This invention relates to re-usable formwork for casting concrete
bodies and more especially (but not exclusively) for forming the end, for the time
being, of a concrete body to be extended in a subsequent casting step (hereinafter
for brevity called the "growing" end). An important application is in constructing
walls below ground level (either to remain so as a "diaphragm wall" or to be exposed
by subsequent excavation of the ground to one side of the wall as a retaining wall).
Diaphragm walls (and retaining walls constructed below ground level)
are generally cast in sections short enough to for the excavation to be supported
by a drilling fluid (typically bentonite slurry), and thus require large numbers
of joints. In some cases the sides of the excavation itself may be sufficient
to contain the concrete, but formwork is normally needed at the growing end of
the casting (or at both ends, where the wall is to be extended in both directions),
so that the end shall be true enough to ensure proper coupling and sealing to
the next section; if the end face is true enough, it may be used to guide the machine
excavating for the next section.
Conventional formwork is extracted by pulling it vertically upwards.
Especially if the wall is deep, the force required to extract it after the concrete
has set may be very high.
In some cases, formwork has been extracted sideways after excavation
for the next section is complete by manipulation of the excavating tool or an auxiliary
In accordance with the invention, reusable formwork for construction
in concrete comprises at least two rigid formwork bodies together forming a containing
surface for concrete to be cast and mechanical means for producing forces across
the containing surface tending to produce relative movement of the said bodies
and sufficient to release one of them from adhesion to concrete cast on the said
Preferably one of the two rigid bodies forms the main structural
member of the formwork (though it may itself be of modular construction) and the,
or each, other rigid body is a plate closing an aperture in that one rigid member.
Preferably there is more than one other rigid body, not only to allow forces to
be distributed over the area of the formwork but also to allow the associated mechanical
means to be actuated successively to release all the parts of the formwork.
Preferably the formwork is of steel, for durability and to allow
the dimensional accuracy needed to ensure there is negligible risk of concrete
leaking between contacting parts.
The said mechanical means are preferably hydraulic jacks, either
direct-acting or using levers, wedges or the like to change the direction of movement.
Other suitable mechanisms (including pneumatic motors and screw jacks) can be
used if preferred, for instance because hydraulic power is not readily available
The containing surface may be generally flat, but preferably it incorporates
at least one projection or recess (for example a vertical rib or channel) to provide
mechanical interlock between the concrete sections.
Where waterproof sealing is required, rubber or other resilient strips
may be inserted partly into longitudinal slots in the formwork so that they may
become embedded in the concrete cast against it and may subsequently be pulled
free from the formwork when it is released from the concrete and in due course
be embedded in the concrete of the next section to be cast.
The invention includes a method of casting a concrete wall or other
body in successive steps including defining the growing end of the body by reusable
formwork comprising at least two rigid formwork bodies together forming a containing
surface for the concrete of a first section to be cast and mechanical means for
producing forces across the containing surface tending to produce relative movement
of the said bodies, casting a first section of the concrete body against the containing
surface and after it has set actuating the said mechnical means to produce forces
sufficient to release one of the two rigid bodies from adhesion to the concrete
and then withdrawing the formwork.
If there are only two said rigid bodies, it may be necessary to apply
an impact after the said one rigid body has been released to release the other.
Another possibility is for the parts to be coupled in a manner allowing relative
movement in both directions (using close-fitting sliding surfaces or a compressible
gasket to maintain a seal against leakage of concrete) and to be actuated first
in a direction to release one (preferably the smaller) of the two rigid bodies
and then in the other direction to release the other.
Preferably, however, there are more than two of the said rigid bodies
and the mechanical means associated with them are actuated sequentially so that
in a first step at least two of the rigid bodies are released from the concrete
and in a second step at least one of them is moved back into contact with the
concrete from which it was released in the first step to transmit forces sufficient
to release the, or each, rigid body that was not released in the first step.
More especially, we prefer to make the formwork with a main rigid
body accounting for a major part of its area and a number (preferably more than
one) of groups of other rigid bodies, each group comprising three such bodies
arranged in line (preferably either horizontally or vertically); the mechanical
means for the outer pair of each (or the) group of three rigid bodies are then
intended to be actuated in the first step to release from the concrete the main
rigid bodies and the central rigid body of each (or the) group, and then those
mechanical means deactivated and the mechanical means for the central rigid body
of each (or the) group acuated to release the outer pair(s) of rigid bodies.
The invention will be further described, by way of example, with
reference to the accompanying drawings in which each of Figures 1-4 is a
diagrammatic plan view of one design of formwork in accordance with the invention
showing successive stages in its preferred mode of operation.
Preferred form of the Invention
The formwork shown in the drawings is of modular construction and
comprises at least two active sections (at top and bottom) spaced apart by plain
sections of the same overall plan section. The active sections have a casing 1
(cross-hatched for clear identification) and the plain sections are of the same
plan shape except for portions 2 which, for clarity, are shown only in Figure 1.
Correponding to these portions 2, the active sections include a pair of similar
rams 3,4 which can be urged outwards by respective hydraulic jacks 5,6 and, in
line between them a smaller ram 7 which can be urged outwards by another hydraulic
ram 8. Membranes 9, 10, 11 ensure that the rams are leakproof in their unactuated
starting positions. Aligned slots are used to hold and position water-stop strips
The formwork is set up in the starting position in an initial excavation,
either contacting the earth at its ends as shown or if the nature of the ground
demands it in conjunction with other formwork, with its back face 12 in nominal
contact with the earth 13 of the next section to be excavated, and concrete 16
(Figure 2) poured to fill the existing excavation 17 and allowed to harden.
A new section of excavation 18 is next formed outside the formwork,
and then hydraulic jacks 5 and 6 are actuated, urging their rams 3 and 4 towards
the concrete and the remainder of the formwork away from it; since the rams have
nowhere to go, this moves the remainder of the formwork and releases both the
body 1 and the ram 7 from adhesion to the concrete. This leaves the formwork adhered
to the concrete only over the area of the rams 3 and 4 and the waterstops 13 embedeed
in the concrete.
Now (Figure 3), the hydraulic jack 8 is actuated to bring its ram
7 back into engagement with the concrete 16; unless the stroke of this ram is greater
than that of rams 3 and 4, they need to be retracted - this can be done simultaneously,
before or after actuation of jack 8, but preferably after as this minimises risk
of damage to the water-stops 13, 14. This action breaks the remaining adhesion
of rams 3 and 4 to the concrete (Figure 4) and the formwork can be easily withdrawn
and repositioned for casting of the next section.
A first alternative design of formwork simply omits the ram 7 and
its jack 8; the procedure is as described to the stage of Figure 2 but the rams
3 and 4 are freed from adhesion to the concrete by applying a few blows to the
formwork by swinging the excavating apparatus or some other adequately massive
A second alternative design of formwork also omits the ram 7 and
jack 8 but in this case the jacks 5 and 6 are bidirectional and the membranes 9,
10, 11 are replaced by comressible gaskets. In this case the first action for the
release of the formwork is to drive the jacks 5 and 6 in the direction to compress
the gaskets, so freeing the rams 3 and 4 from adhesion; the second and final is
to drive them in the other direction to urge the freed rams 3 and 4 into engagement
with the concrete and release the main body 1 of the formwork. The gaskets may
be resilient and reusable (e.g. of a foam rubber) or they could be crushable and
disposable (e.g. of expanded polystyrene).