In this article, you will learn how to properly mix and place concrete.
Refer to part 1 of the article here.
Table of Contents
Unless permission is granted for hand mixing, which should only be for small quantities, all concrete should be mixed by machine.
Batching by volume should be restricted to batches of 0.5 m3 or less.
The “shovelful” method of batching is not recommended.
If hand mixing is allowed, combine the ingredients on a clean board to achieve a uniform color throughout before adding water to achieve the required workability.
Provide detailed written instructions on the quantities of all materials to be batched as well as the batching sequence for each concrete mix that is required.
On small installations, a board with instructions mounted near the plant would be appropriate.
Batching cards may be preferable in large installations.
There are no set rules for batching sequences, but it is important to use the same sequence for each batch. If you don’t, the concrete you make might not be the same consistency.
In general, coarse aggregate should be placed in the hopper first to prevent cement and sand from forming cakes on the hopper surface.
Allow for priming the internal surfaces of the mixer drum during the first batch of material loading.
If no compensation is made, some of the cement and sand content of the first batch will adhere to the inner surface and blades of the drum, and this batch will be too lean.
Reducing the amount of coarse aggregate in the first batch by about 50% is a convenient way to account for this.
Mix the ingredients for a long enough time to ensure even distribution of the ingredients.
Ensure that this time is not less than that specified in the mixer’s installation instructions.
Before discharging a batch, inspect it to ensure that its appearance is uniform. If not, keep mixing until it is.
Before recharging the mixer, remove each batch completely from it.
A mixing time of 1 to 2 minutes is typical for rotating mixers and 30 to 45 seconds for pan mixers.
Care of static batching and mixing plant
Every two days, make sure the mixer is level.
Do not exceed the rated capacity and output of the manufacturer.
When the hopper is empty, turn the weight dial to zero.
Every week, check the measuring mechanisms with known weights across the entire range and adjust them to read correctly.
Once a week, check the speed of the mixer drum and adjust it to the manufacturer’s specifications.
Clean out the mixer drum at the end of each day’s work, whenever there is a material change, or whenever there is a work break.
Keep the batching plant clean and avoid clogging moving parts with cement or concrete.
Always keep the hopper and the ground beneath it clean.
Always keep the weighing pressure pad free of material buildup.
Oil and grease moving parts on a regular basis, and inspect them for wear and fraying of wire ropes in accordance with the manufacturer’s maintenance manual.
The measuring equipment must be accurate to within ± 3% of the specified amounts of cement, water, aggregates, and admixtures, and ± 5% of the specified amounts of admixtures.
The relative mobility or flowing ability of a freshly mixed cement paste or mortar is referred to as consistency.
The primary goal of the consistency test is to determine the amount of water required to produce a cement paste of standard consistency.
The flow table test is used to determine the standard consistency of a mortar.
The normal consistency of OPC ranges from 26 to 33%.
Concrete Setting Times
It is critical that the cement does not set too quickly or too slowly. In the first case, there may not be enough time to transport and place the concrete before it hardens. In the second case, an excessively long setting period tends to slow down the work. It may also postpone the building’s use because it will not be strong enough at the appropriate age.
The amount of time between when water is added to cement and when the paste begins to lose its flexibility is commonly referred to as initial setting.
The time elapsed between the addition of water to the cement and the time at which the paste has completely lost its plasticity and attained sufficient firmness to withstand a specific definite pressure is referred to as the final setting time of cement.
According to BS 12, the initial setting time must be at least 30 minutes and the final setting time must be no more than 10 hours.
Test for setting times
The cement paste is formed and filled in the mould.
The needle is now made to just touch the top surface of the cement paste and fall freely into it.
The initial setting time is the time between mixing the cement and water and when the needle penetrates just above 5 mm from the bottom of the base plate or mould.
Ordinary Portland cement has an initial setting time of 30 minutes.
This time can be increased to 60 minutes for slow-setting cement by adding admixtures or gypsum.
Similarly, another needle with an enlarged 5 mm hollow cylindrical base is used for the final setting time.
The final setting time is the time it takes from mixing the water to when the needle leaves an impression on the surface of the cement but does not penetrate it.
The final setting time of cement (OPC) is typically 10 to 12 hours.
Workability is an important aspect of batched concrete.
Workability is defined asthe ease with which concrete can be mixed, placed, compacted, and finished. It has three properties; mobility, compactability and stability.
Mobilityis the property that determines how easily the concrete can flow into the moulds and around the rebar.
Compactabilityis the property of concrete that determines how easily it can be compacted to remove air voids.
Stability is the property that determines the ability of the concrete to remain as a coherent and stable mass during handling and vibration.
The concrete must be easily worked into the corners and angles of the molds, as well as around any reinforcement, without causing the materials to separate or water to pool on the surface.
Making small changes to the amount of water used when mixing allows you to control the consistency and workability.
These changes are made to account for variations in moisture content in aggregate supplies.
The slump is a measure of workability.
The slump should correspond to the specification and be within the margin of error for each batch in the same structural member.
If it is deemed necessary to use concrete with a slump outside of the tolerances specified, the design engineer must be consulted.