Concrete future - Concrete Mix Design, Quality Control and Specification - page 277

the neighboring structures, which had been already completed and could
not have made this 50:50 call.
Another major project had a number of engineering firms designing the
different phases. The proposed reinforcement for the basement rafts varied
by more than 300% despite each designer being given the same specifica-
tion guidelines. One designer proposed massive perimeter restraint to limit
predicted lateral movement and the others did not. How could there be such
a range of different technical requirements to build essentially the same
structural elements? One factor was the issue of calculating the limiting
crack width. The highest reinforcement volume proposed was required to
reduce the limiting crack width below the 0.2 mm nominated in the design
brief on the basis that it would need to be a watertight structure when the
waterproofing membrane eventually failed. Another important difference
among the proposed designs was the expected stress caused by long-term
shrinkage. The American Concrete Institute (ACI) guidance largely ignores
shrinkage after one year on the basis that the rate of shrinkage will, in
most structural sections, be sufficiently slow that creep would eliminate
any shrinkage stress. BS EN 1992, on the other hand, considers a com-
pleted piled raft as being exposed to end restraint and the reinforcement
required to limit crack widths is significantly higher. End restraint was not
even considered in BS 8007. We are not aware of significant problems with
basements that were previously designed to that code.
BS EN 1992 calculates long-term shrinkage based on the average shrink-
age through the depth of the concrete section. In a raft slab, drying can
only occur from the top surface with the bottom surface often encased in
a membrane and surrounded by water. Gilbert et al. (2012) measured the
shrinkage profile through concrete that was sealed at the base as well as
sealed and restrained at the base as in metal deck. They found a reduc-
tion in shrinkage of sections when evaporation was prevented from the
base using a coating. However, when restrained and sealed, the base of
the section exhibited no shrinkage. A similar effect would be expected to
occur within piled rafts with shrinkage reduced at the base of the raft due
to no evaporation from the bottom and restraint from distributed piles.
Therefore the restrained drying shrinkage at the raft-pile interface may
be much lower than anticipated from the average value calculated using
CIRIA C660/BS EN 1992 and the reinforcement requirement significantly
reduced. It can be very easy for the engineer to err on the side of cau-
tion and overdesign. It provides a greater factor of safety and it is not
his money!
In these types of situations, an important impediment to more sustainable
use of concrete has been a lack of in situ monitoring of structures to verify
the design assumptions and minimise any overdesign. Advances in monitor-
ing technology make it easier to acquire the required data. Accumulating