Tag: Concrete

Hand Mixing Of Concrete

Hand Mixing Of Concrete

The process of mixing the various ingredients of concrete by manual labour, is called hand mixing of concrete. Hand mixing of concrete may be resorted to at small projects which require limited quantity of concrete. Now-a-days machine mixing of concrete is almost done and only in exceptional cases, mixing concrete by hand may be permitted. It may be noted that in hand mixing more cement is required as compared to machine mixing of concrete for obtaining the concrete of same strength. Method of mixing concrete by hand may also be necessitate if the mixer fails to work due to mechanical defects and concreting operation is to be completed. The hand mixing of concrete may be done as detailed below:

hand mixing of concrete

(i) Depending upon the quantity of concrete to be mixed, either construct a suitable platform of bricks with lean concrete or obtain iron sheets of suitable size.

(ii) Spread the required quantity of fine aggregates (sand) on the platform and then spread the required quantity of cement uniformly over the sand layer.

(iii) With the help of shovels, mix the sand and cement in dry state till a mixture of uniform colour is obtained.

(iv) Spread the sand cement mixture once again uniformly on the platform and then spread the required quantity of the coarse aggregates and mix the mixture in dry state thoroughly.

(v) On obtaining a uniform coloured mix with sand, cement and coarse aggregates, make a depression in the centre of the mixed materials.

(vi)  Add 75% of the required quantity of water (based on the water-cement ratio) in the depression and turn the mix towards the middle, with the help of hands or shovels. The mixture is mixed thoroughly till a uniform colour and consistency is obtained.

(vii) Add the remaining quantity of water and continue the mixing process till a concrete of uniform consistency is obtained.

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Guniting or Shotcrete or Shot Concrete

Guniting or Shotcrete

Guniting or Shotcrete: An intimate mixture of cement, sand (or fine aggregate) and water is forced or ejected through a cement-gun and shot into place by means of compressed air. The usual equipment consists of a compressor, spray nozzle and flexible hose pipe. At the end of the hose there is a nozzle to which water under pressure, by a separate connection, is added to form a slurry. Uniformly graded, thoroughly mixed dry materials, with low water/cement ratio, are charged into the gun and shot under pressure by compressed air. Slightly moist sand works better. The usual proportions of cement and fine aggregate are 1:3 or 1:4. The fine aggregate should be well graded up to a maximum size of 10 mm usual size is 4.75 mm downwards. Hard-stones sand should be used. Just only sufficient water necessary for the hydration of cement is used.

There is usually 20 to 30 per cent “rebound” depending upon the wetness of the mixture. A very wet mixture will not stick. While shooting, a nozzle under normal conditions is held at a distance of about 75 to 90 cm from the working face. The surface to be treated must be thoroughly cleaned of any dirt, grease or loose particles and should be fully wetted. The correct No. of gun should be obtained for the maximum size of aggregate or sand to be used. Reinforcement, usually of 80 mm sq. mesh, may be incorporated to withstand structural or temperature stress.

Guniting or Shotcrete

This method is very useful for rehabilitating or reconditioning old concrete, brick or masonry works which have, deteriorated either due to climatic condition or inferior work. It is also used for water-proofing exposed concrete surface or for resisting water pressure on pipes, cisterns, etc., where it forms a very impervious layer.

The bond between old bad concrete with corroded reinforcement and guniting or shotcrete is not assured very effective always, it is sometimes advisable to replace total concrete. Grouting or pressure grouting has to penetrate to some extent into the porous concrete surface exposed. Effectiveness of their surface is considered to last for about five years only.

For bad concrete and corroded reinforcement, clean all the exposed concrete surface with water (in addition to the reinforcement as explained above). Various chemicals with different trade names are now available in the market which can be applied on the old concrete surface for bonding of the old and the guniting or shotcrete. If surface to be concreted is more than 25 mm, expanded metal or wire-mesh can be used.

Advantages of Guniting or Shotcrete:

  1. High Compressive strength of concrete is obtained, a strength ranging from 50 to 65 N/sq. mm is obtained at 28 days (If proper curing is done)
  2. Permeability of concrete is reduced to a greater extent.
  3. Repairing of concrete can be done in a short period of time.

Diasadvantages of Guniting or Shotcrete:

  1. Skilled workmanship required
  2. Machine/Equipment is heavy.


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Cold Weather Concreting- Concreting in Cold Weather

Cold Weather Concreting

During Cold weather concreting, the rate of hardening and setting of concrete is very much retarded when the temperature falls below 21 deg. C. (70 deg. F.). At about 10 deg. C. (50 deg. F.) the action of setting slows down to about one-half of what it is at 21 deg. C. In addition to the slowing down or stopping of hydration and hardening there is also danger of disintegration of unset concrete due to the disruptive effect set up by the expansion of the mixing water as it freezes.

During cold weather concreting shall be abandoned when the temperature falls below 4.5 deg. C, (40 deg. F.). (Use immersion thermometer inserted in concrete near forms or surface for recording temperature.) however, the work is of urgency or importance that must be continued, it can be carried out with complete success provided certain remedial measures and precautions are taken.

The most convenient method is to heat the mixing water and, for very low temperatures to heat the aggregate as well. Heat the mixing water to 66 deg. C. (150 deg. F.). On no account shall the hot water be added to cement alone. Aggregates may be heated to 21 deg. C. Mixer drum may also be warmed. Cement must not be heated when used for cold weather concreting.

Temperatures of fresh concrete exceeding 21 deg. C. (70 deg. F.) are undesirable due to the higher water requirement, and likelihood of cracking when the concrete contracts on cooling, and relatively low strength. For most constructions, the right temperature of concrete at placement is somewhat below 21 deg. C. Concrete with a low water/cement ratio is less liable to damage by frost, and for good resistance to frost it is considered that the average water/cement ratio should not exceed 6.60 (30 litres per 50 kg of cement).

Cold Weather Concreting

Fresh concrete must not be allowed to freeze. If concrete is frozen, setting and hardening ceases. Avoid the use of frozen aggregate. The concrete placed shall be protected against frost by suitable covering. Concrete damaged by frost shall be removed and work redone.

Provide layers of straw of other insulating material on the freshly laid concrete surface as soon as the concrete is hard enough to sustain it without detriment. An insulating layer for covering concrete may be composed of waterproof paper overlaid with a layer of straw and finally with second layer of waterproof paper.

In frosty or other adverse weather conditions, use of colloidal concrete may be considered,

An increase of cement content of the mix by about 20 to 25 per cent, use of rapid hardening cement with an admixture of calcium chloride or, high alumina cement are usually recommended. With high alumina Cement concreting can proceed without any further precautions provided that the temperature is not at freezing point or below and the materials are not frozen.

“Accelerators” are used in cold weather concreting to increase the rate of hardening and thereby reduce the likelihood of failure. They accelerate the hydration of the cement and increase the rate of evolution of heat ; thus the temperature of the concrete is raised and the freezing point of the mixing water is lowered, enabling concreting to be carried out when the air temperature is near or slightly below freezing point.

As far as practicable, the use of accelerators or admixtures should be avoided.

Calcium chloride is the most commonly used material for accelerating hardening of the concrete and is perhaps the most reliable, which may be used up to 2 per cent max : (prefer 1.5 per cent) of the weight of cement. Quantity in excess of this proportion is harmful.

In no circumstances should this chemical be added to high alumina cement. Calcium chloride is a white deliquescent and hygroscopic salt commercially available at low cost in flakes or granular form and delivered in moisture proof bags or airtight drums, and should be stored in a dry place. It is dissolved in the mixing water to which cement is added after wards. Calcium chloride should not be placed in contact with water or mixed dry with aggregate. Calcium chloride shall not be used where reinforcement is provided in the concrete.

The use of calcium chloride approximately halves the setting time; the concrete must be placed in position and finished with the minimum of delay because of the rapid setting.

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Cold weather concreting

See also Hot Weather Concreting

Shoring and formwork


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Hot Weather Concreting – Concreting in Hot Weather

Hot Weather Concreting

Hot Weather Concreting in tropical countries, such as India, air temperatures may rise up to 40-50 deg. C. during summer months. Such temperatures combined with high wind velocity and/or low humidity enhance the rate of evaporation of the water, increase the rate of hydration of cement, which factors reduce the Setting time of the concrete considerably and this is apt to effect adversely the properties of hardened concrete. Concrete placed at high temperatures is prone to excessive contraction due to rapid evaporation of water; there is increased drying shrinkage, which tend to develop cracks soon after placing, even before hardening is complete.

Even though water forms about 1/6 to 1/8 the total weight of concrete, the role it plays, especially during summer months, is of much significance the quantity of water used and its temperature. The quantity of mixing water in a unit of concrete is decided primarily by the work ability desired, besides the role of the maximum size of aggregate. To obtain higher workability more water is required. Due to accelerated hydration of cement and loss of water by evaporation, the concrete is prone to lose workability and to maintain the required consistency water content will have to be increased. But merely increasing the water Content without increasing the cement content will result in decreased strength and durability. Drying shrinkage of concrete is directly proportional to water content. Rapid loss of workability makes compaction difficult. As such, mixing and placing of concrete at high temperatures may not develop its full strength. As for as possible cold water should be used for concreting in hot weather. Rapid hardening cement is not suitable for hot weather concreting.

hot weather concreting

When high temperatures prevail during the summer, it is best to abandon concrete work. Hot weather concreting shall be avoided at temperatures exceeding 38 deg. C. Aggregates shall be stacked under shade and sprinkled with cold water immediately before use. As far as possible all mixing should be done under shade, and mixing time should be the minimum that will ensure quality. Every effort should be made to lessen the time elapsing between mixing and placing. The mixer may be painted white on the outer Side to lessen absorption of heat from sun and air.

Protect the concrete from exposure to sun and drying hot winds with wet gunny bags or hessian cloth as early as possible. In the case of flat surfaces, it may be convenient to use a gabled framework supported across the side-forms and covered with hessain, tarpaulins, or strawmats (which can be made traveling stands). Water curing should be started as soon as possible by ponding.

Setting time Retarding Admixtures:.  Addition of 0.05 percent of sugar by weight of cement, has been found to retard the setting time of concrete by about two hours in outdoor hot weather condition of 44 deg. C. temperatures. For the same water-cement ratio, workability and strength are also improved. For equal strength, the sugar admixed concrete would enable a leaner mix to be used, resulting in about four per cent saving in cement. Over dosage of sugar is harmful. Sugar is added to the mixing water.

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Hot weather Concreting

Concrete Shrinkage – Shrinkage of Concrete

Concrete Shrinkage – Shrinkage of Concrete

Concrete shrinks during setting and drying due to hydration of cement and produces shrinkage cracks. The drying shrinkage increases with an increase in cement content or an increase in water content. Shrinkage is greater with richer mixes (more of cement) and also with aluminious cements. Other things being equal, shrinkage of concrete is almost directly proportional to the amount of-:water in the mix. The type of aggregate used does not generally affect the shrinkage seriously though it has an indirect effect due to the difference of water/cement ratio depending on the type of the aggregate ; with large size of aggregate shrinkage is low. Where shrinkage may give rise to high tensile stresses such as in road slabs, lean dry mixes are desirable. Rich mixtures are uneconomical and are used only for impermeable constructions to ensure water-tightness.



Slump Cone Test and its Advantages – Limitations

Slump Cone Test and its Advantages – Limitations

Although, the slump cone test is not entirely satisfactory since it gives widely varying results and also does not give a true measure of workability but it is of value in the field as a control test and is useful in comparing the consistence of successive batches of concrete made with the same ingredients and is one of the simplest tests to carry out. Provided no change is made in the aggregate or its grading, slump cone tests will indicate whether correct water and cement contents are being maintained. For a given slump and aggregate grading, the water required for unit volume of concrete is constant irrespective of the change of cement content. The amount of slump depends not only on the amount of water in the mix but also on the nature of the aggregate ; rounded stones give a greater slump than angular stones for the same mixture.

The slump cone test should not be used to compare mixes of different proportions or of different types of aggregates. This test is not applicable to lean dry mixes where the water/cement ratio is low as the slump recorded is very small. All aggregates of size 50 mm and above should be removed from the sample concrete before the test.

Slump Cone Test

The apparatus for determining the stump (slump cone) is a steel mould in the form of a truncated cone. Its top diameter is 10cm, the bottom diameter 20 cm, and the height 30 cm, open at both ends and fitted with handles and foot pieces on sides. The cone is placed on a smooth non absorbent surface and freshly mixed concrete is placed in the mould in four successive layers, each layer being rodded 25 times with a bullet-pointed rod 16 mm in diameter and 60 cm long. When tilled to top (after ramming) and top struck level, the mould is immediately with drawn and the slump or subsidence of the concrete measured from a straight edge held across the top of the mould.

“Slump” is the vertical settlement of the concrete after the mould has been withdrawn, i.e., the difference between the height of the mould and the highest point of the subsided concrete.

Slump Cone Test Table

A concrete with 0 to 25 mm slump has very low degree of workability, and with 100 to 175 mm slump a high degree of workability which is not normally suitable for vibrations.

Advantages of Slump Cone Test:

1. It is helpful in detecting the difference in water content of different batches of concrete of the same identical mix.

2. The slump cone apparatus is cheap, Convenient to handle, and moreover it is portable.

Limitations of Slump Cone Test:

1. It is not suitable for concrete in which maximum size of the aggregate exceeds 40 mm.

2. There is no direct relationship between the workability and the value of slump.

3. Different shapes of slump may occur and it is difficult to decide which is the correct value.

4. It is not suitable for dry mixes.

Concrete Bleeding – Bleeding of Concrete

Concrete Bleeding – Bleeding of Concrete

Concrete bleeding is the appearance of a watery scum (also called laitance) on the surface of a concrete after compaction. It is- an indication that there is too much water or deficiency of fine material in the mix, or that too much tamping, floating or troweling has been done. The result is a porous, dusty and weak surface. This scum should be removed. Bleeding makes weak joints between successive lifts in structural work. Concrete bleeding can be reduced by using less water, a finer sand, or by adding a finely ground inert material (stone dust).

The aggregate commonly used are seldom found in a perfectly dry state in the field. Moreover, aggregates have to be washed very often for removing impurities which further add to the moisture content. The moisture content varies considerably from time to time with the changing weather conditions, and this is especially so in the case of sand. The aggregate when dry will absorb water from the concrete and when wet at the surface the mixture will, have excess of water. Therefore, while computing the quantity of water due consideration must be given to the surface conditions of the aggregate that would exist at the time of preparing the mix.

Small size of aggregate need more water than big size and angular aggregate need more than rounded aggregate. In other words, a concrete containing a finely graded aggregate will require more water for a given workability than one containing an aggregate with a coarser grading. Consequently, the more finely graded aggregate, or that containing a larger proportion of fine aggregate and similarly a concrete with angular aggregate will produce a weaker concrete.

Concrete Segregation – Segregation of Concrete

Concrete Segregation – Segregation of Concrete

It is the separating of the coarse aggregate from the rest of the mix or the separating of the cement-water paste from the aggregate. Segregation generally indicates poor aggregate grading or mix design. Segregation may occur in mixes which are too wet or too dry, and most frequently in under-sanded mixes.

Segregation can generally be reduced by altering the water or sand content or by using a finer sand. Even with a mix of satisfactory design, segregation may be caused by mishandling during transport, faulty placing or over-compaction. Segregation leads to lack of uniformity causing honey-combing which reduces the strength and durability of the structure.

If segregation occurs the larger particles of aggregate tend to move to the bottom and this causes undesirable variation of strength through the thickness of the slab.

Sea Water for Mixing Concrete

Sea Water for Mixing Concrete

It is advisable, as stated above, to use clean water fit for drinking purposes for making cement concrete. However, at places where sea water is available in abundance and potable water is costly, the sea water can be used for making cement concrete. The problem of using sea water for mixing concrete has to be studied from the following two aspects:

(1) Strength
(2) Corrosion of reinforcement.

(1) Strength:

Below table shows the analysis of average sea water. It contains about 3.50 per cent of dissolved salts. The approximate percentages of various salts are 78 per cent of sodium chloride, 15 per cent of magnesium chloride and magnesium sulphate and the rest 7 per cent of calcium sulphate, potassium sulphate, etc.

Sea water for mixing concrete

Now all chlorides tend to accelerate the setting of cement and to improve the strength of concrete in early stages. On the other hand, the sulphates tend to retard the setting of cement and to discourage the strength of concrete in early stages.

It is found that the net effect of these two contradictory actions is the fall in strength of concrete to the tune of about 8 to 20 per cent. Hence the sea water can be used for mixing cement concrete for structures where such fall in strength is permissible or where it is possible to correct the same by adjusting water- cement ratio, cement content in concrete, etc.

The sea water tends to develop dampness and efflorescence. Hence it can be adopted for concrete structures where finishing characteristics are not important or where persistent dampness of the surface is permissible.

(2) Corrosion of reinforcement:

It is found that the sea water does not lead to the corrosion of reinforcement, provided the concrete is dense and there is enough cover to the reinforcement.

The minimum cement content for concrete permanently under sea water should be 3 kN per m3 and the minimum cover over the reinforcement should be 75 mm. However it is not advisable to take the risk of corrosion of reinforcement for prestressed concrete and hence the sea water should not be used for making prestressed concrete.

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High Density Concrete

High Density Concrete

High density concrete is a concrete having a density in the range of 6000 to 6400 kg/cu.m. High density concrete is also known as Heavy weight concreteHigh density concrete is mainly used for the purpose of radiation shielding, for counterweights and other uses where high density is required. The high density concrete has a better shielding property, so that it can protect harmful radiations like X-rays, gamma rays, and neutrons.

Whether, High Density Aggregates are used to achieve High Density Concrete ?

Yes, of course. High density aggregates are used to achieve heavy weight concrete. Some of the high density aggregates are, barite, ferrophosphorus, limonite, hematite, ilmenite, magnetite, goethite, steel punchings, and steel  shots. The point to remember is that inorder to achieve this type of concrete, high fixed water content is required.

The selection of the above mentioned aggregates mainly depend upon the physical properties like bulk density, specific gravity, availablity of materials, and its expenses.  In order to achieve workablility, high density the aggregates should be free from dirt, oil or grease stains and other foreign matter. Or else, it will retard the hydration process and the effective bonding of particles. Some of the boron additions like colemanite, borocalcite are used in the prepartion of concrete to improve the shielding properties. These additions may affect the setting time of concrete so, trial mixes should be made and tested depending upon the suitability.

Properties of High Density Concrete:

High Density Concrete

If steel punchings and steel shots are used we can achieve concrete having densities greater than 5000 kg/cu.m

Proportioning, mixing and placing of High Density Concrete:

*  The mix proportions for these high density concrete is same as that of normal concrete.

* Conventional method of mixing and placing is used in high density concrete. The most important thing is to prevent overloading the mixer especially when heavy weight aggregates such as steels are used. Batch sizes should be reduced to 40 to 50% of the allowable mixer capacity. Also avoid excess mixing because it will result in workability and bleeding of concrete.

* Preplaced aggregate methods can be adopted when placing heavy weight concrete. In this method the aggregates are placed in forms , the appropriate grout made of cement, sand and water is pumped over the placed aggregates, so that they can fill the voids inbetween the aggregates. This method prevents the segregation of coarse aggregates also reduces drying shrinkage and helps us to achieve concrete of uniform density and composition.

* Puddling method can also be adopted. In this method, the mortar is placed in forms of 2″ thick and the coarse aggregates are placed over it and vibrated internally. Care should be taken that, the coarse aggregates are distributed evenly.

* Pumping of heavy weight concrete can be adopted only the height is limited. The heavy weight concrete cannot be pumped to larger distances because of their greater densities.

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