Agitating – Mortar mixer type – Methods
Patent
1993-09-03
1998-05-19
Jenkins, Robert W.
Agitating
Mortar mixer type
Methods
366 60, B28C 506, B01F 902
Patent
active
057527687
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to mobile cement mixers and means for controlling the mixing regimen of concrete within such mixers. More particularly it relates to improved means for ensuring that concrete delivered by such mixers is of optimal strength and consistency, and includes means for allowing for the necessity of adjusting its consistency or "slump" from time to time.
BACKGROUND TO THE INVENTION
The quality of concrete is significantly affected by the manner in which it is mixed, and the amount of water added. These factors critically govern the strength of concrete that can potentially be achieved, and the strength of concrete greatly affects it marketability. If strengths over certain thresholds can reliably be assured, concrete can compete economically with steel for large structures.
Concrete specifications for concrete delivered to a job site typically require that the concrete on arrival be guaranteed to achieve a minimum specified strength, 99% of the time. When the variability of concrete strength is large, suppliers must increase the content of portland cement within the concrete sufficiently to statistically ensure that they achieve this result. By narrowing the variability of concrete quality, suppliers would be able, at considerable savings, to reduce the content of portland cement without risking, statistically, failure to meet the required strength criteria. One way of reducing the variability of concrete strength is to control carefully the mixing regime or cycles that the concrete passes through before being placed in position at the job site.
It is now generally customary to have concrete delivered to construction sites in vehicles that have a concrete mixing capacity. Such vehicles are loaded with the correct proportions of sand, stone, cement and sometimes with water at the home-base or batching plant for the mixing trucks. If water is not added at the batch plant, the vehicle is said to be carrying a "dry batch" as opposed to a regular batch that includes water. In the case of a dry batch, the water is eventually added by the driver from the standard water reservoir carried on the truck.
After being charged with all the necessary ingredients, the mixing cycle can commence. In the case of a regular batch containing water this is usually effected before departure, while the vehicle is standing in the batch plant yard. In the case of a dry batch some agitation of the dry mix may occur before or during transit, but mixing proper commences when the vehicle is near the job site and water is added to the dry batch.
The strength of concrete when set, and its workability at the job site, are critically dependent on the mixing regime that is followed. The Truck Mixer Manufacturers' Bureau of the United States of America (TMMB) provides the following recommended criteria for the mixing of a full load of concrete in order to ensure an even consistency and obtain satisfactory strength without "over-working" the concrete: speed in total, including mixing turns. a 6 to 15 cubic yard capacity, a ten foot or so maximum diameter inclined drum with a series of spirally inclined mixing fins on the interior surface.
The foregoing criteria are examples of recommended limits for the target mixing regime for concrete, based on full loads. However, no standards exist that address mixing requirements when the load is less than full.
Another consideration that is not addressed by present standards is that it is very important, once proper mixing is completed, to maintain a minimum degree of further agitation sufficient to prevent separation, (preferably around 11/2-21/2 rpm), but no higher than necessary as this will accelerate the setting of the concrete by over-working it.
These are examples of issues arising from the mixing regime to which concrete may be subjected that significantly affect concrete quality. For the great proportion of deliveries at present, the driver is personally responsible for controlling the rate and duration of rotation of the drum. It is desirable to remove
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