Fresh concrete can be lifted from the ground level to the upper floor level by

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1 3a Concreting Operation 1. Concrete lifting 1.1 Hoists Fresh concrete can be lifted from the ground level to the upper floor level by hoists. However, the further distribution from the hoist to placing points relies upon wheel barrows which is very slow and labour intensive. Concreting with Hoists Concrete skip 1.2 Crane and Bucket The use of crane and bucket/skip is an appropriate means of handling concrete on sites where crane is available. Buckets of 1 to 2 m 3 capacity are most commonly used on building sites. On large mass concrete projects, buckets of up to 6 m 3 capacity may be employed with the discharge gates operated by compressed air. 1.3 Pump and Pipelines Concrete pumps and pipelines are perhaps the most widely used method of moving 1

2 concrete on building sites. The ready availability of mobile pumps, and their relative reliability, makes them an efficient and economical means of transporting concrete, even on quite small sites. Moreover, concrete for high-rise building is normally very suitable for pumping because most high-strength concrete has high cement content and small maximum size aggregate. A wide range of pump types are available, generally trailer or truck mounted. Fixed pumps generally have the highest pumping capacity and are the usual choice for major projects. Modern technology in concrete pumping enables the pumping of concrete over 300 m high in a single lift (a height of 308 m in Central Plaza was a record lift at that time). The rate of delivery (range from 10 to 80 m 3 /h) depends on the type of pump, the pipeline diameter and the distances to be pumped horizontally and vertically. Pump and Pipeline Placing boom Pumping Operation Pipelines must be adequately supported and fixed in position since quite 2

3 substantial forces can be generated as the concrete is forced along the lines. Before actual pumping of concrete, the pump and pipelines must be lubricated by pumping through the pipes with a cement slurry or mortar. Once commenced, concrete pumping must be continuous to avoid blockages in the pipeline. After pumping is completed, the pipelines must be cleaned out as soon as possible to prevent mortar residue which will increase pipe friction and cause blockage. 2. Concreting in Hot Weather Hong Kong lies in a subtropical area. The afternoon temperature raises above 34 C is not uncommon. High air temperatures, particularly when combined with strong winds, can affect the quality of both fresh and hardened concrete in a number of ways: a. High temperature will speed up the hydration of cement which causes reduction in setting time. b. The workability of the fresh concrete will lose quickly due to evaporation and hydration of cement. (Water should never be added to restore the workability.) c. By causing the surface of the concrete to dry prematurely, they can cause cracking; even before the concrete has stiffened and begun to harden (this is known as plastic cracking). d. High temperature will also cause thermal stress in the concrete member. Typical requirement in Hong Kong concerning concrete temperature Temperature before placing: 32 C Maximum concrete temperature after placing: 85 C Maximum temperature different in a concrete member: 25 C 3

4 2.1 Controlling concrete temperature The temperature of fresh concrete may be estimated from the following equation. T = (T a W a + T c W c + 5T w W w )/(W a + W c + 5W w ) where T = temperature of the freshly mixed concrete in C. T a T c T w W a W c = temperature of the aggregates in C. = temperature of the cement in C. = temperature of the mixing water in C. = mass of aggregates including free moisture in kg. = mass of cement in kg. W w = mass of mixing water in kg. (Note: This equation gives approximate results only but is sufficiently accurate for practical purposes. For more accurate results knowledge of the specific heats of the constituent materials is necessary.) Therefore fresh concrete temperature can be controlled by controlling temperature of it constituents: aggregate Aggregates account for the largest quantity in concrete and has the greatest effect to fresh concrete temperature among the other constituents. Methods to reduce aggregate temperature: a. shading stockpile from the sun, b. keeping them moist with sprinkler (the moisture in the aggregate must be accounted when mixing concrete especially for fine aggregate). 4

5 2.1.2 Water Water has high specific heat capacity and also significantly affects fresh concrete temperature. Water tank and pipeline should be shaded from strong sunlight. Chilled water or adding crushed ice will further reduce the fresh concrete temperature Cement Cement does not usually contribute significantly to fresh concrete temperature because of its low specific heat and its relatively small mass in the mix. However, cement temperature as high as 90 C had been recorded from fresh cement leaving a cement factory. Fresh cement shall be allowed to cool before use. Unnecessary rises in temperature should also be avoided, such as painting silos white or other reflective colours to reduce heat from sunlight. 4. Liquid Nitrogen Injection For large and important pours where temperature control is critical, cooling may be achieved by injecting liquid nitrogen directly into the mix on the agitator truck. This is a very effective method but relatively expensive. 5. Admixture Admixtures are very useful to improve the workability of concrete without the addition of extra water and to retard setting. 3. Ready-mix concrete Instead of being batched and mixed on site, concrete can be delivered for placing from a central plant; it is referred as ready-mixed concrete. 5

6 3.1 Advantages of using ready-mixed concrete On small sites, especially in urban area, there are no rooms for stockpiles of aggregates and to set up site mixing plant. In small project where only small quantities of concrete or intermittent placing are required, it is not worth to set up a site mixing plant. In large pours where large quantity of concrete and high supply rate are required, site mixing usually cannot meet the requirement but ready mixed concrete can be supplied from more than one plant. Close quality control of batching in concrete plant reduces the variability of the strength of concrete. 3.2 Advantages of site mixing Proximity of mixing place and placing point reduce the unnecessarily rise in concrete temperature and loss in workability. Reduce the cost of transportation of the concrete. Very often the concrete can be delivered from the site plant to the placing point by pump and pipeline. 4 Curing Methods 4.1 General Methods of curing concrete fall broadly into two categories: - Which minimize moisture loss from the concrete by covering it with a relatively impermeable membrane. - Which prevent moisture loss of continuously wetting the surface of the concrete. The timing of starting curing is critical for maximum effectiveness: 6

7 - For unformed surfaces, curing should be started as soon as possible without marring the finish. - For formed surfaces, curing should be started immediately after striking. 4.2 Impermeable-membrane curing Formwork Leaving formwork in place is often an efficient and cost-effective method of curing concrete, particularly during its early ages. It is desirable that any exposed surface of the concrete (e.g. the tops of beams) be covered with plastic sheeting or kept moist by other means Plastic sheeting Plastic sheets form an effective barrier against water loss. However, the effectiveness is very much reduced if they are not kept securely in place. Curing of concrete by wrapping of plastic sheeting (Source: Standard Australia) Curing compounds Curing compounds are liquids which can be brushed or sprayed directly onto concrete surfaces. They then dried to form relatively impermeable membranes which retard the loss of moisture from the concrete. Spraying of Curing Compound (Source: Standard Australia) Curing compounds should be applied to the surface of the concrete after it has been 7

8 finished, as soon as the free water on the surface has evaporated and there is no water sheen visible. Too early application dilutes the membrane, too late results in it being absorbed into the concrete with a consequent failure of the membrane to form. For formed surfaces, curing compounds should be applied immediately after striking, but the concrete surfaces should be moistened first to prevent absorption of curing compounds into the concrete. Using curing compounds is an effective and cost-effective means of curing concrete, but they may affect the bond between concrete and subsequent surface treatments. 4.3 Water curing Wet coverings Fabrics such as hessian can be used to maintain water on the surface of the concrete. Ponding (Source: Standard Australia) The fabrics should be kept wet with hoses or sprinklers regularly Ponding Flat surfaces such as floors, pavements, flat roofs, etc. may be cured by ponding. A dam or dike is erected around the edge of the slab and water is then added to create a shallow pond. Reference 1. Standard Australia, Guide to concrete Construction (1994), Cement and Concrete Association of Australia 2. Special Topics in Concrete Placement, American Concrete Institute 8