Civil Engineering Reference
In-Depth Information
The tension induced in the structure due to restraint at the construction
joints can therefore be limited. With sufficient testing of concrete such that
the material properties are fully understood, and analysis of temperature
and stress development, the cooling system can be accurately designed to
avoid cracking.
The greatest risk of cracking arises at the construction joints between the
base slabs and the walls of the concrete tunnel structure. The walls are gen-
erally thick enough to generate high temperatures of between 40 and 50°C.
Typically, cooling pipes are located in the core of the walls and extend from
just above the joint to a point up the wall where temperature gradients
have tapered out to a manageable level. They are placed horizontally for
the length of the construction pour. For particularly thick walls, a double
line of pipes may be required, as shown in Figure 11.3. Pipes may be more
closely spaced towards the construction joint where the risk of cracking is
greatest. The pipes are connected into a single network and chilled water
at around 5°C is pumped into the pipes at the lowest level. As water passes
through the pipe network, progressing up the wall, the temperature of the
water will increase. It outlets at a high level in the wall, typically at an
increased temperature anywhere between 25 and 40°C and is passed back
through chiller units to be cooled and passed back into the pipework at the
lower level again. A typical cooling pipe layout is shown in Figure 11.3.
Cooling may not be sufficient on its own and insulation mats may be
required to control heat radiation from the concrete surface. These can be
built into the analysis to determine the insulating properties needed and
the durations for which they should be applied. The cooling can be stopped
when the temperatures drop sufficiently so that there is no longer a risk of
through-cracking. The period for cooling the concrete is typically about
Outlet
Construction joint
Continuous steel pipe
cooling water circuit
Staggered pipe
arrangement
(for wider walls)
Linear pipe
arrangement
Inlet
Figure 11.3
Staggered and vertical cooling pipe layout.
