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77
3.7 The Effect of Concrete Strength · Current AASHTO code provisions may overestimate the
on Bond Performance-- required development length of prestressing strands in
Summary, Conclusions, higher strength concretes.
and Recommendations · I-shaped beams were more susceptible to bond failures
than rectangular beams because of the higher incidence of
The research program involved development length tests web shear cracks developing in I-shaped beams.
on two types of beam specimens. Four types of strands were
employed to cast 43 rectangular-shaped beams and 8 I-shaped Finally, on the basis of the study findings, the following
beams. Both 0.5 in. and 0.6 in. diameter strands were included recommendations are made:
in the testing program. The beam specimens had concrete re-
lease strengths varying between 4 ksi and 10 ksi for both types · AASHTO code equations for transfer length should in-
of beams. Transfer lengths were measured on all beam speci- clude a parameter reflecting the reduced transfer length
mens using the strand end slip of the strands with the help of with increasing concrete release strength. The recom-
clamps attached to the strands. Transfer lengths were also mended equation for transfer length, lt (in.), is
measured using the concrete surface strain measurements.
Fifty flexural tests were carried out on the rectangular beams, 120db
lt = 40db (3.9)
and 14 flexural tests were carried out on the I-shaped beams. fci
Values of load, deflection, and strand end slip were recorded
electronically and manually along with photographic records where
of failure stages and crack patterns. I-shaped beam specimen f ci = release concrete strength in ksi, and
concrete surface strains were measured at 36 in. from the end db = diameter of prestressing strands in inches.
of the beam and vertically at the center of the web. · AASHTO code equations for development length should
Prestressing strand anchorage requirements were assessed include a parameter reflecting the reduced transfer length
using the data collected from the development length tests. with increasing concrete release strength. Further, the flex-
Results from the development length tests were compared ural bond length is reduced by higher strength concrete as
with the NASP pull-out values of corresponding strands. well. The recommended equation for development length
Based on the failure modes during the development length is the following:
tests, the effect of concrete strength on bond performance was 120 225
analyzed. The current AASHTO code requirements for de- ld = + db 100db (3.10)
f ci f c
velopment length of prestressing strands were assessed for
their effectiveness in predicting accurate anchorage require- where
ments. The conclusions from this research are the following: ld = development length (in.),
f ci = release concrete strength in ksi,
· Development length tests can be used to assess the bond f c = design concrete strength in ksi, and
performance of prestressing strands. db = diameter of prestressing strands in inches.
· The ability of a prestressing strand to bond with concrete is · A relatively large database has been collected during the
affected by concrete strength. Increasing concrete strength course of this research project. The data include crack pat-
improves the bond-ability of a given prestressing strand. terns, crack spacing, and surface strain measurements on
· The development length requirement for a particular I-shaped beams. A more detailed analysis should be made
strand is reduced if cast in higher strength concrete. using the information embedded in the summary reports
· The NASP Bond Test provides a good indicator of strand for a better understanding of the failure mechanisms. It is
bond performance in a pretensioned concrete beam. recommended that the Standardized Test for Strand Bond
· The required development length shows a clear relation- be adopted into the AASHTO LRFD Bridge Design Specifica-
ship with the NASP Bond Test values of the prestressing tions. The Standard Test for Strand Bond, formerly known
strand. Higher NASP Bond Test values result in shorter de- as the NASP Bond Test, requires an average pull-out value
velopment lengths. of 10,500 lb with no single test out of a sample of six tests
· Rectangular beams with all types of strands were able to falling below 9,000 lb. These values are established from the
achieve flexural failures at embedment lengths less than or review of the data obtained from the testing reported herein.
equal to the AASHTO-specified development length. Supporting data is found in the NASP Round III test report,
· With increased concrete strength, it is possible to achieve which is incorporated into this report via discussion in pre-
flexural failures at an embedment length less than the vious sections. The Standard Test Method for the Bond of
AASHTO-specified value. Prestressing Strands is recommended to ensure adequate
