QUALITY, RECLAIMED AND RECYCLED WATER

The water used for mixing concrete should be free from any impurities that could adversely affect the process of hydration and, consequently, the properties of concrete. For example, some organic matter can cause retardation whilst chlorides may accelerate the stiffening process and also cause embedded steel such as reinforcing bars to corrode. Other chemicals such as sulfate solutions and acids may have harmful long-term effects by weakening the cement paste by dissolving it. 
It is important, therefore, to be sure of the quality of water. If it comes from an unknown source such as a pond or borehole it should be checked by making trial mixes. The British Standard 3148 specifies the quality of water and gives the procedures for checking its suitability for use in concrete. BS EN 1008, which supersedes BS 3148, had not been finalized at the time this handbook was published.

Drinking water is suitable, of course, and it is usual simply to obtain a supply from the local water utility. But some recycled water is being increasingly used in the interests of reducing the environmental impact of concrete production, and seawater has been used successfully in mass concrete with no embedded steel. The use of seawater does not normally affect the strength of plain Portland cement concrete, but it must not be used for concrete containing embedded metal because of the danger of corrosion of the steel from the chloride content of the water, nor should it be used where white efflorescence could mar the appearance of the work.

The important, and most difficult, issue is the correct assessment of how much water is required. In the sections dealing with aggregates and batching, the variability of moisture content is discussed and in the quest to control free water/cement ratio it is essential to allow for water contained in aggregates. Because suca large proportion of concrete consists of aggregates, a small shift in the moisture content makes a big change in the quantity of water to be added.

Devices based on advanced electronics technology exist for measuring the moisture content of a batch of aggregates and for calculating the free water/cement ratio of concrete during mixing, but most producers rely on the experience of the batcher to judge the point at which the amount of water is correct from the way in which the concrete moves and the sound it makes in the mixer. Any equipment that is capable of indicating the consistence of concrete while it is being mixed assists the batcher in gauging the mixing water with greater speed and accuracy. Where the mixer is powered by an electric motor an ammeter or kilowatt meter accurately indicates the power consumed in mixing the concrete - less power is demanded as concrete becomes more workable. Similarly, truck mixer drums that are turned by hydraulic drive can have the consistence of concrete indicated by a pressure gauge (Figure 3).

Recycled water systems are usually found at large-scale permanent mixing plants such as precast concrete factories or ready-mixed concrete depots where water used for cleaning the plant and washing out mixers after use can be collected, filtered and stored for re-use. Some processes are able to reclaim up to a half of the mixing water in this way.

When reclaiming water for use as recycled mixing water care needs to be taken to avoid impurities including harmful chemicals, oil or organic matter, and any traces of powerful admixtures such as air- entraining agents, retarders or pigments must be diluted to sucan extent that they will have no effect. Any polypropylene or steefibers need to be filtered out and a careful check kept on the amount of suspended fines carried in the water: after all the effort and cost of obtaining clean aggregates, it is not sensible to put an excessive quantity of fines back in the form of dirty water.

Large-volume settlement tanks are normally required. They do not need to be particularly deep but should have a large surface area and, ideally, the water should pass through a series of tanks, becoming progressively cleaner at each stage. Alternatively, the water may be chemically treated, particularly where space is limited, in order to make it suitable for re-use or for discharge into drains in a condition that conforms to statutory requirements.

Measuring the quantity of water

Mixing water is usually measured by volume but, in some plants, it may be more convenient for it to be batched by weight. One liter of clean water weighs exactly one kilogram and so the quantity of water remains numerically the same regardless of whether it imeasured by volume or by mass, but corrections should be applied when water contains fines.


Regardless of the method of gauging the mixing water, it is recommended that the concrete is finally checked by a batcher and/or driver to see that it has the specified consistence and a uniform appearance. This is the most effective way of ensuring that the concrete is thoroughly mixed and has the designed free water content. When the free water content is closely monitored and cement content accurately weighed, the free water/cement ratio is controlled and, therefore, strength, durability and many other essential properties of the concrete are assured.

Artikel Terkait