Lightweight Concrete for High Strength - Expanded Shale & Clay

A.8.5 Martin and Scott, 1976
Martin and Scott re-evaluated previous testing by Hanson and Kaar based on prestressing
strand having an ultimate strength, f pu , of 270 ksi, and commonly encountered strand surface
conditions, and they recommended the implementation of Equation A.7.
l
t
=
80 d
( A.7)
b
A.8.6 Zia and Mostafa, 1977
Based on an extensive review of transfer length testing, Zia and Mostafa proposed
Equation A.8 to account for the effects of strand diameter, initial level of prestress, and concrete
strength at transfer. Equation A.9 was reported to be applicable for concrete strengths from
2,000 to 8,000 psi. This evaluation is reported in Chapter 9.
where
d b = diameter of prestressing strand
f si = stress in prestressing strands just after release
f ci ’ = concrete compressive strength at release (psi)
l
t
f
si
= 1 .5 d 4.6
( A.8)
' b
−
f
ci
A.8.7 FHWA Memorandum, 1988
Based on testing at the University of North Carolina in 1986 where poor transfer and
development length results were recorded, the FHWA issued a memorandum specifying four
interim restrictions, one of which was the restriction from using 0.6-inch prestressing strand.
This restriction prevented the prestressed concrete industry from fully utilizing the benefits of
higher strength concretes in larger members. Several research programs were initiated based on
the FHWA restrictions.
A-12