The use of ready-mixed rather than site-mixed concrete allows for wide variations in demand.
It is recommended that ready-mixed concrete should be supplied from a plant that holds current accredited third-party certification, ensuring that sound practices are followed and systems are in place to maintain high standards of quality and production control.
Batching plants
There are two basic types of batching plant:
'Dry batch' plants
The cement and aggregates are weighed and discharged into the waiting truck-mixer along with most, if not all, of the mixing water, plus any admixture. The concrete is mixed in the truck- mixer drum and any additional water required to obtain the specified consistence may be added either at the plant or, in the case of high consistence concrete, on site.
Thorough mixing is essential to ensure concrete of uniform quality. In transit the mixer drum may rotate slowly at about one or two revolutions per minute to keep the concrete turning over. When the truck-mixer arrives on site, the drum should always be rotated at between 10 and 15 revolutions per minute for at least three minutes and sometimes longer, to ensure thorough mixing before discharge.
Central mixing plants
The cement, aggregates and water plus any admixture are mixed in a central mixing plant before discharge into the truck mixer, which is then used as an agitator. In transit the mixer drum may rotate slowly at about one or two revolutions per minute to keep the concrete turning over. When the truck-mixer arrives on the site the drum should always be rotated at between 10 and 15 revolutions per minute for at least three minutes and sometimes longer, to ensure thorough mixing before discharge.
Exchange of information
Full details of the concrete specification must be submitted by the contractor at the earliest stage, i.e. when quotations are being sought from the supplier.
When several different concrete mixes are used on one contract the essential items to specify for the different types of concrete (designated, designed, prescribed, standard or standardized prescribed) are outlined on pages 24 - 26 and are fully described in BS 5328 / BS 8500.
In addition, the contractor should specify the consistence required for each concrete to suit the proposed placing and compacting techniques. Some concretes of the same strength may need to be supplied at different consistence classes to suit the particular construction. For example, RC30 concrete placed in a sloping
ramp may be required at a slump of 40 mm (consistence class S1) whereas the same strength class in a narrow wall may need a slump of 120 mm (consistence class S3). As the consistence will affect the cement content of designated concretes it is essential
that the consistence required on site is given at the time of the enquiry. If it is not known, it is recommended that a high consistence class should be specified (S3), see Table 8, page 1 7.
Many specifications will also state maximum free water/cement ratios and minimum cement contents required for durability purposes, and it is essential that the supplier is notified about these requirements.
The benefits of using designated concretes include a simple specification process and an assurance that the concrete conforms to British Standard requirements.
Additional requirements should also be given to the supplier when the concrete is for a high-quality surface finish or has to be pumped, because modifications to the concrete proportions may be needed for what might otherwise be a satisfactory concrete for general purposes.
Quotations submitted will usually be accompanied by the supplier's mix design form. This should be carefully checked to ensure compliance with the contract specification and that the proportions are suitable for the intended use and placing conditions. When high quality finishes are required, particular care needs to be taken in assessing the proposed mix design to ensure that the cement content and aggregate proportions and gradings are in accordance with the basic requirements as indicated under Concrete for high quality finishes (page 49).
When a maximum free water/cement ratio is specified, it is essential to check that the amount of water in the design represents a realistic amount appropriate to the consistence required.
On all jobs the contractor and the supplier need to establish a formal communication system and to discuss the planning and ordering procedures in good time before delivery of concrete. This is best done by the contractor nominating one person to be directly responsible for ordering the concrete on a day-to-day basis and for making sure that all is ready on site for the delivery.
Day-to-day ordering
Details of all the concrete to be used on site should always be given to the supplier well in advance. This will help to ensure that when individual loads are ordered, the supplier's dispatch clerk (shipper) will know precisely what is wanted. Orders should be placed at least 24 hours before delivery is required; large pours involving several hundred cubic metres require much longer notice so that the supplier can organize and plan accordingly. When making an order by 'phone, the information given should include the following items:
- Name of the purchaser
- Name and location of site and order reference number if there is one
- Mix reference: each concrete should be given an unambiguous reference that is linked to the full set of specified requirements, designated, designed, prescribed, standard or standardized prescribed concretes are simply referred to by their BS titles. Where prescribed concretes are required, this should be clearly indicated
- Consistence class
- The total amount of concrete of each type required to be delivered
- n The time at which deliveries are required
- n The rate at which deliveries are required and particular requirements for continuity of any pour.