How Mixed Concrete Are Made

Concrete materials for a project are determined by their specified mix design. The mix design provides the list of materials and their acceptable ratios to produce concrete that will meet a PSI rating needed. In addition to determining the “PSI” rating, the list of materials may include special admixtures which can cause the concrete to set up faster, or slower; achieve the an earlier higher “PSI” rating; or the materials may be changed to make them easier to use in dense reinforcing conditions. All of these factors should be considered when discussing the concrete material with the supplier and each mix should have an identified cost. Premix concrete is the blending of the materials which are required for the production of a homogeneous concrete. 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. 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 (strength) are normally provided in the design specifications. The design specifications must be consulted before any batching or mixing takes place. 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. 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. 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.

Chemical Admixtures are incorporated to improve the following premix concrete properties:

• Protect Against Freeze Thaw Cycles
• Mid- Range water reducers
• High-Range water reducers
• High Strength Concrete
• Corrosion Protection
• Set Acceleration
• Strength Enhancement
• Set Retardation
• Crack Control
• Flow ability
• Self levelling

Chemical admixtures are used to improve the quality of premix concrete, transporting, placement and curing. They reduce the cost of construction, modify properties of hardened concrete, ensure quality of concrete during mixing/transporting/placing/curing, and overcome certain emergencies during concrete operations.