John, Ruiz, Floyd, and Hale 1 TRANSFER AND DEVELOPMENT LENGTH AND PRESTRESS LOSSES IN ULTRA-HIGH PERFORMANCE CONCRETE BEAMS

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1 John, Ruiz, Floyd, and Hale TRANSFER AND DEVELOPMENT LENGTH AND PRESTRESS LOSSES IN ULTRA-HIGH PERFORMANCE CONCRETE BEAMS Word count: 3830 Number of figures and tables: 9 Emerson E. John Graduate Research Assistant 4190 Bell, 1 University of Arkansas Fayetteville, AR eejohn@uark.edu Fax: Tel.: Edmundo D. Ruiz Graduate Research Assistant 4190 Bell, 1 University of Arkansas Fayetteville, AR edmundoruizc@hotmail.com Fax: Tel.: Royce W. Floyd Graduate Research Assistant 4190 Bell, 1 University of Arkansas Fayetteville, AR rfloyd@uark.edu Fax: Tel.: W. Micah Hale (corresponding author) Associate Professor 4190 Bell, 1 University of Arkansas Fayetteville, AR micah@uark.edu Fax: Tel.:

2 John, Ruiz, Floyd, and Hale ABSTRACT Results from a project that measured the transfer and development length and prestress losses of beams cast with ultra-high performance concrete (UHPC) are presented. Seven beams measuring 6.5 in. by 12 in. by 18 ft. were cast. The beams had compressive strengths that ranged from approximately 27 ksi to 29 ksi. The results are compared with the calculated values derived from the American Association of State Highway and Transportation Officials Load and Resistance Factor Design (AASHTO LRFD) bridge design specifications, National Cooperative Highway Research Program (NCHRP) Report 603, and the University of New South Wales (UNSW) design guidelines for prestressed UHPC beams. The research findings indicate that measured transfer and development lengths are shorter than those predicted using the various design guidelines and recommendations developed for conventional concrete and high-strength concrete. However, measured transfer lengths correlate well with those predicted using the NCHRP 603 (neglecting f c limitations) and the University of New South Wales recommendations and guidelines. The AASHTO LRFD specifications overestimated transfer length and development length and prestress losses (refined method) by approximately 50 percent. The data also show that the contribution of creep and shrinkage in prestress losses is minimal. Key words: Transfer length, development length, prestress losses, ultra-high performance concrete

3 John, Ruiz, Floyd, and Hale INTRODUCTION For centuries engineers and researchers have developed new materials and approaches to solve engineering challenges. In the area of bridge construction, finding new ways to construct structurally efficient and durable bridges that will require minimal maintenance at a reasonable cost is one of those challenges. There is a new development in the concrete industry, ultra-high performance concrete (UHPC). This material possesses compressive strengths in excess of 20 ksi [1] and has very low permeability which enhances its durability. The use of this material in the construction of precast, prestressed bridge girders has many benefits. These include improved resistance to chloride penetration, increased span length for a given girder depth and spacing, reduction in the depth and number of girders for a given span and a reduction in the dead load carried by the substructure [2]. The number of bridges, be it highway or pedestrian, constructed to date which used UHPC is comparatively low. The first reported use of UHPC in highway bridge construction was in France in 2001 [2]. Since then, the state of Iowa has used UHPC in the construction of two bridge structures [3]. Several other pedestrian and highway bridges were completed in Europe, Asia and Australia during this decade [2]. UHPC is a new product and for this reason there are limited design methodologies that can be used by engineers to aid in the implementation of a UHPC project. Therefore, the objective of this paper is to present experimental data for transfer and development lengths and prestress losses in prestressed UHPC beams, the results of which can add to the existing knowledge of the performance of UHPC structural elements. BACKGROUND UHPC was developed in France in the 1990s [3]. It is a mixture of fine sand, cement, silica fume, quartz flour and steel fibers. The proportions are determined based on optimized granular packing of the different constituent materials [1]. Typically, UHPC has water to cementitious materials (w/cm) ratio of 0.15 to 0.19 [3]. The inclusion of steel fibers enhances the ductility of the concrete and is added to the mixture in amounts of approximately 2 percent by volume [3]. The compressive strength of UHPC can be in excess of 20 ksi depending on the mixing and curing procedures [1]. Researchers at Ohio University completed a study of the bond of prestressing strands in UHPC [4]. The investigation focused on determining the development length for 0.50 in., Gr. 270, low-relaxation prestressing strands cast in UHPC. Different lengths of strands were cast into blocks of UHPC, then the strands were pulled until slip or strand rupture. Embedment lengths (L E ) were 12, 18, and 24 in. The researchers reported strand fracture occurring before significant strand slip and thus concluded that the development length of the prestressing strands when cast in UHPC is less than 12 in. Graybeal [5] investigated the development length of prestressing strand in UHPC prestressed I-Girders. The AASHTO Type II girders contained 0.50 in., Gr. 270, lowrelaxation strands. Results from one of the tested beams showed that strand rupture occurred prior to slip under flexural tensile loading at an embedment length of 48 in. In another beam test, rupture of the prestressing strands occurred prior to slip at an embedment length of 37 in. which was in a region of very high shear stresses. These results indicate that the

4 John, Ruiz, Floyd, and Hale development length of 0.50 in., Gr. 270, low-relaxation prestressing strands in UHPC is less than 37 in. Hegger and Bertram [6, 7] examined the shear capacity and the anchorage behavior of pretensioned strands in UHPC. Their results indicated that transfer lengths ranged from 10 to 12 in. Measured prestress losses were 18 to 24 percent of the initial stress in the strands. Almansour and Lounis [2] stated that total prestress losses can be estimated as 17 percent of the ultimate strength of the prestressing tendons. This gives a value for total prestress losses of approximately 46 ksi for Gr. 270 strands RESEARCH OBJECTIVE The objective of this research is to measure the transfer and development lengths and prestress losses in UHPC beams and compare the results to values derived from design specifications and guidelines from the literature. This empirical procedure is carried out with the aim of evaluating the accuracy of the equations found within the literature in predicting transfer and development lengths and prestress losses. EXPERIMENTAL PROGRAM Seven UHPC beams with dimensions of 6.5 in. by 12 in. and 18 ft. were cast. For each beam, the transfer length, development length, and prestress losses were measured. The beams contained two 0.60 in., Gr. 270, low-relaxation prestressing strands located 10 in. from the extreme compression fiber. The beams contained no mild reinforcement for shear or flexure. Ductal was the UHPC mixture used and was supplied by Lafarge North America Inc. The mixture proportion is shown below in Table 1. The steel fibers were straight steel wire fibers, also supplied by Lafarge North America, with a diameter of in. and a length of 0.50 in. The fibers have a minimum tensile strength of 377 ksi and an elastic modulus of 29,000 ksi. The beams were batched and cured at the University of Arkansas Engineering Research Center. The UHPC was mixed in a rotating drum mixer, and the beams were cured at 40 o C for 4 days then at 60 o C for 3 or 4 days. The beams were cast during the winter months which affected the duration of the heat treatment. The mixing, casting and curing procedures are discussed in detail in earlier publication by the authors [8]. The beams are shown in Figure 1. TABLE 1 UHPC Mixture Proportion Materials UHPC Proportions (lb/yd 3 ) Ductal premix 3696 Water 219 Steel fibers 263 HRWR 51

5 John, Ruiz, Floyd, and Hale FIGURE 1 Finished UHPC beams after strand release and curing. Transfer Length Evaluation After curing, the beams were instrumented with Detachable Mechanical Strain Gage (DEMEC) targets placed at both ends of the beams on both faces, for a distance of 44 in. at 4 in. intervals. The targets were placed at the center of gravity of the prestressing strands. Readings were taken immediately before release of the prestressing strands, within one to two hours after release and periodically up to 28 days. Development Length Evaluation Evaluation of development length (l d ) was performed using flexural load tests. A point load was applied to the beams at predetermined distances (L E ) from the beam end. Linear voltage displacement transducers (LVDTs) were used to measure strand slip. A LVDT was attached to each strand at both ends of the beam being tested. Readings from the LVDTs were continually monitored through the data acquisition system to detect the beginning of any strand slip. A diagram of the test set up is shown below in Figure 2. The behavior of the strands at failure determined whether L E was longer or shorter than the development length. Evidence of strand slip occurring before the full moment capacity was reached signified that the L E is shorter than the development length. A subsequent test then used a longer embedment length. The opposite also applies. That is, if there were no strand slip after the beams have attained their nominal moment capacity, then a shorter L E was used in a subsequent test. Where both flexural failure and strand slip occur simultaneously, L E equals the development length of the prestressing strands.

6 John, Ruiz, Floyd, and Hale FIGURE 2 Development Length Test Configuration. Prestress Loss Evaluation To measure prestress losses, the beams were instrumented with vibrating wire strain gages (VWSG). The VWSG were placed at the center of gravity of the prestressing strands and located at midspan in six of the seven beams. For the seventh beam, the gage was located slightly off center, approximately 8 ft from one end. Readings were taken before strand release, immediately after strand release, and periodically for approximately 60 days after casting. RESULTS Material Properties The material properties of the seven beams are shown below in Table 2. Also shown in Table 2 is the day when the prestressing strands were released. Due to weather related issues and an accidental overdose of HRWR, the strands were released at 4 or 8 days of age. The compressive strength at release (f ci ), measured using 4 x 8 in. cylindrical specimens, ranged from ksi to ksi. At the time of testing, the researchers did not have access to an end grinder, and therefore the cylinders were tested using end caps containing neoprene pads. This explains the large amount of scatter in the data. Replicate cylinders were end ground and tested and these results are labeled f c in the table below. These cylinders were tested approximately five months after the beams were cast. Excluding specimens from Beam UHPC-3, which never recovered from an overdose of HRWR, the compressive strengths were between ksi to ksi. The modulus of elasticity for beams UHPC-1, 5, 6, and 7 ranged from 7750 ksi to 8500 ksi. These were measured following the release of prestress.

7 John, Ruiz, Floyd, and Hale TABLE 2 Measured Compressive Strengths of the UHPC Beams Average Compressive Strengths (ksi) Modulus of Beam Release Neoprene pads End grinding Elasticity (ksi) f ci f c E c UHPC-1 4-day UHPC-2 a 8-day UHPC-3 a 8-day UHPC-4 4-day UHPC-5 4-day UHPC-6 4-day UHPC-7 4-day a - accidental overdose of high range water reducer Transfer Length Transfer lengths were determined using the 95% average maximum strain (95% AMS) method reported by Russell and Burns, 1997 [9]. Using this method, the transfer length is defined as the distance from the end of the beam to the point where 95% of the average maximum concrete strain is measured. The measured transfer lengths for the live ends and dead ends of the beams are shown in below in Table 3. Also shown in the table are transfer lengths calculated using predictions equations. The first equation shown in the table is from AASHTO LRFD specifications [10]. AASHTO LFRD also allows for approximating transfer length using 60d b which is the second equation shown. The third column of predicted values was calculated using the results from NCHRP Report 603 [11] but neglecting the 9 ksi limit on f ci. The resulting transfer length equation (Equation 1) is shown below: l t 120 = d ' f ci In the design guidelines for prestressed reactive powder concrete (RPC), also known as UHPC, Gowripalan and Gilbert (referred to UNSW from hereafter) estimate transfer length between 20 and 40d b [12]. This range is independent of the strand diameter. They explained that for the design of the anchorage zone, it is recommended that the lower end of the range be selected as the length over which the concentrated prestressing force is transferred to the concrete. This is a conservative value which will result in the largest transverse tension within the anchorage zone. Conversely, a check for stresses on a cross section near the end of the beam or a check for shear strength of such a section should use a transfer length value closer to the upper end of the range. b [1]

8 John, Ruiz, Floyd, and Hale TABLE 3 Measured and Predicted Transfer Lengths Beam Measured Predicted Values (in.) f pe (ksi) Values (in.) at (f days pe /3)d b 60d b live dead 20d b end end 3 40d b UHPC UHPC UHPC UHPC UHPC UHPC UHPC = AASHTO Eqn. 2 = NCHRP Report 603 Eqn. 3= University of New South Wales Eqn. Development length The results of the development length tests are summarized in Table 4. In the table are values for nominal capacity (M n ), maximum moment (M max ) attained by the beams during testing, and moment (M slip ) when strand slip occurred. Two methods were used to calculate nominal moment capacity; strain compatibility and the UNSW guidelines. It can be seen that the values obtained using strain compatibility are overly conservative. This can be explained by the fact that the strain compatibility analysis does not account for the material s ability to carry load in tension while the UNSW procedure does. The values of M n obtained from the UNSW procedure are much closer to the measured maximum moment (M max ). Development lengths were calculated using the AASHTO LRFD Bridge Design Specifications (Equation 2) and the recommendations of NCHRP Report 603 (Equation 3). Once again, the compressive strength limitations are neglected in the NCHRP equation. The measured and predicted values are shown in Table 5. l d 2 κ f ps f pe db [2] ld = + db 100d b [3] ' ' f ci f c

9 John, Ruiz, Floyd, and Hale TABLE 4 Summary of Flexural Tests Result Beam L E (in) M n (kip-in) 4 M n (kip-in) 5 M slip (kip-in) M max (kip-in) UHPC UHPC * UHPC b UHPC UHPC UHPC UHPC no slip occurred * exceeded the capacity of the load actuator b results from the low compressive strength reported in Table 2 4 strain compatibility analysis 5 UNSW guidelines TABLE 5 Comparisons of Calculated Versus Measured l d Beam f pe f ps l L (ksi) (ksi) E (in) d (in) Measured AASHTO NCHRP UHPC < UHPC UHPC < UHPC < UHPC < UHPC < UHPC < Prestress Losses Shown below in Table 6 are the section properties, material properties, measured and calculated elastic shortening, and the measured and calculated prestress losses for the beams. The material properties and cross-sectional properties were used to calculate the losses using the refined method in the AASHTO LRFD. The modulus of elasticity at strand release, E ci, was calculated using recommendations from Graybeal s study (Equation 4). E ci = c [4] 46,200 f '

10 John, Ruiz, Floyd, and Hale Table 6 Measured and Calculated Prestress Losses Beam Specimen Properties UHPC-1 UHPC-2 UHPC-3 UHPC-4 UHPC-5 UHPC-6 UHPC-7 Measured Concrete Properties Unit Weight (lb/ft 3 ) f ci (ksi) f c (ksi) E c (ksi) Calculated Concrete Property E ci (ksi) Cross-Section Properties A tr (in 2 ) y tr (in) I tr (cm 4 ) General Information Age (days) VWSG Loc. (ft) Measured Losses Elastic Shortening (ksi) Total Losses (ksi) Total Losses + RE Predicted Losses Elastic Shortening (ksi) RE (ksi) Total Losses (ksi) Measured Losses/Predicted Losses Elastic Shortening (ksi) Average 0.74 Total Losses + RE Average 0.55 DISCUSSION OF RESULTS The measured transfer lengths obtained from this research ranged from 12 in. to 16 in. at the live end of the beam and from 11 in. to 18 in. at the dead end. Comparisons of these values to those obtained using UNSW equations indicate that Gowripalan and Gilbert s [12] range of 20d b to 40d b for transfer length to be a very good estimate. The measured values are also in good agreement with the values obtained from the NCHRP method when the lower bound of 40d b and the 9 ksi limit on f ci are neglected. On average, transfer length values based on AASHTO Specifications (60d b ) are more than twice of the measured values. The AASHTO LRFD equation is overly conservative which is expected since the equations are not intended for members cast with UHPC. During the flexural tests, six of the seven beams failed in flexure. Strand slip was observed in UHPC-1-3 and 5 at embedment lengths of 25, 25, and 35 in., respectively. Slip

11 John, Ruiz, Floyd, and Hale occurred when the applied moment was in excess of M n (using strain compatibility) by more than 50%. When using UNSW guidelines, slip occurred at moments 8 and 26% greater than the calculated capacity. The results indicated that the development length is between 25 and 35 in. which is similar to the 37 in. obtained by Graybeal [5]. The measured l d values represents less than 50% of the l d estimates based on the AASHTO Specifications but correlate fairly well with l d estimates based on NCHRP report 603 (neglecting the strength limitations and the limit of 100 d b ). Measured and predicted elastic shortening were approximately 25 percent less than the predicted values. It must be noted that the modulus of elasticity at release, E ci, was calculated, not measured, which affects the predicted elastic shortening. In terms of the total prestress losses, the measured losses were almost half of the predicted values, but these are also affected by the modulus of elasticity at release. Shown in Figure 3 are the prestress losses over time. From the graph, it is apparent that there is little change in loss after 28 days of age, meaning the effects of creep and shrinkage are minimal. The total measured and predicted losses represent approximately 7 and 12% of the prestress value (75%F u ). The total measured and predicted values of prestress losses were in excess of 50 percent less than the estimates given by Hegger and Bertram [6, 7] and Almansour and Lounis [2] Figure 3 Prestress Losses with Time

12 John, Ruiz, Floyd, and Hale CONCLUSIONS The research program investigated the bond performance and prestress losses in UHPC using 7 rectangular beams. The measured values of transfer and development lengths and prestress losses were compared to estimated values based on the AASHTO LRFD Bridge Design Specifications, the University of New South Wales (UNSW) Guidelines, and recommendations from NCHRP Report 603. The research reveals that there is an improvement in bond performance between prestressing strands and UHPC. This is evident from an average measured transfer length of 14in. and a development length less than 35in. The range of 20 to 40d b for transfer length in UHPC, outlined in the UNSW design guidelines, appears to be a good estimate. However, a more conservative approach would be application of the NCHRP recommended equation for l d neglecting the lower limit of 40d b and the compressive strength limitations. Prestress losses due to elastic shortening can be reasonably approximated using the AASHTO LRFD Bridge Design Specifications. Based on the losses measured in this research, it appears that creep and shrinkage contribute little to total losses. Gowripalan and Graybeal mentioned creep and shrinkage strains in UHPC are significant during the period of steam treatment. Thereafter, both creep and shrinkage strains are reduced to negligible amounts. A similar conclusion was reached in this research based on the results summarized in Figure 3. ACKNOWLEDGEMENT Thanks to Edmundo Ruiz, Nam Do, Blake Staton, Andy Tackett, and Royce Floyd for performing the tests outlined in this research. Also, thanks to the Arkansas State Highway and Transportation Department and the Mack Blackwell Transportation Center for providing some of the support for the research. Thanks to Lafarge North America Inc. and Insteel Industries Inc. for providing the materials used in the study. REFERENCE 1. Graybeal, B. Material Property Characterization of Ulta-High Performance Concrete. Publication FHWA-HRT FHWA, U.S. Department of Transportation, Almansour, H. and Z. Lounis. Design of Prestressed UHPFRC Girder Bridges According to Canadian Highway Bridge Design Code. Publication NRCC-51413, National Research Council Canada, November Bierwagen, D. and A. Abu-Hawash. Ultra-high Performance Concrete in Iowa. HPC Bridge Views, Sept/Oct 2009, Issue 57, pp Lubbers, A. Bond Performance between Ultra-High Performance Concrete and Prestressing Strands. Master of Science Thesis, College of Engineering and Technology of Ohio University, August 2003, pp Graybeal, B. Structural Behavior of Ultra-High Performance Concrete Prestressed I- Girders. Publication FHWA-HRT FHWA, U.S. Department of Transportation, August 2006.

13 John, Ruiz, Floyd, and Hale Hegger, J. and G. Bertram. Shear Carrying Capacity of steel Fiber Reinforced UHPC. Proceedings, Second International Symposium on Ultra-High Performance Concrete, Kassel, Germany, Hegger, J. and G. Bertram. Anchorage Behavior of Pretensioned Strands in Steel Fiber Reinforced UHPC. Proceedings, Second International Symposium on Ultra- High Performance Concrete, Kassel, Germany, Ruiz, E., Do, N., Staton, B., and Hale, W. Preliminary Investigation into the Use of UHPC in Prestressed Members. Proceedings, PCI National Bridge Conference, Phoenix, AZ, Russell, B. W. and N. H. Burns. Measurement of Transfer Lengths on Pretensioned Concrete Elements. Journal of Structural Engineering, May 1997, pp AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALS. AASHTO LRFD Bridge Design Specification. 2007, 4th Edition 11. Ramirez, J. and B. Russell. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Publication NCHRP 603, Transportation Research Board, Washington D.C, May Gowripalan, N. and R. I. Gilbert. Design guidelines for RPC prestressed concrete beams. The University of New South Wales, January 2000.