Bond Behavior of Reinforcing Steel in Ultra-High Performance Concrete

CHAPTER 2. EXPERIMENTAL INVESTIGATION

INTRODUCTION

The research discussed herein focuses on the assessment of bond performance of deformed bar in field-cast grout. This is an ongoing research program at the FHWA Turner-Fairbank Highway Research Center as part of a larger effort focused on developing innovative connection details for prefabricated bridge components. This report mainly presents the results of the bond behavior between deformed bar and UHPC. Direct tension pullout tests were conducted. The experimental setup is presented in this chapter.

Details of the UHPC formulation investigated in this study, included the proportioning of the UHPC material and its compressive strength properties, are presented first. Then the deformed steel bar properties are reported. Both normal strength Grade 60 bar, including uncoated and epoxy coated, and high strength Grade 120 uncoated bar were used in the study and their yield strength, tensile strength, and deformation properties are reported. Next, the details of the specimen preparation and the pullout tests configuration are presented. Finally, the design philosophy of the test matrix is introduced.

UHPC FORMULATION

The UHPC used for this research is a product produced by Lafarge North America. The specific product tested is Ductal JS1212 and mix proportions are shown in Table 2.

Table 2. UHPC mix design.

As shown in Table 2, this UHPC formulation contains premix power, water, Premia 150 (a modified phosphonate plasticizer), Optima 100 (a modified polycarboxylate high-range water-reducing admixture), Turbocast 650A (a non-chloride accelerator), and steel fibers. The steel fibers included in this mix design were nondeformed, cylindrical, high-tensile strength steel. They have a diameter of 0.008 in. (0.2 mm) with a length of 0.5 in. (12.7 mm). The steel tensile strength is specified to be greater than 290 ksi (2000 MPa). The steel fibers have a thin brass coating which provides lubrication during the drawing process and provides corrosion resistance for the raw fibers. A constant steel fiber content of two percent by volume was used in this study.

UHPC COMPRESSIVE STRENGTH

Alongside each batch of pullout test specimens, a set of three compression test cylinders were cast. All cylinders were 3 in. (76.2 mm) nominal diameter with approximately 6 in. (152.4 mm) lengths. These cylinders were cast at the same time as the pullout test specimens. After each cylinder mold was filled, the cylinder was briefly vibrated on a vibrating table to assist in the release of entrapped air. The cylinders were then finished with a magnesium hand float and covered in plastic. The cylinders were cured alongside the pullout test specimens in the ambient laboratory environment.

The compressive mechanical testing was completed through modified version of the ASTM C39 Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens.⁽¹¹⁾ The employed test method has been engaged multiple times in the past.^(6,12,13) From the standpoint of the ASTM C39 test method, the load rate was increased from 35 psi/second (0.24 MPa/second) to 150 psi/second (1.0 MPa/second) due to the high compressive strength of UHPC and the duration of test that would result from the slower load rate.

The compressive strength for each of UHPC batch is reported in Chapter 3, alongside the pullout test results. The UHPC used in this study had an average compressive strength of 13.5 ksi (93 MPa) at one day, with a minimum of 11.7 ksi (81 MPa) and a maximum of 14.2 (98 MPa) for 33 specimens tested in 11 batches. It had an average compressive strength of 19.4 ksi (133 MPa) at seven days, with a minimum of 18.5 ksi (128 MPa) and a maximum of 20.5 (141 MPa) for 21 specimens tested in 7 batches. It had an average compressive strength of 21.3 ksi (147 MPa) at 14 days, with a minimum of 20.3 ksi (140 MPa) and a maximum of 22.2 (153 MPa) for six specimens tested in two batches. The majority of the tests in this study were conducted at one day or seven days after casting.

REINFORCING STEEL

The properties of reinforcing steel used in this study are reported in this section. The bar type and size used in the study are presented first, then the yield strength and tensile strength for each bar are reported. The deformation properties and rib pattern of the reinforcing bar are also included in this section.

Three types of reinforcing bar were tested in this study, including normal strength Grade 60 uncoated and epoxy coated bar and high strength Grade 120 uncoated bar. All the Grade 60 uncoated and epoxy coated bar meet the specification of ASTM A615⁽¹⁴⁾, and will be referred to as uncoated and epoxy coated bar, respectively; all the high strength Grade 120 uncoated bar meets the specification of ASTM A1035⁽¹⁵⁾, and will be referred to as A1035 bar in later discussions. The Grade 120 high strength bar is manufactured by MMFX Technologies Corporation. The bar sizes tested in the study include uncoated bar No.5, epoxy coated bar No.5 and No.8, and A1035 bar No.4, No.5, and No.7. The steel bar mechanical properties were tested following ASTM A370⁽¹⁶⁾. Two bars were tested for each nominal size used in the pull out specimens. Strain was measured with an 8 in. (203 mm) extensometer. The tests were conducted under displacement control. For the Grade 60 bars, the free-running rate of separation of the crosshead was adjusted to have a loading speed of 0.003 in. per min per inch until the strain in the extensometer reached 0.1%; then speed was adjusted to 0.002 in. per min per inch and the specimen was loaded until the strain reached 1%, and at the this point, the extensometer was removed; the test was then continued at a speed of 0.03 in. per min per inch until the bar fractured. For the Grade 120 bars, the free-running rate of separation of the crosshead was adjusted to have a loading speed of 0.006 in. per min per inch until the strain in the extensometer reached 0.5%; then speed was adjusted to 0.003 in. per min per inch and the specimen was loaded until the strain reached 2.5%, and at the this point, the extensometer was removed; the test was then continued at a speed of 0.03 in. per min per inch until the bar fractured. The yield strength was determined using the 0.2% offset method. The tensile stress versus strain curve for each type and size reinforcing bar is presented in Figure 1, and the yield and tensile strengths are listed in Table 3. In general, as shown in Figure 1 and Table 3, the uncoated has a yield strength of approximately 75 ksi (517 MPa) and tensile strength of 118 ksi (814 MPa) while the yield and tensile strength of epoxy coated bar are 70 and 108 ksi (483 and 745 MPa), respectively. The A1035 bar has a yield strength (0.2% offset method) of approximately 130 ksi (896 MPa) and tensile strength of 170 ksi (1172 MPa). All of the bar tested in this study exhibited nearly identical stress-strain response from the initiation of loading through the attainment of a tensile stress of 68 ksi (469 MPa).

Figure 1. Graph. Tensile stress strain response of reinforcing bars.

Table 3. Properties of Reinforcing Steel

^† Per ASTM A370.
*Per ASTM A615 and A1035.
**Per ACI 408R-03⁽¹⁷⁾ and ACI 408.3R-09⁽¹⁸⁾ for calculation of relative rib area.
Note: 1 in. = 2.54 cm, 1 ksi = 6.895 MPa.

The rib pattern for each type and size of the reinforcing bar used in the study is demonstrated in Figure 2. The A1035 high strength bar and the normal strength uncoated and epoxy coated bar were chosen to have similar rib patterns, as shown in Figure 2. The bar deformation, rib height and spacing, were measured following ASTM A615 and ASTM A1035. The height was determined from measurements made on three deformations. The spacing was determined by measuring the length of a minimum of ten spacings and dividing that length by the number of spaces included in the measurement; the average of two measurements was reported. The relative rib area, calculated as the ratio of the bearing area of bar deformations to the shear area between the deformations per ACI 408R-03⁽¹⁷⁾, is also presented in Table 3. As shown in Table 3, they all had similar relative rib areas, mostly in a range of 0.083 to 0.088 with the exception of A1035 No.4 and No.7 bars having a relative rib area of 0.074 and 0.099, respectively.

Figure 2. Photo. Reinforcing bar rib pattern.

TEST SETUP AND PROCEDURES

Direct tension pullout tests, with a novel test specimen design and associated loading apparatus, were conducted in this study. The test setup was developed so as to mimic the tension-tension lap splice configuration that may be encountered in a field-deployed connection system.

The pullout tests speicmens were UHPC strips cast on top of precast concrete slabs, as shown in Figure 3. The No.8 bars extended 8 in. (20.3 cm) from the pre-cast concrete slab. UHPC strips were cast on top of the precast slab with the No. 8 bars in the center of the strips. Each tested bar was situated so as to be embedded into the UHPC strip and located between two of the No. 8 bars.

Each precast concrete slab has dimensions of 4 × 8 × 1 ft (1.2 × 2.4 × 0.3 m) (width × length × depth) and the spacing between the extended No.8 bars in the longitudinal direction (along the UHPC strip as shown in Figure 4) is either 8 or 12 in. (20.3 or 30.5 cm). More details of specimen layout on the precast slab are illustrated in Figure 4.

In Figure 3 and Figure 4, notations were assigned to represent dimension parameters, including c_so for the clear side cover, 2c_si for the clear spacing between the testing bar and the extended No. 8 bars, l_d for the embedment length of testing bar measured from the top surface of the UHPC strip to the end of the testing bar, and l_s for the lap splice length measured from the end of the testing bar to the end of extended No. 8 bars. These are also the main factors that will be investigated for their effect on bond strength in this study. The notations of c_si, c_so, l_d, and l_s are adopted from ACI 408 R-03 “Bond and Development of Staright Reinforcing Bars in Tension.”⁽¹⁷⁾

Figure 3. Illustration. Overall configuration of test specimens.

Figure 4. Illustration. Pullout test specimens layout.

The pullout tests were conducted using the fixture showing in Figure 5 and Figure 6. A hydraulic jack (Enerpac^® Cylinder, Model RRH1508) was placed on a steel chair, and the steel chair stands on the precast slab. When a pullout force is applied, the fixture reacts against the precast slab. With such a setup, the reinforcing bars being tested as well as the extended No. 8 bars are both placed in tension. The UHPC surrounds these bars transfers the loads between them. This test setup simulates structural configurations wherein lap spliced reinforcement is loaded in tension.

Tests were conducted by applying a load to the free end of the embedded reinforcing bar. The load was applied under a closed-loop displacement control by adapting a servo valve into the system, which was attained with an MTS Flex Test GT controller uing a linear variable differential transformer (LVDT) for feedback. The jack was run at a constant displacment rate of 0.2 in./min (5 mm/min) as measured by the LVDT which captured the displacement at the bar chuck relative to the top of the jack. The top of the jack was approximately 30 in. (76 mm) above the precast slab and the bar chuck started approximately 36 in. (91 mm) above the top of the precast slab. A load cell located between the jack and the bar chuck measured the load applied to the bar. The bar displacement was measured at a location of approximately two inches (5.1 cm) above the top surface of UHPC strip, as shown in Figure 7. Three LVDTs were arranged in 120-degree angle and the average displacement of the three LVDTs were used to offset the possible bending of the loaded bar. The load cell and all LVDTs were calibrated to the MTS DUC conditioner. The load cell used in this study was Strainset^® model FL100U(C)-2DGKT (S/N 08905-7) univeral flat load cell. The LVDTs used in this study were Omega^® model LD300-150 (for displacement control) and Omega^® model LD320-25 (for displacement measurement at the bottom).

Figure 5. Illustration. Loading setup.

Figure 6. Photograph. Loading setup.

Figure 7. Photograph. Displacement measurement via three LVDTs.

SPECIMEN PREPARATION

The UHPC materials tested in this study contained 2% (by volume) steel fibers. Casting technique can influence the dispersion and orientation of the fiber reinforcement. In this study, the UHPC strips were prepared using plywood forms. Two cast orientations were compared, as shown in Figure 8. The first orientation involved casting the specimen on its side as shown in Figure 8a and Figure 8b; the second orientation involved casting the specimen upright as shown in Figure 8c and Figure 8d, where the slab was placed with a small slope of approximately 1.5 degree to facilitate the flow of the UHPC. For both orientations, the UHPC was first poured in from one end and allowed to flow until the forms were mostly filled. Thereafter, the UHPC was poured in from the middle locations. The orientation of the casting was not observed to have a significant effect on bond behavior as will be discussed in Chapter 3. For the large majority of test specimens, the upright casting orientation was selected.

Figure 8. Photograph. UHPC strip casting setup and orientation: (a) side pour setup; (b) side pour casting; (c) upright pour setup; and (d) upright pour casting.

The actual dimension of c_so, 2c_si, l_d, and l_s were measured. The side covers can be easily measured by taking the distance from the sides of the UHPC strip to the bar under consideration after forms are removed; the least side cover is reported as c_so. The clear spacing between the testing bar and adjacent two No. 8 bars was determined by taking the difference between the spacing of the extended No. 8 bars before casting and the spacing of the testing bars after casting; the smaller value of the spacing to adjacent No.8 bars is used as 2c_si. The actual embedment length (l_d) was determined by substracting the exposed length of the bar after casting from the original length of the bar and the spliced length (l_s) was calcuated as l_d – (strip height – 8 in.), where the 8 in. (203 mm) is the length of No. 8 bars that is extended outside the precast slab.

The forms were normally removed at 22 ± 1 hours after casting so the one–day testing can start at 23 ± 1 hours after casting.

TEST MATRIX

The objective of this study was to evaluate the bond behavior between the deformed reinforcing steel (lap spliced) and UHPC. The primary parameters investigated included the embedment length of reinforcing steel, concrete side cover, bar spacing, UHPC compressive strength, type of deformed bar, and size of deformed bar. Throughout the study, in order to better assess the influence of a particular variable, each individual parameter was varied while others remained constant. Each of the above mentioned variables will be evaluated and the results are presented in Chapter 3. The test matrix is provided in each section where the individual parameter is evaluated in Chapter 3.