Transit Mixed Concrete

Transit Mixed Concrete

It is also called dry batched concrete because all the basic ingredients including water are charged directly into the truck mixer. The mixer drum is revolved fast at charging speed during the loading of the material and after that it continues rotating at a normal agitating speed. In this type of RMC also, three variations are possible as given below :

 (i) Concrete mixed at job site: While being transported towards the destination, the drum is revolved at a slow or agitating speed of 2 rpm but after reaching the site just before discharging the material, it is revolved at maximum speed of 12 to 15 rpm for nearly 70 to 100 revolution for ensuring homogeneous mixing.

(ii) Concrete mixed in transit: The drum speed is kept medium during the transit time, i.e. approximately 8 rpm for about 70 revolutions. After 70 revolutions, it is slowed down to agitating speed i.e. 2 rpm till discharging the concrete.

(iii) Concrete mixed in the yard: The drum is turned at high speed of 12 to 15 rpm for about 50 revolutions in the yard itself. The concrete is then agitated slowly during transit time.

Types of Ready Mix Concrete

Types of Ready Mix Concrete

There are three types of Ready mix and Concrete depending upon the mixing of the various ingredients as given below :

a) Transit mixed concrete

b) Shrink mixed concrete

c) Central mixed concrete

Methods of curing concrete

Methods of curing concrete

Various methods of curing are adopted in the field as given below :

(a)         Replenishing the lost water

(i)          By immersion in water

(ii)         By ponding

(iii)       By sprinkling

(iv)        Using saturated coverings, e.g. jute bags etc.

(b)         Preventing loss of moisture

(i)          Using curing compounds

(ii)         Using impermeable membrane coverings

Out of above methods, immersion may be the ideal method of curing but it is not practicable always. It is restricted to laboratories or pre-cast units where small units like cubes or PSC sleepers etc. can be immersed in water. Ponding is suitable only for flat surfaces but not for vertical or overhead surfaces. Sprinkling is the most commonly used method in the field but it requires a large quantity of  water. Saturated covering is a better method which requires less quantity of water. Curing compounds are effective for concrete with high initial w/c ratio. Impermeable membrane is also having the same effect as of curing compounds. Out of these methods, the most widely used method in the field is saturated coverings using jute bags.

Quality of Aggregates

Quality of Aggregates

The aggregate should be free from impurities like deleterious material, salt contaminations and alkali reaction. These are elaborated as under :

(a) Deleterious Material: Impurities and deleterious material like clay, silt, coal, lignite, mica, shale etc. interfere with the process of hydration and prevent bonding of aggregate with cement matrix.

(b) Salt contamination: This is the main problem mainly with sea dredged aggregate. These aggregates should not generally be used for PSC works. Even if these are permitted for PSC works, the limits should be as given in “Concrete Bridge Code”(CBC) :

As per CBC:

Limit of chloride – 0.04% by weight of FA or

 0.02% by weight of CA

Limit for sulphate – 0.4%by weight of Total Aggregate

(c) Alkali Reaction: Some aggregates contain active silica which may react with alkalies present in cement like sodium oxide, potassium oxide etc. Alkaline hydroxide derived from alkalies in the cement may attack these constituents and gels of “unlimited swelling” is formed which results in cracking. This problem can be overcome if:

1.Low alkali cement is used (cement with alkali content < 0.6%)

2.Ground Pozzolana is added to the cement so that Pozzolana react with alkalies before they attack aggregates.

Good Construction Practices

Good Construction Practices

In order to improve the workmanship of concrete, the following good practices be adopted in the field :

1. To prevent leakage of cement slurry through shutter plate joints, 10 mm thick sponge should be used.

2. Cover block should have the comparable strength, durability and permeability as that of main concrete.

3. Sequencing of concrete work should be finalized in advance so that the first layer does not set before the next layer is placed over it. This can be ensured by carefully planning the following items :

 (a) Capacity of mixers

(b) Initial Setting Time of cement

(c) Man power to be deputed

4. Cold joints should be avoided by forward flowing method of concreting.

5. Provide proper construction joint at the end of days work. Lattice of cement, which has risen up to the surface due to vibration, should be removed before next days’ concreting because it is highly permeable. It should be removed by wire brush and aggregates should be exposed without dislodging in such a manner that 1/3 of aggregates should be visible on the surface. This is called ‘Hacking of joints’. Never sprinkle cement slurry before next days’ concrete.

6. For important structures, full scale mock-up trial should be conducted to test the working of equipments, cover, behavior of plasticizers, workability and the final product.

Advantages of Ready mix Concrete

Advantages of Ready mix Concrete

1.        Reduction in cement consumption by 10-12% due to better handling and proper mixing. Further reduction is possible if mineral admixtures or cementations materials are used.

2.        Since RMC uses bulk cement instead of bagged cement, dust pollution will be reduced and cement will be saved.

3.        Conservation of energy and resources because of saving of cement.

4.        Environment pollution is reduced due to less production of cement.

5.        With better durability of structure, their overall service life of increase and there is a saving in ‘Life Cycle Cost’.

6.        Quality assurance due to mechanical handling and uniformity of processes.

7.        Eliminating or minimizing human error and reduction independency on labor.

8.        General benefits like speedy work, stability of structures etc.

  Ready mix Concrete is a tailor-made concrete which improves durability and sustainability. Instead of purchasing the raw materials by individuals and experimenting every time with handling and proportioning, it would be a far better idea to entrust all these activities to some expert supplier who is having a professional acumen.

Statistical interpretation of cube results

Statistical interpretation of cube results

A number of factors can influence the results of the cube test like material, their proportions, various processes like mixing, compaction and curing and finally the testing procedure of cubes. Even the results of different cubes cast from the same concrete at the same time, cured and tested in similar way may also show different results. Therefore, for evaluation of the test results, the help of statistical approach is required.

 When the test results of a large number of cubes, prepared under similar conditions, are plotted on a histogram, the results are found to follow a definite “bell shaped curve” known as “Normal Distribution Curve”. It will be further observed that a large number of cubes will have the strength near the mean value and the no. of cubes having more or less strength will reduce progressively depending upon the difference from the mean value. This deviation in strength represents quality control in the field.

Importance of Cube Test

Importance of Cube Test

Although cube strength does not represent the actual strength of concrete in the structure because of many reasons as explained in the forthcoming paragraphs, yet the importance of cube testing can’t be underestimated because of the following reasons:

(i)         It indicates the potential strength of the mix.

(ii)        It detects the variation in quality control at site.

(iii)       It helps in determining the rate of gain of strength of concrete.

(iv)        It helps in determining the time of removal of form work.

 However as explained above, the cube strength does not guarantee the same strength in the structure because of the following reasons:

 (i)          The concrete in cubes does not pass through the misadventures of transportation, placement, compaction and curing. All these processes are much superior in cube as compared to the structure.

(ii)         Its shape is different from the shape of structures. Therefore the shape factor also plays an important part in strength. The strength of the actual structure is approximately 2/3rd of cube strength for the same concrete.

Concrete Mix Design

Concrete Mix Design

It is a process of selecting suitable ingredients and determining their relative proportions with the objective of producing concrete of having certain minimum workability, strength and durability as economically as possible.

Design of a mix not only needs the knowledge of properties of all ingredients and the properties of concrete in plastic condition, but it also require wider knowledge and experience of concreting. Without experience, a person will be like a novice and inexperienced cook equipped with a recipe book. Good food and good concrete both can’t be produced without experience.

Types of Mix Design

A mix can be designed in two ways as explained below :

1. Nominal Mix

2. Design Mix

Durability of Concrete

Durability of  Concrete

Durability is defined as the capability of concrete to resist weathering action, chemical attack and abrasion while maintaining its desired engineering properties. It normally refers to the duration or life span of trouble-free performance.

Different concretes require different degrees of durability depending on the exposure environment and properties desired. For example, concrete exposed to tidal seawater will have different requirements than indoor concrete.

Concrete will remain durable if:

The cement paste structure is dense and of low permeability

Under extreme condition, it has entrained air to resist freeze-thaw cycle.

It is made with graded aggregate that are strong and inert

The ingredients in the mix contain minimum impurities such as alkalis, Chlorides, sulphates and silt

Factors affecting durability of concrete

Durability of Concrete depends upon the following factors

Cement content

Mix must be designed to ensure cohesion and prevent segregation and bleeding. If cement is reduced, then at fixed w/c ratio the workability will be reduced leading to inadequate compaction. However, if water is added to improve workability, water / cement ratio increases and resulting in highly permeable material.

Compaction

The concrete as a whole contain voids can be caused by inadequate compaction. Usually it is being governed by the compaction equipments used, type of form works, and density of the steelwork

Curing

It is very important to permit proper strength development aid moisture retention and to ensure hydration process occur completely

Cover

Thickness of concrete cover must follow the limits set in codes

Permeability

It is considered the most important factor for durability. It can be noticed that higher permeability is usually caused by higher porosity .Therefore, a proper curing, sufficient cement, proper compaction and suitable concrete cover could provide a low permeability concrete

Factors affecting the performance of Admixture

Factors affecting the performance of Admixture

The various factors affecting the performance of admixture are as given below :

1.   Type of super-plasticizer: The admixture will be more effective if molecular  weight of the super-plasticizer is high.

2.   Dosages: The quantity of admixture should be optimum. Excess of admixture may cause segregation or bleeding. It may also cause excessive retardation. The optimum dose should be estimated by trials.

3.   Compatibility with cement: All admixtures may not produce same results with different cements. Therefore before using any admixture, its compatibility with cement has to be established. Properties of cement like fineness, chemical-composition, C3A content etc. affect the performance of admixture. Therefore, trials have to be made before finalizing an optimum dose of admixture.

4.   Mix Design: All the constituents of mix affect the performance of the super-plasticizer as given below:

(a)Water: More water in the mix improves the physical interaction and dispersion of admixtures.

(b)Coarse Aggregate: Proportion and grading of CA influence the performance of admixture.

(c)Fine Aggregate: Proportion, grading and silt content also influence the performance.

(d)Cement: Its fineness, C3A content influence the performance. Higher C3A reduces efficiency of admixture.

(e)Other Admixtures: Presence of other admixtures also influence the performance of super-plasticizers.

Therefore proper trials before actual use are very vital for effectiveness of admixture.

5.   Other factors: Certain other factors like temperature and humidity at the time of concreting also affect the performance of the super-plasticizers. High temperature and less humidity lower the efficiency of admixture. Drum mixers are not considered ideal for mixing admixtures, instead we should use pan or compulsive shaft mixers.

Important observations regarding concrete curing

Important observations regarding concrete curing

(1)  Curing should be started earliest as possible.

(2)  For the portion of concrete which is covered with form work, the curing should be started as soon as the form work is removed.

(3)  On exposed surface, it should be started when concrete has sufficiently hardened such that it doesn’t get disturbed by curing.

(4)  Ensure uninterrupted curing. If it is discontinued for any reason, the reaction of hydration will be stopped permanently. The partial hydration makes the capillary pores discontinuous and water can’t enter the concrete even if the curing is started again.

(5)  High strength concrete should be cured at an early age.

(6)  There is a widespread belief that humid climate is sufficient and curing is not required in rainy season.

(7)  The person generally entrusted for curing is the most unskilled person. He doesn’t appreciate the importance of curing. In fact he believes that curing is a process of wastage of water time and money.

(8)  It can’t be made a measurable item in the contract.

Therefore the best and practical method to ensure proper curing is the education of the person who is responsible for curing. Once he understands the importance of curing, he would certainly ensure it.

Concrete Mixing

Concrete Mixing

The purpose of proper mixing is to ensure that mass should become homogeneous, uniform in color and uniform in consistency.

There are two types of mixing that are adopted in the field i.e. Hand Mixing and Machine Mixing

(a) Hand Mixing It is done manually and thus inferior to machine mixing as it can’t give homogeneous and uniform concrete.

(b) Machine Mixing It is an efficient and economical way of mixing in which homogeneous and uniform mixing can be ensured. It can further be divided into two categories as given below:

(i)Batch Mixing – The mixing of concrete is done in batches.

(ii)Continuous Mixing – The mixing is done continuously till the plant is working. All the ingredients are fed through screw feeder in a continuous manner. It is used in large works such as dams and bridges.

Proportioning

Process for Concrete Production Proportioning

It is the relative quantity of each ingredient to make the desired concrete. It is decided based upon the calculations of mix-design. The proportioning should be such that the resultant mass should be compact with minimum voids and the required strength should be achieved.

Production of Concrete

Production of Concrete

Production of concrete involves two distinct activities. One is related to ‘material’ and the other to ‘processes’. The material part is generally taken care by everybody, but the involved processes in the production of concrete are often neglected. Therefore no wonder that it is the ‘process’ which is responsible for good or bad quality of concrete. If we take care of processes, the quality of concrete will be improved automatically without incurring any extra expenditure as the major expenditure has already been made in procurement of material. In order to ensure the quality, it is very important to have a knowledge of each and every process.

Process for Concrete Production

The various process involved in concrete production are as given below :

1)   Proportioning

2)   Mixing

3)   Transportation

4)   Placement

5)   Compaction

6)   Curing

Concrete Aggregate

Concrete Aggregate

Aggregates give body to the concrete. They also reduce shrinkage and effect overall economy. Since aggregate is cheaper than cement, it is economical to put as much aggregates as practically possible. Not only the use of more volume of aggregate in concrete is economical, it also provides higher volume stability to the concrete.

Generally they occupy 60-70% of the total volume of concrete. At the same time the aggregates should be strong because the weak aggregates can’t make strong concrete and they may limit the strength of concrete. Therefore the selection of aggregate becomes very vital.

 Earlier aggregates were viewed as an inert ingredient of concrete but now their importance has been understood and these are no more considered inert. Their physical, chemical as well as thermal properties greatly influence the properties of concrete.

High alumina cement

High alumina cement

This cement has a very high rate of strength development. In this type of cement, up to 80% of total strength is achieved in one day only. Lime and bauxite are used as raw material for producing this cement. Because of large proportion of bauxite, this cement is called high alumina cement. The rate of heat generation is also proportionately high i.e. 2.5 to 3 times of rapid hardening cement. The form work should be stripped-off as early as possible to prevent built-up of high temperature.

This cement is resistant to sulphate attack due to inert quality of alumina gel and absence of Ca(OH)2.

Expansive cement

expansive cement

A cement which when mixed with water forms a paste that tends to increase in volume, after setting, to a significantly greater degree than Portland cement paste does; used to compensate for volume decrease due to shrinkage or to induce tensile stress in reinforcement. Classified as Type K: Contains anhydrous aluminosulfate burned simultaneously with a Portland cement composition, or burned separately when it is to be inter-ground with Portland cement clinker or blended with Portland cement, calcium sulfate, and free lime. Type M: A mixture of Portland cement, calcium aluminates cement, and calcium sulfate. Type S: A Portland cement containing a large computed tri calcium aluminates content, modified by an excess of calcium sulfate above usual optimum content.