Civil Engineering Reference
In-Depth Information
Inclusion into the mix at between 5 and 10% provides a wide range of
performance enhancements. Firstly it increases the cohesiveness of the fresh
concrete and improves the bond of the sprayed concrete to the proposed
surface. This cohesiveness also has a big impact on rebound, reducing it from
about 40% to less than 10% on typical mixes. Any 'rebounded' material
is unusable once sprayed so considerable cost savings can be made. It also
reduces the amount of dust created by the spraying action. The cohesiveness
of the mix then improves the build-up characteristics, allowing increased
thicknesses of sprayed material. Collectively this reduced dusting and
lower rebound enables more efficient production and improved working
conditions.
For wet processes, improved pumpability is noticeable. Silica fume is often
added to conventional concrete as a pumping aid and this enhancement is
also true for sprayed concrete.
A number of benefits accrue when using silica fume. These include:
• Increased compressive, flexural and tensile strength and increased bond
to the surface and reinforcement,
• Reduced permeability,
•
Improved frost- and chemical-resistance.
Tests have shown strengths in excess of 100 MPa can be obtained using
a quality silica fume. Similarly, early-age strengths of 1 MPa at only 2 hours
can be achieved using accelerators. The natural increased cohesiveness of a
silica fume mix can also have an impact in lowering the required accelerator
additions.
For special applications higher dosages can be considered and suitable
trial mixes undertaken to test suitability. Major suppliers will be able to
advise on mix details, provide sample material for testing and a range of site
support services (storage, pumps, etc.) if required.
The quality standard for silica fume is BS EN 13263 (BSI, 2005) and this
should be considered as mandatory when considering a likely source.
13.7 Spraying procedures
As a bond is required in the case of repairs or strengthening work, the substrate
must be clean before spraying of concrete can commence. If the substrate
surface is porous it may need wetting down to kill excessive suction from the
concrete being sprayed. In dry spraying the w/c ratio of the sprayed concrete
is already very low and suction from the interface surface might reduce it to
below the critical level for complete hydration of the cement. However, there
should be no free water left on the surface before spraying commences.
Before applying dry sprayed concrete, the nozzleman should direct the
spray away from the works until he judges that the concrete coming out of
the nozzle is of an acceptable quality.
