Precautions of Concreting | Concrete Technology

In this article we will discuss about the precautions to be taken in hot weather and cold weather concreting.

Precautions of Concreting in Hot Weather:

To improve the quality of concrete, it is necessary that the temperature of the freshly mixed concrete should be as low as possible. It can be obtained by keeping the temperature of its ingredients as low as possible.

Aggregates of Concrete:

The temperature of aggregates can be minimised by adopting the following measures:

1. The aggregates should be stockpiled in shade.

2. The temperature of aggregates can be lowered by sprinkling water over them in stock pile. The evaporation of sprinkling water will cool the aggregates.

3. If possible aggregates may be cooled by spraying of cold air over them just before the batching.

Mixing Water of Concrete:

The temperature of the mixing water has the greatest effect on the temperature of the concrete as its specific heat is 1.0 while that of aggregate is 0.22 i.e., the specific heat of water is 5 times more that of aggregate. In practice it is easier to control the temperature of mixing water than other ingredients. The weight of water used in concrete is lesser than other ingredients.

Methods of Lowering Water Temperature:

The temperature of water may be lowered by the following methods:

1. The temperature of mixing water may be lowered by insulating or shading pipe line and tanks.

2. Use of cold mixing water will result in the good reduction of concrete temperature. As per IS 7861-1971, the effect of cold water at 5°C on the concrete tem­perature has been shown in Fig. 18.1.

3. If the ambient temperature is very high, the use of cold water will not be fully effective. In such situations ice may be used as a part of mixing water. Crushed ice may be put directly into the mixer. It should be ensured that ice is completely melted by the time mixing is complete. Fig. 18.2 shows the possible reduction of concrete temperature by the substitution of ice as part of mixing water.

Production and Conveyance of Concrete:

1. To keep the temperature of concrete below 40°C at the time of placement, the temperature of aggregates, water and cement should be maintained as low as possible. At some locations maximum placement temperature should be much lower than 40°C.

2. The required mixing time of concrete in hot weather should be minimum.

3. To reduce the length of delivery to the minimum, the concrete mixer must be positioned as near to the place of deposition as possible.

4. Cold water should be sprinkled on subgrade, form work and reinforcement etc. before placing the concrete. The reinforcement projecting out of the concrete should be kept cool by any practical means or covered.

5. To reduce the time interval between successive layers or lifts, concrete should be placed compara­tively in thin layers. However the thickness of concrete layer should not be so thin as to get dried up very soon.

6. After finishing the concrete surface it should be covered effectively to prevent the loss of moisture from the concrete,

7. Rapid hardening and drying of fresh concrete can be checked by protecting it from direct sun rays. In such situations, work should be started in the afternoon, so that the hydration of plastic concrete may take place during relatively cooler night. The temperature record of air, the concrete coming out from the mixer, the concrete placed and the temperature of the concrete during the early period of hydration should be maintained. It will help to regulate the temperature for future concreting. The concrete surface should not be allowed to become dry and at the same time water should not be applied before the final setting of concrete. The best practice is to cover the concrete with moist covering for 24 hours and then apply water by spray or ponding.

8. For curing, covering the top surface with gunny bags, or hessian cloth or with wet burlap is desir­able. The gunny bags should be squeezed fully, so that water does not drip on the fresh concrete. Covering of finished floors with wet gunny bags may not be suitable, for flooring whose finishing is of primary importance. In such cases the use of moisture retaining materials has been found better as it has a definite cooling effect.

9. Wet-curing must commenced at the earliest possible time. If the concrete is effectively moist covered, the ponding may be commenced after 24 hours in case of floor, roof or pavements. Curing by ponding has been found better than covering with burlap, gunny bags. Sprinkling of water should not be applied till the concrete has set fully.

10. Surface sealing compounds should not be used as they lose some of their efficiency in hot weather. If they are to be used, they should be used after 24 hours of water curing.

11. Black surfaces should be white washed to reduce their heat absorbing capacity.

12. In massive structures such as dams, the temperature of hardened concrete can be controlled by circulating cold water or brine through copper pipes embedded in concrete at the pre-determined distances or 1 m distance at the time of placing con­crete.

13. Due to evaporation of water, cracks will be developed on the surface of concrete.

Proportioning of Concrete Mix:

The mix should be so designed that it may contain minimum content of cement consistent with other functional requirements. As far as possible in hot weather concreting, cement of lower heat of hydration should be preferred to those having greater fineness and greater heat of hydration. In hot weather con­creting use of water reducing or set retarding admixtures has been found useful. However accelerators should not be used.

Precautions of Concreting in Cold Weather:

1. Preparations for concreting in cold weather may be completed well in advance of severe conditions. To protect the mixing and batching plants from wind effects, wind breakers should be provided. To cover the surface of the concrete, tarpaulins, plastic sheets and other covering and insulating materials should be made available at the site. Steam generating plants and other nece­ssary equipment should also be available at the site.

2. At the point of placing concrete the temperature of the concrete should not be less than 5°C. The concrete should be placed quickly and the top surface of the concrete be covered with an insulating material.

3. Before placing any concrete, all ice, snow and frost should be removed from the form work and reinforcement etc., and the temperature be raised as close as practicable to the temperature of the fresh concrete to be placed. No concrete should be placed on a frozen sub grade or on frozen materials. In case concrete is to be placed over an permanently frozen ground either it may be covered with a dry granular material for sufficient depth or the sub grade materials may be thawed upto a sufficient depth such that it will not freeze back upto the concrete.

4. When the air temperature is near or below freezing point, heated enclosures are commonly provi­ded. Enclosures may be made of wood, canvas, fibre insulating boards, ply wood etc. These enclo­sures may be heated by steam, jet steam pipes and other types of heaters.

5. In cold weather concreting, in order to preserve the heat of hydration and protect it from freezing, the concrete surface should be covered as soon as the concrete is placed. The covering materials may be polythene plastic sheets, water proof paper, tarpaulins, sacks and clean straw blankets of about 5.0 cms thickness. Along with these materials air gap can also be provided as insulation. If possible insulating material may be placed against form work before concreting.

6. During placement of concrete, tarpaulin or any other readily movable coverings supported on form work should follow closely the placing of concrete, so that only a small area of finished surface is exposed to outside air These tarpaulins should be so used that hot air may be circulated freely on the surface. Layers of insulating materials may also be used for the protection of concrete against freezing action.

7. During freezing or near freezing conditions water curing is not necessary as loss of moisture from concrete due to evaporation of water will be greatly reduced in such cold conditions.

8. For concrete cast in. insulated from work it is necessary to cover the member completely to retain the sufficient water in the concrete for hydration of cement. After removing the form work and insulation, the member immediately should be covered with plastic sheet or tarpaulin etc., properly lapped and made air tight. In no condition, the concrete released from insulated from work, be saturated with cold water. After discontinuing the protective measures the surface temperature of the concrete should be gradually adjusted to the air temperature.

9. Low pressure wet steam curing has been found best for heating the enclosures as well as for moist curing of concrete. Early curing with liquid membrane forming compounds may be followed with heated enclosures. It is better to cure first with steam curing during the initial period of protection and then using a curing compound after the protection has been removed and the air temperature is above freezing point.

10. Form work should not be removed till the concrete has achieved strength of at least twice the stress to which the concrete may be subjected at the time of removal of form work. In the normal conditions, where ambient temperature does not fall below 15°C and where ordinary port-land cement is used and adequate curing is done, the removal of form work period shown in Table 18.4 may be adopted. It is a guide line. For lower temperatures and other cements it may be suitably modified.

11. In cold weather concreting form work other than steel provides better protection. The form work should not be removed until the end of minimum period or a little more.

12. During cold weather, the record of outside air temperature, temperature of concrete at the time of placing, and general weather, day, and time should be maintained.