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.
