Civil Engineering Reference
In-Depth Information
and the likely effects of the layering on the permeability and durability of
the sprayed concrete repair.
Figures 2.13
and
2.14
illustrate the typical
appearance of sprayed concretes in thin sections.
2.3.3 Non-proprietary hand-applied mortars based on Portland cement
Repair materials of this type tend to be applied on a small scale. They range
from dense, low-porosity mortars that might be expected to be as durable
as carefully mixed and applied proprietary mortars through to very low
durability mortars that carbonate rapidly and have high water/cement and
aggregate/cement ratios. An example of a repair mortar of this type is shown
in
Figure 2.15.
2.4 Investigating the condition of the bond between a
concrete repair and the concrete substrate
2.4.1 The effects of differing methods of surface preparation on
bond strength and repair durability
Many failures of concrete repairs result from weaknesses of the bond
between the repair material and the substrate. Petrographic examination can
provide much information about the nature of the concrete surface and the
method of preparation of the concrete surface.
Most repairs have a roughened substrate surface that is intended to
provide a key for the bond with the repair material. The roughening of the
concrete substrate is most commonly carried out either by mechanical means
using a method such as scabbling or by water-jetting.
Figure 2.15
Photomicrograph showing the development of crystals of secondary
crystals is indicative of moisture ingress and recrystallisation of the cement hydrates.
The ettringite crystals are needle-like and occur on the right side of the field of view
within a void that contains the resin used during the preparation of the thin section.
The left side of the field of view is occupied by moderate-porosity paste containing
bright portlandite crystals and small quantities of unhydrated Portland cement.
