Lightweight Concrete for High Strength - Expanded Shale & Clay

2.8.10 Mitchell, Cook, Khan and Tham, 1993
Mitchell et al. based their research on determining the effect of concrete strength on
transfer length. They examined strand diameters of 3/8-inch, 1/2-inch and 0.6-inch used to
prestress rectangular sections made using normal weight concretes having strengths from 4,500
to 12,900 psi at 28 days. The method of strand release was gradual which was reported to
provide a better comparison to the sudden strand release used in most prestressing operations.
Based on their results, Mitchell et al. proposed Equation A.11.
l
t
=
3
0 .33 f
si
d
b
( A.11)
'
f
ci
A.8.11 Buckner, 1994
Buckner noted that inconsistencies existed between the methods used by various
researchers to determine transfer length. After an evaluation of results, Buckner concluded that
the 95 Percent Average Mean Strain (AMS) Method provided the best overall prediction of
transfer length. Furthermore, he recommended Equation A.12 for predicting transfer length.
Buckner noted that 1250/E ci was about 3 for compressive strengths greater than 3,500 psi; thus,
his findings endorsed Equation A.9 as well.
where
E ci =
l
t
=
1250 f
concrete modulus of elasticity at release (psi)
E
ci
si
d
b
( A.12)
A.8.12 FHWA Study, 1996
In addition to the numerous studies aforementioned, the FHWA also published findings
from their own study addressing the bond of prestressing strand in NWC and HPC. The FHWA
study addressed the following variables:
1. Concrete compressive strength at 28 days, f c ’
2. Square root of concrete compressive strength at 28 days, √f c ’
A-14