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60
Transfer Lengths at Release vs. Normalized NASP Test
Transfer Lengths at 240 d
50 Design Curve for Transfer Length
Power (Transfer Lengths at Release vs. Normalized NASP Test)
Power (Transfer Lengths at 240 d)
Transfer Length at Release (in.)
Power (Design Curve for Transfer Length)
40
y = 103.17x-0.45
R2 = 0.49
30
20
y = 97.2x-0.5
y = 67.78x-0.46
10 R2 = 0.58
0
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0
Normalized NASP Value (kips)
Figure 3.32. Transfer lengths versus normalized NASP bond values for Strands A/B and
plotted together at release and at 240 days.
Finally, in Figure 3.32, the transfer lengths at release and at improving bond, as do the increasing NASP Bond Test val-
240 days after release are plotted against the normalized ues. The proposed design equation shown indicates that
NASP Bond Test value. The normalized NASP Bond Test transfer length can be obtained by dividing 97.2 in. by the
value is obtained from Equation 3.1. In Equation 3.1, the nor- square root of the normalized NASP Bond Test value. The
malized value can be obtained because the ratio of the NASP normalized NASP Bond Test value factors in the same factor
Bond Test Value in concrete to the standard NASP Bond Test for the square root of the concrete strength. The proposed
value (in mortar) is essentially equal to one-half of the square design equation will provide a transfer length of 60 strand
root of the concrete strength at 1 day. In this manner, data diameters for concrete strength of 4 ksi. Increasing concrete
from strands with widely dissimilar NASP Bond Test values strengths will reduce the proposed transfer length in propor-
can be plotted on the same chart and the results compared. In tion to the square root of the concrete strength.
Figure 3.32, we see that the power regression curve fits
through both sets of data. The data set shown with lower
3.5 Development Length Tests
NASP Bond Test values, toward the left side of the chart, are
the data derived from Strand D; data with higher NASP Bond Measured transfer and development lengths of prestress-
Test values, toward the right side of the chart, are obtained ing strands are indications of the quality of bond between the
from Strands A/B. The power regression curve shows a best strand and concrete. The research conducted as part of
fit with an exponent of -0.46. NCHRP Project 12-60 and described earlier resulted in five
Also plotted on Figure 3.32 is the curve that corresponds to overarching conclusions:
the proposed equation for transfer length. The normalized
NASP Bond Test value is obtained from Equation 3.1. For ex- 1. The Standard NASP Bond Test method provides a reliable
ample, for a concrete strength of 4 ksi, the transfer length and repeatable method to test for the bond performance
should be 60 strand diameters. For 0.5 in. diameter strand, of prestressing strands. Results were found to be repeat-
the transfer length would be 30 in. The data illustrated in able at different testing sites.
Figure 3.32 show that transfer lengths are shortened with 2. The Standard NASP Bond Test is able to determine ac-
increasing NASP Bond Test values. However, it is not pro- ceptable quality levels for bond of prestressing strands.
posed to shorten the transfer length equation as a function of This ability is demonstrated by the correlation of NASP
NASP Bond Test values. It is worth noting, however, that the Bond Test results with measured transfer lengths. In-
data illustrated in Figures 3.12, 3.13, and 3.32 clearly show creases in measured transfer lengths correlate directly with
that increases in concrete strength have a similar effect in decreases in bond performance, as measured by the NASP
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Bond Test. The development length tests reported in this measured on each beam end, either directly by measuring
chapter supplement the findings from the NASP Bond concrete surface strain or indirectly by measuring strand
Tests and transfer length measurements. end slip before release.
3. The NASP Bond Test can be modified by testing strand · I-shaped beam specimens. In all, eight I-shaped beam
bond performance in concrete instead of mortar. The specimens were fabricated with target concrete release
modified NASP Bond Tests in concrete demonstrate that strengths of 6 ksi and 10 ksi. These beams were 24 ft in
increases in concrete strength result in improving bond length and designed to be tested at each end. Transfer
performance. The results develop a strong statistical cor- lengths were also measured on these beams prior to devel-
relation, and the best fit indicates that bond strength opment length testing.
improves in proportion to the square root of concrete
strength. 3.5.1.1 Terminology
4. Concrete strength influences the bond of prestressing steel
with concrete. In the NASP Bond Tests (modified), in- The testing program terminology was as follows.
creasing concrete strengths resulted in increasing bond
strength between strand and concrete. In beams where Embedment length, le. For the purposes of this research
transfer lengths were measured, increasing concrete and generally in the broader literature, the embedment length
strength correlated to shortening transfer lengths. The is the length of bond provided from the beginning of bond
measured pull-out forces from the NASP Bond Tests (usually at the end of the beam) to the critical section of the
established that bond strength improves in proportion to beam. The critical section in these tests is generally under-
the square root of concrete strength at release. stood to be the section where maximum moment occurs. In
5. The transfer length data further establish that measured this testing program, the embedment length is the distance
transfer lengths decrease in inverse proportion (approxi- from the end of the beam to the point of loading, which
mately) to the square root of concrete strength. The influ- corresponds to the point of maximum moment.
ence of concrete strength and NASP Bond Test value Development length, ld. Development length of prestress-
correlates with the inverse of strand end slip measure- ing strands is the minimum distance from the free end of the
ment, which is a direct indicator of transfer lengths. strand over which the strand should be bonded to concrete so
that the section under consideration achieves its full nominal
The development length tests are necessary to determine the capacity.
following:
Flexural bond length. The flexural bond length is meas-
· Whether the NASP Bond Test can be used as a predictor of ured from the section where the prestressed force is fully
strand bond performance in flexural applications, effective (at the end of the transfer length) to the critical
· The minimum acceptable level of bond performance as section. In these tests, the flexural bond length is equivalent
measured by the NASP Bond Test, and to the embedment length minus the transfer length. Often,
· What modifications are necessary to the LRFD develop- the flexural bond length is used in conjunction with the
ment length equation to account for variations in concrete development length, ld . In that case, the embedment length is
strength. the development length minus the transfer length.
3.5.1 Testing Program 3.5.1.2 Beam Identification System and
Section Properties
The experimental program consisted of the flexural tests
on two types of beam specimens: Each beam carries a unique identifying name. The system
for identification is described in Figure 3.14. The identifica-
· Rectangular beam specimens. In all, 43 rectangular beam tion system indicates the following beam characteristics:
specimens were fabricated with target release concrete shape (rectangular [R] or I-shaped [I]), strand source, strand
strengths varying from 4 ksi to 10 ksi. Rectangular beams size, nominal concrete strength at release, and specimen
were cast with two prestressing strands at a depth of 10 in. number in a series. The section properties and materials are
in a beam 12-in. deep. Both 0.5 in and 0.6 in diameter described in the sections under transfer length.
strands were used. The rectangular beams were 7 ft in Rectangular beams 17 ft in length were fabricated with
length and designed to be tested independently at each end two strands in each beam. Longitudinal top steel was in-
to assess the development length of embedded strands. cluded in the cross section to provide additional compres-
Prior to development length testing, transfer lengths were sion reinforcement and to ensure under-reinforced flexural
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conditions at capacity. As shown in Figures 3.15 and 3.16, 3.5.1.3 Loading Geometry
#3 stirrups, or "ties," were provided on 6-in. centers. The
Both rectangular and I-shaped beams were designed to be
nominal flexural capacity, Mn, of the rectangular beams var-
tested on both ends, enabling a distinct development length
ied from about 700 k-in. for the lower strength concrete
test at each beam end. The loading geometry varied from end
(nominal 4 ksi at release) to approximately 754 k-in. for the
to end so that a different embedment length was tested at each
10 ksi (release) concrete.
end. Embedment lengths varied for each test and were cho-
Four-strand beams were cast for transfer length measure-
sen depending upon results from prior tests.
ments with two strands in the bottom of the cross section and The typical loading geometries for rectangular beams with
two strands at the top of the cross section. Four-strand beams 0.5-in. strands are shown in Figure 3.33. The geometry shown
were not tested for development length and are not discussed for the south end corresponds with an embedment length of 58
in this chapter. in., which is approximately 80 percent of the computed devel-
Figure 3.18 shows the cross-section of the I-shaped beam opment length requirement. The geometry shown for the north
with the reinforcement details. Each I-shaped beam was cast end corresponds with an embedment length of 73 in., which is
with a length of 24 ft. Top flanges were reinforced longitudi- approximately equivalent to the AASHTO LRFD and ACI re-
nally with two #3 bars that ran the length of the beam. Trans- quirements for development length. Rectangular beams with
verse reinforcement in the top flange consisted of #3 bars at 0.6-in. strands required longer embedment lengths than those
9-in. centers over the beam length. Stirrups were made from shown in Figure 3.33. The two testing lengths, 73 in. and 58 in.,
#3 bars with standard 90° hooks and spaced at 7-in. centers. were established through testing programs conducted by Rose
Stirrups were arranged so that the legs alternated directions. and Russell (1997) and Logan (1997).
Horizontal reinforcement was placed in the webs of each end The typical loading geometries for the I-shaped beams are
of each I-shaped beam. shown in Figure 3.34. The geometry illustrated is typical for
Figure 3.33. Typical loading geometry for rectangular beams.
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Figure 3.34. Typical loading geometry for I-shaped beams.
beams made with 0.5 in. strands. The test on the south end to the pretensioned concrete test beam. The loading geometry
shows an embedment length of 58 in. The north end shows was arranged so that constant bending moment is applied
development length test geometry for an embedment length between the two load points. In this picture, the beam that is
approximately equal to the LRFD and ACI requirements, 72 being tested is a rectangular beam. It is supported by a pin on
in. As with the rectangular beam series, tests on I-shaped the near end and a roller at the far end.
beams with 0.6 in. diameter strands required longer embed-
ment lengths than those shown in Figure 3.34.
3.5.1.5 Instrumentation
3.5.1.4 Test Frame The following instrumentation was used.
The test frame was designed to perform flexural tests on both Electronic data acquisition. Load, hydraulic pressure,
rectangular beams and I-beams. The photograph in Figure 3.35 beam deflection, and strand end slips were measured and
shows the test frame with a beam in position for testing. The test recorded by an electronic data acquisition system. Data were
frame has four sides that form a rectangular "frame." Load is sampled and recorded at regular intervals without manual
applied through a hydraulic actuator (attached to the top hori- prompting. The rate of sampling was fixed at 1.0 Hz, which
zontal beam in the frame) to a spreader beam (attached to the provided smooth transition of load, displacement, and strand
bottom of the actuator). The spreader beam distributes loading end slip values. The data were stored on a laptop computer
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and were then available for analysis. During each development
length test, data were also recorded manually in the event that
electronic data were corrupted by unforeseen circumstance.
Load. Load was measured electronically with a load cell
placed between a spherical head under the hydraulic actuator
and the spreader beam. The load cell can be seen in Figure
3.35 just above the steel loading beam. Load was applied
hydraulically, and the hydraulic pressure was also monitored
electronically by a pressure transducer. The pressure trans-
ducer also sent electronic signals to the data acquisition sys-
tem for monitoring and recording. A hydraulic pressure gage
was employed during the test for visual observations and
manual recording.
Deflection. Wire transducers with a range of 30 in. and
accuracy ±0.005 in. were used to determine the vertical de-
flection. Deflection was measured below the center of the
loading point. Two wire transducers were used to measure de-
flection, one on each side of the beam, so that any twisting of
the beam would be taken out when computing the average be-
tween the two sides. Data from the wire transducers were Figure 3.36. Wire transducers
recorded and stored electronically. In addition to the elec- (foreground) and a dial gage.
tronic data, a dial gage with a precision of one one-thousandth
of an inch was used to manually record deflection readings.
The dial gage was also used to monitor displacements when
Strand end slip. Linear voltage displacement transducers
the testing switched from load-controlled testing to displace-
(LVDTs) were used to measure strand movement relative to the
ment controls. The wire transducers and the dial gage are
concrete. The LVDTs had a stroke limit of 1.0 in. and recorded
shown in Figure 3.36.
strand end slips to one one-thousandth of an inch. Clamps were
attached to the strands, and LVDTs were mounted on these
clamps at a location providing an initial reading of approxi-
mately 0.9 in. with an error of (±0.003 in. Strand end slips were
measured and recorded for each strand on the "test" end. The
photograph in Figure 3.37 shows the LVDTs clamped to strands
to measure strand end slip relative to concrete.
At the far end of the beam, or at the end of the beam
opposite the end being tested, strand end slips were measured
by a mechanical deflection gage with an electronic readout.
The device and arrangement are shown in Figure 3.38.
Measurements with a precision of ±0.005 in. were possible
using this technique.
3.5.1.6 Testing Procedure
For each test, the instrument readings were initialized prior
to the application of external load. Load was then applied to
beams in regular load increments. Load was applied manually
by an hydraulic pump. At all load increments, values of load,
displacement, and strand end slips, as well as DEMEC read-
ings (wherever applicable) were noted and recorded manually.
Figure 3.35. Test frame with a In addition to electronic data being stored at the 1-Hz refresh
rectangular beam readied for testing rate on the data acquisition system, data were recorded man-
rupture. ually. Once cracking began, cracks were marked with perma-
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until failure. Failure was defined by the beam's inability to
sustain or maintain load with increasing deflections or by
abrupt failures of the concrete or strand.
Throughout the test, manual readings at every load
increment were noted along with any significant develop-
ment such as first flexural crack, first shear crack, appear-
ance of flexure-shear crack, first strand end slip, concrete
spalling, concrete crushing, and any audible developments.
Written summaries of each development length test appear
in the appendices. Detailed progress of each test was docu-
mented and is included along with significant photographs
and data plots in Appendices C through G. Also, plots of
moment versus deflection, strand end slip versus deflec-
tion, and shear versus average shear strain were plotted
from the acquired data. Shear strains were measured from
Figure 3.37. LVDTs clamped to strands to measure DEMEC target points attached to the webs of I-shaped
strand end slip relative to concrete. beams. Shear stress was determined by dividing the shear
force applied by the product of the web width and the beam
depth.
nent markers as soon as they were observed. The loads at
which the cracks first appeared were noted alongside of the
cracks. Photographs were taken at regular intervals to record 3.5.2 Experimental Results
cracking patterns. from Development Length Testing
As displacements became larger with smaller increments in
All together, 50 flexural tests were performed on rectangu-
load, the manual system of loading switched from regular
lar beams and 14 tests on I-shaped beams. All of these tests
load increments to regular displacement increments. This
were carried out at the Civil Engineering Laboratory at OSU.
was done arbitrarily by the researchers conducting the test.
Most of the beams were tested on both ends. For each beam
At each load or displacement increment, manual readings
test, the embedment length was determined on the basis of
of hydraulic pressure and beam displacements were made.
various factors, including the AASHTO development length
Additionally, manual and electronic instruments were
equation with changes to account for prior results, concrete
checked to determine whether strand end slip had occurred
strength, or strand bond strength. In this section, Tables 3.23
during the prior loading increment. Loading was continued
through 3.27 report the results from development length test-
ing. 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 devel-
oped; no reduction in flexural capacity was assumed for
embedment lengths provided that are less than the calcu-
lated development length;
Figure 3.38. Mechanical deflection gage arrangement · Maximum beam deflection;
for measuring strand end slips at the beam end · Maximum strand end slip; and
opposite to the test end. · Classification for each type of failure.
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Table 3.23. Development length test results on rectangular beams containing Strand D.
fc fc Avg. NASP Avg. Lt Max.
Avg. Lt @ Actual Failure Deflection
@ 56 Pull-Out (56-Day or Span %Mn End Failure
Beam End Release Le Moment @ Failure
Release Days Value @ Test) (in) Slip Mode
(in) (in) (kip-in) (in)
(psi) (psi) (lb) (in) (in)
RD-4-5-1-N 4,033 7,050 6,890 32.79 38.54 73 162 804 115 3.4 0.00 Flexure
RD-4-5-1-S 4,033 7,050 6,890 31.02 42.28 58 132 759 108 1.6 0.35 Bond
RD-4-5-2-N 4,033 7050 6,890 42.19 63.05 73 162 831 119 2.7 0.40 Flexure
RD-4-5-2-S 4,033 7,050 6,890 49.71 51.81 58 132 513 73 2.5 0.57 Bond
RD-6-5-1-N 6,183 8,500 6,890 30.24 49.75 73 162 797 111 2.5 0.06 Flexure
RD-6-5-1-S 6,183 8,500 6,890 28.07 45.26 58 132 788 109 2.0 0.18 Flexure
RD-6-5-2-N 6,183 8,500 6,890 25.60 44.24 73 162 735 102 2.0 0.01 Flexure
RD-6-5-2-S 6,183 8,500 6,890 29.22 48.27 58 132 724 100 2.0 0.25 Bond
RD-6A-5-1-N 7,960 11,420 6,890 35.4 39.94 73 162 794 106 2.3 0.00 Flexure
RD-6A-5-1-S 7,960 11,420 6,890 29.1 37.16 58 132 805 108 2.5 0.08 Flexure
RD-6A-5-2-S 7,960 11,420 6,890 20.08 40.07 58 132 778 104 1.9 0.02 Flexure
RD-8-5-1-N 8,570 13,490 6,890 20.15 39.08 73 162 811 107 2.6 0.00 Flexure
RD-8-5-1-S 8,570 13,490 6,890 20.15 34.54 58 132 805 106 2.6 0.08 Flexure
RD-8-5-2-N 8,570 13,490 6,890 13.67 37.38 58 132 775 102 2.2 0.08 Flexure
RD-8-5-2-S 8,570 13,490 6,890 17.30 50.41 58 132 813 107 2.0 0.00 Flexure
RD-10-5-1-N 9,711 14,470 6,890 26 30.24 58 132 821 108 2.1 0.00 Flexure
RD-10-5-1-S 9,711 14,470 6,890 13.57 27.14 46 120 819 107 2.6 0.00 Flexure
RD-10-5-2-N 9,711 14,470 6,890 14.85 22.30 58 132 788 103 1.9 0.00 Flexure
RD-10-5-2-S 9,711 14,470 6,890 18.23 22.03 46 120 794 104 1.9 0.01 Flexure
3.5.2.1 Tabulated Beam Test Results--Rectangular Table 3.23 also reports the maximum strand end slip that
Beams occurred during testing, which corresponds to the maximum
strand end slip measured at the time the beam failed, whether
Table 3.23 reports the results from development length tests a flexural failure or a bond failure. Note that it is not uncom-
on rectangular beams made with Strand D. Strand D was the mon for strand end slips to be measured even though a beam
0.5 in. strand with the lower NASP pull-out value, 6,890 lb. fails in flexure. For example, RD-4-5-2-N failed in flexural at
Concrete strengths at release varied from a target of 4 ksi to a a load that exceeded its nominal capacity by 19 percent.
target of 10 ksi. 56-day concrete strengths ranged from 7.05 ksi Further, the beam achieved adequate ductility as demon-
to 14.47 ksi. Table 3.23 reports only three bond failures, all strated by 2.7 in. of overall deflection while sustaining capac-
occurring with lower strength concretes. Also, all of the bond ity. However, the measured strand end slip was 0.40 in. This
failures occurred at an embedment length of only 58 in., which finding is consistent with other research that has been
is approximately 80 percent of the ACI- and AASHTO- performed to date. More notably, the results in Table 3.23
prescribed development lengths. Of the three bond failures, demonstrate that the measured strand end slips decrease
two occurred at an applied moment that matched or exceeded measurably with increasing concrete strengths. At higher
Mn, the nominal flexural capacity for the beams. Table 3.23 concrete strengths, strand end slips did not occur. Overall,
also shows that at higher strengths, in general, flexural failures the results support a conclusion that higher concrete
were observed in all tests. For example, two ends of the beams strengths result in increasing bond strength and reducing the
with 14.47 ksi concrete were tested with an embedment length required development lengths. Detailed testing summaries
of only 46 in., or approximately 63 percent of ld. In these cases, on each development length test are found in Appendix C.
the development length test resulted in flexural failures with- Table 3.24 reports the results from development length
out bond slip (beams RD-10-5-1-S and RD-10-5-2-S). tests on rectangular beams made with Strands A/B. Strands A
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Table 3.24. Development length test results on rectangular beams containing Strands A/B.
fc fc Avg. NASP Avg. Lt Max.
Avg. Lt @ Actual Failure Deflection
@ 56 Pull-Out (56-Day or Span End- Failure
Beam End Release Le Moment %Mn @ Failure
Release Days Value @ Test) (in) Slip Mode
(in) (in) (kip-in) (in)
(psi) (psi) (lb) (in) (in)
RA-6-5-1-N 6,183 8,500 20,950 19.2 33.70 73 162 790 110 2.1 0.00 Flexure
RA-6-5-1-S 6,183 8,500 20,950 18.2 30.03 58 132 800 111 2.1 0.00 Flexure
RA-6-5-2-N 6,183 8,500 20,950 16.5 28.00 58 120 772 107 1.5 0.00 Flexure
RA-6-5-2-S 6,183 8,500 20,950 15.01 23.50 46 120 777 108 1.5 0.00 Flexure
RA-6A-5-1-N 7960 11,420 20,950 17.74 26.54 73 162 769 103 2.4 0.00 Flexure
RA-6A-5-1-S 7,960 11,420 20,950 17.68 28.55 58 132 770 103 1.7 0.00 Flexure
RA-6A-5-2-N 7,960 11,420 20,950 24.51 31.75 58 132 788 105 1.9 0.00 Flexure
RA-6A-5-2-S 7,960 11,420 20,950 22.03 29.38 46 120 788 105 1.7 0.01 Flexure
RA-8-5-1-N 8,570 13,490 20,950 13.3 24.91 58 132 829 109 1.7 0.01 Flexure
RA-8-5-1-S 8,570 13,490 20,950 13.5 22.54 46 120 832 110 1.9 0.00 Flexure
RA-10-5-1-N 9,711 14,470 20,950 24.27 24.34 58 132 788 103 1.7 0.00 Flexure
RA-10-5-1-S 9,711 14,470 20,950 9.69 13.14 46 120 796 104 1.7 0.00 Flexure
RB-4-5-1-N 4,033 7,050 20,210 18.42 22.10 73 162 776 111 1.9 0.00 Flexure
RB-4-5-1-S 4,033 7,050 20,210 18.49 20.51 58 132 802 114 2.0 0.00 Flexure
RB-4-5-2-N 4,033 7,050 20,210 21.12 22.52 73 162 721 103 2.4 0.00 Flexure
RB-4-5-2-S 4,033 7,050 20,210 22.46 23.75 58 132 748 107 1.7 0.00 Flexure
Table 3.25. Development length test results on rectangular beams containing Strand A6 (0.6-in. diameter).
fc fc Avg. NASP Avg. Lt Max.
Avg. Lt @ Actual Failure Deflection
@ 56 Pull-Out (56-Day or Span End- Failure
Beam End Release Le Moment %Mn @ Failure
Release Days Value @ Test) (in) Slip Mode
(in) (in) (kip-in) (in)
(psi) (psi) (lb) (in) (in)
RA-4-6-1-N 4,033 7,050 18,290 33.42 41.82 88 192 1084 114 3.0 0.00 Flexure
RA-4-6-1-S 4,033 7,050 18,290 24.96 28.87 70 156 964 102 2.7 0.00 Flexure
RA-4-6-2-N 4,033 7,050 18,290 30.24 37.66 73 162 1011 107 2.4 0.13 Flexure
RA-4-6-2-S 4,033 7,050 18,290 29.35 33.19 58 148 921 97 3.0 0.33 Bond
RA-6-6-1-N 4,855 8,040 18,290 29.73 40.85 88 192 1012 104 2.5 0.00 Flexure
RA-6-6-2-N 4,855 8,040 18,290 31.65 52.18 73 162 1001 103 2.1 0.02 Flexure
RA-6-6-2-S 4,855 8,040 18,290 30.1 49.37 58 148 913 94 2.7 0.41 Bond
RA-6-6-3-N 4,855 8,040 18,290 25.83 44.96 88 192 1046 108 2.6 0.00 Flexure
RA-8-6-1-N 5,413 8,220 18,290 28.21 45.48 88 192 1008 103 2.4 0.00 Flexure
RA-8-6-2-N 5,413 8,220 18,290 28.2 46.35 73 162 1007 103 2.0 0.01 Flexure
RA-8-6-2-S 5,413 8,220 18290 25.7 42.37 58 132 988 ~101 2.5 0.14 Bond
RA-10-6-1-N 9,150 14,610 18,290 20.03 29.98 88 192 1084 102 2.8 0.00 Flexure
RA-10-6-2-N 9,150 14,610 18290 15.62 26.79 73 162 1070 101 2.5 0.00 Flexure
RA-10-6-2-S 9,150 14,610 18,290 21.78 30.70 58 148 1083 102 2.4 0.00 Flexure
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Table 3.26. Development length test results on I-shaped beams containing 0.5 in. diameter strands.
Measured fc fc Avg. NASP Max.
Maximum Maximum
Overall @ 56 Pull-Out Le Span End- Failure
Beam End Moment %Mn Deflection
Depth (h) Release Days Value (in) (in) Slip Mode
(kip-in) (in)
(in) (psi) (psi) (lb) (in)
IB-6-5-1-N 24 5,810 9,350 20,210 58 166 3,526 82 1.1 0.04 Shear
IB-6-5-1-S 24 5,810 9,350 20,210 72 222 3,980 98 3.1 0.03 Flexure
IB-10-5-1-N 24 7,615 13,490 20,210 54 168 4,282 102 2.0 0.03 Flexure
IB-10-5-1-S 24 7,615 13,490 20,210 58 180 4,196 100 1.6 0.02 Flexure
ID-6-5-1-N 24 5,492 9,840 6,890 72 222 3,538 82 2.5 0.80 Bond
ID-6-5-1-S 24 5,492 9,840 6,890 88 270 3,280 81 3.5 0.75 Bond
ID-10-5-1-N 24 8,225 14,160 6,890 88 270 4,026 92 5.2 0.08 Flexure
ID-10-5-1-S 24 8,225 14,160 6,890 72 222 4,039 92 3.7 0.75 Bond
and B were used interchangeably in this beam series as the ment length test are found in Appendix D, for Rectangular
two strand samples tested with approximately the same NASP Beams Made with Strands A and B.
Bond Test value. Concrete strengths at release varied from a Both Tables 3.23 and 3.24 report results on beams made
target of 4 ksi to a target of 10 ksi. 56-day concrete strengths with air-entrained concrete. The development length test
ranged from 7.05 ksi to 14.47 ksi. Table 3.24 reports no bond results on the air-entrained beams closely match from beams
failures. These results demonstrate that the NASP Bond Test made with 6-ksi concrete without air entrainment. In other
is a good predictor of the ability of strands to perform in pre- words, all of the ends tested with air-entrained concrete failed
tensioned applications. At concrete strengths above 4 ksi, em- in flexure, with strand end slip in only a few cases. These
bedment lengths as short as 46 in. were tested. All of these results mirrored the results of the development length tests
tests also resulted in flexural failures without any strand end without air entrainment.
slip. All of the flexural failures occurred at an applied moment Table 3.25 reports the results from development length
that matched or exceeded Mn, the nominal flexural capacity tests on rectangular beams made with 0.6 in. diameter strand.
for the beams. Detailed testing summaries on each develop- The strand is called Strand A6. Strand A6 had an NASP Bond
Table 3.27. Development length test results on I-shaped beams containing 0.6 in. diameter Strand A6.
Measured fc fc Avg. NASP Max.
Maximum Maximum
Overall @ 56 Pull-Out Le Span End-
Beam End Moment %Mn Deflection Failure Mode
Depth (h) Release Days Value (in) (in) Slip
(kip-in) (in)
(in) (psi) (psi) (lb) (in)
IA-6-6-1-N 24.125 4,381 8,990 18,290 75 156 3,267 81 1.7 0.05 Shear @ opposite end
IA-6-6-1-S 24.125 4,381 8,990 18,290 91 188 4,387 109 2.8 0.12 Flexure
IA-6-6-2-N 24.125 4,381 8,990 18,290 88 270 4,125 102 3.2 0.13 Shear
IA-10-6-1-N 24.25 10,480 14,990 18,290 58 166 4,243 103 1.2 0.05 Shear @ opposite end
Flexure w/ Strand
IA-10-6-1-S 24.25 10,480 14,990 18,290 72 222 4,620 112 2.5 0.03
Rupture
IA-10-6-2-N 24.375 10,590 14,930 18,290 72 222 2,983 73 0.9 0.00 Shear @ opposite end
IA-10-6-2-S 24.375 10,590 14,930 18,290 88 270 4,559 111 5.7 0.00 Flexure
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Test value of 18,290 lb, which is interesting as it falls between Strand D. Table 3.26 reports three bond failures out of
the higher and lower NASP Bond Test values for the 0.5 in. four tests on I-shaped beams made with the lower bond per-
strands tested. Concrete strengths at release varied from a tar- former, Strand D. The fourth flexural test resulted in a
get of 4 ksi to a target of 10 ksi. The range for 56-day concrete flexural failure; this beam was made with the higher strength
strengths was 7.05 ksi to 14.61 ksi. concrete. Bond failures occurred at both the lower concrete
For 0.6 in. strands, the ACI and AASHTO development strength and the higher concrete strength. Unlike the rectan-
length provision would require a development length of ap- gular beams, bond failures of Strand D occurred at lengths
proximately 88 in. From Table 3.25, it can be seen that several equal to and exceeding the ACI and AASHTO development
of the tests were performed at an embedment length of 88 in., length design equation. At the lower concrete strength, 9.48
which roughly corresponds to 100 percent of the AASHTO re- ksi, bond failures occurred at embedment lengths of 72 and
quired development length. At the embedment length equal 88 in. At the higher concrete strength, 14.16 ksi, one bond
to the required development length of 88 in., all of the beam failure occurred at an embedment length of 72 in. The flex-
specimens failed in flexure, regardless of concrete strength. ural failure had an embedment length of 88 in. These results
This would indicate that the strand performance was adequate support two primary conclusions:
and suitable for making pretensioned concrete beams.
Other tests on beams made with Strand A6 were conducted 1. The strand with an NASP Bond Test value of 6,890 lb is
at an embedment length of 72 in., which roughly corresponds inadequate to develop the tension necessary to support
to 80 percent of ld. This was done intentionally to mirror the flexural failures as intended, and
80 percent of ld that was tested for 0.5 in. strands. Addition- 2. Higher concrete strength can improve the bond between
ally, note that some tests were conducted at an embedment prestressing steel and concrete.
length of 58 in., which is about 55 percent of ld.
Three bond failures occurred in the tests on rectangular Strand B. Table 3.26 reports results of four tests done on
beams made with Strand A6. Notably, all three bond failures beams made with Strand B. In the four tests, none of the
occurred at embedment lengths of 58 in., which is consider- beams failed in bond. The highest strand end slip measured
ably shorter than the required development length. The three was 0.04 in. Of the four failures, one was a shear failure and
bond failures occurred in beams made with the three lower the other three were flexural failures. Three of the four tests
concrete strengths, with nominal release strengths of 4 ksi, were conducted with embedment lengths of 52, 54, and 58
6 ksi, and 8 ksi. In contrast, the fourth beam, made from con- in., lengths which are significantly less than the development
crete with a nominal release strength of 10 ksi, failed in flex- length prescribed by ACI and AASHTO. These results sup-
ure when tested at an embedment length of 58 in. The results port one of the primary conclusions, i.e., that strand with a
of these tests would support the conclusion that increasing high NASP Bond Test value, in this case 20,210 lb, will
concrete strength improves the bond performance of pre- provide bond that exceeds the implicit requirement of the
stressing strands. Detailed testing summaries on each devel- development length design equations.
opment length test are found in Appendix N, for Rectangu-
lar Beams Made with 0.6 in. Strands A, or Strand A6. Strand A6. Table 3.27 reports the results from develop-
ment length tests on I-shaped beams made with 0.6 in.
strands. Strand A6 was the only 0.6 in. strand cast in beams.
3.5.2.2 Tabulated Beam Test Results--I-Shaped
It has an NASP pull-out value of 18,290 lb. Four beams were
Beams
made, two with a target release strength of 6 ksi and two with
Table 3.26 reports the results from development length a target release strength of 10 ksi. These casts achieved the tar-
tests on I-shaped beams made with 0.5 in. strands. Strand D get release strength of 10 ksi, and 1-day strengths measured
was the 0.5 in. strand with the lower NASP pull-out value, 10,590 lb. The range for 56-day strengths was 8,990 and
6,890 lb, and Strand B possessed the higher NASP Test value 14,910 lb. Of the seven beam ends tested, three ends failed in
of 20,210 lb. Two different concrete strengths were employed, shear at the end opposite the "test" end. The larger diameter
concrete with a target release strength of 6 ksi and concrete strands required longer testing spans, and the beams were not
with a target release strength of 10 ksi. The beams were made able to overcome the damage sustained during tests on the
in pairs, and the release strength of 10 ksi was not achieved. south end when tests were performed on the north end.
56-day concrete strengths ranged from 9.35 ksi to 14.16 ksi, Of the four tests that would qualify as development length
which is very near the target design strengths of 10 and 15 ksi. tests, one resulted in shear failure whereas the other three
Detailed testing summaries on each development length test tests resulted in a flexural failure. None of the failures resulted
are found in Appendix F, for I-Shaped Beams Made with 0.5 from bond failure. At the lower strength, some strand end
in. strands, including both Strand B and Strand D. slips were measured and observed; however, these strand end
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slips were consistent with behavior that was noted in previ- Flexural failures of rectangular beams. A typical flexural
ous testing and did not prevent the strands from develop- failure is observed from the test on the south end of Beam RB-
ment tension adequate to support flexural failures at, or 4-5-1. The rectangular beam contained two 0.5 in. strands,
exceeding, the nominal flexural capacity. Detailed testing with a Strand B designation and a 56-day concrete strength of
summaries on each development length test are found in 7.05 ksi. The embedment length for this test was 58 in.,
Appendix G, for I-Shaped Beams Made with 0.6 in. strand, or about 80 percent of the AASHTO design requirement for
Strand A6. 0.5 in. strands. Strand B had a relatively high NASP Bond Test
value of 20,120 lb.
The moment versus deflection curve is found in Figure
3.5.3 Discussion of Development Length
3.39. Note that the beam achieves its nominal flexural ca-
Test Results
pacity, Mn, and that it also displays the ability to sustain the
Development length tests must be conducted to failure, moment under large deflections. Additionally, for this
and the type of failure observed determines whether the em- beam, strand end slips remained small or the strand did not
bedment length provided was adequate to ensure proper slip at all. The beam failed in flexure as the concrete in the
strand development. Three distinct types of beam failures compression zone crushed. A photograph of the beam at
were observed in the conduct of the development length tests: failure is shown in Figure 3.40.
(1) flexural failure, (2) bond failure, or (3) shear failure.
Flexural failures of I-shaped beams. The test on the south
end of I-shaped beam IA-10-6-1 provides a good example of
3.5.3.1 Types of Failure--Flexure
a flexural failure. In this test, one of the strands ruptured in
Flexural failures are characterized by two primary criteria: tension, an obvious indicator that the strand was able to fully
develop the tension necessary to resist the flexural capacity.
1. The beam is able to resist a flexural moment that ap- The embedment length for this test was 72 in., which is
proaches and often exceeds the nominal flexural capacity approximately 80 percent of the ACI and AASHTO required
(strength), and development length for 0.6 in. strands. The NASP Bond Test
2. The beam is able to undergo large deformations while sus- value for Strand A6 was 18,290 lb.
taining its capacity for resistance (ductility). The moment versus deflection curve is found in Figure
3.41. Note that the beam achieves its nominal flexural capac-
Flexural failures of the beam specimens were typically ity, Mn, and that it also displays the ability to sustain the load
characterized by the crushing of concrete at the top of the under large deflections. The beam failed at a moment of 4,620
cross section where the compression zone exists. The beams kip-in., which exceeded the calculated Mn by about 12 per-
were designed to be under-reinforced, which ensures that the cent. In this beam, the strands slipped a small amount as
strands themselves will experience large strains at flexural loads increased to capacity; the maximum strand end slip
failure. Even so, crushing of the concrete is the most common measured was 0.03 in. This small amount of strand end slip is
failure mode. In one or two specimens of this test series, also consistent with many of the flexural failures that occur
strands ruptured in tension. It should be noted that some during development length testing.
strand end slip can be observed even during a flexural failure. The beam failed when one of the strands ruptured in ten-
The strands consistently exhibit an ability to develop strand sion. Strand rupture was accompanied by a loud noise. The
tension even with small amounts of slip. However, when cracking pattern at failure is shown in the photograph shown
larger amounts of slip are observed, often the result is a bond in Figure 3.42. The cracking pattern is typical for I-shaped
failure. When strand end slips are observed, the determina- beams. There are two distinct regions of cracking. Flexural
tion of whether the failure is a flexural failure with adequate cracking is predominant in the regions of maximum mo-
strand bond or a bond failure is based on whether the beam ment. These cracks are distinguished by a vertical propaga-
meets the two criteria listed above for a flexural failure. tion near the bottom fibers of the beam. Web shear cracking
Beams that failed in flexure also showed considerable duc- occurs in the webs within the shear span of the tested end.
tility, with deflection increasing dramatically with sustained These cracks are distinguished by their diagonal nature. It was
loads or with some incremental load increases. In some fl- uniformly observed that web crack propagation was limited
exural failures, strand fractures occurred. Typically, strand to the webs of the I-shaped beams until loads approaching
fractures occurred in beams made with higher strength flexural capacity were applied. As loading increased, the web
concrete. In these cases, failures did not cause crushing of cracks would propagate into the bottom "bulb" of the
concrete at the top surface; rather, the applied moments were I-shaped beam. Additionally, the photograph in Figure 3.42
large enough to cause the strands to rupture in tension. shows inclined flexural cracks that propagate vertically from
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1200 1.50
1.35
1000
1.20
1.05
800
Mn = 705.3 kip-in
Moment (kip-in)
0.90
End-slip (in.)
600 0.75
Moment vs Deflection 0.60
400
0.45
0.30
200
0.15
Strand End Slip vs Deflection
0 0.00
0 0.5 1 1.5 2 2.5 3 3.5 4
Deflection (in.)
Figure 3.39. Moment versus deflection and strand end slip for Beam RB-4-5-1-S.
the bottom of the beam, but then incline as the crack Oftentimes, although not always, bond failures can be
approaches and enters the webs. abrupt and occur without warning. However, it is generally
noted that test beams failing in bond demonstrate some
measure of gradual failure; that is, they possess an ability to
3.5.3.2 Types of Failure--Bond
sustain some load through large deformations. However,
Failures of pretensioned bond are characterized by the bond failures nearly always occur at loads less than the calcu-
following two primary markers: (1) an inability to develop lated nominal flexural capacity, Mn.
resistance to meet its design capacity and (2) excessive strand
end slip. Bond failures in rectangular beams. A typical bond failure
is observed from the test on the south end of Beam RD-4-5-2.
The rectangular beam contained two 0.5 in. strands. The con-
crete strength at 56 days was 7.05 ksi. The embedment length
for this test was 58 in., or about 80 percent of the AASHTO
design requirement for 0.5 in. strands. The beam contained
strands from the sample Strand D, which possessed a rela-
tively low NASP Bond Test value of 6,890 lb.
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.
Mn for this beam was 705 kip-in., and the beam's actual ca-
pacity was 513 kip-in., as measured during the test. In re-
viewing the load versus deflection curve and the strand end
slip curve, it is apparent that the strand started slipping very
soon after flexural cracking first occurred. The beam was
RB-4-5-1-S unable to resist loads that were much larger than the cracking
moment, and strand end slips continued to increase with
Figure 3.40. Concrete crushing in the compression additional beam deflections. At a total deflection of about
zone of Beam RB-4-5-1-S. 3 in., the compression block at the top of the beam exhibited
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5000 1
4500 0.9
Mn = 4110 kip-in
4000 0.8
3500 0.7
Strand End Slip (in.)
Moment vs Deflection
Moment (kip-in)
3000 0.6
2500 0.5
2000 0.4
1500 0.3
1000 0.2
500 Strand End Slip vs Deflection 0.1
0 0
0 0.5 1 1.5 2 2.5 3 3.5 4
Deflection (in.)
Figure 3.41. Load versus deflection and strand end slip for IA-10-6-1 South.
crushing failure. The cracking pattern and the crushing fail- four ends tested, bond failures occurred on the beams where
ure of the beam can be viewed in Figure 3.44. Note the one the embedment length was only 58 in. The companion beam
wide flexural crack, which is often a characteristic of bond to Beam RD-4-5-2 (south end), described above, was Beam
failures. Because the beam was unable to achieve its nominal RD-4-5-1 (south end). It also failed in bond but at a load
flexural capacity and because the beam exhibited excessive equal to the nominal flexural capacity. Still, the beam exhib-
strand end slips, this test was classified as a bond failure. ited excessive strand end slip during the test, and the failure
It should be noted that two rectangular beams were con- was not particularly ductile in that the beam was unable to
structed with Strand D and a targeted release strength of 4 ksi. sustain its resistance through large deformations. A descrip-
These beams are the RD-4-5-1 and RD-4-5-2 beams. Of the tion of that test and all other development length tests can be
found in the appendices to this report.
Bond failures in I-shaped beams. In development length
tests on I-shaped beams made with 0.5 in. strands, three bond
failures occurred. All of the bond failures occurred in beams
made with Strand D, the strand with the lower NASP Bond
Test value of 6,890 lb. Of the three tests that failed in bond, two
ends failed at embedment lengths of 72 in. and 88 in. These
were two ends of the same beam that had a release strength of
5,490 psi and a 56-day strength of 9,840 psi. 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.
Figure 3.42. Cracking pattern for Beam Test IA-10-6-1 Beam ID-6-5-1 (south end) shows a typical bond failure.
South, at strand. This I-shaped beam contained five 0.5 in. strands; the con-
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1200 1.50
1.35
1000
1.20
1.05
800
Strand End Slip (in.)
Mn = 705.3 kip-in
Moment (kip-in)
0.90
600 0.75
0.60
400
0.45
Moment vs Deflection
0.30
200
Strand End Slip vs Deflection 0.15
0 0.00
0 0.5 1 1.5 2 2.5 3 3.5 4
Deflection (in.)
Figure 3.43. Applied moment versus deflection and strand end slip for Beam RD-4-5-2-South.
crete strength at 56 days was 9.84 ksi. The embedment length kip-in. was only about 81 percent of its calculated nominal
for this test was 88 in., or about 120 percent of the AASHTO flexural capacity. In reviewing the results from the test, it is
design requirement for 0.5 in. strands. The beam contained apparent that the incidence of web shear cracking coincided
strands from the sample Strand D, which possessed a rela- with the initial strand end slips. Strand end slips continued to
tively low NASP Bond Test value of 6,890 lb. increase with increased beam loadings and increased beam
The moment versus deflection curve is found in Figure deflections. The test was concluded at a total deflection of
3.45. The moment versus deflection curve illustrates that the about 3.5 in., when it was apparent that deflections were in-
beam was unable to reach its nominal flexural capacity, Mn. creasing without further increase in beam capacity. The
The results indicate that the beam's flexural capacity of 3,280 cracking pattern and the crushing failure of the beam can be
viewed in Figure 3.46. The photograph shows one flexural
crack under the loading point that became very wide under
load. The excessive width of the crack is further evidence of
bond failure in the prestressing strand. Because the beam was
unable to achieve its nominal flexural capacity and because
the beam exhibited excessive strand end slips, this test was
classified as a bond failure.
3.5.3.3 Types of Failure--Shear Failure
Two shear failures occurred in I-shaped beams; no shear
failures occurred in the rectangular beams. Prior research has
shown that significant interaction can exist between shear
and bond behaviors, especially in I-shaped beams with nar-
row webs (Kaufman and Ramirez 1988). In these beams,
shear behavior is improved considerably by the inclusion of
Figure 3.44. Cracking pattern of bond failure for horizontal mild reinforcement within the webs and extend-
Beam RD-4-5-2 (South). ing for the first 96 in. from each end of the I-shaped beam.
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5000 1
4500 0.9
Mn = 4051.6 kip-in
4000 0.8
3500 0.7
Strand End Slip (in.)
Moment (kip-in)
3000 0.6
2500 0.5
Moment vs Deflection
2000 0.4
1500 0.3
1000 0.2
Strand End Slip vs Deflection
500 0.1
0 0
0 0.5 1 1.5 2 2.5 3 3.5 4
Deflection (in.)
Figure 3.45. Applied moment versus deflection and strand end slip for Beam ID-6-5-1 (South).
An example shear failure is observed from the test on Beam moment versus deflection curve follows a pattern indicative
IA-6-6-2 (north end). This I-shaped beam contained four 0.6 of a flexural failure. The curve also shows that the beam was
in. strands; the concrete strength at 56 days was 8.99 ksi. The unloaded and then reloaded a second time. Web shear
embedment length for this test was 88 in., or approximately cracking and flexural cracking occurred at the same load
equal to the AASHTO design requirement development increment, corresponding to a moment of about 2,400 kip-
length for 0.6 in. strands. The beam contained strands from in. Strand end slips did not occur with the initial web crack,
the sample Strand A6, which possessed a NASP Bond Test but soon followed.
value of 18,290 lb. Also, this beam was dropped and damaged One of the interesting things about this test is that the shear
during handling at the prestressing plant. Several cracks re- failure occurred as the beam had reached its nominal flexural
sulted from the dropping of the beam. capacity. The large deformations also suggest that strand
The moment versus deflection curve and the strand end yielding was probably occurring, and, as the test on the beam
slip versus deflection curve are shown in Figure 3.47. The was being conducted, a flexural failure was indicated. How-
ever, as one can view in the photograph shown in Figure 3.48,
the beam failed suddenly and violently with a diagonal com-
pression failure of the web. The shear failure shows that even
though the beam is failing in shear, the strand possesses bond
adequate to develop the beam's capacity.
3.5.3.4 Summary of Development Length Tests
There are three key issues:
1. Whether the NASP Bond Test can be used as a predictor
of strand bond performance in flexural applications,
2. What the minimum acceptable level of bond performance
is as measured by the NASP Bond Test, and
3. What modifications are necessary to the LRFD develop-
Figure 3.46. Cracking patterns at the maximum load ment length equation to account for variations in concrete
(failure) of Beam ID-6-5-1 (South). strength.
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5000 1
4500 0.9
Mn = 4040 k-in
4000 0.8
3500 0.7
Shear Failure
Strand End Slip (in.)
Moment (kip-in)
3000 0.6
2500 0.5
2000 Moment vs Deflection 0.4
1500 0.3
1000 0.2
Strand End Slip vs Deflection
500 0.1
0 0
0 0.5 1 1.5 2 2.5 3 3.5 4
Deflection (in.)
Figure 3.47. Applied moment versus deflection and strand end slip for Beam Test IA-6-6-2
(North).
The NASP Bond Test as a predictor of strand bond per- bond failures at shorter embedment lengths. In I-shaped
formance in flexural applications. Two 0.5 in. strands were beams made with Strand D, the strand failed in bond even at
tested in beams. Strand D had an NASP Bond Test value of lengths in excess of the ACI and AASHTO design require-
6,890 lb and Strands A and B had an NASP Bond Test value ments for development length. In other words, Strands A and
exceeding 20,000 lb. In the rectangular beams made with B, which have relatively high NASP Bond Test values, demon-
Strands A or B, no bond failures were experienced, even at rel- strated excellent bond characteristics. In contrast, Strand D,
atively short embedment lengths. In I-shaped beams made with a relatively low NASP Bond Test value, demonstrated
with Strand B, no bond failures were experienced, even at em- poor bond characteristics. The results clearly show that the
bedment lengths shorter than the AASHTO design require- NASP Bond Test can distinguish between strands with good
ment for development length. In contrast, both rectangular bonding behavior and strands with poor bonding behavior.
beams and I-shaped beams made with Strand D experienced
The minimum acceptable level of bond performance as
measured by the NASP Bond Test. To determine a mini-
mum level of bond performance as measured by the NASP
Bond Test, results from the testing program conducted in the
NASP Round III testing program are required. However, the
results from the testing described in this chapter clearly indi-
cate that the minimum value for the NASP Bond Test should
be greater than the value measured on Strand D, 6,890 lb, but
need not be as strong as the bond value measured on Strands
A and B, which exceeded 20,000 lb.
Modifications necessary to the LRFD development
length equation to account for variations in concrete
strength. The results clearly show that increases in concrete
strength bring about improvements in strand development.
Strand D, which failed in bond at lower concrete strengths,
was still able to fully develop adequate tension at the higher
Figure 3.48. IA-6-6-2 (North) at shear failure. concrete strengths.
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3.5.4 Discussion of Test Results The variables for development length tests in this research
were embedment length, concrete strength, and the type of
This section includes analysis in three primary areas: strand. These parameters were changed for flexural tests on
both rectangular and I-shaped beam specimens.
1. What influence does concrete strength have on the devel- The current ACI/AASHTO equation does not include the
opment length for pretensioned prestressing strands? concrete strength parameter for calculating transfer and de-
2. What is the proper expression for development length? velopment length. However, results obtained during the flex-
3. What should be the minimum NASP Bond Test Value of ural tests strongly suggest that the anchorage ability of the
the prestressing strand for achieving adequate anchorage? strands is improved as concrete strength increases. The next
section reports on the effects of increasing concrete strength
The NASP Bond Tests in concrete clearly demonstrate that on the results obtained during the flexural tests.
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 3.5.4.2 Direct Tabular Method
strand was shortened as concrete strength increased. In this
Table 3.28 summarizes the results from development
section, the results from development length tests are ana-
length tests performed on Strand D cast in rectangular beams.
lyzed to determine the influence of concrete strength. Based
In the Tables 3.28 through Table 3.30, the letter "F"' denotes
on the analysis, certain modifications to the current
a flexural failure, and the letter "B" denotes a bond failure. In
AASHTO equation for development length are recom-
Table 3.28, the results indicate that for embedment lengths of
mended. Comparisons among flexural test results are used to
73 in. and concrete release strengths of about 4 ksi (56-day
assess the validity of such recommendations.
strength of 7 ksi), Strand D was able to develop the necessary
tension to achieve a flexural failure in the beam. However, at
3.5.4.1 Evaluating Development Length an embedment length of 58 in. and tested at the opposite ends
from the Flexural Tests of the same beams, Strand D failed in bond.
The embedment length of 73 in. corresponds to 100 per-
The development length is the length for which the strand cent of the development length prescribed in the AASHTO
must be fully bonded to ensure strand anchorage adequate to
develop the tension stress necessary to support the nominal Table 3.28. Development length tests on rectangular
flexural capacity of the cross section. The development length beams with 0.5-in. Strand D (average NASP pull-out
is distinguished from the embedment length, which is the value = 6,870 lb).
length of bond that is actually provided. In the course of test-
ing, a specific embedment length may be longer or shorter fc fc
than the strand's development length. If a beam test results in @ 56
Beam No. Release Days Embedment Length (in)
a bond failure, then one must conclude that the embedment (psi) (psi)
length provided was shorter than the required development 46 58 73
length. Conversely, if a beam test results in a flexural failure, RD-4-5-1 4,033 7,050 B F
then one can conclude that the embedment length provided RD-4-5-2 4,033 7,050 B F
was longer than the required development length. Each inde-
RD-6-5-1 6,183 8,500 F F
pendent beam test therefore becomes a single data point that
can indicate whether the embedment was sufficient. In most RD-6-5-2 6,183 8,500 B F
cases, it is difficult to discern from a single test what the "true" RD-6A-5-1 7,960 11,420 F F
development length must be.
RD-6A-5-2 7,960 11,420 F F
Ideally, the "true" value of development would be when the
flexural test results in simultaneous flexural, shear, and bond RD-8-5-1 8,570 13,490 F F
failures (Meyer 2002). Research that varies the embedment RD-8-5-2 8,570 13,490 F, F* -
length between the values corresponding to complete flexural
RD-10-5-1 9,711 14,470 F F -
failure and the values corresponding to complete bond failure
can get closer to identifying the "true" development length. RD-10-5-2 9,711 14,470 F F -
Based on prior test results, the embedment length can be F = Flexural failures
systematically lengthened or shortened for the purpose of brack- B = Bond failures
eting the test results. In this manner, an accurate picture for de- * Both ends were tested at an embedment length of 58 in. Both ends failed in
flexure.
velopment length may be obtained through multiple beam tests.
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Table 3.29. Development length tests on rectangular The tests demonstrate that, had the concrete strength been
beams with 0.5-in.Strands A/B (average NASP 7 ksi, the development length required for Stand D would be
pull-out value for A = 20,210 lb and for B = 20,950 lb). less than 73 in. but greater than 58 in. The dark line in the
table separates the zone of bond failures from the zone of flex-
fc fc
Beam No.
ural failures. The test results clearly show that the strand bond
RLS 56 Days Embedment Length (in)
(psi) (psi) improves in development length applications with increases
46 58 73
in concrete strength.
RB-4-5-1 4,033 7,050 F F Table 3.29 shows the results from development length
RB-4-5-2 4,033 7,050 F F tests performed on beams made with Strands A/B. The re-
RA-6-5-1 6,183 8,500 F F sults show that (1) Strands A/B bonded better with concrete
RA-6-5-2 6,183 8,500 F F than Strand D, and (2) the bond of Strands A/B improved
RA-6A-5-1 7,960 11,420 F F
as concrete strength increased. The dark line in the table
separates the zone of bond failures from the zone of flexural
RA-6A-5-2 7,960 11,420 F F
failures.
RA-8-5-1 8,570 13,490 F F
Table 3.30 summarizes the results of beam tests on rectan-
RA-10-5-1 9,711 14,470 F F
gular beams made with 0.6 in. strands. The current AASHTO
F = Flexural failures
B = Bond failures
expression gives a development length requirement equal to
88 in. for 0.6 inch diameter strands. Test results show that
flexural failures occurred at lengths of 88 in. and 73 in. for all
LRFD Bridge Design Specifications, while the embedment concrete strengths. The results also show that bond failures
length of 58 in. corresponds to 80 percent of the code- occurred for the three concrete strengths when an embed-
specified value. Important to the purposes of this research, ment length of 58 in. was tested. However, when Strand A6
the bond of Strand D demonstrates marked improvement as was cast in concrete with a release strength of 10 ksi and a
concrete strengths increase. At a concrete strength of 11 ksi, 56-day strength of over 14 ksi, the strand was able to develop
Strand D was able to develop the necessary tension at the required tension force at an embedment length of 58 in.
embedment lengths of either 58 in. or 73 in. The test results The dark line in the table separates the zone of bond failures
indicate that for Strand D, cast in 11 ksi concrete, the devel- from the zone of flexural failures. These results show clear
opment length required is equal to or less than 58 in. Further, improvements in strand bond behavior with increasing con-
in Beams RD-10-5-1 and RD-10-5-2, Strand D was able to crete strength.
develop its tensile force in only 46 in. of bonded length. These The current ACI/AASHTO equation does not include the
tests indicate that for Strand D cast in 14 ksi concrete, the concrete strength parameter for calculating transfer and
development length required is equal to or less than 46 in. development length. However, results obtained during the
Table 3-30. Development length tests on rectangular
beams with 0.6-in. Strand A6 (average NASP pull-out
value = 18,920).
fc fc
Beam End RLS 56 Days Embedment Length (in)
(psi) (psi)
58 70 73 88
RA-4-6-1 4,033 7050 F F
RA-4-6-2 4,033 7,050 B F
RA-6-6-1 4,855 8,040 F
RA-6-6-2 4,855 8,040 B F
RA-6-6-3 4,855 8,040 F
RA-8-6-1 5,413 8,220 F
RA-8-6-2 5,413 8,220 B F
RA-10-6-1 9,150 14,610 F
RA-10-6-2 9,150 14,610 F F
F = Flexural failures
B = Bond failures
