Concrete mixing simply means to mix the ingredients that make up concrete. The ingredients for a conventional concrete mix include cement, sand, coarse aggregates and water. An admixture may or may not be added depending upon necessity.
It should be kept in mind that apart from the quality of materials used, the differentiating factor between a good-quality concrete and a bad one is the mixing method employed and the handling of concrete.
Given this, proper care and a lot of consideration is required while mixing concrete.
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Prerequisites of Mixing Concrete
The production of concrete is not a one-step process that involves some random mixing of components. To produce a homogenous, workable and good concrete mix, you should have in hand knowledge about the following,
- What grade of concrete is being targeted?
- What is the mix design of the concrete you are targeting?
- Is concrete to be produced in bulk quantities?
- Which structural/non-structural member is to be cast?
- Will manual mixing suffice on site or is mechanical mixing of concrete required?
Once you have answers to the above question, the next step is how to mix concrete? For this, the following paragraphs will have your queries cleared.
Why is Proper Mixing Important for Concrete?
Concrete is a heterogeneous mixture and its properties keep fluctuating with even the tiniest disparities in the ingredients. Just like when you make a cake batter, you ensure the ingredients are adequately mixed to get a uniform output, the same goes for concrete mixes.
If you undermix the components of concrete, you will end up with segregation and bleeding in the wet mix and honeycombing or excessive voids in the hardened concrete.
With improper mixing, a part of your concrete will dry out earlier than the other, creating voids in the internal structure and making it prone to collapse just when made serviceable. Hence, you must ensure that after dry mixing the ingredients, water reaches every part of the mix and the ingredients are shaken up well in the presence of water.
Types of Mixing
Dry mixing means to add the dry concrete ingredients first and mix them up prior to the addition of water. The order of placing the ingredients for conventional concrete is to add the coarse aggregates first, followed by fine aggregates (sand) and finally cement.
The dry ingredients should be mixed for 1 to 3 minutes either in a mixer or manually, depending upon the method employed.
The dry mix can be stored even for months with proper precautions. This is because the coarse and fine aggregates have a long shelf-life. However, owing to the addition of cement, the shelf life of a dry mix becomes limited to a few months, after which the cement quality does not remain same.
Wet mixing of concrete is followed by the dry mixing process. Water is added in the dry mix and the ingredients are thoroughly mixed to produce a workable concrete mix.
Once water is added to concrete, the process of hydration is commenced. Therefore, once you add water, you cannot take too long to handle concrete. The mixing time for preparing a wet mix is to be regulated and is probably the most critical stage of concrete production.
If proper time is not allotted, the mix will be a failure. If more time if given to mixing the ingredients, you will end up with concrete hardening in the mixer while you transport it. Therefore, casting a balance in the mixing time is the key to an efficient mix.
Manual Mixing vs Mechanical Mixing
Manual or Hand Mixing
Manual concrete mixing is done when small quantities of concrete are needed. However, care is to be exercised during the process of mixing. Here is the right method for mixing concrete by hand.
Manual mixing is relatively less efficient and thorough but for small-scale concreting activities, employing a mechanical mixer is not always cost-effective. You may then resort to manually mixing the ingredients with due care.
For manual mixing, materials are batched by volume using small wooden boxes called Farma boxes or any other standard measure like buckets. Since this process is relatively less efficient than weight batching, 10% extra cement is to be added in the mix to keep an upper margin of safety.
The boxes should be of the same size and filled to the same level. To get the best out of manual mixing, cement and sand should be thoroughly dry-mixed first such that the color becomes uniform.
The dry mix is then spread uniformly on the bed of coarse aggregates or crushed stones.
A small depression is made at the center of the spread mix and one-fourth of the measured quantity of water is poured into the depression. This is followed by a thorough mixing of dry ingredients with addition of remaining water intermittently until the concrete attains a uniform color and desired workability.
In manual mixing, it is practically impossible to produce two batches of concrete with exactly same specifications or properties despite their mix design being the same. This is because precision and control over external parameters get lessened when you mix the ingredients by hand.
Mechanical Mixing in Concrete Mixers
Mechanical mixing of concrete is done using mixers which entail a number of types suiting their use. Mixing concrete constituents in a mixer is the most efficient way of producing a better-quality concrete mix at a faster rate and having a homogenous color and texture.
The size of the mixer should be such that it can easily incorporate and mix the ingredients for one batch of concrete. Mixers with capacity of 200 liters are the most popular in use.
Prior to feeding dry ingredients into the mixer, you must ensure that the mixer is clean, free from dents on the inner surface and the blades properly aligned.
For mixers without hopper, one forth quantity of water is added into the mixer first, followed by half the quantity of total coarse and fine aggregates. Following this, the full batched quantity of cement is added and then the remaining quantities of coarse and fine aggregates along water are added.
For mixers with hopper, the dry, batched ingredients are fed one by one, starting with coarse aggregates, fine aggregates and finally cement is added. The dry mixing of ingredients should be carried out and then water is added gradually.
Just like manual mixing, once water is added, the regulation of mixing time should be carefully done to get the best results.
Mixing Time for Concrete
Concrete should be mixed for an optimum time to accrue maximum benefit. The time of mixing can range from half minute to two minutes.
It is observed experimentally that increasing the mixing time of concrete up to 2 minutes increases the compressive strength but beyond this time, the strength graph becomes almost constant and no significant improvement is observed.
The reason for the increase in strength can be attributed to improvement in the workability of the mix. This is because with more mixing time, the abrasion and attrition of coarse aggregates takes place, increasing the fines content and enhancing concrete workability.
For machine mixing, the mixing time is more related to the mixer capacity. Greater is the drum capacity, more is the mixing time and the range is generally between one and a half to two and a half minutes.
In general, the mixing time of concrete depends on the following factors;
- Compressive strength of concrete required
- Water to cement ratio
- Batch size
- The type and quality of raw materials used
- The standard guidelines being followed
- The type of mixing method being employed (mechanical mixing, hand mixing, etc.)
Mixing concrete for too long period may result in early evaporation of water and formation of excess fines. This may also lead to an early setting and hardening of concrete, leaving you with lesser time for its handling on site.
How Will You Know Your Concrete is Well-Mixed?
A few indications of a well-mixed concrete mix are as follows;
- The color of the mix will be uniform and homogenous.
- The raw materials will appear to be mixed thoroughly, with cement-sand paste covering every coarse aggregate particle all-around.
- There is no appearance of a thin water film over the mixed concrete. If that happens, it indicates bleeding water which results from inefficient mixing of the concrete ingredients.
- A well-mixed concrete will also not segregate when dropped from a heigh of 1 ft.
- Your concrete, if well-mixed, will make it easy for you to handle it, compact and finish it.
Factors Affecting Mixing of Concrete
The following factors must be heeded to while you mix concrete.
- Compressive strength of concrete or its mix design controls the proportion of ingredients which in turn decide the quality of the concrete mix. A higher water to cement ratio intended in the mix design means you want a flowable concrete mix with a higher slump. Therefore, based on the mix type intended to achieve (stiff mix, high slump mix, etc.), the mixing of concrete ingredients ought to be regulated.
- Workability of concrete is greatly improved if proper mixing is carried out. This is because if every aggregate particle is properly coated with the cement matrix, the particle-particle friction will reduce and the aggregates will easily glide over each other producing flowable concrete.
- The mixing method used controls the homogeneity of mix. Oftentimes, when manual mixing is carried out, the resulting mix falls short of desired characteristics. Therefore, hand mixing should only be carried out for small-scale work.
- The type of mixer used (if you go for mechanical mixing) affects the quality of concrete. Some mixers batch ingredients by weight and some others by volume, so, the possibility of getting exactly the same mix is repealed.
- Mixing time is probably the governing factor for your mix. You don’t have to undermix the ingredients nor should you overmix them.
- Mixing speed of mixer should also be monitored otherwise, the mixer blades tend to exert large abrasive forces on the aggregate particles, generating fines out of them.
Precautions for Concrete Mixing
- Do not estimate the amount of ingredients to be added as raw materials for concrete; a proper mix design is a must to get the desired results.
- When going for volume batching of concrete, care should be taken while measuring the ingredients. The standard measuring tool used (farma boxes, buckets, etc.) should all be of the same size and filled to the same level.
- The mixing drum should be clean and free from dents.
- Check for proper alignment of blades in the drum.
- The mixing time should be at least 2 minutes once water is added in the dry mix.
- The setting time and hardening time of concrete should be considered while you mix the ingredients. The concrete should be placed within 30 minutes after mixing, so that proper compaction and finishing is attained.
- It should be ensured that the cement you use in the concrete mix has not passed its shelf life, otherwise, the required strength results will not be attained.
- When you mix concrete with hand, make sure the surface where mixing is carried out is levelled and non-porous.
Frequently Asked Questions
What is a conventional concrete mix?
Concrete mixes can be tailored for use by adding chemical admixtures, supplementary cementitious materials, or proportioning the ingredients in a certain unconventional way. However, the general mix we refer to as ‘concrete’ only includes cement, sand, coarse aggregates and water. Therefore, for ordinary purposes, this type is used and is therefore, the conventional concrete mix.
What is meant by batching of concrete?
Batching of concrete means to proportion the ingredients of the mix in order to get the target characteristics. This includes weighing the required quantity of cement, coarse aggregate, fine aggregates and water required for a particular quantity of concrete.
Batching may be done by weight or by volume, however, the former is preferred and is more accurate.