Mixing and Transporting Concrete

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 process of placing concrete.

2.0 Batching and Mixing Concrete

Mixing concrete is simply defined as the "complete blending of the materials which are required for the production of a homogeneous concrete" (Young, 267). This can vary from hand to machine mixing, with machine mixing being the most common.

However, no successful mixture can be achieved without the proper batching of all materials. Batching is the "process of weighing or volumetrically measuring and introducing into the mixer the ingredients for a batch of concrete" (Kosmatka, 94). Quality assurance, suitable arrangement of materials and equipment, and correct weighing of the materials are the essential 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. The design specifications must be consulted before any batching or mixing takes place.

    1. Components of Mixing

      2.  

      There are many components of mixing that need to be considered in order to ensure that a uniform concrete mixture can be achieved. Location, shape and angle of the mixing blades, shape of the mixing chamber, speed of rotation, and horsepower must all be taken into account. It is paramount that each batch is consistently mixed to design specifications so the concrete’s final strength is not compromised.

    2. Classifications of Mixers

      3.  

      There are essentially three classifications of mixers: the drum mixer, pan mixer, and continuous mixer. Each of these mixers can be further classified as batched or continuous, free-falling or forced movement, and stationary or portable.

    3. Mixing Time
                    Once the appropriate mixer has been chosen, it is necessary to determine the mixing time. This is the duration of
                    time it takes to mix concrete, once the mixer is fully charged with all the materials. Charging is an important step
                    because it gives the materials an opportunity to pre-blend. The type and condition of the mixer, speed of rotation,
                    size of the charge, and nature of the materials all determine the correct mixing time. The mixing time is not
                    standard for each batch. For example, a drum mixer with a small diameter creates 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 concrete from the mixing plant to the construction site. Keep in mind that not all concrete is mixed on the actual construction site and could require some significant travel. This is most common for ready-mixed concretes. The main objective in transporting concrete is to ensure that the water-cement ratio, slump or consistency, air content, and homogeneity are not modified from their intended states.

3.1 Important Factors in Choosing Transportation

There are many elements of transporting that need to be considered in order to ensure that a mix does not change its state as specified in the contract. The two key goals when transporting concrete from the mixing plant to the construction site are to prevent segregation and to not reduce the workability of the mix. This transportation process must be well thought out and organized efficiently. As a general rule of thumb, thirty to sixty minutes of transportation are acceptable on small jobs. At a central or portable ready-mix plant, concrete should be discharged from a truck mixer or agitator truck within two hours. If non-agitating transporting equipment is used, this time is reduced to one hour. All delays must be avoided in order 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 consideration when choosing a mode of transport for your concrete. If you choose the wrong mode of transportation, your concrete could be segregated, which would in effect, make it useless. Therefore it is essential that adequate thought be given to the type of transportation you actually need.

3.2 Categories of Transportation

There are many modes of transportation as shown below:

    1. Wheelbarrow or motorized buggy
    2. Truck mixer
    3. Bucket or steel skip
    4. Chute
    5. Belt conveyor
    6. Concrete pump
    7. Pneumatic placer
4.0 Conclusion

As indicated above, the successful mixing and transporting of concrete requires the careful management of many factors. Once the appropriate mixing and 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 and Inspection PavementsMaterials and General.
        Detroit: American Concrete Institute, 1997.

Dobrowolski, Joseph A. Concrete Construction Handbook (4th Edition). New York: McGraw-Hill, Inc., 1998.

Kosmatka, Steven H. And William C. Panarese. Design and Control of Concrete 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 and Practice (6th Edition). London: Edward Arnold, 1991.

Popovics, Sandor. Fundamentals of Portland Cement Concrete: A Quantitative Approach, Vol.1: Fresh Concrete. New York:
        John Wiley & Sons, Inc., 1982.