Lightweight Concrete for High Strength - Expanded Shale & Clay

If significant strand end slip over occurred during the test, it was impossible to accurately
estimate what the level of stress was at a point along the strand based on CSS data. Strand end
slip indicates there is a lack of adequate bond between the strand and concrete; near perfect bond
is necessary for CSS to be a meaningful predictor of an increase in strand stress. A telling
depiction of the effect of strand end slip can be seen in Figure C.25 for test G1B-East, which
shows the stress level in the strand to be 260 ksi near the end of the girder and only 227 ksi at the
point of load application. Extensive shear cracking in the transfer length region during test G1B-
East initiated a strand slip failure resulting in strand end slip of 0.75 inches. The slip allowed the
shear cracks to expand causing CSS values in that region to become extremely large. When used
to predict strand strain, the values were found to be erroneous.
300
Stress in Strands (ksi)
250
200
150
100
50
0
0 10 20 30 40 50 60 70 80
Distance From Girder End (Inches)
Strand Yield Stress at Load Point
Strand Maximum Stress at Load Point
Strand Maximum Stress
Effective Prestress
Crack Locations
Figure C.25 G1B-East Strand Stress vs. Distance from Girder end
In tests where strand slip was less than 0.01 inches, the CSS data appeared consistent and
was used to determine the strain and the stress in the prestressing strand. Two trends became
evident in examining these plots. First, in every case where strand slip was under 0.01 inches,
there was an observed increase in strand stress in the transfer length region above the level of
effective prestress. The effect was far more pronounced with shorter embedment lengths.
C-16