Proper mixing of concrete ingredient is very much necessary as it affects the quality of concrete in its fresh state as well as in hardened state. A concrete is said to be well mixed if it fulfills the following requirements.
Concrete mix should be uniform in colour
Concrete should achieve proper consistency for which it is designed
Complete blending of all concrete ingredients
Cement paste should cover all the surface of the aggregate
Hand mixing is adopted for small works and where the quantity of concrete used is small. The following procedure is followed to mix concrete by hand.
Step-1 [Mixing Platform]
Clean and water tight platform should be used for mixing concrete. Construct a platform of bricks or lean concrete or iron sheets, of a size as per requirement of the volume of concrete to be mixed. Commonly a platform made of iron sheet having 3m2 in size with strips or kerbs are used.
Step-2 [Dry Mixing of Cement & Sand]
Spread out the measured quantity of sand uniformly on the mixing platform. Dump the cement on the sand and distribute it uniformly. Mix the sand and cement with the shovels, turning the mixture over and over again until it is uniform in colour.
Step-3 [Dry Mixing of C.A. With Cement-Sand Mix]
Spread out the sand-cement mixture once again uniformly on the mixing platform. On top of this, spread the measured quantity of C.A (coarse aggregate). Don’t dump at one place; otherwise the bigger particles will tend to roll out. Mix again at least three times by shoveling from the center to side, and then back to the center and again to the side.
Step-4 [Addition of Water]
Make a depression in the middle of the mixed pile and pour slowly into half to three-quarters of the total quantity of water required. While the material is turned in towards the centre with shovels, add the remainder of the water slowly, turning the mixture over and again until the colour and consistency are uniform throughout the pile. This will indicate that all ingredients are thoroughly combined. The water should be poured slowly into the mix. This is best done by means of a gardener’s water can fitted with a rose-head. Throwing water from a bucket all at a time will result in its running away and taking cement with it.
Time of mixing does not exceed 3 minutes
Mixing platform is cleaned at the end of the day’s work, so that it is ready for use the next day.
Check for Consistency
A rough estimation of consistency of concrete can be made by doing the following simple & quick field test.
Take a handful of concrete and squeeze it tightly in the hand. If the concrete is mixed properly, then after releasing the hand pressure, it will retain its shape. Also the surface of the sample will be moist without water dripping out of it.