two different processes; tempering and quenching techniques. The nominal yield and nominal ultimate strengths are again 420 and 620 MPa respectively. Tensile tests were performed with different rebar diameters for each steel manufacturing type. The ratios, mean to nominal, for yield strength “γy” and the ratio of mean to nominal ultimate strength “γu” were computed and given in Table 3 for different rebar diameters. As can be noticed, the values of γy for rebars prepared by the tempering process are very close to 1.00, while the values of γy for rebars prepared by quenching process are much higher and can reach up to 1.43. However the values of γu for both types of steel rebars are within reasonable range (1.00 to 1.15). These results indicate that rebars produced by the quenching process exhibit high values of yield strength as compared to the nominal value. Typical stress-strain curves for 16-mm and 20-mm rebars manufactured by both tempering and quenching processes are shown in Figures 1a and 1b respectively. There is a significant variation in yield strength whereas the ultimate strengths are quite close. The quenched process delivered a yield stress much higher than the nominal value. In addition, the rebars develop a yield plateau until a strain less than or equal to 0.02, which is then followed by nonlinear hardening until the ultimate strength is reached at about 0.10 strain. It is anticipated that such high values of yield strength, will have a significant effect on the flexural behavior of beams.
Figure 1a Typical stress-strain curves for 16-mm rebars.
Figure 1b Typical stress-strain curves for 20-mm rebars.