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Prevention of Concrete Scaling
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| Example of scaled concrete surface. |
Scaling is defined as a general loss of surface mortar or mortar surrounding
the coarse aggregate particles on a concrete surface. This problem
is typically caused by the expansion of water due to freezing and
thawing cycles and the use of deicing chemicals; however, properly
specified, produced, finished, and cured quality concrete need not
suffer this type of deterioration. There is a distinct chain of responsibility
for the production of scale resistant concrete.
Specifying (Architect/Engineer)
First, the concrete must be properly specified for its intended
service environment. For severe climates where it is to be expected
that concrete in a wet condition will undergo cycles of freezing
and thawing and the application of deicing chemicals;
- Concrete should be specified with a minimum compressive strength
of 28 MPa (4000 psi),
- Contain an appropriate volume of air (entrapped and entrained),
- Have a minimum cementitious content of 335 kg/m3
(564 lb/yd3), and
- Maximum water-cement ratio of 0.45.
These minimum specification requirements have produced positive results
for many decades. Proportioning and Delivery (Ready
Mixed Concrete Producer)
Secondly the concrete as proportioned and delivered by the ready
mixed concrete producer must comply with the aforementioned specification
requirements. It is the responsibility of the concrete producer
to properly proportion the concrete mixture to comply with the specification
and produce a workable mixture using locally available materials.
The proposed mixture must have sufficient workability that the concrete
contractor can place, consolidate, and finish in a timely fashion.
One challenging issue for the concrete producer is choosing the
correct air content for the mixture. The appropriate air content
is dependent on the maximum coarse aggregate size as shown in the
following table.
Recommended
Total Air Content for Concrete |
Nominal maximum size aggregate,
mm (in.) |
Air Content, percent* |
Severe exposure**
(Class F2 and F3) |
Moderate Exposure#
(Class F1) |
Mild Exposure## |
<9.5 (3/8) |
9 |
7 |
5 |
9.5 (3/8) |
7 1/2 |
6 |
4 1/2 |
12.5 (1/2) |
7 |
5 1/2 |
4 |
19.0 (3/4) |
6 |
5 |
3 1/2 |
25.0 (1) |
6 |
4 1/2 |
3 |
37.5 (1 1/2) |
5 1/2 |
4 1/2 |
2 1/2 |
50 (2)‡ |
5 |
4 |
2 |
75 (3)‡ |
4 1/2 |
3 1/2 |
1 1/2 |
*Project specifications
often allow the air content of the concrete to be
within -1 to +2 percentage points of the table target values.
**Concrete exposed to wet-freeze-thaw conditions, deicers, or
other aggressive agents (ACI 318 Exposure Class F2 and F3).
#Concrete exposed to freezing but not continually
moist, and not in contact with deicers or aggressive chemicals
(ACI 318 Exposure Class F1)
##Concrete not exposed to freezing conditions, deicers,
or aggressive agents.
‡These air contents apply to the total mix, as for the
preceding aggregate sizes. When testing these concretes, however,
aggregate larger than 37.5 mm(1 1/2 in.) is removed by handpicking
or sieving and air content is determined on the minus 37.5 mm
(1 1/2 in.) fraction of mix. (Tolerance on air content as delivered
applies to this value.) |
Finishing and Curing (Concrete Contractor)
After concrete has been placed, consolidated, and struck-off, the
concrete finisher is charged with using proper finishing practices
to achieve an aesthetically pleasing yet durable concrete surface.
The typical recommendations for finishing are as follows:
- Bullfloat, darby, edge, and make initial grooves prior to the
onset of bleeding.
- Allow excess bleed water to evaporate from the concrete surface
before floating the surface and making the second pass with edging
and grooving tools.
- Allow time for initial stiffening of the concrete to take place.
- Make a final pass with edging, grooving, and floating operations.
(Avoid steel troweling exterior concrete surfaces, especially at
later ages of stiffening. This can reduce entained air and may promote
scaling.)
- Allow additional stiffening.
- Use appropriate broom to produce the desired texture (soft bristle
for fine and medium textures, stiff bristle for coarse texture).
After the final texture is established concrete, must be cured to
assure that the surface will achieve the desired properties to be
durable.
Curing methods may include:
- Moist curing with water, fogging, or sprinkling of absorptive
coverings.
- Spray applied curing compounds.
- Plastic sheet cover (may cause discoloration on exposed concrete
surfaces).
And finally, exterior concrete should be allowed to air dry for
30 days prior to exposure to freezing and thawing cycles. In addition,
deicer chemicals should not be used on the new concrete surface
for a period of 3 months after the concrete has been placed—longer
if the concrete is placed during the fall or winter seasons.
References:
Kosmatka, Steven H., Kerkhoff, Beatrix, Panarese, William C., Design
and Control of Concrete Mixtures,
EB001, Portland Cement Association, 2002, 372 pages.
Collins, Terry C., Panarese, William C., Bradley, Bentley J., Concrete
Finisher’s Guide,
EB122, 2006, 88 pages.
Concrete Slab
Surface Defects: Causes, Prevention, Repair,
IS177, 2001, 16 pages
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