BATCHING
Batching is the process of measuring the composition of a concrete mixture by volume or mass and introducing it into the mixture. Traditionally, ingredients are made by volume, but most regulations require ingredients to be made by mass rather than volume. The accuracy of concrete material measurement is as follows.
Cement:
When the cement content of the ingredient exceeds 30% of the balance capacity, the measurement accuracy should be within 1% of the specified mass. If the measurement quantity is less than 30%, that is, for smaller batches, the measurement accuracy should be within 4% of the required quantity.
Aggregation:
If the measured value exceeds 30% of the scale, the measurement accuracy should be within 1%. If the measured value is less than 30%, the measurement accuracy should be within 3%.
Water:
Water is measured by volume, 1 liter=1 kilogram. If it is water, the measurement accuracy should be within 1%.
Admixtures:
The accuracy requirements for mineral admixtures are the same as those for cement. For chemical admixtures, their accuracy and water usage requirements are the same. The accuracy of mineral admixtures is the same as that of cement, as they partially replace cement. Due to the fact that chemical admixtures are liquid or added to water, their accuracy is the same as that of water.
MIXING
The mixing operation includes rotation or mixing, with the aim of applying cement slurry to the surface of all aggregate particles and covering all components of the concrete to a uniform mass; This uniformity shall not be disturbed by the process of discharge from the mixer.
Batch mixer
The usual type of mixer is a batch mixer, which means mixing a batch of concrete and discharging it before adding more material to the mixer. There are four types of intermittent mixers.
Flip drum mixer:
A tilting drum mixer is a type of mixer that tilts the mixing drum to discharge materials. The drum is conical or bowl shaped, and the internal blades are quickly discharged without separation. Therefore, these mixers are suitable for low workability mixtures and mixtures containing large-sized aggregates.
Disc mixer:
A disc mixer is a forced mixer, different from a drum mixer that relies on the free fall of concrete inside the drum. The disc mixer consists of a disc that rotates around its axis and one or two star shaped blades that rotate perpendicular to the axis of the disc.
Double drum mixer:
Double drums are sometimes used for highway construction. There are two drums in series here, where the concrete is mixed in one drum for a portion of the time, and the remaining mixing time is transferred to the other drum before unloading.
Continuous mixer:
These are automatically provided by a continuous weighing and batching system.
In any mixer, sufficient material exchange must be carried out between the various parts of the chamber in order to produce uniform concrete. The efficiency of the mixer can be measured by the variation of the mixed sample. ASTM stipulates that samples should be collected from approximately 1/6 and 5/6 points of the batch discharge, and the performance difference between the two samples should not exceed any of the following:
Concrete density 1 pound/cubic foot
Air content 1%
When the average value is less than 4 ", the slump is 1"
1.5 "when the average value is less than 4 to 6"
Sieve 4 intercepts aggregate% 6%
Compressive strength 7 days, 3 cylinders 7.5%
Mixing time:
It is important to understand the minimum mixing time required to produce concrete with uniform composition and reliable strength.
The mixing time or cycle should be from all cementitious materials and aggregates entering the mixing drum until the concrete is removed.
The mixing time depends on the type and size of the mixer, the rotational speed, and the quality of component mixing during the mixer loading period. Generally speaking, a mixing time of less than 1 to 1.25 minutes will result in significant compositional heterogeneity and significantly lower strength; Mixing for more than 2 minutes will not result in significant improvements in these performance.
