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• Index, Structural Behavior of Ultra-High Performance Concrete Prestressed I-Girders

Index, Structural Behavior of Ultra-High Performance Concrete Prestressed I-Girders

CHAPTER 3. GIRDER MATERIAL PROPERTIES

3.1 Test Specimen Casting, Harvesting, and Preparation

The initial UHPC AASHTO Type II girder test program included a limited amount of work on material characterization. During the casting of the two girders, which will be discussed in chapter 4, three cylinders and three prisms were cast for future testing. The cylinders had a 76-mm (3-inches) diameter, and the prisms were 51 by 51 by 279 mm (2 by 2 by 11 inches). The limited availability of test specimens increased the importance of each specimen. It also forced the use of specimens harvested from undamaged areas of the girders after the girder tests were completed. Both the cores and prisms were cut from the girders. The method used is described below.

The cast cylinders and prism molds were filled according to standard fiber reinforced concrete casting methods. These specimens were treated to the same curing environment that was used for the girders throughout the fabrication process. The prisms required no special preparation, while the cylinders had their ends cut and ground to ensure parallel loading surfaces. This process resulted in 76-mm (3-inches) diameter cylinders with approximately 127-mm (5-inches) lengths.

After the girder tests were complete, cores and prisms were harvested from Girder 28S. The web and top flange at the east end of Girder 28S were not damaged during either the testing of this girder or of its parent girder, Girder 80F. Six cores with approximately 102-mm (4-inches) diameters were obtained by drilling vertically through the top flange of the girder between the strands. An additional 16 prisms were cut from the web of the girder. These prisms had a nominal cross section of 51 mm by 51 mm (2 inches by 2 inches), were approximately 406 mm (16 inches) long, and had a long axis that ran parallel to the length of the girder. Finally, five additional cores were drilled from the web of the girder. These cores had diameters of 70 mm (2.75 inches).

The preparation of the harvested cores included grinding the ends of the cores, measuring the cores to ensure parallel ends, and measuring their lengths and diameters. This method was similar to the method used within the larger material characterization program. The preparation of the harvested prisms was somewhat more extensive than would be expected for cast prisms. Because these prisms were cut from a large block of UHPC using a diamond saw, the cross-sectional dimensions of the prisms varied and had to be measured. The cut prisms were measured at eight locations along their lengths on all four faces of the prism. The average cross-sectional dimensions for each prism were then calculated.

3.2 UHPC Compression Testing

In total, 14 compression tests were completed on cylinders or cores composed of UHPC that was associated with the 2 AASHTO Type II girders. In general, these compression tests were performed in a 4,450-kN (1,000-kip) capacity Forney test machine. The lone exception was cylinder "3," which was tested in a 1,780-kN (400-kip) capacity Universal testing machine. The cylinders were all loaded at approximately 240 kPa/sec (35 psi/sec) as specified in American Society for Testing and Materials (ASTM) C39.⁽¹⁶⁾ The cylinders were all loaded directly to failure. The cores underwent two unloads between 40 percent and 10 percent of their projected strengths before being loaded to failure.

Most of the compression specimens were instrumented with resistance-based strain gages prior to testing. These gages were applied either axially or circumferentially on the outside of the cylinder. In general, four axial gages and two transverse gages were used on each specimen to capture the strain response while eliminating errors caused by uneven loading. Table 4 lists the strain gage configuration applied to each specimen. Gages with a 12.7-mm (0.5-inch) length were used in all instances except for cylinder "3," which had axial gages with a 25.4-mm (1-inches) length.

Table 4 provides the results from the cylinder and core tests. With the exception of core CW-5, the results show relatively little scatter. These results indicate that the compressive strength is 200 MPa (29 ksi), the modulus of elasticity is 52.4 GPa (7,600 ksi), and the strain at peak strength is 0.0043. In terms of transverse behavior, the Poisson's ratio is 0.18 with a transverse modulus of elasticity of 29.4 GPa (42,600 ksi).

These UHPC compression tests also reiterated a number of the qualitative findings reported in the associated UHPC material characterization report.⁽²⁾ Namely, the cores and cylinders tended to exhibit a dramatic decrease in load-carrying capacity soon after the peak load was reached. However, the specimens remained largely intact throughout the failure. Again, this is due to the presence of the steel fiber reinforcement.

3.3 UHPC Flexural Prism Testing

Monotonic flexural prism testing was successfully completed on two cast and six cut prisms associated with the AASHTO Type II girders. The prisms all were tested in four-point bending in MTS load frames. As previously mentioned, the prisms all had nominal cross section dimensions of approximately 51 by 51 mm (2 by 2 inches). The span lengths ranged from 152 to 305 mm (6 to 12 inches). The cast prisms were oriented in the load frame such that the top and bottom of the prism as cast became the back and front of the prism as tested.

The load rate varied depending on the particular specimen being tested. Some prisms underwent essentially static loading wherein the load level was increased slowly enough so that it took minutes before the cracking load was reached. Other prisms were loaded much more rapidly, at up to 6.7 kN/sec (1,500 lb/sec).

All prisms were instrumented with at least two resistance-based, bonded strain gages on their top and bottom flanges. The gages had either a 12.7-mm (0.5-inch) or a 25.4-mm (1.0-inch) gage length and were centered on the midspan of the loading. The gages on the bottom flange served the dual purpose of indicating the strain level before first cracking and recording the first cracking event.

The results from these tests are presented in table 5. The cracking strain listed is the average strain recorded in the tension flange of the prism at first cracking. The two values of cracking stress shown are calculated via different, but equally viable, methods. In the left column, the cracking stress is calculated using the cracking strain and the assumptions of pure bending and the applicability of Hooke's law. The right column calculates the cracking stress based on the load at cracking, the cross-sectional dimensions, the loading configuration, and the assumption of pure bending. The value presented in the right column corresponds to the tensile stress that would be calculated based on the ASTM standard test methods for prism flexure testing.^(17,18) Because these prisms all were tested on relatively short spans, it is unlikely that either of these methods of calculating the cracking stress is truly accurate. The cracking strain results show that the strain at first cracking is approximately 0.0003. The cracking stress results indicate that an average cracking stress between 15 and 16.5 MPa (2.2 and 2.4 ksi) could be expected depending on the method used to calculate the result.

Because these prism flexure tests were not controlled based on the actual deflection of the prisms, direct comparisons from postcracking results obtained here cannot be compared with the results presented in the associated UHPC material characterization report.⁽²⁾ However, these results are still relevant because they provide information related to the qualitative postcracking response of UHPC. The load versus crosshead deflection response of cast prism "1" is shown in figure 2. The deflection is normalized based on the deflection at first cracking. Note the continued increase in load after first cracking occurs, until a peak load greater than twice the cracking load is reached. In general, the shape of this load-deflection response is very similar to the responses presented in the prism testing section of the associated report.⁽²⁾

†"A" indicates axial strain gage. "T" indicates transverse gage.
1 MPa = 145 psi
1 mm = 0.039 inch
1 GPa = 145 ksi

† Based on pure bending, Hooke's Law, the cracking strain, and an assumed modulus of elasticity of 52.4 GPa.
‡ Based on loading configuration, cross-sectional dimensions, and load at first cracking assuming pure bending.
1 MPa = 145 psi
1 mm = 0.039 inch

Figure 2. Graph. Third-point loading response of a 2-inch by 2-inch prism on a 9-inch span.