Repairing the Surface of Concrete - casting defects and minor damage
1.
Introduction
With very few exceptions, concrete elements are formed by a casting process:
the fresh concrete is placed and com-pacted in formwork or a mould which
is removed when the
concrete has hardened. The quality of any cast product depends on material
selection, equipment used and work-manship. Surfaces of acceptable quality
and accuracy are
difficult to achieve and, depending on functional and aesthetic requirements,
some repairs may be necessary.
Repairs may also be necessary where surfaces are damaged in service.
This publication is intended to assist those responsible for specifying,
supervising or carrying out such repairs.
Types of defects and their causes are given. Repairs of various types
and the use of different techniques are described. The selection of materials
for repairs is discussed in an appendix.
Only relatively superficial repairs are discussed. Repairs to effect structural
changes and repairs of cracks are outside the scope of the publication.
Repair materials are limited to formulations based on portland cement.
2. Types of defects
Minor defects may be categorized as follows:
- Small cavities such as those formed by tie-bolts and by impact damage to the surface.
- Honeycombing, ie concrete in which mortar is partially or completely absent. Honeycombing is usually caused by the use of concrete which contains too much stone,or by segregation of the stone and mortar.
- Misplaced surfaces which are usually the result of incorrectly aligning or positioning formwork or move-ment of formwork while the concrete was being placed. Under- or overfilling of formwork may also be a cause.
- Blowholes, ie relatively small voids of roughly
spherical shape which often occur in surfaces formed by formwork or
moulds. They are usually caused by
inadequate compaction of the thin layer of concrete in
contact with formwork.
Repairs to small cavities, honeycombing and misplaced surfaces are discussed
in section 3. Filling of blowholes is dealt with in section 4.
3. Repairs to small cavities, honeycombing and misplaced surfaces
3.1 Basic types of repair materials
Repairs are carried out usually after some drying, and con-sequent shrinkage,
of the concrete has taken place. Repair material therefore tends, when
it dries out, to shrink relative to the concrete being repaired. Cracking
of the repair, which may result from this relative shrinkage, is functionally
and aesthetically unacceptable. Repair materials are therefore formulated
in order to reduce the likelihood and severity of cracking. This is done
by reducing potential drying shrinkage and possibly by reducing the tendency
to crack as a result of a given shrinkage strain.
Two approaches are possible:
- The use of “semi-dry” mixtures of cement, water and aggregate which are compacted by being rammed into position.
- The use of plastic mixtures of cement, water, polymer emulsion and aggregate (polymer-modified mixtures) which are applied by means of a trowel or spatula.
The implications of these approaches are discussed below.
3.1.1 Semi-dry mixtures
Semi-dry mixtures have, because of their low water content, very low potential
drying shrinkage and consequently a low tendency to crack. But water content
is critical: if too low, thorough compaction is impossible; if too high,
the repair may slump or crack in the fresh state or crack in the hardened
state. These mixtures can be used only in confined spaces such as cavities.
For effective use, semi-dry mixtures must be applied by trained, experienced
and skilled operators.
3.1.2 Plastic mixtures
The inclusion of a suitable amount of specially formulated polymer in
a cement-water-aggregate mixture has the following important effects:
- the amount of water required for a given consistence is reduced.
- the hardened material, which incorporates a three-dimensional network of coalesced polymer, exhibits greater creep and toughness.
Consequently, potential
drying shrinkage is relatively low; tensile stresses resulting from restrained
shrinkage are low (because of low shrinkage and high creep); cracking
is signifi-cantly reduced (because of low stresses and good toughness).
Polymers are usually based either on styrene-butadiene rubber (SBR) or
acrylic, and are used in emulsion form.
Plastic mixtures are relatively easy to apply.
3.2 Principles of repairing
3.2.1 Timing
Repairs to new concrete should be done as soon as possible after formwork
has been removed. This reduces differential shrinkage and improves bond
between original
concrete and repair material.
3.2.2 Edges to repairs
Feather edges to repairs must be avoided. Where neces-sary the outline
of a repair should be cut with a masonry cutting disc or saw to ensure
a square edge. See Figure1.
3.2.3 Surface preparation
Good adhesion between original concrete and repair is essential. To ensure
good adhesion, the surface of the original concrete must be strong, rough
and clean. Any loose or weak material must therefore be removed. It is
important to remove concrete in such a way that the remaining concrete
is damaged as little as possible. Sharp chisels driven by relatively light
hammers are suitable. Sand-blasting, which can remove small volumes of
concrete, is an excellent means of achieving a rough surface free of loosely
adhering material. Before doing the repair, all dust and detritus must
be removed from the surface. Washing with clean water may be used for
vertical and near-vertical surfaces. An industrial vacuum cleaner is an
effective means of cleaning all surfaces.
3.2.4 Mechanical fixing
In cases where repairs are large, say in excess of 0,1m2, and especially
where persons could be injured by falling fragments, it is prudent not
to rely solely on adhesion between repair and background concrete but
to provide mechanical fixing.
Such fixing should be in the form of corrosion-resistant metal devices
such as screws or rods. Austenitic stainless steel is preferable. Fixing
devices, which are shown in Figure 2, should be installed after surface
preparation is complete but before the surface is cleaned.
3.2.5 Bonding
To ensure good adhesion of fresh concrete or mortar to a substrate of
hardened concrete, the substrate should have enough suction to absorb
the water film at the interface but should not desiccate the repair material.
This condition of limited suction can be achieved in different ways, depending
on the age and denseness of the concrete.
If the concrete is fairly young - say within 48 hours of being placed
- it should be sufficient to allow the surface to become visibly dry.
Figure 1: Edges to repairs - use saw cuts to avoid feather edges
Older concrete should be assessed for absorptiveness by wetting the surface: if water is rapidly absorbed, absorptive-ness may be regarded as high; if water is hardly absorbed, absorptiveness may be regarded as low.
Concrete with high absorptiveness should be saturated
for some hours before repairs are to be carried out. Surface water must
then be removed and the surface allowed to
become visibly dry.
Repairs must be started as soon as this state is achieved.
Concrete with low absorptiveness does not require prewet-ting and should
be repaired in a dry state.
The substrate should be primed with a slurry immediately before the repair
material is placed.
Priming slurry should be a mixture of equal volumes of cement and dry
plaster sand with sufficient water to achieve a paint consistence. (Neat
cement paste is difficult to mix and is therefore not recommended.) Polymer
emulsion may be added to the mixing water - 1 part emulsion to 2 parts
water is usually satisfactory.
Figure 2: Mechanical fixing devices
Primer slurry must be applied as a thin
coating to the substrate using suitable brushes.
Do not allow primer to accumulate in depressions in the surface.
Do not allow primer to dry before applying repair material. Priming must
therefore be done immediately ahead of repairing and is best done in a
small area at a time.
3.2.6 Compaction
Good compaction is essential. Semi-dry mixes must be com-pacted by heavy
tamping.
Plastic mixes must be applied with heavy pressure on the trowel or spatula.
3.2.7 Finishing
Where appearance is important, repairs should be finished to match the
texture of the surrounding concrete. (Colour matching is discussed in
section A1 of the appendix.)
Finishing tools include wood floats, steel trowels, sponges, wire and
nylon brushes, etc.
3.2.8 Curing
Repairs must be moist cured for at least seven days. Plastic sheeting,
fixed along the edges to the concrete with pressure-sensitive tape, is
an effective way of trapping moisture and ensuring good curing. Good quality
membrane-forming curing compounds may also be used.
After completion of the curing period, polymer-modified repairs must be
allowed to dry out completely before being subject to wet conditions.
This allows the emulsion to
coalesce and so become water-resistant. Note that drying may be retarded
if curing compounds are used for curing and not removed from the surface.
3.3 Repair methods
The information in this section is arranged according to type of defect.
Defects dealt with are small cavities, honey-combing, and misplaced surfaces.
3.3.1 Small cavities and zones of honeycombed concrete
Remove all weak, soft or honeycombed material to expose hard, sound concrete.
Ensure that feather edges will not be formed in the repair; use saw cuts
at least 10 mm deep to outline the repair if necessary.
Install mechanical anchoring devices if necessary.
Remove all debris and dust from the surface to be repaired.
Check surface absorptiveness, control if necessary and prime surface as
described in section 3.2.5.
Repairing may be done using one of the following techniques:
Semi-dry mixture
Depending on the depth of the cavity to be filled the mix proportions
by volume are:
* A fairly coarse sand, with some plaster
sand blended in if necessary
Mix the cement and aggregates dry until the colour is uni-form. Add water
in small increments while mixing until the mix is cohesive: a lump squeezed
in the hand and should remain in one piece on release of pressure.
Place the mix in a continuous operation
but in layers not exceeding 20 to 30 mm thick and ram it into place using
the end of a piece of reinforcing rod or hardwood. If the surface of the
repair is visibly wet when compaction is complete, the mixture is too
wet and repair must be removed and redone with a slightly drier mix. A
mix containing too much water will shrink and crack or pull away from
the surrounding concrete.
Moist-cure the repair for at least seven days.
Plastic mixture
Use a ratio, by volume, of 1 part of cement to 2 parts dry sand. Sand
should be a fairly coarse sand which has been passed through a 2,36 mm
sieve. If necessary, plaster sand may be substituted for some of the coarse
sand to improve workability.
Mix cement and sand dry until the colour is uniform.
Mixing liquid should consist of 2 parts water to 1 part polymer emulsion
or the ratio recommended by the manufacturer.
Add mixing liquid gradually while mixing until a trowelable consistence
is achieved. Thorough mixing is essential with this type of mixture.
Apply the mixture, using a suitable trowel or spatula, in layers not exceeding
20 mm thick. If the repair depth exceeds 20 mm, the repair should be built
up in 20 mm layers applied on successive days. Roughen the surface of
intermediate layers to ensure good adhesion of the next layer. Cover each
layer with plastic sheeting until the next layer is applied.
If the cavity to be filled is of uneven depth, filling should be done
in such a way that the final layer is of uniform thickness. See Figure
3.
Figure 3: Filling a cavity to ensure that final layer is of uniform
thickness
3.3.2 Surface out of position
Ensure that feather edges will not be formed in the repair; use saw cuts
at least 10 mm deep to outline the repair if necessary.
Minimum thickness of repair is about 10 mm
Install mechanical anchoring devices if necessary.
Such devices should be spaced at intervals not exceeding ten times the
thickness of the repair.
Roughen the surface of the concrete. Abrasive blasting is recommended.
Remove all debris and dust from the surface.
Check surface absorptiveness, control if necessary and prime surface as
described in section 3.2.5.
Use a plastic mixture to build up the surface to the required plane and
texture to match the surrounding concrete.
Mix proportions and method are as given in section 3.3.1.
Blowholes
No surface preparation is necessary but it is important to fill blowholes
immediately after formwork is removed. Do not pre-wet the concrete because
water left in the blowholes dilutes and weakens the repair material.
Use of a mixture of:
- volume cement
- volumes plaster sand (loose and dry)
plus enough mixing liquid to achieve
a slurry consistence.
Mixing liquid consists of 2 parts water and 1 part polymer emulsion or
the ratio recommended by the manufacturer.
No priming is required. Work the repair mix over the whole area with a
sponge until holes are filled. Immediately after-wards, remove excess
material from the surface with a wet sponge, taking care not to obscure
any surface texture.
Spray surface lightly with water using a mist sprayer and
cover immediately with plastic sheeting. Keep covered for seven days and
then allow to dry slowly.
Appendix
Selecting materials for repairs
Repair materials are mixtures of cement, water and aggre-gate, with the
possible inclusion of a polymer emulsion. Each of these materials is discussed
below.
A.1 Cement
Cement should comply with SANS 50197-1 strength class 42,5N or higher.If the recommended curing procedure is followed, then any SANS 50197-1 cement could be used.
Because repairs tend to be darker than the original concrete when they
dry out, white portland cement may be substituted for about a third of
the grey material if a colour match is required. The optimum substitution
ratio should be deter-mined by test: carry out a repair in an unimportant
area and assess colour once the repair material has hardened and dried.
A.2 Water
Use potable water from a municipal supply. Water from other sources may
be used if shown by testing to be suitable.
A.3 Aggregate
Particle Size
Maximum particle size must not exceed one quarter of the thickness (or
least dimension) of the repair. An indication of particle size for various
thicknesses is as follows:
* These are standard sieve sizes
Particle Shape
Ideally, particles should be spherical. If such materials are unavailable,
particles should be roughly cubical or “chunky”. Flaky and elongated
particles should be avoided.
Grading
Sand used for repair work should be graded: particle size should range
from dust to the largest size. Sands with particles all of similar size
produce mixes with poor workability and high water requirement. Such sands
should be blended with other sands to improve grading.
Types
The following aggregate types, blended where necessary, may be found suitable:
Plaster sand
Useful for blending with a coarser sand to improve work-ability or on
its own for making priming slurry and for filling blowholes.
Concrete sand
Naturally derived pit or river sands and well-shaped crusher sands, with
coarse particles removed by sieving if neces-sary and blended with a finer
plaster sand. (Pit sands
consisting of decomposed granite usually have a high water requirement
and should therefore be avoided unless suitability can be demonstrated
by testing.)
Stone
Pebbles and crushed stone with well-shaped particles as described above
(see Particle shape).
A.4 Polymer emulsions
Polymer emulsions formulated for use with cement are obtainable from specialist
suppliers. Such emulsions should be based on styrene butadiene rubber
(SBR) or acrylic. Do not use polymer emulsions formulated as paint.
Emulsions based on polyvinyl acetate (PVA) are best avoided unless the
repair can, after curing is complete, be kept in a permanently dry state.
PVA emulsions, after drying
and coalescing, are unstable in wet conditions.
A.5 Proprietary formulations for repairs
Polymer-modified formulations that need only a specified amount of water
to be added and mixed are available from specialist manufacturers. Although
such formulations tend
to be more expensive per unit volume than repair materials made up with
ingredients obtained separately, they are convenient to use and should
be considered especially for
smaller jobs or when close supervision is not possible.
Cement &
Concrete Institute
PO Box 168, Halfway House, 1685
Tel (011) 315-0300 Fax (011) 315-0584 e-mail info@cnci.org.za website
http://www.cnci.org.za
Published by the Cement & Concrete Institute, Midrand, 1997, reprinted
1999, 2001, 2003, 2006
©Cement & Concrete Institute