AS 1379 The specification and supply of concrete
AS 1478 Chemical admixtures for concrete, mortar and grout
AS 1478.1 Admixtures for concrete
AS 2758 Aggregates and rock for engineering purposes
AS 2758.1 Concrete aggregates
AS 3582 Supplementary cementitious mater- ials for use with Portland and blended cement
AS 3582.1 Fly ash
AS 3582.2 Slag – Ground granulated iron blastfurnace
AS 3582.3 Silica fume
AS 3600 Concrete structures
AS 3648 Specification and methods of test for packaged concrete mixes
AS 3972 Portland and blended cements
6.1 PLANT MIXED
6.1.1 General
It is beyond the scope of this Guide to discuss, in any detail, the plant and equipment used in the manufacture of concrete. Figure 6.1 (page 6.3) illustrates the layout of a typical concrete batching plant from which the following elements may be noted.
6.1.2
Bins and Silos
Bins and silos are used to store the various maximum size aggregates used in the manufacture of concrete as well as the cementitious materials and liquid admixtures. Some ground storage may also be provided for coarse and fine aggregates. The following general requirements apply to all storage units:
They should be so constructed as to prevent contamination from other materials and, in the case of aggregate bins and silos, to keep the different aggregate types and maximum-sized materials from intermingling.
Aggregate storage units should facilitate the free drainage of the materials and be provided with a means of minimising segregation.Chapter 6 Manufacture of Concrete
Figure 6.1 Typical medium-sized concrete batching plant
Each storage unit should be provided with a means of actively controlling the discharge from the unit.
Bins and silos to contain cement and other cementitious materials must be designed and constructed to keep the contents dry and to promote the complete discharge of the contents.
Bins and silos which are used to store more than one type of constituent shall be capable of being cleaned out thoroughly and inspected internally.6.1.3 Weighing Equipment
All weighing equipment in central batching plants should be provided with a visual weight-indicating device that is clearly visible to the operator in control of the equipment, and which should be graduated to a scale compatible with the accuracy
of the production process. The weighing equipment itself should be accurate to ±0.4% of the maximum scale value when statically loaded. The equipment should also be routinely checked for accuracy, at least monthly for cement, three monthly for aggregate and six monthly for water.
6.1.4 Liquid-Dispensing Equipment
All liquid-dispensing equipment, e.g. for dispensing admixtures, should also be equipped with a visual metering device that is clearly visible to the operator. The equipment should be capable of metering the volume, or weight, of liquid to an accuracy of at least 2% of the indicated value. As with weighing equipment, calibration of the equipment should be undertaken at least every six months, or more often if required by the equipment. Liquid-dispensing equipment should be cleaned between changes in types of liquid including changes in brands of the same type of liquid and at a frequency not less than that recommended by the manufacturer.6.1.5 Mixers
General
A variety of mixers, ranging from the simple tilting- drum mixer, used almost universally to produce bricklaying mortar on housing sites, to the more sophisticated split-drum mixers, used on major concrete road projects, are available to mix concrete.
Although the tilting-drum mixer and its companion, the horizontal-drum mixer, were once widely used to mix concrete on construction sites, they have now largely been replaced by the inclined-drum mixer and the split-drum mixer. Indeed, the great bulk of concrete used in construction today is mixed in inclined-drum mixers mounted on trucks, the so- called truck or transit mixers.
These receive accurately batched materials from a central batching plant and, operating at mixing speed, mix the concrete en-route to the site. The complex blade and fin system operates in such a way that, when the direction of rotation of the drum is reversed, the mixed concrete discharges continuously from the drum.
Many concrete suppliers incorporate central mixers in plants, partly for the increased control over the mixing of very high strength concretes, partly for the additional pollution (dust) control which can be achieved.
The availability of split-drum mixers (a high energy, high efficiency mixer), that can mix concrete with maximum size aggregates as large as 150 mm in 60–70 seconds, has accelerated this trend.
Chapter 6 Manufacture of Concrete
Guide to Concrete Construction 6.4
Continuous mixers, e.g. pugmills, are also being incorporated into central plants to handle the drier mixes required for some forms of roadwork, e.g. roller-compacted concrete.
Mixers must be maintained in a clean condition, and produce a uniformly mixed batch. Mixing time is specified for stationary and truck mixers and batch volumes limited to rated capacity. Mixers must achieve results determined by the mixer performance test which checks the uniformity of mixing through a concrete batch. This is carried out annually.
New Zealand Standard Requirements
NZS 3104 sets out a number of requirements to govern the performance of both batch and continuous mixers. These include:
A requirement that batch mixers have mounted on them an identification plate which provides information on:
the gross internal volume of the mixing chamber (m3);
the rated mixing capacity (m3);
the recommended minimum –
number of revolutions of the mixer required to achieve uniformity in the concrete, or
mixing time (in minutes) at a given rotational speed of the mixer (in revolutions per minute);
if the mixer is designed to be used as an agitator –
the recommended capacity of the mixer used as an agitator, and
the recommended speed of the mixer (in revolutions per minute) when used as an agitator;
Limits on the capacity of the mixer to no more than 65% of the gross internal volume of the bowl when used as a mixer, and no more than 80% when used as an agitator unless testing in accordance with the Standard permits a higher figure.
Procedures for determining or confirming the minimum mixing time or number of revolutions at mixing speed for batch mixers.Continuous mixers are also required by AS 1379, to carry an identification plate, in this case indicating both the name of the manufacturer, and the maximum discharge rate in tonnes per hour.
6.1.6
Production and Delivery
NZS 3104 also sets out a number of requirements governing the production and delivery of concrete from batching plants.
The following are of particular note:
All materials other than liquids have to be proportioned by mass, i.e. weigh batched. (A limited number of mixes may use volume proportioning – see Chapter 20).
The quantity of each ingredient in a batch should be measured within the tolerances shown for each ingredient in Table 6.1.Table 6.1 Accuracy of Measurement (from NZS 3104) Material Volume Cement ±1.0% Water (mass or volume) ±2.5% Aggregates (mass) ±2.0%
Any fraction provided that total mass of aggregate is within 1% Admixture
(mass or volume)
±10.0% of amount specified
Water may be added to a mixed batch of concrete, prior to its complete discharge, only if the following relevant conditions are satisfied:
The supplier’s approval is obtained. (Only the supplier’s representative can add water or admixtures to a mixed batch prior to its discharge. This is because the supplier is responsible for the quality of the concrete up to the point of acceptance of delivery. The addition of water and/or admixtures will affect the quality of the concrete and may cause it to fail to meet the specified properties.)
Immediately after the water is added, the mixing bowl is rotated for 30 revolutions at mixing speed, or for such time as is necessary to re- establish the uniformity of the mix.
If water is added once discharging has commenced, this fact is noted on the identification certificate for the batch.
If a maximum water-cement ratio has been specified, the quantity of water added does not cause this ratio to be exceeded.Chapter 6 Manufacture of Concrete
Discharge of all concrete from the batch should be completed within 90 minutes of mixing having been commenced (or sooner if proper placement and compaction cannot be achieved). This requirement may be waived, however, if the consistence of the concrete can be maintained for a longer period without the addition of extra water to the mix.
Unless otherwise specified, concrete at the point of delivery should have a temperature of not less than 5°C nor more than 35°C. (For additional precautions in cold or hot weather see Chapter 12 Hot- and Cold-Weather Concreting.)6.1.7 Records
An essential feature of a Quality system is the maintenance of records. Ready mixed concrete production entails the following records related to quality:
Material Supply and Plant Certificates
Plant tests on aggregates and sand which also record testing frequency and evidence that the results are monitored and action taken when necessary.
Test certificates for weighing and testing equipment, including scale checks by plant personnel as specified.
Cement certificates from cementmanufacturers.
Order Records
Technical requirements of the purchaser taken with the order including:(a) Nominated compressive strength. (b) Nominal aggregate maximum size. (c) Slump required on delivery.
(d) Proposed compaction method if other than high frequency vibration.
(e) Whether the concrete is to be pumped (f) Any other requirements.
Delivery Records
Type of cement and nominal maximum aggregate size.
Specified strength and delivery slump ordered.
Date and time of completion of mixing.
Quantity delivered.
Any other special requirements as ordered by the customer.Mixing Records
A record must be kept at the plant which identified for each batch the following: