Civil Engineering Reference
In-Depth Information
The first Easiforms were built
between the wars, and consisted of
no-fines clinker concrete. Those built
in the 1920s had cavity walls in
which only the inner leaf was clinker.
The Boswells of the late 1920s had
precast concrete corner columns
providing permanent profiles for the
in situ poured cavity walls.
High-rise examples for the most
part consisted of a concrete frame
and no-fines panel infilling, rendered
on the outside. They should present
no problems in identification. The
most numerous dwellings in this
category are those by Wimpey.
Occasionally too, the surveyor
may well come across what at first
sight would be described as a
rendered wall, but which on closer
inspection, together with the
absence of day joints and other
shuttering marks, and the lack of a
very coarse aggregate, would be
revealed as a pumped and sprayed
in situ finish.
Main performance
requirements and defects
Choice of materials for structure
No-fines concrete typically
consisted of a single size
3
⁄
4
inch
coarse aggregate in a cement slurry.
In practice the cement content
ranged widely - samples measured
by BRE investigators ranged from
5-20%.
Mixes for dense concrete will vary
widely. See the note on accelerators
in the section on durability.
Figure 2.59
The shell of a typical no-fines house
Characteristic details
100 mm
Basic structure
A typical loadbearing no-fines
concrete shell is shown in Figure
2.59. External walls in low-rise
dwellings built before 1951 were
commonly 12 inches thick; those
built 1951-64, 10 inches; and those
after 1964, 8 inches. The walls were
mostly built off a conventional brick
masonry base on strip foundations.
Reinforcement was incorporated,
either in bar or mesh form. The first
floor was supported off steel corbels
cast into the loadbearing walls, or
steel beams on padstones set into the
no-fines; many dwellings had dense
reinforced concrete eaves and ring
beams. High-rise dwellings mostly
had a reinforced concrete frame with
infilling panels in no-fines.
However, there are other kinds of
in situ construction, sometimes
hybrids partly framed, such as the
Universal system. This system
consisted of solid external walls, cast
in situ using clinker and gravel
aggregate concrete. The walls were
faced on internal and external
surfaces with asbestos cement
sheeting used as permanent
shuttering. Pressed steel channel
stanchions set at 4 foot centres
temporarily supported the
permanent shuttering and roof
framing before the concrete walls
were poured. Horizontal
reinforcement in bar form was
threaded through holes in the
stanchion webs
(125)
.
Strength and stability
Minor cracking will be seen on a
number of no-fines dwellings,
caused by drying shrinkage of the
no-fines material and subsequent
changes in environmental conditions.
It has no structural significance.
Minor variations in original
specifications are not considered to
affect structural adequacy.
Reinforcement generally has been
found to be in good condition, with
slight surface rusting only. Concrete
quality achieved in the dense
components was variable, and
carbonation will be found to have
penetrated to the reinforcement in
many cases.
In a few cases where gable walls
have an external leaf of brickwork,
the wall ties may be corroding.
No instances of structural
inadequacy were identified by BRE
during the investigation of Easiform
dwellings, although cracking in the
external walls of some houses has
occurred due to corrosion of the
embedded reinforcement. In general
this cracking is of a minor nature in
post-1946 construction but is more
substantial in a few of the oldest
dwellings (50 years)
(126)
.
Rainwater
penetrates
into the outer
zone of the
no-fines wall
Figure 2.60
Rainwater penetrating cracks in the
external render tends to find its way out
again without penetrating through the wall
thickness
