3. Mixing Duration and Guidelines

Now, we will discuss about the next type of power mixer which is tough mixer. So, it resembles a trough shaped drum. So, you can see the schematic picture of the trough mixer. You can have a single shaft or double shaft trough mixer. Early, pan mixer, the shaft was vertical but here you can see, the shaft is horizontal. You have horizontal arrangement of shaft. So, on the shaft, you can see, the paddles will be arranged spirally along the shaft. The paddles are arranged spirally along the shaft. Either you can have paddle arrangement or you can even have a wave shaped arms arrangement. So, that will also give you better mixing. So, whenever you go for twin shaft mixer, you can see, the intensity of mixing will be very good because in the intersections zone, you will have more amount of turbulence that results in good intensity of mixing.

In this chunk, the mixing duration is highlighted as a critical factor in achieving a homogeneous concrete mix. Different types of mixers require different mixing durations, with power mixers needing less time than free fall mixers. This variance also depends on the capacity of the mixer machine. The guidelines provided by IS 4925 outline minimum mixing times for various mixer types and capacities. If mixing is too short, the mix won't be homogeneous; too long, and the concrete may start to set, leading to inefficiencies.

Think of baking a cake. If you don't mix the ingredients long enough, the batter will be lumpy, leading to an uneven texture. But if you mix it too long, the cake can become tough. Similarly, in concrete mixing, the goal is to find that perfect mixing duration to achieve a smooth and uniform consistency.

So, this is what I told you. This table is available. It is taken from IS 4925 which gives you a guideline for concrete batching and mixing. So, you can see that the mixing duration is given in seconds. It varies for different types of mixer machines. Say, for instance, if you go for non-tilting reversible drum type mixer, the mixing duration is more. So, generally, for non-titling mixer, it is more. When compared to the pan type mixer and the shaft mixer, you can see that the mixing duration is more for the non-tilting reversible drum type mixer. So, similarly, for different capacities of mixer machines, you can note the minimum mixing time. This is a minimum mixing time given. We should never go below this.

This chunk discusses specific guidelines provided in IS 4925, which lists the required mixing durations for various types of mixers. For example, non-tilting reversible drum mixers require longer mixing times compared to pan mixers. Adhering to these guidelines is essential, as failing to meet them can hinder the quality and consistency of the concrete mix, similar to how strict timing in baking is crucial for the right texture.

Consider a school pacing a lesson plan. If the teacher speeds through a topic without allowing enough time, students may remain confused and unprepared for tests. Conversely, spending too much time on one subject while ignoring others might prevent them from understanding key concepts. Achieving the right pace in teaching is like achieving the right mixing duration in concrete preparation.

So, generally, if you go for a lesser duration, then what is needed? What will happen? You will not get a homogenous mix that is a main problem and if you go for a very high duration, mixing duration also is not good because as you know that as you add water to the concrete, your cement starts setting. So, it will result in loss of water from the concrete due to evaporation that will affect your workability of the concrete. So, extended mixing duration is also not good with respect to this setting and the workability of the concrete.

This chunk elaborates on the consequences of not adhering to the prescribed mixing durations. Mixing for too short a time leads to uneven batches, while excessive mixing may cause the concrete to set prematurely or become less workable due to water loss. Both scenarios can result in a final product that doesn't meet structural standards or workability requirements.

Imagine watering a plant. If you only sprinkle a few drops, the plant won't get the nourishment it needs; it will remain wilted. However, if you overwater, the roots might rot and the plant could die. Proper water management is akin to finding the right mixing duration—too little or too much can lead to problems.

So, that is why there is an optimum mixing duration for every mixer machine. So, we should go with a manufacturer recommendation. Guidelines for minimum mixing time for each batch is given in the IS 4925. So, it will apply the mixing time actually starts after all the materials are added into the concrete mixer machine except the full quantity of water is added in the mixer drum that means your mixing time starts counting after all the materials are added into the mixer machine other than the full quantity of water because commonly you can see that the water, you add it in parts.

This chunk addresses the specific guidelines regarding when to start counting the mixing time. Mixing time is measured after all components, except the full amount of water, are added to the mixer. This is because water is often added in stages, especially when using superplasticizers, to maintain the concrete's workability. Following these guidelines ensures proper hydration and mixing processes.

Think of making a smoothie. If you add all the fruits and solids and then slowly pour in the liquid, you're giving it a chance to blend well. If you pour the liquid in first and then add all the solids, the mix might not combine well, leading to an uneven smoothie. Similarly, in concrete mixing, the timing of adding water matters significantly.

So, once the concrete is made, you have to take the samples from different portions of the batch and you should check for the uniformity. For that also, the codes are giving you the guidelines. You take the samples and check for the fresh density; check for the air content, slump, coarse aggregate content, you do the sieve analysis and check the coarse aggregate content. This, the difference between the samples should not be more.

In this chunk, the importance of quality control after mixing is highlighted. It is crucial to take samples from various parts of the mixed batch to ensure that the concrete is uniform in quality. Specific measurements such as fresh density, air content, and slump must be evaluated. Adhering to these checks helps maintain the integrity of the concrete and ensures it meets required standards.

Imagine baking a batch of cookies. If you just take a cookie from one side of the tray to taste, you may not get an accurate sense of the whole batch——some cookies might be burnt while others are perfectly baked. Sampling from different sections ensures you have a good understanding of the overall quality, just like checking concrete uniformity.

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