Reducción las series históricas del periodo 1965-2007 a 1992- 1992-2007

The objective of all supply and handling must be to provide concrete in sufficient quantity, of acceptable quality and of suitable workability at the point of placing to enable the construction to proceed systematically and economically. Determination of the means of batching, mixing and handling concrete are generally critical decisions made in the precontract planning stages. The plant selected will be based on a number of factors:

site location and available space; service availability;

scale of operation; location of batching plant; distance of transport;

type of concrete to be produced; required rate of placement; ground conditions;

tide or flow conditions in or over water;

detached operations (jetty, tunnel construction, etc.); vertical and horizontal travel.

There will also be purely economic considerations, such as available equipment, cost of hire or purchase and possibility of further use on successive contracts.

The type and size of batching plant ranges from the “coffee pot” mixer used so effectively by the small builder and contractor, through tilting drums, reversing drums, pan and paddle mixers, to the huge electronically controlled plant which can be worked through a shift with little or no manual intervention. Certain types of construction and various techniques demand particular means of handling and in some instances the means of handling combines with the method of achieving compaction, as in placing using pneumatic placers, spray application of concrete and similar processes. The supervisor should install and maintain control on a weekly and day-to-day basis in the course of construction to ensure continuity of concrete supply. Points such as maintenance of equipment and selection of operators depend on the supervisor and it is his attention to detail which ensures that the plans prepared in the precontract and planning stages can be taken to a successful completion.

Receipt of materials on site

The whole process of supply begins with the acceptance and storage of concrete materials. The quality of the materials is generally specified by reference to British Standards and Codes together with the local specification governing the sampling and testing procedures which must be adopted on site. Regular checks should be maintained as set down in the local specification and certificates obtained for compliance of materials to the established and relevant Standards.

Batching

Batching is the term used to describe the process of collecting together and measuring, preferably by weight, the correct quantities of each of the materials specified to be included in the concrete mix. Batching includes the proportioning, according to the mix design, of some of the following materials:

Aggregate—coarse, fine, graded, single sized, “all-in”, lightweight, dense, natural and man-made Cement—Ordinary Portland, rapid-hardening Portland, sulphate-resisting Portland, Portland blastfurnace,

white, low-heat Portland, masonry, waterrepellent, hydrophobic Portland and high alumina

Admixtures—accelerating, retarding, water-reducing, accelerating water-reducing, retarding

waterreducing and air-entraining

Water

The batching process may be simply carried out using small balance type weigh hoppers in conjunction with mobile mixing plant, or automated in terms of measurement by load cells incorporated in weigh hoppers as part of a complete storage and dispensing system. In this instance, proportioning may also be carried out by load cells in conjunction with moving feed belts. An essential requirement of weigh batching equipment is

that the cements and aggregates must be weighed separately, as aggregates are generally in a moist condition.

Mixing

Types of mixer are clasified by BS 1305:1974 into four groups identified by the initial letters:

T = tilting drum

NT = non-tilting drum

R = reversing

P = forced action

The main characteristics of these mixers are as follows:

Tilting drum mixers (T) are suitable for medium strength concrete in the capacity range 100–200 litres

and for producing mass concrete with large (150 mm) aggregate in 3 and 4 m3 _{batch size. Materials are} poured into the drum and discharged by tilting the mixer drum. Front to rear mixing is good provided the specified drum angle is maintained (generally 20–30° to the horizontal). To avoid clogging of the mixer, the drum water inflow should be started prior to the dry materials being loaded into the drum. To prevent loss during charging, the cement should be sandwiched between aggregate and sand in the weigh hopper. Material left sticking to the blades will impede the mixing action and hence effect the quality of concrete produced. The blades lift the concrete to the top of the drum and allow it to fall, encouraging mixing.

Non-tilting drum mixers (NT) are very rarely seen today. Materials are poured into the drum and

discharged by a chute which pivots into the mixing drum, catching material as it falls from the top. They are not good for high strength or lean mixes and have cycle times of about 3 minutes—the discharge is particularly time-consuming. The BS output capacity range for these units is 200–750 litres.

Reversing drum mixers (R) have two openings and two sets of blades. Materials are loaded at one side

and efficiently mixed by the one set of blades. When the rotation of the drum is reversed, the second set of blades empties the concrete out of the other side. These machines are good all-round mixers and are found

on many medium sized sites. They can cope with most concrete, except lean and sticky mixes and range between 200–500 litres output capacity.

[The 3 previous mixers are known as “free fall” mixers where a certain amount of gravity is used in mixing]

Forced action mixers (P), as their name implies, only use mechanical power to combine the constituents.

Because of this, mixing is much more thorough and all types of concrete can be produced. This makes them the most versatile, but at the same time the most expensive in energy consumption and mechanical wear. Capacity ranges from 200 litres–3 cubic metres.

There are two types of pan mixer:

(a) stationary pan with rotating blades on central axis; (b) rotating pan with rotating blades on an eccentric axis.

Forced action pan type mixer with planetary action—this equipment combined with central visual control ensures continuous consistency of concrete supply (Liner-Croker Limited)

The pan is mounted horizontally and discharge is through doors on the pan floor. Materials are usually weighed whilst the previous batch is being mixed. With the fast discharge, the complete cycle time will be between 1−1½ minutes. Another type of forced action mixer is the trough mixer which has paddles rotating about a horizontal axis set in a semi-circular trough. Other parameters are similar to the pan type.

Mixer outputs

The amount of concrete a machine will produce depends on its type, the workability of the concrete and the cycle time. Theoretical outputs in m3_{/hours are given in the following table for various machines. Actual} outputs are generally about half theoretical, but theoretical outputs can be attained when required.

Location of equipment

The location of equipment on the construction site must be related to site access, the flow of materials in the process, the location of areas of greatest demand, and a number of other factors, the most critical of which must be access for vehicle delivery of materials such that handling of the fresh concrete can be carried out using the simplest and most economic means. This may mean the adoption of dumpers, trucks, conveyors, pumps and placers, skips handled by tower crane, hoists or a combination of one or several of these measures. The location of storage, handling and mixing equipment should be determined by the geography of the site, access to main roads or, in the case of rail supply of materials, access to railway siding. It is usual to pave the area adjacent to the plant and, in the smaller operation, the mixer may well be set up on a part of the permanent access road system. In an ideal situation loaded dumpers and so on should run downhill from the plant and some degree of elevation ensures that aggregate bins drain effectively, standardising stocks in hand. Where small plant is concerned, a mixer set up on a sleeper built bay will enable skips and dumpers to be located below the discharge chute to receive freshly mixed concrete batches running out from below the plant by gravity.

The location will, of course, be related to the area of the site where the maximum demand is going to occur in the course of construction. It may be desirable to locate ancilliary plant in strategic positions of considerable demand. For example, with a substantial raft to cast, it may prove economic to set the mixer up on a platform such that it can discharge directly to the point of placing or to chutes which will be used to distribute the concrete. An extension of this system in work ad jacent to water and in the maintenance of river and canal walls and bridges, is the barge mounted plant complete with storage bins for aggregate and a

The effluent treatment unit which recycles the cleaning water forms a useful addition to the readymixed plant—four vehicles can be washed at a time and the aggregate re-used (except for quality concrete) (Batching Plant Limited)

small hoist for discharge into form work. Massive vessels loaded with aggregate and cement together with a batching plant are occasionally used to supply concrete to sites in extremely difficult locations, such as the construction of oil platforms and similar, where distances are too great to allow economic transport by pumping.

As a general rule, services should be taken to the plant rather than the plant located at available supplies of water and electricity. Water supply to the plant must be provided from tanks fed by a substantial mains or by pump from bowsers. Air at the required pressure will be provided by a local compressor. Electricity supply must be guaged to the demand and the establishment of a supply sub-station at site may be necessary. Drainage must also be considered as a substantial amount of water is required in washing down. In the case of the larger operation, a proper wash bay should also be incorporated for cleaning vehicles used to transport the concrete about the site. Larger plants today incorporate a control room providing sound working conditions for the operator as well as protecting control equipment. Winterised plant will incorporate insulated bins and discharge equipment as well as a supply of hot water or steam. In plant designed for use in hot countries, the cabinet protecting equipment may be pressurised to exclude dust and where plants are located in exposed sites in very hot countries and concreting is carried out in extremes of heat, ice dispensers may be incorporated into the plant, discharged crushed ice as part of the water introduced to the mix.

The location of batching plant in precast operations, whether on site or in a purpose-built factory, will be determined by access, means of transport, and will generally be so located that the operator has a view of the making bays. Here again the plant must be located in such a way as to minimise the distance to be travelled by the fresh concrete to the points of maximum demand. High demand is generally met by the use of “bullets” or high speed skips running on monorail.

Storage of aggregates

Aggregates will normally be stocked in heaps between bin walls or dividers. Where possible the bin bottoms should be paved to prevent contamination of aggregate by soil or surface water. The paving should drain to a sump or away to an adjacent soakaway. Bin dividers should be of substantial concrete—sleepers

or precast elements serve well. Walls should be able to withstand the considerable force arising from the materials in stock as well as possible impact from delivery vehicles or the bucket of the boom scraper. Most large plants are now fitted with boom scrapers, manually operated or latterly automatic. Automatic scrapers are controlled by sensors which determine the level of bin stocks and switch the scraper accordingly. In this instance the point of delivery of aggregate must be guarded either by sensor or by the provision of a switch overriding the automatics. Operatives must also be prevented from inadvertently walking into the area swept by the scraper.

Delivery to hoppers is generally by conveyor or sometimes by direct discharge into receiving hoppers. Receiving hoppers such as those which feed conveyors must be guarded with a grillage which protects the worker and guards against debris being discharged into the conveyor. Elevated bins provide storage for aggregates in 400–500 tonne lots. Bins are usually provided for four or six different materials. Again the bins must be covered by grilles to prevent operatives falling through or into the aggregates. Although this is a well known hazard, there are still many accidents and some fatalities reported due to the omission of such safety features. Sand stocks provide a particular hazard as a person is easily suffocated, even when only partially immersed in sand. The material often cavitates at the discharge door and collapses with dire results should a man venture onto the surface.

Stocks should be prevented from becoming contaminated—in extreme conditions from aggressive dust and salts and in rural situations from wind blow leaves and rubbish or mud splashed by passing vehicles. It is generally advisable to stock two or three days supply of aggregate so that they drain down and standardise. In extreme weather stocks should be heated and protected from freezing by insulation. The cut- off doors on bins are particularly prone to freezing, especially where pneumatics are used as motive power. All receiving hoppers should be clearly marked with the size to be tipped. Quality control can be drastically effected by even a part load wrongly tipped.

Control panel for a modern batching plant—the display panel indicates which mix is being prepared and the state of stocks, a print of the actual mix proportions and quantities is delivered for each batch (Cornelly Equipment Company Limited)

Storage of cement

In the case of bagged cement, adequate protected storage space should be provided. High stacks should be avoided and rotation of stocks is essential. It is generally sufficient on the smaller site to allocate an “elephant” tent whilst a Nissen hut may be appropriate where larger stocks are to be maintained. The cement should be stacked on duckboards with sufficient space for access where necessary without damage to the bags.

Bulk supplies of cement are generally stored in silos with gravity or screw feed to the mixer. The silo itself must be watertight. The pipes for introduction of the material must be clearly marked and located such that the hoses from delivery vehicles can be easily attached, avoiding sharp curves. The inlets should be marked with the type of cement stored in the silo to avoid wrong delivery of mixing materials. The filters on the exhaust must be cleaned often, to avoid excessive dust being expelled when filling is in progress.

Storage of admixtures

Whilst small quantities of admixtures may be stored in drums and dispensed by the mixer man, it is essential that admixtures used in large production runs should be dispensed by a properly calibrated device. The majority of admixture suppliers offer equipment which has been designed specifically for this purpose and their advice should be sought at the time of installation of main plant. The dosage for each batch must be clearly stated on the batch card to which the operative refers when setting up for a particular mix. The admixture should be discharged into the mixing water as it is introduced into the mixture. Where an admixture is added manually, slightly longer mixing time should be allowed.

It must be noted that admixtures containing chloride must not be used in prestressed or reinforced concrete.

The large batching and mixing plant

This equipment, suited to the larger scale building or civil engineering contract, is in fact a concrete production factory in its own right. The mixer will range from 1–4 m3 _{capacity, fed by a boom scraper from} elevated bins or ground level storage bays. Consumption of aggregate is considerable, 150 tonnes or more per hour. Good access roads for tipper trucks and cement lorries delivering bulk cement to an elevated storage hopper are essential for efficient production. Cement is screw-fed or blown to a weigh hopper separate from that in which aggregates are weighed. In more modern equipment the batching plant is controlled automatically by programmes contained in micro-processors which receive data from various sections of the plant. A typical control system is linked to:

1. the electronic load cells used in each of the plant batch weighing systems 2. electro-pneumatic circuits controlling the flow of water/cement and aggregate 3. all motors and limit switches

4. an automatic boom scraper to control the stocks throughout in direct relation to the selected mix design 5. a memory bank which stores information necessary to handle 100 or more different mix designs 6. a print-out producing a permanent record of date, time, mix time and actual batch weights 7. a stock control system providing records of the state of stocks at any given time.

The controls are such that at any one time in the batching and mixing cycle, one mix is being weighed, one is being elevated and one is being mixed by a high speed compulsory pan mixer. Facilities are normally

provided for the use of hot water, or possibly steam, in the mix. Dispensers allow the accurate dosage of admixtures and additional silos will accommodate pfa if required. A number of techniques have been employed for the measurement of water, ranging from electronic sensors to watt meters recording the electrical consumption of the plant as a means of assessing workability (and, indirectly, wate content). A recent technique uses an electronic device measuring variations in moisture content in the aggregate as it is discharged into the mixer. In this case the amount of water added to the batch is automatically adjusted to compensate for varying moisture content of the aggregate from stock.

In automated supply, such as in the on-site production of precast elements, the transporting equipment may be automatically activated. Each time a batch of concrete is mixed, a skip is filled and travels at high speed by monorail to the mould or machine where it is discharged into the receiving hopper. Automated supply can also be used to produce varying graded batches of concrete where, for example, cement replacement by pfa is automatically adjusted according to the location of that batch within the production