|Batching of Cement Concrete:|
Batching of concrete is the process of asuring concrete mix ingredients either by volume method or by mass calculation and introducing them into the mixture by proper scientific way. In tradition way or productionpurpose batching is done by volumetric process but most of the time specifications require that the batching process is to be done by mass calculation rather than volume calculation. Accuracy for measurement of concrete materials/ingredients is as mentioned belows:
Cement /Binder preparation:
When the quantity of cement or binder to be batched which exceeds 25% of scale capacity or quantity, the measuring accuracy of concrete should be within 1% of required mass of cement. If the measuring quantity of cement concrete is less than 25% i.e. for smaller quantity of batches then the measuring accuracy should be within 3% of the required quantity of cement.
Aggregates(Coarse /Fine aggregate) :
If the measurement of aggregate is more than 25% of the scale capacity or quantity then the measuring accuracy of aggregate should be within 1%. If measurement quantity is less than 25% then the measuring accuracy of aggregate should be within less than 2%.
Water can be measured in volumetric quantity as 1 litre = 1kg or in mass quantity. In case of added water content, the measuring accuracy of water should be within 1%.
Admixtures/ Additive compound:
In case of mineral type of admixtures same accuracy as that required for cement or binder material.For chemical type of admixtures( poly carboxylate based) purpose the same accuracy as that required for water content. In case of use of mineral admixtures the accuracy is same as that of cement or binder because it is used as partial replacement or supplement of cement material. As chemical admixtures are used in liquid form or added to the water therefore its measurement of accuracy is same as that of water content.
Mixing operation of cement concrete:
A good mixing operation of cement concrete consists of rotation or stirring, the objective being to coat the surface the all aggregate particles with cement paste or binder paste, and to rotate all the mixing ingredients of the cement concrete into a uniform mass quantity; this uniformity must not be disturbed or turbulated by the process of discharging from the mixer pan.
The general type of mixer is a batch type mixer, which means that one batch of concrete is mixed properly and discharged the mixing ingredients material before any more materials are put into the mixer. There are basically four types of batch mixer.
1. Tilting drum mixer:
A tilting base type concrete drum mixer is one such type drum mixer in which mixing take place is tilted for discharging. The drum is conical shaped or bowl shaped with consisting internal vanes, and the discharge is so much rapid and unsegregated condition so that, these mixers are very much suitable for concrete mixes of low workability and for those containing large sizeof aggregate.
2. Non tilting drum mixer:
The non tilting type drum is one in which the central axis of the mixer is always kept horizontal, and discharge of cement concrete take place by inserting a chute into the drum or by reversing the direction or rotation condition of drum or mixer. Because of slow rate of discharge of concrete, some segregation may occur of mixed concrete.
3. Pan/Bowl type mixer:
The pan type mixer is a forced–action type mixer, it is basically as distinct from drum type mixer which relies on the free fall of the cement concrete inside the drum. The pan type mixer consist of a circular,cylindrical pan rotating about its vertical axis with one or two stars paddles rotating about vertical axis of pan of the machine.
Dual drum mixer:
A dual drum type of mixer is sometimes used in highway construction purpose. Here are two drums exists in series condition, now cement concrete being mixed part of the time in one drum and then being transferred to the other drum for the remainder of the mixing time before discharging the concrete.
In these type of mixer are automatically by a continuous weigh-batching systematic process.
Charging of the mixer:
There are no such specific generalized rules and process in the order of feeding the mixing ingredients into the pan mixer as this depend on the properties of the mixer and mix quality. Generally a very small quantity of water is added first in the mixer, followed by all the solids materials and ingredients. If it is possible maximum part of the water should also be fed during the same time of mixing, the remainder material being added after the solids inclusions. However during mixing, when using very dry type mixes in the drum mixers it is very much necessary to add the coarse aggregate just at that time after the small quantity of initial water added in order to ensure that the added aggregates surface is sufficiently wetted condition .Uniformity of Mixing in any kind of mixer, it is very much essential that a sufficient quantity of interchange of materials or ingredients occurs between parts of the chamber portion, so that as a result a uniform concrete is produced properly. The mixing efficiency of the pan mixer can be thoroughly measured by the variability of the mixing samples from the overall mix.As per ASTM prescribtion the samples are to be taken from about points 1/6 and 5/6 of the discharge point of the batch and the difference in the physical properties of the two different samples should not exceed any of the following as mentioned below,
Density of cement concrete 1 lb/ft³
Air content is 1%
Slump value 1″
whenever average is less than 4″1.5″
when average is less than 4 to 6″% of aggregate retained on 4 No. sieve 6%Compressive strength of concrete 7 day,
3 cylinders 7.5%
Mixing time of cement concrete:
It is most important to know the minimum mixing time as necessary to produce a mass concrete of uniform integrated composition/combination, and of reliable strength value. The adequate mixing time or period should be measured from time all the cementing/binding materials and aggregates are inside mixer drum till taking time out the concrete.
Overall mixing time of concrete positively or totally depends on the type of mixer and size of the mixer drum, on the speed of rotation of mixer, and on the overall quality of blending and mixing of ingredients or materials during charging of the mixer pan. In generally, a adequately mixing time of less than 1 to 1.28 minutes, total produces of mixing is appreciable non-uniformity in composition of concrete and a significant value of lower strength; mixing time which beyond 2 minutes already causes no significant enlightment or improvement in these properties.
Prolong type mixing:
If mixing of concrete continue over a long period, the evaporation of water from the mixes may occur, with a consequent decrease in workability of concrete and an increase in strength of concrete. Now secondary effect is grinding of the aggregate, particularly effectively if soft; the grading thus becomes very finer and the workability lower condition. In case of such air entrained type of concrete, a prolong mixing of concrete reduces the percentage of air content.
Ready mixed concrete:
Apart from batched and mixed concrete on site condition, the different concrete whenever is delivered for placing from a central plant and also ready to use is called ready mix concrete. It is now referred to as ready-mixed or pre-mixed concrete. This special type of concrete is used with extensively abroad as it offers various advantages in comparison with other methods of manufacture of controlled concrete :
Close quality control of batching plant which reduces the variability or change of the desired properties of hardened concrete. RMC generally use on congested construction sites or in highway construction where there is little/narrow space for a for set up a site mixing plant and aggregate stockpiles.
The use of agitator or rotating type of trucks carry the ready mix concrete to ensure take care in transportation of concrete, thus all type of prevention segregation of concrete and also maintaining workability convenience whenever small quantities of concrete or intermittent placing of concrete is required.
There are two categories of ready-mixed concrete:
central plant-mixed and transit mixed or truck mixed concrete one such type of concrete.
In the case of first category, all mixing of ingredients is done in a central concrete plant and then Concrete is transported in an agitator truck or transit mixer.
In the case of second category, the ingredients materials are batched at a central concrete plant but are mixed in a truck or transit mixer.In case of concrete Placing and compaction of concrete the operation of placing and compaction are totally interdependent and are carried out simultaneously at site level. They are the most important for the purpose of ensuring the general requirements of concrete strength, concrete impermeability and the durability of hardened concrete in the actual concrete structure.
As per placing is concerned, the main objective is to deposit accumulate the concrete as near as possible to its final position or destination so that the segregation can be avoided and the concrete can be fully compacted in side of form work. One of the main aim of production of good concrete placing can be stated with quite simply.
It is established to get that the concrete into such a position at a at a certain speed, and in a controlled condition, that allow it to be compacted concrete properly.
To achieve proper placing of concrete following rules and norms should be kept in mind:
The cement concrete should be placed in uniform thickness and layers, not in large heaps or sloping condition of layers.The thickness of the concrete layer should be sufficiently compatible with the method of vibration of concrete so that the entrapped air can be easily removed from the bottom of each layer.The rate of placing and of compaction of concrete should be equal in all respect. If it proceed too much slowly, the mixes may stiffen so that it is no longer sufficiently workable condition. On the other side water ever be added to concrete that is setting. It need to be kept in mind that , if too quickly done, it might be race ahead of the compacting gang or labour, to making it impossible for them and to do their job properly.Each and every layer of concrete should be fully compacted before placing over the next one layer, and each subsequent different layer should be placed while the underlying layer on concrete is still plastic condition, so that monolithic construction of structure is achieved.The collision between concrete and the formwork or reinforcement should be avoided as much as possible.In case of deeper sections, a long downward pipe always ensures the accuracy of location of concrete and minimum segregation of concrete.Once the concrete has been placed properly, it is ready to be compacted now . The actual purpose of compaction of concrete is to get minimise of the air voids or air bubbles that are trapped inside the loose concrete.
The necessity of Compaction:
It is very much important to compact the concrete fully and adequately because:
Air voids and bubbles reduce the strength of the concrete. For every 1% of entrapped air content, the strength of concrete falls down by somewhere between 6 and 8%. It means that the concrete containing a more 6% air voids due to incomplete compaction may lose as much as one third of its actual normal strength.Air voids increase concrete’s permeability value. Also it reduces its durability of concrete. If the concrete is not properly dense condition and impermeable condition, it will not be watertight condition.Moisture and air content are more likely to penetrate into the reinforcement and causing it to rust and corrosion. The required bond strength will not be achieved and the reinforced steel member will not be as strong as in good concrete condition.Air bubbles and voids produce blemishes on struck surfaces discretely. For example, blowholes and honeycombing might occur in such condition. So fully compacted concrete is dense than other, strong and durable also; badly compacted concrete will be more porous, weak and prone to rapid deterioration than good concrete. Stiff or hard mixes contain far more air than workable concrete. It is one of the main reasons why at low-slump value concrete requires more compactive force than one with a higher slump concrete – the compaction of concrete needs to continue for a longer time period, or more equipment has to be used during compaction.
The small air bubbles that are entrained are comparatively small and spherical in shape, also increase the workability of the mixes, reduce the bleeding, and increase frost resistance condition. In case of entrapped air on tends to be irregular in shape and is detrimental to the strength of the mix and concrete. It is require to remove this air content that the concrete must be properly compacted.
Methods of Compaction of cement concrete vibration:
To compact concrete apply the energy to it so that the mixes becomes more fluid condition. Air trapped in it can then rise up to the top and escape quickly. As a result of that, the concrete becomes consolidated easily, and left with a good dense material , after proper curing of concrete, develop its full strength value and durability stage.
Vibration is the easiest and quickest method of supplying the energy into the concrete. In case of manual techniques such as rodding r tampering are only suitable process for small projects. Various types of concrete vibrator are available at market for use on site concreting .Paker Vibrators poker, or immersion type , vibrator is the most popular and well known of the appliances used for compacting of concrete. Because it works directly inside the concrete and can be moved around easily all portions.
Sizes of vibrator:
Nozzles with diameters ranging between 25 to 75mm are easily available, and these are the suitable for most cases in reinforced concrete work. Larger size pokers or nozzle are also available – with diameters up to 150mm – but these types are for mass concrete in heavy civil engineering construction work.
Area of action:
When a nozzle vibrator is under operating condition, it will be effective over a circular centred area on the poker or nozzle. The actual distance from the poker to the outer edge of the circle is known as the radius of action of nozzle. However, it observed that the actual effectiveness of any nozzle depends on the workability condition of the concrete and the characteristics of the vibrator machinery itself also. In generally , the bigger size of the nozzle and the higher its amplitude value, the greater will be the radius of action area. It is always better to judge from civil engineers own observations, as concreting work proceeds on site, the effective radius of the nozzle are operating on the concrete which are compacting. The length of time it takes for a electric or diesel operated vibrator to compact concrete fully depends on the workability condition of the concrete. The lesser workable mix, the longer it must need to be vibrated.