Civil Engineering Reference
In-Depth Information
General
When blended bagged materials are being used, they are mixed with water
on site and fed into the spraying machine. For large quantities, the mortar
can be supplied ready-mixed or from a site mixer.
The mixing water can be accurately controlled and, with the use of
water-reducing plasticisers, a w/c ratio of less than 0.45 is easily achievable.
Sprayed concrete is a cement-rich material, although the surface area of the
sand is high. Cement contents are normally in the range 350-450 kg/m³ and
28-day strengths can be expected to be 30-60 MPa.
It is normal for the sprayed concrete contractor to design the mix so that
he can take into account the performance of the wet concrete when it is being
pumped and when it is in position, such as minimising the rebound, to produce
a mix which will also meet the specified requirements of strength and durability.
With this process, rebound is less than that produced by the dry process.
Equipment
The wet process uses either piston or worm pumps to deliver the concrete to
the nozzle in a dense stream. Compressed air is introduced at the nozzle to
project and compact the concrete onto the substrate. A set accelerator may
also be added at the nozzle to give the in-place material an early mechanical
strength, which can minimise sloughing off.
For the low output normally required for repairs (up to 4 m³/h) a worm
pump would be suitable and this can take a maximum aggregate particle size
of 4 mm. This output may seem very low to the conventional concretor but
it produces an acceptable output of sprayed concrete. A double piston pump
(in the range 4-25 m³/h and capable of taking 20 mm aggregate), would be
used for larger applications.
