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Studying the effect of embedded length strength of concrete and diameter of anchor on shear performance between old and new concrete

  • Rana F. Yousef , Haitham H. Muteb and Ayoob A. Ibrahim EMAIL logo
Published/Copyright: February 21, 2023
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Curved and Layered Structures
From the journal Curved and Layered Structures Volume 10 Issue 1

Abstract

This article illustrates the specifications required to accurately design, specify, and install embedded anchor bolts between old and new concrete composite specimens for concrete repair or reinforcing of collapse concrete a research hotspot. The concrete slabs are facing a major challenge with deterioration, especially for reinforcement corrosion caused mainly by severe cycles of various chemical attacks. In this research, the impact of using contact plates between composite specimens was investigated by testing grouped specimens, thereby the models were divided into two groups, which tested under static load. The findings of a series of tests conducted to evaluate the structural behavior of shear connections (by pushout test) by including many parameters; the diameter (8, 12 and 16 mm), bounding between different compressive strength should be changed [normal concert (NC) mixes , ultra-high performance fiber concrete (UHPFC), and self-compacting mortar (SCM)]. Also, the embedded length of bolts was varied from 70, 130, to 190 mm. These parameters were studied individually in two groups. The first group was without contact plate and the second group was with contact plate. Experimental findings were obtained and reported, including the failure modes, maximum resistance, slippage capacity, and load–slip characteristic responses of the connections. Based on the obtained data, a relationship between the studied parameters was investigated. Experimental findings showed that the ultimate strength of rough surface specimens (without contact plate) was about 31% greater than that of smooth surface specimens (with contact plate), and obviously, all pushout specimens failed due to stud shank failure.

Keywords: bolts connector; pushout tests; shear capacity; load–slip curve; embedded length

1 Introduction

Concrete composite structures have indeed been widely employed to make use of the advantages of mixing two distinct building materials and exercising each benefit. Shear connectors, which are used to transmit applied loads to concrete in composite beams, are steel components that are cast into concrete or placed after the concrete has hardened. Individual elements are also transported to the construction site and installed in the appropriate location in precast concrete construction, thus connecting them to the main structure or other auxiliary elements. Every load acting on these elements is routed through their anchorages, making them an essential component of the structural support system. Concrete anchors can fail in two ways: brittle failure when the concrete cracks and ductile failure in the anchor shaft due to the concrete’s brittleness. The anchor system suddenly cracks due to concrete failure. As a result, the concrete anchor system must make a sensible decision about the concrete’s cracking strength.

The study data of beams and pushout experiments were examined, and it was found that the ultimate load of the bolts was frequently lower in the pushout testing than in the beam test [1]. Primary factors such as influences of high-strength anchor bolt behavior on bolt diameter, embedment length, clear cover, and bearing area were investigated [2]. The shear effectiveness of stud connections in composite beams made of high-strength steel (fy = 690 MPa) and ultra-high performance concrete (UHPC) was examined, and the test parameters were the diameter (13/19 mm) and stud configuration (single-stud/grouped-stud) [3]. It also suggests testing large cast-in-place anchor bolts in concrete with a shear test [4]. To evaluate the shear performance of big anchors, defined as those with an embedment depth or diameter greater than 25 in. (635 mm) or a diameter greater than 2 in. (50 mm), which neither ACI 318-08, Appendix D, nor ACI 349-06, Appendix D, address it. Also, it was interested in learning more about the safety of such anchors when used in nuclear power plants as an exception, and the findings would be used to determine whether or not the design formulae already in use for smaller anchors could be used for big anchors.

Based on experimental findings, an FEM analysis of pushout tests with group studs was performed [5]. Also, it was investigated how group studies fail to progress [6]. A paper was published in 2015 that detailed the suggested rubber-sleeved stud’s mechanical behavior [7]. Eighteen pushout test specimens, including shear connectors with various rubber sleeve dimensions, were constructed and evaluated. Nonlinear finite element models have also been created to simulate the failure of shear connectors under shear stress. In 2015, the static behavior of stud shear connections installed in elastic concrete was examined [8]. The following four distinct rubber content levels were considered: 0, 5, 10, and 15%. His findings reveal that when the rubber component in the stud raises, the stud ductility also improves dramatically. In 2017, it was focused on the numerical analysis of a pushout test using crumb rubber concrete slabs rather than regular concrete slabs [9]. During the pushout testing, the stress mechanism of a single stud inserted in several slabs of rubber-mixed concrete was investigated, as well as the impact of a welding flaw on a single stud’s behavior was investigated. A parametric investigation including various rubber compositions and stud dimensions is presented.

In 2018, the capacity of a single cast-in headed anchor to break through steel fiber-reinforced concrete was explored [10]. Anchors with a diameter of 30 mm, an embedment depth of 240 mm, and an edge distance of 150 mm were tested. Fiber volume fractions of 0.33, 0.67, and 1.00% were investigated. Static push-out tests were performed on one set of welded stud specimens and four groups of bolted connection specimens. The shear efficiency of innovative bolted connections (short bolt, long bolt, and coupler) was compared to that of traditional welded headed studs. Furthermore, the failure mode and significant mechanical behavior were investigated [11]. Also, the pushout experiments conducted to examine the shear behavior of large-diameter studs (22/30) mm in typical steel UHPC composite beams was investigated [12]. The results show that all specimens failed due to stud shank failure, while the UHPC slab had no visible cracks. Using pushout tests, the static and fatigue shear performance of short studs with a diameter of 13 mm in UHPC was examined [13]. The test findings showed that no apparent fractures appeared on the UHPC slab surface, and for the stud with a 2.7 aspect ratio, the failure mode of a steel-UHPC pushout specimen was still stud shank failure.

Standard stud shear connectors and epoxy resin shear connectors have equivalent contact shear capacities [14]. Epoxy resin shear connectors typically fail due to the strength of the concrete or the adhesive layer, whereas magnesium phosphate cement (MPC) shear connectors typically fail because of the debonding of the binder and steel. In order to fully use the advantages of both adhesive and metal shear connections, they can be used in combination. In several simulations of a demountable steel–concrete bolted connection in pushout testing, the friction force at the steel–concrete interface contributes to the shear resistance of the bolted connector in the tests [15]. The load–slip curves and the fracture of bolts in pushout tests may be roughly predicted by utilizing damage models of the bolt and concrete materials and accounting for an acceptable friction coefficient. On the bolted connection’s shear performance, finally, the effects of the concrete damage model, clearance in the bolt hole, and pretension of the short bolt were discussed. A concrete damage model applicable to element characteristic length is utilized to represent the pushout tests. The effects of friction coefficient, concrete softening behavior, hole clearance, and pretension are investigated with regard to the shear performance of pushout specimens.

The primary goal of this study is to examine mechanical characteristics of the suggested bolts, which were developed within the original concrete during casting. The mechanical joining (by bolts) specimens provided enhanced comparison of the ultimate carrying capacity and cracking loads for chemical connecting specimens, which motivated us to perform this research on the bolts and to study several variables that could affect the association [16].

Two groups of push-out tests are created and implemented herein to investigate the differences in behavior between specimens with thin plates between old and new concrete and specimens without plates. Shear connectors with varying diameters, embedded lengths, and multi-strengths of new concrete examples were all constructed and tested for all groups, and their shear performance and failure modes, including slip performance, shear load carrying capacity, shear stiffness, and load–slip curve, were all observed and compared reciprocally using the bolted connector groups test.

2 Experiment program

All of the tested concrete specimens consisted of three parts. The two side pieces are poured 28 days after the middle piece is melted, and the bolt is fixed during casting. The structural behavior of shear connections (by pushout test) and many studied parameters that have been considered are the concrete strength, diameter’s bolts, and embedded length of bolts.

2.1 Test specimens

In the present research on shear connectors, an experimental investigation is regularly conducted by pushout tests. In this study, a center column specimen with cross-sectional sizes of 200 mm × 200 mm and a length of 500 mm was constructed and tested. All specimens were reinforced with four 10 mm longitudinal steel bars and 4 mm steel bars served as tie reinforcement with a spacing of 100 mm, and the bolts were fixed inside them during the casting process. After a 28-day maturation period, the side columns were poured with the same dimensions as the first column. Details of the samples that were prepared for the thrust testing are shown in Figure 1.

Figure 1 Dimensions of pushout specimens. (a) Front view; (b) top view.
Figure 1

Dimensions of pushout specimens. (a) Front view; (b) top view.

The models were tested on two groups of pushout tests; the primary variables for each group of pushout experiments are displayed in Table 1. The first group was the contact surface between the new and old concrete without a barrier, and the second group was the contact surface with a cast roughness. Thin steel sheets were used between the old and new castings in the second group (smooth surface). Samples were casted for each group using the following:

  1. Different bolt diameters of 8, 12, and 16 mm for connectors with an embedded length equal to 70 mm.

  2. For (12 mm) diameter bolt, various embedded lengths (70, 130, and 190 mm) were investigated by pushout tests.

  3. For 12 mm diameter bolt with an embedment length of 70 mm, the effect of concrete strength for the overlayed concrete with normal, self-compact, and high strength was considered. Invention process is presented in Figure 2.

Table 1

Specimen designation

Group Specimen l m (mm) ϕ (mm) of the bolt Type of mixture, for side columns Surface condition
I PN130Ø16 130 16 NC Smooth Surface
PN130Ø12 130 12 NC Smooth surface
PN130Ø8 130 8 NC Smooth surface
PN190Ø12 190 12 NC Smooth surface
PN70Ø12 70 12 NC Smooth surface
PH130Ø12 130 12 UHPFC Smooth surface
PS130Ø12 130 12 SCC Smooth surface
II N130Ø16 130 16 NC Rough surface
N130Ø12 130 12 NC Rough surface
N130Ø8 130 8 NC Rough surface
N190Ø12 190 12 NC Rough surface
N70Ø12 70 12 NC Rough surface
H130Ø12 130 12 UHPFC Rough surface
S130Ø12 130 12 SCC Rough surface

NC = normal concrete, UHPFC = ultra-high-performance fiber concrete, SCM = self-compacting mortar.

Figure 2 Fabrication process of test specimens.
Figure 2

Fabrication process of test specimens.

2.2 Material properties

All of the materials utilized to make the normal concrete (NC)mixes substrate, as well as the overlay material blends, were readily available in the area. In this research, Portland limestone cement (PLC) was used. In the substrate concrete mixes, Al-Ekhaider sand was used as a fine aggregate with a maximum particle size of 5 mm. The nominal size of the fine aggregate in an ultra-high performance fiber concrete (UHPFC) mix ranges from 150 to 600 µm. To achieve the required grading standards, the sand is sieved to create this. The fine aggregate complies with the standards and is pure, free of loam and clay. The substrate concrete mixes used coarse aggregate up to a maximum size of 12.5 mm, which satisfied the specifications [17]. Micro steel fiber (MSF), superplasticizer (SP), and silica fume are all components of the UHPFC (steel fiber [SF]). The MSF has a length of 13 mm and a diameter of 0.2 mm, and it is rust and oil-free. Straight brass-coated MSF is the type of MSF used in this application. To fluidize the dry mix, the SP employed was a third-generation SP for concrete and mortar known commercially as Hyperplast (SP-PC200); it is a chloride-free aqueous solution of modified polycarboxylic polymers with long chains that meet the requirements [18]. The SF utilized in this investigation had a particle size of 0.1–1 µm, which is roughly 100 times smaller than the normal cement particle, and it met the physical and chemical parameters [19]. In self-compacting mortar (SCM) mixes, limestone powder (LP) with a maximum particle size of 125 µm was used as a filler to enhance the paste volume and lubricate the aggregate particle surfaces. The uniaxial compressive strength of standard concrete was evaluated using 150 mm cubes, while for UHPFC and SCM, 50 and 100 mm cubes were utilized. The amounts of the ingredients used in the NC substrate, UHPFC, and SCM mixes, as well as the average compressive strength of concrete, are shown in Table 2. Also, the details of the reinforcing steel bars, bolts, and SFs are shown in Tables 3 and 4. In this research, all the concrete specimens were constructed on the same day from the same batch.

Table 2

Mix proportions of NC substrate and different overlay materials, kg/m3

Constituents PLC FA CA Water MSF SF LP SP-PC200 Mix strength grade, MPa at 28 days Young’s modulus (GPa)
NC 450 723 1,010 200.5 33.65 38
SCM 625 1,185 208 138 64 60.23
UHPFC 780 920 150 157 200 40 150 51
Table 3

Experimental mechanical property of steel

Material Diameter (mm) and type Yield strength fy (MPa) Tensile strength fu (MPa) Young’s modulus (GPa) Elongation rate (%)
Reinforcement ∅ 10 , deformed 436 592 192 18
∅ 4 , smooth 275 385 192 17
Bolts 374.9 457.8 198 19
Table 4

Steel fiber parameters

Fiber type Strength (MPa) Modulus (GPa) L (mm) D (µm) D/L
Straight steel fiber 2,500 200 16 200 80

Note: L = length of steel fiber; D = diameter of steel fiber.

2.3 Test setup and loading procedure

The specimens were tested using a hydraulic testing machine as shown in Figure 3 with a capacity of 1,000 kN and the loading rate was 0.1 kN/s. The longitudinal slip between the midsection and each side column was measured continuously during loading by using two linear variable differential transformer (LVDTs) of 1/100 mm accuracy. A loading steel disc was installed on top of the pushout specimens in order to transfer a uniformly distributed load. Each pushout specimen was equipped with two displacement transducers to monitor the vertical slip between the middle part and the external parts. The welded steel plate held the two displacement transducers in the same vertical position on either side of the middle part. The representative slip for each specimen in the analysis of the slip data that followed was determined by taking the average of the two vertical displacements.

Figure 3 Test setup.
Figure 3

Test setup.

3 Experiment results

To compare the comparative response of the specimen’s slips, the Pushout test result data and observations are used. This comparison was made to confirm the impact of the connection methods used on the composite structures’ structural performance. According to the ultimate load carrying capacity, load–slip curves, shear stiffness, and (Pu/Su) of the specimens, the structural performance has been assessed.

3.1 Failure modes

According to previous research, there are typically three different failure modes for pushout specimens mixed modes, concrete failure with studs pulling out, and stud shank failure. The failure mode of the pushout specimens for two groups was stud shank failure on one side, as shown in Figure 4. The two sides of the studs did not fail at the same time because it was difficult to confirm that the stress condition of the studs on both sides of the specimen was exactly the same during the test. The bolts yielded and fractured but the maximum concrete stress of the concrete element was not attained and no concrete crack happened in the surface of the specimens. In the final failure modes of the connection pushout specimens, the specimen only exhibits a few minor cracks and no local concrete crushing event. The concrete under the bolt coupler is under substantially lower local compressive stress than that at bolts connecting the two columns because the bolt embedded in the concrete column magnifies the interface area between the bolt leg and the surrounding concrete. Also, the reinforcement will yield at the peak load if the resistance to shear of the shear connections is greater than the shear resistance from cohesion. If the resistance of the connections across the interface is less than the resistance given by cohesion, the connectors will yield after the peak load.

Figure 4 Failure mode of specimens.
Figure 4

Failure mode of specimens.

3.2 Load–slip curves and stiffness

As the applied weight grew, slip between the concrete blocks occurred. As illustrated in Figure 3, the average slip from two LVDTs is shown versus the load per connector. The load–slip curves depict the static behavior of shear connectors, which is an essential feature of the connection in pushout tests and crucial for determining interfacial slip and composite beam ultimate bearing capacity. Pushout experiments and load–slip curve fitting formulas have been performed by several researchers. In the testing, there are three different parts to the load–slip curves. The first part is the elastic portion. The second part is the plastic portion, and the third part is the descending portion. The load–slip curves in the elastic part have a nearly linear relationship at first. The slip is tiny, and the studs have a lot of shear rigidity. With increased stress, the curves in the plastic section display a new branch with a softer slope. The stud shear stiffness decreases constantly as the slip grows rapidly while the load increases slowly. After the maximum load, the specimens fail abruptly, and the declining portion of load–slip curves is steep and brief. There is no discernible declining component in the load–slip curves. Figure 5 depicts the load–slip curves. The stiffness of a stud connector is described in this work as the load at 0.2 mm relative slip. The shear strength and characteristic slip of the bolts are summarized in Table 5. Pu is the greatest load per bolt during the testing, Su is the equivalent slip, S max is the ultimate slip, and k is the stiffness of the connection, which is determined as the secant slope at a slip of 0.2 mm [20].

Figure 5 Load–slip curves of pushout rough surface specimens. (a) Alteration strength of new concrete, (b) alteration diameters of bolts, (c) alteration embedded length, and (d) percentage of differences between rough surface specimens with reference model N130Ø12.
Figure 5

Load–slip curves of pushout rough surface specimens. (a) Alteration strength of new concrete, (b) alteration diameters of bolts, (c) alteration embedded length, and (d) percentage of differences between rough surface specimens with reference model N130Ø12.

Table 5

Pushout test results for group I rough surface specimen

Specimen P u (kN) S u (mm) S max (mm) Failure mode
N130Ø12 169.3 1.58 2.02 Stud shank failure
H130Ø12 202.8 1.28 1.6 Stud shank failure
S130Ø12 135 0.95 1.26 Stud shank failure
N130Ø16 278.54 2.17 2.97 Stud shank failure
N130Ø8 70 1.2 1.4 Stud shank failure
N70Ø12 91.8 1.46 1.64 Stud shank failure
N190Ø12 176.7 1.53 1.87 Stud shank failure
Average 160.6

Note: Pu = shear capacity of the pushout specimen; Su = interfacial slip at maximum load; S max = maximum interfacial slip.

3.3 Ultimate shear strength and slip

The load–slip curves of two sets of bolted connection (rough surface and smooth surface) specimens are compared in Figures 5 and 6, respectively. Tables 5 and 6 summarize the shear bearing capacity and characteristic slip for these two sets.

Figure 6 Load–slip curves of pushout for smooth surface specimens. (a) Alteration strength of new concrete, (b) alteration diameters of bolts, (c) alteration embedded length, and (d) percentage of differences between of smooth surface specimens with reference model PN130Ø12.
Figure 6

Load–slip curves of pushout for smooth surface specimens. (a) Alteration strength of new concrete, (b) alteration diameters of bolts, (c) alteration embedded length, and (d) percentage of differences between of smooth surface specimens with reference model PN130Ø12.

Table 6

Pushout test results for group II smooth surface specimen

Specimen P u (kN) S u (mm) S max (mm) Failure mode
PN130Ø12 126 2.65 3.24 Stud shank failure
PH130Ø12 142.7 1.45 1.76 Stud shank failure
PS130Ø12 90 1. 5 1.7 Stud shank failure
PN130Ø16 179.5 3.2 4 Stud shank failure
PN130Ø8 57.71 1.8 2.3 Stud shank failure
PN70Ø12 60 2.3 2.72 Stud shank failure
PN190Ø12 128.8 2.4 2.7 Stud shank failure
Average 111

Note: Pu = shear capacity of the pushout specimen; Su = interfacial slip at maximum load; S max = maximum interfacial slip.

3.3.1 Group I (rough surface [as a cast] specimens)

To achieve a suitable strength match, this research evaluated the shear strength and ductility of stud connections in NC–NC, NC–UHPFC, and NC–SCM composite structures with constant diameter for bolts (12 mm) and embedded length equal to 130 mm. The test results showed that specimens with UHPFC are 16.5 and 33.4% larger than specimens with NC and SCM, as shown in Figure 5(a). On the other hand, normal concrete specimens have an interfacial slip at maximum load that is 19 and 40% greater than that of UHPFC and SCM specimens, respectively.

The internal porosity of UHPFC is decreased as a result of optimal particle grading and the elimination of large aggregates; also, the compressive strength is considerably enhanced with greater durability, resulting in increased stud shear capacity and stiffness, but because of its high elastic modulus, the ductility of studs would be decreased. The addition of SFs increases the UHPC matrix’s toughness and increases the tensile strength and ductility of UHPFC, which works to stop or slow the development and propagation of cracks.

According to the testing data, the aggregate tends to restrain the cement paste’s shrinkage and helps the surfaces stick together better, so the SCM specimens produce the least ultimate load but their stiffness is greater than that of normal concrete specimens, and the current design formulae are valid for anchors in ordinary plain concrete but not for the shear capacity of anchors in SCM. As a result, when the UHPFC board was constructed with the studs inside it, they failed due to brittleness and after the maximum load the curve had a short and sharp falling branch, but the NC specimens showed ductile behavior of the studs.

When the quantity of steel crossing the contact had shear resistances lower than adhesion, the fracture between the interfaces developed and a sudden slide occurred. A sudden slide was still seen when the connector’s resistance was about equal to the adhesion, but the sustained load was roughly equal to the peak load. When the shear connections’ resistance became considerably larger than the adhesion as the fracture developed, the steel began to gradually accept the load until it yielded. There were no unexpected lapses in these testing [21].

The shear bearing capacity of the bolted connector is compared to that of changing the diameter of bolts (8, 12, and 16 mm) with embedded length (130 mm) and normal concrete. As shown in Figure 5(b), the ultimate strength of specimens with bolt by diameter (16 mm) is about 39.2 and 75% higher than it is for (12 and 8 mm) diameter bolt specimens, respectively. It can be observed that the shear capacity was significantly influenced by the stud diameter. However, the ultimate slip value for specimens with bolt size of 16 mm was 27 and 44.7% higher than the ultimate slip value for specimens with bolt diameters of 12 and 8 mm, respectively. As a result, the slip of the specimens was more significantly impacted by the change in stud diameter. A stud connection can be deemed ductile if its maximum slip is at least 6 mm, according to the criteria of the standards specified in EC4 [22]; for this reason, the stud failure exhibited brittleness characteristics since during this research, the specimens in each group had a maximum slippage that was on average less than 6 mm.

The bolted connector's shear bearing capacity is compared to standard concrete with different embedded lengths (70, 130, and 190 mm) and bolt diameters (12 mm). As shown in Figure 5(c), the ultimate strength of specimens with an embedded length of 190 mm is about 4.1 and 48% larger than specimens with embedded lengths equal to 130 and 70 mm, but the ultimate slip value for specimens with an embedded length of 130 mm is greater than specimens with embedded lengths of 190 and 70 mm by 3.2 and 7.6%, respectively. When the embedded length of the bolts is shorter than 130 mm, the peak load increases dramatically before remaining almost constant. This pattern suggests that just extending the bolt length will not raise the peak pushout load indefinitely. The bolt length for which a longer length results in just a small change in the pushout load is known as the critical anchoring length, and this value is crucial for optimizing support parameters and maximizing environmental advantages. In these pull-out experiments, it can take a length of 130 mm to be the essential anchoring length [23].

3.3.2 Group II (smooth surface specimens)

The shear bearing capacity of the bolted connector is compared with the change in strength of new concrete for smooth surface specimens with a constant diameter for bolts (12 mm) and a constant embedded length (130 mm). As shown in Figure 6(a), the shear capacity of specimens with UHPFC is about 11.7 and 37% larger than the specimens that used NC and SCM for new concrete, respectively. UHPFC has a strong bond to the NC substrate and is well suited for overlay applications; as a result, when using UHPFC for specimens without plates between his parts (group I), it achieves satisfactory results; also, for NC–NC and NC–SCM specimens, the bond strengths were superior to those of specimens with thin plates between the contact surfaces (group II) [24]. So, the slip at peak load for NC specimens is about 45.3 and 43.4% larger than that of UHPFC and SCM specimens, respectively.

With different diameter bolts (8, 12, and 16 mm) with embedding lengths of 130 mm, the bolted connector's shear bearing capacity is compared. The ultimate strength of specimens with bolts that are 16 mm in diameter is about 29.8 and 67.8% more than the ultimate strength of specimens with bolts that are 12 and 8 mm in diameter, respectively. However, as seen in Figure (6b), the slip value for specimens with bolts with a diameter of 16 mm is higher than the slip value for specimens with bolts with a diameter of 12 and 8 mm by 17.2 and 43.8%, respectively.

The shear bearing capacity of the bolted connector is compared with the change in the embedded length (l m = 70, 130, and 190 mm) with constant diameter of bolts (12 mm) and normal concrete. As shown in Figure 6(c), the ultimate strength of embedded length (l m = 190 mm) specimens is about 2.2 and 53.4% larger than specimens with embedded length equal to 130 and 70 mm, respectively, but the ultimate slip value for specimens with embedded length (130 mm) larger than the ultimate slip value for specimens with embedded length (190 and 70 mm) by 9.4 and 13.2%, respectively. As shown in Figure 6(d), the slip between the old and new concrete was more affected by the change in stud diameter.

3.4 Shear stiffness

Slippage would appear between old and new concrete during the pushout test. By taking the secant line on the load–slip curve, it is possible to determine the shear stiffness down a specific load. A highly prominent factor of bolts is the specimen’s shear stiffness. Typically, the elastic shear stiffness is determined by the slope of the secant line at a slip of 0.2 mm [25,26]. The elastic stiffness values of specimens for the two groups are shown in Figure 7 and Table 7.

Figure 7 Shear stiffness of the pushout specimens vs different parameters. (a) Alteration strength of new, (b) alteration diameters of bolts, and (c) alteration embedded length.
Figure 7

Shear stiffness of the pushout specimens vs different parameters. (a) Alteration strength of new, (b) alteration diameters of bolts, and (c) alteration embedded length.

Table 7

Summary of shear stiffness of pushout specimens

Specimen Slip at 0.2 mm
Load (kN) Load per stud (kN) Slip (mm) k (kN/mm)
Group I rough surface
N130Ø12 9.68 2.4 0.2 12
H130Ø12 40 10 0.2 50
S130Ø12 28.8 7.2 0.2 36
N130Ø16 25 6.25 0.2 31.3
N130Ø8 5 1.25 0.2 6.25
N70Ø12 6.7 1.7 0.2 8.5
N190Ø12 12.8 3.2 0.2 16
Average 23
Group II smooth surface
PN130Ø12 7.1 1.8 0.2 9
PH130Ø12 15.6 3.9 0.2 18
PS130Ø12 11.5 2.9 0.2 14.5
PN130Ø16 10.81 2.7 0.2 13.5
PN130Ø8 3 0.75 0.2 3.75
PN70Ø12 3.8 0.95 0.2 4.75
PN190Ø12 9.5 2.4 0.2 12
Average 10.8

3.4.1 Shear stiffness at group I (rough surface specimens)

A comparison of H130Ø12, S130Ø12, and N130Ø12 specimens showed that the shear stiffness of H130Ø12 specimens was 76 and 28% greater than that for N130Ø12 and S130Ø12 specimens, respectively.

In comparison to compressive strength elastic concrete, the diameter has minimal effect on the ductility and stiffness of studs. The shear stiffness values of N130Ø16 specimen are higher than those for N130Ø12 and N130Ø8 specimens by 61.7 and 80%, respectively. However, the shear stiffness values of N190Ø12 specimen are higher than those of N130Ø12 and N70Ø12 specimens by 25 and 47%, respectively.

3.4.2 Shear stiffness at group II (smooth surface specimens)

In this research, we study the elastic stiffness of bolts of smooth surface specimens also, and we found that it is lower than that of the specimens with rough surface by 53%. As a consequence, the smooth surface decreases elastic shear stiffness, allowing the bolts to retain their position and orientation while preventing adhesion between the parts. Calculations for the deflection of composite beams may need to take into account the slip at the steel–concrete interface due to the much lower stiffness of connections.

The shear stiffness of specimens with smooth surfaces PH130Ø12, PS130Ø12, and PN130Ø12 was compared, and the shear stiffness of PH130Ø12 was 50 and 19.4% higher than that of specimens PN130Ø12 and PS130Ø12, respectively. The shear stiffness values of PN130Ø16 specimen are higher than those of PN130Ø12 and PN130Ø8 specimens by 33.3 and 72.2%, respectively. The shear stiffness values of PN190Ø12 specimen are higher than those of PN130Ø12 and PN70Ø12 specimens by 25 and 60.4%, respectively. With an increase in new concrete strength, bolt diameters, and embedded length of the bolts, the shear stiffness of each bolted connector shows an increasing tendency. The correlation coefficient R2 for the linear regression analysis between the shear stiffness and the variables is shown in Table 8.

Table 8

Correlation coefficient R 2 for the linear regression analysis between the shear stiffness and variables

Variables R 2
Group I Group II
The strength of new concrete 0.8072 0.8251
Diameters of bolts 0.9111 0.998
Embedded length of the bolts 0.9985 0.9902

3.5 (Pu/Su) of the specimens

The influence of the variables considered herein in this study on the stiffness of specimens is shown in Figure 8.

Figure 8 Relationship between shear stiffness and a variables (a) diameter ∅ (mm), (b) strength of new concrete (kN/m2), and (c) embedded length of bolts (mm).
Figure 8

Relationship between shear stiffness and a variables (a) diameter ∅ (mm), (b) strength of new concrete (kN/m2), and (c) embedded length of bolts (mm).

3.5.1 Effect of concrete strength

From Figure 8(a), it is noted that the concrete strength has noticeable effect on the shear strength for roughened surface rather than that for smooth surface. This is due to the friction stress beside the dowel action effect. For the studied cases, it is found that the friction stress has contribution of about 56% for the shear stiffness, while the dowel action has 51.7%.

3.5.2 Effect of diameter’s bolts

Figure 8(b) shows that bolt diameter has a significant impact on shear strength for roughened surfaces, which is higher than for smooth surfaces. The friction stress, in addition to the dowel action impact, is to blame. The friction stress contributes roughly 54.6% of the shear strength in the examined situations, whereas the dowel action contributes just about 42.8%.

3.5.3 Effect of embedded length of bolts

From Figure 8(c), the sensitivity to embedment length is more on the shear strength for smooth surface (51.4%) as compared to that of roughened surface (45.3%). This is due to the steel layer at smooth surface specimens between the concrete parts that operate on the bolt attribution.

It is also obvious that upon increasing the embedment length of bolt for smooth surface specimens from 70 to 130 mm and from 130 to 190 mm, the stiffness of specimen change by 45 and 11.5%, respectively. However, for rough surface specimens when the embedment length of bolt is increased from 70 to 130 mm and from 130 to 190 mm, the stiffness of specimens increased by 41.2 and 7%, respectively.

4 Conclusions

In this study, pushout specimens were prepared and tested. The effects of the diameters, embedded length of bolts, and strength of concretes on shear performance were investigated. It is possible to draw the following conclusions. The current conclusion, which is based primarily on experimental data from these tests, recommends for further study to be conducted in the future.

  1. The failure mode of all the pushout specimens was stud shank failure. After the bolts have fractured, there are only a few tiny fractures under the coupler with no local concrete crushing phenomena. By increasing the contact area between the bolt shank and the surrounding concrete, the coupler implanted in the concrete of the bolted connection specimen reduces the local compressive stress of the concrete directly beneath it.

  2. The shear capacity of specimens without plates between parts (rough surface specimens) was 31% greater than that of specimens with plates (smooth surface specimens), obviously, because of the friction stress in addition to the dowel action.

  3. Good results for UHPC are obtained for pushout specimens of both groups; at rough surface H130Ø12 specimen is about 16.5 and 33.4% larger than N130Ø12 and S130Ø12 specimens, respectively. Also at smooth surface, PH130Ø12 specimen is about 11.7 and 37% larger than the PN130Ø12 and PS130Ø12 specimens, respectively.

  4. The shear capacity of specimens can be greatly increased by increasing the diameter of studs. At group I, the shear capacity of N130Ø16 specimens is about 39.2 and 75% larger than N130Ø12 and N130Ø8 specimens, respectively, but at group II the shear capacity of PN130Ø16 specimens is about 29.8 and 67.8% larger than PN130Ø12 and PN130Ø8 specimens, respectively. In the first group, the influence of diameter at shear capacity is greater than in the second group.

  5. Shear capacity of N190Ø12 specimens is about 4.1 and 48% larger than N130Ø12 and N70Ø12, respectively, that for rough surface, but for smooth surface the shear capacity of PN190Ø12 specimens is about 2.2 and 53.4% larger than PN130Ø12 and PN70Ø12 specimens, respectively. From the results it can be seen that when the embedded length of the bolts is less than 130 mm, the peak load increases dramatically before stabilizing. This pattern shows that just extending the bolt length will not raise the max pushout load indefinitely. In these pushout investigations, we can employ a critical bolt length of 130 mm.

  6. N130Ø12 specimens at group I have an interfacial slip at maximum load that is 19 and 40% greater than that of H130Ø12 and S130Ø12 specimens, respectively. So, the slip at peak load for PN130Ø12 specimen at group II is about 45.3 and 43.4% larger than that of PH130Ø12 and PS130Ø12 specimens, respectively.

  7. Effect of strength of concrete on ultimate slip at smooth surface specimens is clearer than rough surface specimens, this is because the effect of the force of bolt adhesion to the concrete in the second group is what works without friction between the parts.

  8. However, each bolted connector specimen’s max slip rises as bolt diameter increases. So, at first group the ultimate slip value for specimens with bolt sizes of 16 mm was 27 and 44.7% larger than the ultimate slip value for specimens with bolts diameters of 12 and 8 mm, respectively. But the second group slips at maximum load value for specimens with 16 mm diameters of bolts larger than the ultimate slip value for specimens with diameters of bolts of 12 and 8 mm by 17.2 and 43.8%, respectively. In the first group, the influence of diameter is greater than that in the second group.

  9. In group I, the specimens with an embedded length of 130 mm had ultimate slip larger than specimens with embedded lengths of 190 and 70 mm by 3.2 and 7.6%, respectively. But the ultimate slip value for specimens in group II with an embedded length of 130 mm is larger than the ultimate slip value for specimens with embedded lengths of 190 and 70 mm by 9.4 and 13.2%, respectively. It was also noted that embedded length has the least influence on ultimate slip rather in both groups.

  10. In this investigation, we looked into the elastic stiffness of bolts of smooth surface specimens as well, and we discovered that they are 53% lower than those of rough surface specimens.

  11. N130 ∅ 12 and S130 ∅ 12 specimens had shear stiffness that was 76 and 28% lower than H130∅12, respectively. For group II, the shear stiffness of PN130 ∅ 12 and PS130 ∅ 12 specimens was 50 and 19.4% lower than he shear stiffness of PH130 ∅ 12 specimen, respectively. From the results, a higher effect is observed for compressive strength in the shear stiffness at first group, and this is very logical because of the friction between the parts at this group in addition to dowel action.

  12. For rough surface, the shear stiffness of N130Ø16 specimens was greater than of N130Ø12 and N130Ø8 specimens by 61.7 and 80%, respectively. But shear stiffness at smooth surface of PN130Ø16 specimens is higher than those of PN130Ø12 and PN130Ø8 specimens by 33.3 and 72.2%, respectively.

  13. However, the shear stiffness values of N19012 specimen are higher than N130Ø12 and N70Ø12 specimens by 25 and 47%, respectively. So, shear stiffness values of PN190Ø12 specimen greater than those of PN130Ø12 and PN70Ø12 specimens by 25 and 60.4%, respectively.

  14. As an inorganic binder, UHPFC has strong bonding performance when used with concrete but has weak adhesion when used with smooth steels, which results in the building’s inability to use its bearing capacity. It may be able to increase by roughening the steel surface’s ability to support the shear connection or using an improved (UHPFC) bonding effectiveness with the surface of steel, which will be the future study. It is suggested that additional research should be done on the possibility of using metal shear connectors in addition to adhesive shear connectors to fully capitalize on each technology’s advantages.

  1. Funding information: The authors state no funding involved.

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Conflict of interest: Authors state no conflict of interest.

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Received: 2022-02-05
Revised: 2022-08-29
Accepted: 2022-09-26
Published Online: 2023-02-21

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/cls-2022-0184
Keywords for this article
bolts connector; pushout tests; shear capacity; load–slip curve; embedded length
Creative Commons
BY 4.0

Articles in the same Issue

  1. Research Articles
  2. Investigation of differential shrinkage stresses in a revolution shell structure due to the evolving parameters of concrete
  3. Multiphysics analysis for fluid–structure interaction of blood biological flow inside three-dimensional artery
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