Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete (2008) / Chapter Skim
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From page 26... ...
The results show that strand development length requirements shorten with increased concrete strength. Additionally, the development length tests provide the data to support the recommendation for minimum threshold values from the Standard Test Method for the Bond of Prestressing Strands.
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From page 27... ...
The May 2004 protocols were used for NCHRP Project 12-60 for the purpose of further refining the NASP Bond Test. Some refinements in protocol were made to develop the final version found in Appendix H and titled, "Standard Test Method for the Bond of Prestressing Strands." For this research, minor changes were made to the NASP Test procedures that were used in NASP Round III research.
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From page 28... ...
The loading frames used in the Round III trials were more "flexible" when compared with the frame used in the current NCHRP research, which is more "rigid." Because the NASP Bond Test protocols require a displacement rate, the rigidity of the test apparatus affects the loading rate. Therefore, the Standard Test Method for the Bond of Prestressing Strands limits the loading rate to 8,000 lb/min for 0.5 in.
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From page 29... ...
Figure 3.2 shows a schematic of the Standard Test Method for the Bond of Prestressing Strands. Additional details for the NASP Bond Test are shown in Figure 3.3.
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From page 30... ...
NASP Test specimen strand end slip measurement. 3.2.2 Reproducibility of the NASP Bond Test Between Sites The NASP Bond Test was performed on specific strand samples at Purdue and OSU.
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From page 31... ...
3.2.3 Recommendation for the Standard Test Method for the Bond of Prestressing Strands The NASP Bond Test performed in this research program was conducted on ten 0.5 in. diameter and two 0.6 in.
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From page 32... ...
The modified NASP Bond Test was conducted in concrete to understand the effects of varying concrete strengths on the bond of prestressing strands. The test procedure was identical to the NASP Bond Test protocols discussed in Section 3.2 except that concrete with varying strengths was used instead of the standard cement-sand mortar.
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From page 33... ...
Concrete mixture proportions for transfer and development length testing and for the NASP Bond Test in concrete.
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From page 34... ...
Note that the NASP Bond Test pull-out value for the standardized test in mortar is 20.95 kips for NCHRP Strand A and 20.21 kips for Strand B Also, note that the regression plots cross the 4 ksi concrete strength at a corresponding NASP Bond Test (modified)
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From page 35... ...
Please note that the NASP Bond Test pull-out value for the standardized test in mortar is 18.29 kips for the NCHRP Strand A6, and that the regression plots cross the 4 ksi concrete strength at a corresponding NASP Bond Test (modified) value of about 17.5 kips.
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From page 36... ...
diameter strand. Concrete strength at 24 hr from the modified NASP Bond Test is plotted against normalized NASP values.
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From page 37... ...
These significant results are used later in the recommendation for transfer and development length code expressions. Also note that the modified NASP Bond Test in concrete nearly matches the Standard NASP Bond Test if the concrete strength is only 4 ksi, as compared to the requirement for mortar strength of 4,500 to 5,000 psi.
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From page 38... ...
Number of transfer length beams and research variables employed. 3.4 Measured Transfer Lengths versus Varying Concrete Strengths and Varying NASP Bond Test Values The research aims at assessing the effects that varying concrete strength can have on strand bond.
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From page 39... ...
Figure 3.19 shows the depth micrometer measuring strand end slips immediately after prestress release. Strand end slips are directly related to measured transfer lengths, as shown in Figure 3.20.
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From page 40... ...
Fabrication of rectangular beams. 23 1.5 2 10 20.5 3 6.5 20 2324 3 # 3 stirrups at 7" c/c # 4 bars with standard hooks 2" c/c for 96" from ends 4 bars at north end and 2 bars at south end # 3 bars 4" c/c shape for internal hoop reinforcement for 72" from end # 3 bars on deck at 9" c/c and 2 bars throughout the length Prestressing strand Mild steel reinforcement Figure 3.18.
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From page 41... ...
Table 3.8 reports transfer lengths only on strands located at the bottom of the cross sections. Table 3.8 reports a transfer length for each
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From page 42... ...
Table 3.11 includes the beam number, the measured transfer length for each strand, the average transfer length for Strand D by concrete strength, the standard deviation of the transfer lengths, and 1-day and 56-day concrete strengths. Table 3.12 reports transfer lengths measured on Strand D placed in top locations of four-strand beams.
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From page 43... ...
As in Tables 3.8 through 3.12, measured transfer lengths are reported for each strand, the average and standard deviation are reported for each beam, along with concrete strengths at release and at 56 days. Table 3.13 includes data collected from strands located in the bottom bulbs on the I-shaped beams only.
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From page 44... ...
(Although Strand A and Strand B represent two different sources of strand, their NASP Bond Test values were very similar; therefore, the data from the two strands are treated as part of one data set.) Two regression curves are shown in Figure 3.24; one shows the best fit for data derived from the DEMEC gage, and the other shows the best fit for the data derived from strand end slip measurements.
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From page 45... ...
Summary of transfer lengths at release for Strand D in top locations. Beam Number Location North South X(kips)
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From page 46... ...
Table 3.15. Transfer length at release measured by DEMEC gage and strand end slip for 0.5-in.
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From page 47... ...
Transfer Length at release measured by DEMEC gage and strand end slip for 0.6-in. Strand A6.
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From page 48... ...
Figure 3.23. Transfer lengths measured by DEMEC gage versus transfer lengths measured by strand end slip.
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From page 49... ...
North South North South North South North South Beam Number Average W & E Average W & E Average W & E Average W & E RD4-5-1 32.78 31.02 RD4-5-2 42.19 49.70 RD6-5-1 30.24 28.07 43.00 38.57 46.82 44.37 49.75 45.26 RD6-5-2 25.60 29.22 36.79 39.87 41.99 44.72 44.24 48.27 RD6A-5-1 35.40 29.10 37.39 34.47 39.40 36.41 39.94 37.16 RD6A-5-2 20.48 20.08 26.26 35.24 30.73 39.37 32.39 40.07 RD8-5-1 20.15 20.15 28.34 26.55 32.66 30.33 39.08 34.54 RD8-5-2 13.66 17.30 34.14 46.08 36.82 47.73 37.38 50.41 RD10-5-1 26.03 16.85 26.31 25.27 26.45 26.51 30.24 27.14 RD10-5-2 14.85 18.23 17.47 20.16 18.71 22.30 22.30 22.03 Table 3.19. Change in transfer lengths over time for bottom 0.5-in.
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From page 50... ...
Transfer Length from 240 Days from Strand End Slips (in.) North South North South North South North South Beam Number and Location Average W & E Average W & E Average W & E Average W & E RD8-5-1-T (Top)
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From page 51... ...
However, there is a direct relationship between the NASP Bond Test values in concrete and the square root of concrete strengths. Figures 3.12 and 3.13 show a strong correlation between the NASP Bond Test value and the square root of concrete strength.
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From page 52... ...
Transfer length versus for Strand A6 (0.6 in) in rectangular beams.
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From page 53... ...
Note that the transfer lengths for Strand D are considerably longer than those for Strands A/B. Recall that Strand D had a NASP Bond Test value of 6,890 lb, whereas both Strands A and B had NASP Bond Test values in excess of 20,000 lb (see Table 3.3)
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From page 54... ...
It is worth noting, however, that the data illustrated in Figures 3.12, 3.13, and 3.32 clearly show that increases in concrete strength have a similar effect in improving bond, as do the increasing NASP Bond Test values. The proposed design equation shown indicates that transfer length can be obtained by dividing 97.2 in.
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From page 55... ...
The development length tests are necessary to determine the following: • Whether the NASP Bond Test can be used as a predictor of strand bond performance in flexural applications, • The minimum acceptable level of bond performance as measured by the NASP Bond Test, and • What modifications are necessary to the LRFD development length equation to account for variations in concrete strength. 3.5.1 Testing Program The experimental program consisted of the flexural tests on two types of beam specimens: • Rectangular beam specimens.
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From page 56... ...
Four-strand beams were cast for transfer length measurements with two strands in the bottom of the cross section and two strands at the top of the cross section. Four-strand beams were not tested for development length and are not discussed in this chapter.
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From page 57... ...
The test on the south end shows an embedment length of 58 in. The north end shows development length test geometry for an embedment length approximately equal to the LRFD and ACI requirements, 72 in.
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From page 58... ...
During each development length test, data were also recorded manually in the event that electronic data were corrupted by unforeseen circumstance.
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From page 59... ...
These tables report on the following parameters: • Concrete strength at release; • Concrete strength at 56 days; • Average NASP Bond Test value for the strands contained in the beams; • Embedment length for each individual test; • Test span; • Failure Moment, which is the maximum applied moment measured during the test; • Percentage of the Failure Moment to the nominal flexural capacity, Mn, as determined by strain compatibility. The calculation for Mn assumes that the strands are fully developed; no reduction in flexural capacity was assumed for embedment lengths provided that are less than the calculated development length; • Maximum beam deflection; • Maximum strand end slip; and • Classification for each type of failure.
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From page 60... ...
Table 3.23 reports only three bond failures, all occurring with lower strength concretes. Also, all of the bond failures occurred at an embedment length of only 58 in., which is approximately 80 percent of the ACI- and AASHTOprescribed development lengths.
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From page 61... ...
Development length test results on rectangular beams containing Strands A/B. Beam End c f @ Release (psi )
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From page 62... ...
All of the flexural failures occurred at an applied moment that matched or exceeded Mn, the nominal flexural capacity for the beams. Detailed testing summaries on each development length test are found in Appendix D, for Rectangular Beams Made with Strands A and B
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From page 63... ...
Notably, all three bond failures occurred at embedment lengths of 58 in., which is considerably shorter than the required development length. The three bond failures occurred in beams made with the three lower concrete strengths, with nominal release strengths of 4 ksi, 6 ksi, and 8 ksi.
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From page 64... ...
The NASP Bond Test value for Strand A6 was 18,290 lb. The moment versus deflection curve is found in Figure 3.41.
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From page 65... ...
The moment versus deflection curve is shown in Figure 3.43. The moment versus deflection curve illustrates that the beam was unable to reach its nominal flexural capacity, Mn.
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From page 66... ...
On the higher strength beam, with a 56-day concrete strength of 14.16 ksi, a bond failure occurred at an embedment length of 72 in., and a flexural failure occurred at an embedment length of 88 in. These tests demonstrated that Strand D, with an NASP Bond Test value of 6,890 lb, was inadequate in its ability to bond with concrete and satisfy the design requirements implied in the ACI and AASHTO expressions for development length.
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From page 67... ...
The beam contained strands from the sample Strand D, which possessed a relatively low NASP Bond Test value of 6,890 lb. The moment versus deflection curve is found in Figure 3.45.
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From page 68... ...
The moment versus deflection curve and the strand end slip versus deflection curve are shown in Figure 3.47. The moment versus deflection curve follows a pattern indicative of a flexural failure.
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From page 69... ...
Strand D had an NASP Bond Test value of 6,890 lb and Strands A and B had an NASP Bond Test value exceeding 20,000 lb. In the rectangular beams made with Strands A or B, no bond failures were experienced, even at relatively short embedment lengths.
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From page 70... ...
The NASP Bond Tests in concrete clearly demonstrate that concrete strength can exert great influence over the bond of strand with concrete. This trend was also demonstrated in measured transfer lengths as the transfer length for a given strand was shortened as concrete strength increased.
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From page 71... ...
However, when Strand A6 was cast in concrete with a release strength of 10 ksi and a 56-day strength of over 14 ksi, the strand was able to develop the required tension force at an embedment length of 58 in. The dark line in the table separates the zone of bond failures from the zone of flexural failures.
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From page 72... ...
The NASP Bond Test can be modified to perform the test in concretes with varying concrete strengths. However, the NASP Bond Test values used in the discussions regarding minimum Bond Values are pull-out strengths obtained from the standardized NASP Bond Test performed in mortar.
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From page 73... ...
, longer transfer lengths, and longer development length requirements than Strands A/B. Figure 3.49 shows that bond failures occurred in rectangular beams with embedment lengths of 58 in.
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From page 74... ...
There are no bond failures occurring in the region where embedment length exceeds the calculated development length using the proposed equation. The tests support the proposed equation for development length.
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From page 75... ...
First of all, however, it was important to establish a correlation between the NASP pull-out test values and the bond performance of the same strands in transfer and in development length tests. Russell and Brown (2004)
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From page 76... ...
with respect to the concrete strength and the provided embedment lengths. There are no bond failures occurring in the region where provided embedment length exceeds the calculated development length using the proposed equation.
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From page 77... ...
• Rectangular beams with all types of strands were able to achieve flexural failures at embedment lengths less than or equal to the AASHTO-specified development length. • With increased concrete strength, it is possible to achieve flexural failures at an embedment length less than the AASHTO-specified value.
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From page 78... ...
anchorage at embedment lengths equal to or higher than AASHTO code development length provision for normalstrength concretes. • The effect of admixtures on the transfer and development length tests should be studied, with more development length tests carried out while changing the proportions of different admixtures in the concrete.
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From page 79... ...
The factor representing the contribution of confining reinforcement across potential splitting planes is Ktr. The variable cb represents the spacing or cover dimension, calculated using either the distance from the center of the bar (or wire)
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From page 80... ...
The concrete strength continued to increase after 28 days and achieved a strength of 17 ksi at 56 days. The specimens began to be tested after they reached a 15-ksi uniaxial compressive strength.
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From page 81... ...
3.8.5 Beam End Displacement The applied load versus deflection at the tip of the overhang response for Specimens I-1 to I-6 is shown in Figure 3.59. Load represents the average of the two values from the actuators.
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From page 82... ...
82 (a) Specimen I-1 (c)
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From page 83... ...
All the gages on the longitudinal reinforcement showed strains in excess of the bar yield strain before reaching peak load. In the gages placed on the stirrups in the constant moment region, the measured maximum strain was around half of the bar yield strain in Specimens I-4 and I-5 and almost equal to the I-1 0 10 20 30 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Displ.
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From page 84... ...
The Orangun, ′fc 84 Jirsa, and Breen study -- together with contributions on bond of reinforcement from ACI Committee 318 and ACI Committee 408 that were meant to simplify the provisions for calculating development length of straight bars in tension -- led to Equation 12-1 in the 318 Code (ACI 2005) , which is Equation 3.12 herein: (3.12)
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From page 85... ...
In the new format, a basic development length, ldb, was determined and then modified by appropriate factors to obtain the required anchorage length, ld. ′fc ′fc ′fc μ ϕu u o d V j = ∑ Gage Location I-1 I-2 I-3 I-4 I-5 I-6 Longitudinal Bar 3,405 2,370 3,100 10,300 10,750 5,960 Transverse Reinforcement N/A N/A N/A 1100 875 1,910 Table 3.37.
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From page 86... ...
The basic development length was modified to reflect the influence of cover, spacing, transverse reinforcement, casting position, type of aggregate, and epoxy coating. The basic development lengths remained essentially the same as in the 1971 edition of the ACI Code and the current AASHTO LRFD Bridge Design Specifications with the exception of the equation for #18 bars, ′fc ′fc ′fc ′fc l l f fd db= *
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From page 87... ...
shows the comparison of test maximum stress to calculated stress in the bar using Equation 3.15 (AASHTO LRFD Bridge Design Specifications) on uncoated bars for the specimens reported by ACI Committee 408 (2003)
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From page 88... ...
Figure 3.61. Comparison of bond efficiency with concrete strength.
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From page 89... ...
and the design equation in the AASHTO LRFD Bridge Design Specifications, Equations 3.11 and 3.15, respectively, overestimated the bar stress in several of the bottom cast specimens in the ACI 408 Committee Database, especially for specimens with concrete compressive strength higher than 10 ksi. However, the calculated result proposed by ACI 408 Committee, Equation 3.18, resulted in fewer cases where the ratio of test to calculated stress was less than 1.0.
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From page 90... ...
Other researchers' concrete strengths vary from 4 to 10 ksi. The relationship between the bond efficiency (the ratio of test stress to stress calculated using 318 Code [ACI 2005]
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From page 91... ...
In summary, although the factors are the same in both the ACI Code and AASHTO LRFD Bridge Design Specifications with respect to development and splice length of tension of epoxy-coated reinforcement, the same differences observed in the case of uncoated bars for the calculation of tension development length remain. 3.9.3 Experimental Program 3.9.3.1 Test Specimens The experimental program covers the testing of 12 beam splice specimens reinforced with epoxy-coated bars.
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From page 92... ...
As shown in Table 3.38, all the bars in specimens with 3db concrete cover had a calculated stress greater than 60 ksi. The specimens were reinforced in the overhang region to prevent premature shear failures outside of the test region.
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From page 93... ...
ASTM A615 Grade 60 reinforcing bars were used for both longitudinal and transverse reinforcement. The yield strength, calculated by a 0.2-percent offset from tensile tests of samples of the reinforcing bars, was 70.3 ksi and 74 ksi for the #6 and #11 bars, respectively.
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From page 94... ...
Near the peak load, splitting horizontal cracks appeared along the longitudinal bars in the splice region. Finally, the deformations pushed the concrete away from the bar by wedge action.
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From page 95... ...
Specimen II-18 Figure 3.67. Typical failure crack pattern for beam-splice specimens reinforced with epoxy-coated bars.
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From page 96... ...
. 3.9.4.2 Load versus End Displacement Characteristics The applied load versus deflection at the tip of the overhang response for Specimens II-7 to II-10 and II-17 and II-18 is shown in Figure 3.68.
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From page 97... ...
, increasing the concrete strength resulted in increases in both maximum stress and deflection at failure. Effect of Minimum Amount of Transverse Reinforcement in Higher Strength Concretes.
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From page 98... ...
3.9.6 Design Recommendation When the spliced bar stress was calculated using 318 Code (ACI 2005) , without a limitation on the square root of the compressive concrete strength, only Specimen II-14 with 3db concrete cover had a ratio of test maximum stress to calculated stress of less than 1.
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From page 99... ...
The use of transverse reinforcement over the splice region increased the ACI-calculated stress, causing a decrease in the ratio of test maximum stress to ACI-calculated stress, and this tendency was consistent with the tendency of the tests conducted under NCHRP Project 12-60. It is interesting to note as well that for the studies in the literature, the range of stress ratios in the specimens with epoxy-coated bars and companion specimens with uncoated bars was similar, as shown in Table 3.41.
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From page 100... ...
Epoxy-coated hooked bars consistently developed lower anchorage capacities and load-slip stiffness than companion uncoated hooked bars. The companion hooked-bar specimens that had ties in the beam-column joint region improved both the anchorage capacity and loadslip behavior of both coated and uncoated bars.
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From page 101... ...
, for deformed bars in tension terminating in a standard hook specified in Article 5.10.2.1 shall not be less than the following: • The product of the basic development length and the applicable modification factor or factors, as specified in Article 5.11.2.4.2; • 8.0 bar diameters; or • 6.0 in. Basic development length, lhb, for a hooked-bar with yield strength, fy, not exceeding 60.0 ksi shall be taken as: (3.23)
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From page 102... ...
The 318 Code (ACI 2005) anchorage requirements for uncoated bars anchored by a combination of standard hook and straight embedment length were based on the test results of Marques and Jirsa (1975)
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From page 103... ...
The average coating thickness measured with a dry film thickness gage for all epoxycoated bars was around 12 mils. 3.10.3 Experimental Results 3.10.3.1 Load versus Slip Behavior and Cracking Pattern Pull-out load versus slip responses for Specimens II-9, II-10, III-17, and III-18 are shown in Figure 3.75.
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From page 104... ...
, respectively, it can be observed that the increase in concrete compressive strength from about 13.5 ksi to 16.5 ksi resulted in an increase in pull-out strength. The same increase in concrete strength in the case of the specimens anchoring #11 bars, III-18 and III-20, also resulted in an increase in pull-out strength (see Figure 3.75[d]
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From page 105... ...
, the tendency was the same as in Series I specimens reinforced with black bars. In Series III, the specimens anchoring #11 bars reached failure stress levels less than or equal to the calculated stress values, yielding a ratio of test to calculated stress ranging from 0.83 to 0.98 while the specimens
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From page 106... ...
106 (a) Specimen I-1 (c)
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From page 107... ...
Comparison of maximum pull-out bar stress compared with calculated stress using the 318 Code (ACI 2005) method with a modification factor of 0.7 (ksi)
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From page 108... ...
3.10.4 Summary and Conclusions Based on the review of over 40 specimens in the literature and the results from 21 tests of hooked bar anchorages in beam-column specimens with normal-weight concrete strengths up to 16 ksi, the following conclusions can be drawn: • The approach in the 318 Code (ACI 2005) provision for anchorage of bars terminated in standard hooks in tension, black and epoxy-coated, can be extended to concrete compressive strengths up to 15 ksi.
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From page 109... ...
In the #11 hooked bar specimens, the ratios of measured stress to calculated stress were 0.85 to 0.88. • Transverse reinforcement in the anchorage length of a bar terminated with a standard hook improves the maximum pull-out strength and load versus slip behavior.
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