Experimental investigation and design of corroded stud shear connectors

Abstract

In this study, the effect of corrosion on the static behavior of stud shear connectors was investigated. Two series of push test specimens having different stud diameters were fabricated according to Eurocode 4. The test specimens were first corroded to different corrosion rates by electronic accelerating method. Static loading tests were then performed to obtain the load-slip curves and ultimate strengths of corroded test specimens. Material properties of the concrete and steel studs used in the test specimens were also measured. Actual corrosion rates were measured from the studs obtained from the tested specimens. Design methods specified in current design standards are used to predict the design strengths of test specimens. It is shown that the design strengths are unconservative for corroded test specimens. Therefore, a new design equation is proposed which enables the designer to consider the effect of corrosion.

Keywords

corrosion loading test push test specimen stud shear connector

Introduction

Corrosion is the major cause of deterioration of concrete structures (Asami and Kikuchi, 2003; Bazant, 1979; Caré et al., 2008; Duffó et al., 2004; Vassie, 1984). Therefore, effect of the corrosion is crucial for predicting the serviceability and durability of concrete structures. In many cases, corrosion occurs at the steel which lacks protection although this steel is embedded in the concrete. Investigation of steel and concrete composite bridges indicated that corrosion is easy to occur on the interface between steel and concrete, as shown in Figure 1 (a steel girder from a 30-year composite bridge in Purdue University, USA). Normally, shear connectors are used at the steel and concrete interface to provide interaction between the concrete slab and steel girder. Headed stud shear connectors are the most common type of shear connectors and are used in composite bridges. The behavior of the stud connectors has been broadly investigated by many researchers (Kim et al., 2011; Lam and Ellobody, 2005; Mirza and Uy, 2010; Nie et al., 2008; Smith and Couchman, 2010; Xue et al., 2008). The deterioration in the strength of stud connectors due to fatigue damage has also been reported (Coughlan, 1987; Dai and Richard Liew, 2010; Dogan and Roberts, 2012; Johnson, 2000; Lin et al., 2013; Oehlers, 1990; Wang et al., 2014). However, there is no research reported about the deterioration in the strength of stud connectors due to corrosion. In addition, there may be deterioration in stiffness and ductility of stud connectors. Therefore, the effect of corrosion on the behavior of stud shear connectors was investigated in this study.

Figure 1.

A steel girder from a 30-year composite bridge.

Most of the studies on shear connectors have been obtained from various types of “push-out” or “push” test (Johnson, 2004). The flanges of a short length of steel I-section are connected to two small concrete slabs. Current Eurocode 4 (EN 1994-1-1:2004) has specified the details of the standard push test. In this study, the standard push test was also used to study the static behavior of the corroded stud shear connectors. Based on the test results, load-slip curves and ultimate strengths of specimens having different corrosion rates were evaluated. In addition, the suitability of current design equations for corrode stud shear connectors was also evaluated.

Experimental investigation

Test specimens

Test specimens were fabricated according to the standard push test specimen specified in Annex B of Eurocode 4 (EN 1994-1-1:2004). Slabs of 150 mm thickness were used, and bond at the interface between the flanges of steel beam and the concrete slab was prevented by greasing the flange. Each of both concrete slabs was cast in the horizontal position, as is done for composite beams in practice, and the push test specimens were air-cured. Figure 2 shows the details of the specimens. The test specimens were labeled so that the nominal stud diameter and expected corrosion rate could be identified from the label. For example, the labels “D10.0-5A” and “D16.0-10B” define the specimens as follows:

The first letter indicates the nominal diameter of the stud, where the prefix letter “D” refers to diameter.

The following three digits (10.0 and 16.0) indicate the nominal diameter of the studs in millimeter.

The following one (5) or two digits (10) are the expected corrosion rates of stud in percentage.

The last letter “A” or “B” indicates the repeated test.

Figure 2.

Details of push test specimens.

Material properties and measurements

Three concrete cubic specimens were prepared at the time of push test specimen casting to determine the concrete strength of the push test specimens. Table 1 summarizes the material properties of concrete at 28 days. Two kinds of studs with nominal diameters of 10.0 and 16.0 mm are used in this study. Tensile tests for the stud material were conducted. The yield stress from the tensile tests was determined by 0.2% strain because the steel for studs generally does not show clear yielding point. Table 2 summarizes the material properties of stud material. Quality control of welding process is a very important factor since the effect of welding quality may cover the effect of corrosion. Therefore, welding trials were carried out to obtain proper and reliable welding quality.

Table 1.

Material properties of concrete.

Specimen	E_c (MPa)	f_cu (MPa)
1	3.31 × 10⁴	43.9
2	3.36 × 10⁴	44.3
3	3.38 × 10⁴	45.2
Average	3.35 × 10⁴	44.5

Table 2.

Material properties of stud material.

Specimen	Elastic modulus (MPa)	Yield stress (MPa)	Tensile strength (MPa)	Elongation (%)
10.0 mm	1.94 × 10⁵	275	392.3	13.6
16.0 mm	1.97 × 10⁵	270	391.7	13.5

Accelerating corrosion process

All specimens, except the uncorroded one (control specimen), were immersed in a 5% NaCl solution for 3 days after cured for 28 days, and then the direction of current of about 0.2 µA/cm² was arranged for accelerating stud corrosion; studs worked as the anodes, while a piece of stainless steel positioned in the solution served as cathode, as shown in Figure 3. The I-section steel beam was isolated by epoxy resins so that corrosion only occurs at the stud and steel–concrete interface, as shown in Figure 4. The corrosion time of each specimen was determined based on the expected corrosion rate. Faraday’s theory is used to calculate the corrosion time. The calculated results are shown in Tables 3 and 4 for series D10.0 and series D16.0, respectively. It should be noted that the actual corrosion rates of test specimens may differ from the expected corrosion rates.

Figure 3.

Set-up of electronic accelerating corrosion.

Figure 4.

Electronic accelerating corrosion of push test specimens.

Table 3.

Expected stud corrosion rate and actual corrosion time of D10.0 series.

Specimen	Electronic density (mA/cm²)	Expected corrosion rate (%)	Corrosion time (days)
D10.0-0A	0.2	0	0
D10.0-0B
D10.0-5A	0.2	5	31
D10.0-5B
D10.0-10A	0.2	10	62
D10.0-10B
D10.0-15A	0.2	15	93
D10.0-15B
D10.0-20A	0.2	20	124
D10.0-20B
D10.0-25A	0.2	25	153
D10.0-25B
D10.0-30A	0.2	30	186
D10.0-30B

Table 4.

Expected stud corrosion rate and actual corrosion time of D16.0 series.

Specimen	Electronic density (mA/cm²)	Expected corrosion rate (%)	Corrosion time (days)
D16.0-A	0.2	0	0
D16.0-B
D16.0-5A	0.2	5	19.6
D16.0-5B
D16.0-10A	0.2	10	39.2
D16.0-10B
D16.0-15A	0.2	15	58.8
D16.0-15B
D16.0-20A	0.2	20	78.4
D16.0-20B
D16.0-25A	0.2	25	98
D16.0-25B
D16.0-30A	0.2	30	117.6
D16.0-30B

Loading test setup and procedure

Corroded push test specimens were loaded in a hydraulic testing machine with a 2000-kN capacity. The slabs are bedded onto the lower platen of the testing machine, and load is applied to the upper end of the steel section, as shown in Figure 5. Slip between the steel member and the two slabs is measured using linear variable differential transformer (LVDTs).

Figure 5.

Loading test setup.

The testing procedure complied with the method specified in Eurocode 4 Annex B. The load should first be applied in increments up to 40% of the expected failure load and then cycled 25 times between 5% and 40% of the expected failure load. In this study, the expected failure load of corroded specimens is difficult to be assumed; therefore, the load is first applied in increments up to 30% of the failure load of specimens having 5% expected corrosion rate less. Subsequent load increments were then imposed such that failure does not occur in less than 15 min and the approximate loading rate is 0.5 mm/min. The longitudinal slip between each concrete slab and the steel section was measured at each load increment.

Test results

Measurement of stud corrosion rate

The corroded studs were retrieved from the failed specimens and the corrosion product was cleaned using a corrosion-inhibited HCl solution (Bertoa et al., 2008). The area loss of the steel rebar (ΔA) was estimated afterward by subtracting the post-corrosion area from the measured pre-corrosion area. The post-corrosion area of stud was calculated using the measured diameter of the shank of the stud. The measured diameter of the shank was used to calculate the corrosion rate of each stud (ψ) as ψ = (A−ΔA)/A%. The average corrosion rate of eight studs is taken as the corrosion rate of each push test specimen, as shown in Figure 6. It is shown that the measured corrosion rates of push test specimens are different from those expected corrosion rates. There is no corrosion occurrence between the interface of concrete slab and steel beam.

Figure 6.

Corroded stud shear connectors: (a) D10.0 series corroded studs and (b) D16.0 series corroded studs.

Static behavior

The static behavior of stud connectors can be described using load-slip curve and ultimate strength. In this study, the effect of corrosion on static behavior of stud was investigated.

Load-slip curves

The load-slip curves of test specimens D10.0 series and D16.0 series are shown in Figures 7 and 8, respectively. Since the failure mode of all specimens is stud failure, the load-slip curves only could be measured up to the point of ultimate strength. It is shown that the initial stiffness of specimens decreases with the increment of corrosion rate for both series specimens. Initially, the ductility of specimens increases when the corrosion rate increases. However, the ductility of specimen decreases when the corrosion rate further increases, as shown in Figures 7 and 8.

Figure 7.

Load-slip curves of D10.0 series specimens.

Figure 8.

Load-slip curves of D16.0 series specimens.

Ultimate strength

In this study, the failure mode of all push test specimens is stud failure. Figure 9 shows typical stud failure of the test specimens. The ultimate strengths of test specimen series D10.0 and D16.0 are shown in Tables 5 and 6, respectively. It is shown that the ultimate strengths of test specimens decrease when the corrosion rate increases. The maximum ultimate strength reduction rate of test specimens D10.0 series and D16.0 series are 64% and 50%, respectively. It means that the corrosion has significant effect on the ultimate strengths of test specimens.

Figure 9.

Typical stud failure.

Table 5.

Measured diameter and corrosion rates of D10.0 series corroded studs.

Specimen	Average measured diameter (mm)	Expected corrosion rate (%)	Measured corrosion rate (%)
D10.0-5A	9.622	5.0	2.60
D10.0-5B	9.487	5.0	5.31
D10.0-10A	9.270	10.0	9.59
D10.0-10B	9.263	10.0	9.74
D10.0-15A	9.288	15.0	9.25
D10.0-15B	9.198	15.0	11.00
D10.0-20A	9.171	20.0	11.52
D10.0-20B	8.774	20.0	19.02
D10.0-25A	8.671	25.0	20.90
D10.0-25B	8.632	25.0	21.62
D10.0-30A	9.086	30.0	13.16
D10.0-30B	8.889	30.0	16.88

Table 6.

Measured diameter and corrosion rates of D16.0 series corroded studs.

Specimen	Average measured diameter (mm)	Expected corrosion rate (%)	Measured corrosion rate (%)
D16.0-5A	15.955	5.0	0.62
D16.0-5B	15.523	5.0	5.87
D16.0-10A	15.135	10.0	10.51
D16.0-10B	15.208	10.0	9.65
D16.0-15A	15.953	15.0	14.39
D16.0-15B	14.886	15.0	13.43
D16.0-20A	14.377	20.0	19.25
D16.0-20B	14.469	20.0	18.22
D16.0-25A	14.273	25.0	20.42
D16.0-25B	14.485	25.0	18.04
D16.0-30A	14.091	30.0	22.44
D16.0-30B	13.988	30.0	23.57

Design method

Current Eurocode 4 (EN 1994-1-1:2004) provides design method for the shear strength of stud, as shown in equations (1) and (2). The design shear resistance of a headed stud in accordance with Eurocode 4 (EN 1994-1-1:2004) should be determined from

P_{Rd} = \frac{0.8 f_{u} d^{2}}{4}

(1)

P_{Rd} = 0.29 α d^{2} \sqrt{f_{ck} E_{cm}}

(2)

whichever is smaller, with

α = 0.2 (\frac{h_{sc}}{d} + 1) for 3 \leq \frac{h_{sc}}{d} \leq 4

α = 1 for \frac{h_{sc}}{d} > 4

where d is the diameter of the shank of the stud, E_cm is the elastic modulus of the concrete slab, f_u is the specified ultimate tensile strength of the material of the stud, f_ck is the characteristic cylinder compressive strength of the concrete at the age considered, and h_sc is the overall nominal height of the stud.

It should be noted that the partial factor is not considered in the calculation.

The design strengths predicted using Eurocode 4 (EN 1994-1-1:2004) are compared with test results in Tables 7 and 8. It is shown that the design predictions agree with the test results of uncorroded test specimen well. However, the design predictions are generally unconservative for test results of corroded specimens. Therefore, new equation with the consideration of corrosion effect was proposed. Since the failure mode of all test specimens in this study is stud failure, the proposed equation is only for stud failure mode, as shown in Equation (3). Comparison in Tables 7 and 8 indicates that the proposed equation is able to predict the test results with reasonable accuracy. The comparison between the test results and design predictions is also shown in Figures 10 and 11

P_{Rd} = \frac{0.8 (f_{u}) {d_{c}}^{2}}{4 ((100 - ψ) / 100)}

(3)

where d_c is the diameter of the shank of the corroded stud and ψ is the corrosion rate of the stud in percentage.

Table 7.

Comparison of ultimate strengths obtained from D10.0 series test results with predictions.

Specimen	Measured corrosion rate (%)	Ultimate strength (kN)
		Test (P_test)	EC4 (P_EC4)	Proposed (P_Pro)
D10.0-0A	0.00	254.6	197.2	197.2
D10.0-0B	0.00	218.0	197.2	197.2
D10.0-5A	2.60	259.2	197.2	187.1
D10.0-5B	5.31	236.4	197.2	176.8
D10.0-10A	9.59	193.7	197.2	161.2
D10.0-10B	9.74	143.4	197.2	160.7
D10.0-15A	9.25	137.7	197.2	162.4
D10.0-15B	11.00	132.7	197.2	156.2
D10.0-20A	11.52	157.9	197.2	154.4
D10.0-20B	19.02	153.5	197.2	129.3
D10.0-25A	20.90	153.1	197.2	123.4
D10.0-25B	21.62	94.05	197.2	121.1
D10.0-30A	13.16	127.7	197.2	148.7
D10.0-30B	16.88	119.6	197.2	136.2

Table 8.

Comparison of ultimate strengths obtained from D16.0 series test results with predictions.

Specimen	Measured corrosion rate (%)	Ultimate strength (kN)
		Test (P_test)	EC4 (P_EC4)	Proposed (P_Pro)
D16.0-A	0	652.7	504.1	504.1
D16.0-B	0	610.2	504.1	504.1
D16.0-5A	0.62	666.8	504.1	497.9
D16.0-5B	5.87	512.0	504.1	446.7
D16.0-10A	10.51	531.2	504.1	403.7
D16.0-10B	9.65	384.1	504.1	411.5
D16.0-15A	14.39	455.5	504.1	369.5
D16.0-15B	13.43	461.4	504.1	377.8
D16.0-20A	19.25	410.7	504.1	328.7
D16.0-20B	18.22	425.4	504.1	337.1
D16.0-25A	20.42	401.0	504.1	319.2
D16.0-25B	18.04	537.6	504.1	338.6
D16.0-30A	22.44	330.0	504.1	303.2
D16.0-30B	23.57	339.0	504.1	294.5

Figure 10.

Comparison of design strengths using proposed equation with test results of D10.0 series specimens.

Figure 11.

Comparison of design strengths using proposed equation with test results of D16.0 series specimens.

Conclusion

Experimental investigation of steel and concrete composite push test specimens with corrosion deterioration was conducted in this study. Two series of push test specimens having different stud diameters were tested. The test specimens were first electronic accelerating corroded and then loaded to failure. Based on the test results, the effect of corrosion on the load-slip curves and ultimate strength was studied. It is shown that the corrosion of stud has significant effect on the ultimate strengths of test specimens. Test results obtained from the loading tests were compared with design strength predicted by current Eurocode 4. It is shown that the design strength was unconservative for corroded specimens. New design equation with reasonable accuracy was proposed, which enables the designer to consider the effect of corrosion.

Footnotes

Appendix 1 Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research work described in this paper was supported by National Key Technology R&D Program (2011BAJ09B03) and research project from Science and Technology Department of Zhejiang Province (2015C33005).

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