7. |
CONCRETE PLACEMENT, FINISHING AND CURING |
7.1 |
Unfavourable Conditions |
7.1.1 |
Concrete shall not be placed on frozen ground, nor shall concrete be placed in unfavourable conditions which may be detrimental to the quality and finish of the concrete in the structure unless adequate precautions have been taken. Unfavourable conditions shall be deemed to include low temperatures (below 5 o C with temperatures descending, or below 2 o C with temperatures ascending), hot, dry conditions, excessively wet conditions making it impracticable to work and finish the concrete adequately. |
7.2 |
Preparation for Concreting |
7.2.1 |
Immediately before any concreting is commenced all builders must ensure that all dirt, dried mortar and other refuse has been removed by brushing or by hosing and that construction joint faces have been prepared in accordance with 5.6. (Edges and faces against which new concrete or mortar is to be placed must be scrabbled). The inside of timber forms shall be wetted with clean water immediately prior to placing concrete. |
7.3 |
Handling and Placing |
7.3.1 |
The addition of water to the mixed concrete at the site is strictly forbidden. |
7.5 |
Compaction |
7.5.1 |
Vibrators to be Used. Unless approved otherwise compaction of all concrete, including floor slabs, shall be carried out principally by the use of immersion vibrators. Immersion vibrators shall be used in all sections for which their use is practicable. |
7.5.5 |
General . Irrespective of the method of application, vibration of the concrete shall be such that expulsion of entrapped air and settlement of the concrete is visibly evident over all areas of the surface and shall be maintained until this action ceases and until coarse aggregate at the surface is embedded. Vibration shall not be prolonged beyond the time at which this condition is reached. |
7.6 |
Finishing (including finishes) |
7.6.4 |
No work shall be carried out where there is free surface water. Application of dry cement or sand to absorb free water is not permitted. |
7.7 |
Curing and Protection |
7.7.1 |
General. Immediately after placement, concrete shall be protected from premature drying, excessively hot or cold temperatures and mechanical injury. |
7.7.2 |
Unformed Surfaces. For concrete surfaces not in contact with the forms, one of the following procedures shall be applied on completion of concrete finishing operations as soon as it is possible to do so without damaging the surface: |
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(a) |
Ponding or continuous sprinkling. |
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(b) |
Application of absorbtive mats or a layer of sand and maintaining continuously wet. |
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(c) |
Continuous application of a mist spray. |
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(d) |
Application of a curing compound. |
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(e) |
Covering with an impermeable sheet such as polyethylene in a manner to prevent undue loss of water from the concrete. |
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COMMENTARY |
C7 |
CONCRETE PLACING, FINISHING AND CURING |
C7.1 |
Unfavourable Conditions |
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(a) |
General . NZ has a temperate climate and one does not generally suffer from the extremes of cold or heat prevailing in some other countries. |
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(b) |
Cold Weather Concreting . Precautions that may be taken in unfavourable cold conditions referred to above, incude the following: |
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- Use of air entrainment.
- Use of lower slump concrete.
- Use of an approved accelerating admixture.
- Use of water reducers.
- Increased cement content.
- Protection of aggregates against frost.
- Use of hot water or heated aggregates.
- Avoidance of placing concrete on a frozen subgrade.
- Protecting the surface of the freshly placed concrete from frost until the strength of at least 3.5 mPa has been achieved - this is usually a period of not less than two days.
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(c) |
Hot Weather Concreting . High temperatures result in more rapid hydration of cement, and hence early stiffening of the concrete, greater mixing water demands, increased evaporation of mixing water, reduced strengths, and large volume changes; and greater chances of plastic cracking. |
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- Elapsed time mixing and placing should be minimised.
- Exposure of mixers and agitators to the hot sun, while waiting to be unloaded, should be ;minimised.
- Use suitable retaring admixtures.
- Avoid excessive mixing.
- Wet forms, reinforcing, subgrade and surrounding areas by spraying with water shortly before placing.
- Speed up placing.
- Use fog sprays shortly after placement.
- Give prompt curing and protect exposed surfaces from drying out.
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(d) |
Temperatures rise in mass concrete due to heat from the hydration of cement. In each concrete pour the temperature rise resulting from the heat of hydration of cement, being largely adiabic, can be considerable, leading to relatively large volume contraction and tension in the hardened concrete as it eventually cools. |
C7.4 |
Concrete Placing Under Water |
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Tremie Concrete . Placement by concrete pump or through a tremie are the safest methods provided the discharge point is buried throughout the discharge period, as failure in this respect is disastrous. |
C7.6 |
Finishes (including finishes) |
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The timing of surface finishing is critical. If attempted too soon, crazing may occur. If attempted too late, extra work is necessary and in some cases the specified surface finish is not achieved. The correct timing depends on the cement content and workability of the concrete. |
C7.7 |
Curing and Protection |
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Curing concrete is a very important part of the building process because all useful properties such as strength, water tightness, wear resistance and volume stability improve with age as long as conditions are favourable for continued hydration of the cement. The improvement is rapid at early stages but continues more slowly for an indefinite period.
Hydration proceeds relatively slowly when temperatures are low. The necessary duration of curing is dependent upon the type of cement, mix proportions, required strength, size and shape of the concrete mass, weather and future exposure conditions. Since all the desirable properties of concrete are improved by curing, the curing period should be as long as practicable in all cases, but especially for concrete made with cements of slow strength gain characteristics. |
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Provided you and your staff follow the above, you can be assured of a quality job for both you and your clients. |
