1.0 Introduction
The successful placement of concrete is dependent upon careful mixing,the proper equipment, and adequate transportation. This site will define,analyze, and demonstrate the importance of each in the overall processof placing concrete.
2.0 Batching and Mixing Concrete
Mixing concrete is simply defined as the "complete blending of the materialswhich are required for the production of a hom*ogeneous concrete" (Young,267). This can vary from hand to machine mixing, with machine mixing beingthe most common.
However, no successful mixture can be achieved without the properbatching of all materials. Batching is the "process of weighing orvolumetrically measuring and introducing into the mixer the ingredientsfor a batch of concrete" (Kosmatka, 94). Quality assurance, suitable arrangementof materials and equipment, and correct weighing of the materials are theessential steps that must be completed before any mixing takes place.
The types and volume of materials, the mix design, and the end result(i.e. strength) are normally provided in the design specifications. Thedesign specifications must be consulted before any batching or mixing takesplace.
- Components of Mixing
- Classifications of Mixers
- Mixing Time
There are many components of mixingthat need to be considered in order to ensure that a uniform concrete mixturecan be achieved. Location, shape and angle of the mixing blades, shapeof the mixing chamber, speed of rotation, and horsepower must all be takeninto account. It is paramount that each batch is consistently mixed todesign specifications so the concretes final strength is not compromised.
There are essentially three classificationsof mixers: the drum mixer, pan mixer, and continuous mixer. Each ofthese mixers can be further classified as batched or continuous, free-fallingor forced movement, and stationary or portable.
time it takes to mix concrete, once the mixer is fully charged with allthe materials. Charging is an important step
because it gives the materials an opportunity to pre-blend. The type andcondition of the mixer, speed of rotation,
size of the charge, and nature of the materials all determine the correctmixing time. The mixing time is not
standard for each batch. For example, a drum mixer with a small diametercreates a greater velocity than a drum
mixer with a large diameter, therefore, the mixing time would be decreased.However, if the goal is stiffer
concrete, a longer mixing time is required.
3.0 Transporting Concrete
Transporting the concrete mix is defined as the transferring of concretefrom the mixing plant to the construction site. Keep in mind that not allconcrete is mixed on the actual construction site and could require somesignificant travel. This is most common for ready-mixed concretes. Themain objective in transporting concrete is to ensure that the water-cementratio, slump or consistency, air content, and hom*ogeneity are not modifiedfrom their intended states.
3.1 Important Factors in Choosing Transportation
There are many elements of transporting that need to be considered inorder to ensure that a mix does not change its state as specified in thecontract. The two key goals when transporting concrete from the mixingplant to the construction site are to prevent segregation and to not reducethe workability of the mix. This transportation process must be well thoughtout and organized efficiently. As a general rule of thumb, thirty to sixtyminutes of transportation are acceptable on small jobs. At a central orportable ready-mix plant, concrete should be discharged from a truck mixeror agitator truck within two hours. If non-agitating transporting equipmentis used, this time is reduced to one hour. All delays must be avoided inorder prevent honeycombing, as shown in Figure 1,or cold joints.
Many factors determine which type of transportation is most suitable.Type and constituents of the concrete mix, size and type of construction,topography, weather conditions (i.e. temperature, humidity, wind speed),location of the batch plant, and cost are all taken into considerationwhen choosing a mode of transport for your concrete. If you choose thewrong mode of transportation, your concrete could be segregated, whichwould in effect, make it useless. Therefore it is essential that adequatethought be given to the type of transportation you actually need.
3.2 Categories of Transportation
There are many modes of transportationas shown below:
- Wheelbarrow or motorized buggy
- Truck mixer
- Bucket or steel skip
- Chute
- Belt conveyor
- Concrete pump
- Pneumatic placer
As indicated above, the successful mixing and transporting of concreterequires the careful management of many factors. Once the appropriate mixingand transporting equipment is chosen and the important factors are addressed,you are ready to move on to the next critical step, known as placing.
Information compiled by Greg Vinci
5.0 References
ACI Manual of Concrete Practice, Part 2: Construction Practices andInspection PavementsMaterials and General.
Detroit: American ConcreteInstitute, 1997.
Dobrowolski, Joseph A. Concrete Construction Handbook (4thEdition). New York: McGraw-Hill, Inc., 1998.
Kosmatka, Steven H. And William C. Panarese. Design and Control ofConcrete Mixtures(13th Edition). Illinois:
Portland Cement Association,1998.
Mindess, Sidney and J. Francis Young. Concrete. New Jersey: Prentice-Hall,Inc., 1981.
Murdock, L.J., K.M. Brook, and J.D. Dewar. Concrete: Materials andPractice (6th Edition). London: Edward Arnold, 1991.
Popovics, Sandor. Fundamentals of Portland Cement Concrete: A QuantitativeApproach, Vol.1: Fresh Concrete. New York:
John Wiley & Sons, Inc.,1982.
