Building construction Archives

2Aug, 2020

Why Does Concrete Need Reinforcement ?

Why we provide Reinforcement ? It is more important for all aspects. In this, we talk about why it is important and why we choose steel as reinforcement.

The purpose of providing reinforcement in R.C.C. is :

To take up all the tensile stresses develop in the structure.
Increase the strength of concrete sections.
To make the sections thinner as compare to plain concrete section.
To prevent the propagation of cracks develop due to temperature and shrinkage stresses.

Although steel is the most suitable reinforcing material yet rusting of steel is the biggest disadvantage. If possible is porous or if cover to the reinforcement is not sufficient, steel gets rust and loses strength.

Characteristics of a Reinforcing Material

The reinforcing material should have the following characteristics :

It should be cheap and easily available.
Also, have more durable.
It should be able to develop a good bond with concrete so that the stresses are easily transfer from one – material to another.
Should have high tensile strength.
It should have high modulus of elasticity.
It should have co-efficient of linear expansion nearly same as that of concrete so the temperature stresses are not develop.
Easy to cut bend or weld.
It should be free from loose mill scales, loose rust and coats of paints, oil, mud or other coatings which may destroy or reduce bond.

SUITABILITY OF STEEL AS A REINFORCING MATERIAL

Steel fulfills almost all the characteristics require for a reinforcing material. Hence it is the most suitable reinforcing material. Steel is use as a reinforcing material because of the following reasons :

It has high tensile strength.
Also, have highly elastic.
It can develop good bond with concrete as its co-efficient of expansion is nearly equal to that of concrete ( i.e., 11.7 * 10 /°C of steel and 9.9 x 10 /°C of concrete).
It is easily available in India.

FORMS OF STEEL REINFORCEMENT

Steel can be provided in the form of:

Mild steel plain bars conforming to IS: 432 (Part 1) – 1982
(6) High strength deformed steel bars conforming to IS: 1786 – 1985
(c) Hard-drawn steel wire fabric conforming to IS: 1566 – 1982
(d) Structural steel conforming to Grade A of IS: 2062 – 1992 .

Mild Steel Plain Bars.

Mild steel is the most ductile amongst all kinds of steel. The mild steel bars are plain round and hot roll bars. These bars have plain surface and have sufficient bond with concrete. Also, These bars need hooks and bends at their ends for anchorages. These bars are bent easily and are weldable. Mild steel has a definite yield point. Plain mild steel bars (Fe 250 grade) are less commonly used in reinforced concrete, because they have less strength (250 N/mm2 yield strength) and cost approximately the same as high strength deformed bars. However, they are use in practice in situations where nominal reinforcement is to be provide.

The mild steel bars can yield on overloading. It results in an excessive cracking in concrete, thus giving sufficient warning before failure. Due to higher ductility, mild steel bars can be economically used in blast and earthquake resistant structures.

High Yield Strength Deformed Bars (HYSD Bars).

These bars are obtain by subjecting the mild steel plain bars to cold working by tensioning and twisting. The cold working process increases both the yield as well as the ultimate strength of steel. As the name indicates the high yield strength deformed bars have much high yield point, but the yield point is not define. The yield stress or characteristic stress is given by 0.2% proof stress.

These bars have some type of projections (or deformations or lugs or ribs) on the surface which act as a key for the concrete and check the slipping of bars. The lugs generally have an average spacing not exceeding 0.7 dia.(where dia. – Nominal diameter of bar). The height of lugs is at least 0.04 dia. to 0.05 dia.

The lugs must be present around at least 75% of nominal bar perimeter. The deformation are place so that the angle to the axis of the bar is not less than 45 degree. Generally, longitudinal ribs are also present on the surface of the bar.

HYSD Bars of Different Grades

The different grades of HYSD bars are :

Fe 415 grade,
And, Fe 500 grade
Fe 550 grade

Fe refers to ferrous metal and the number following it refers to yield strength of steel in N/mm2.

Advantages of HYSD Bars.

Due to high yield strength, the amount of steel require is considerably reduce.
The twist or deform bars provide a better bond with concrete than mild steel bars. Due to increase bond strength. Deform bars do not need end hooks and require reduce length for overlaps etc. It results in further saving of steel.
During the twisting process the defects, if any, in mild steel are notice and they are reject. Thus, each bar is severely test for defects.

Disadvantages of HYSD bars

With increase in stress level the cracking tendency increase. Hence for Fe 500 or higher grade of steel it becomes necessary to use high grade concrete.
Due to reduction in tension steel the design shear strength of concrete is reduce.

12Jul, 2020

Testing of Hardened Concrete

Testing of hardened concrete plays an important role in controlling and confirming the quality of cement concrete works. Systematic testing of raw materials, fresh concrete and hardened concrete are inseparable part of any quality control programme for concrete, which helps to achieve higher efficiency of the material used and greater assurance of the performance of the concrete with regard to both strength and durability. The test methods should be simple, direct and convenient to apply.

How to determine the workability of concrete?

One of the purposes of testing hardened concrete is to confirm that the concrete used at site has developed the required strength. As the hardening of the concrete takes time, one will not come to know, the actual strength of concrete for some time. This is an inherent disadvantage in conventional test. But, if strength of concrete is to be known at an early period, accelerated strength test can be carried out to predict 28 days strength. But mostly when correct materials are used and careful steps are taken at every stage of the work, concretes normally give the required strength.

The tests also have a deterring effect on those responsible for construction work when testing of hardened concrete is done. The results of the testing on hardened concrete, even if they are known late, helps to reveal the quality of concrete and enable adjustments to be made in the production of further concretes. Tests are made by casting cubes or cylinder from the representative concrete or cores cut from the actual concrete. Knowledge of the strength of concrete in structure can not be directly obtained from tests on separately made specimens.

Compression Test

Compression test is the most common testing conducted on hardened concrete, partly because it is an easy test to perform, and partly because most of the desirable characteristic properties of concrete are qualitatively related to its compressive strength.

The compression test is carried out on specimens cubical or cylindrical in shape. Prism is also sometimes used, but it is not common in our country. Sometimes, the compression strength of concrete is determined using parts of a beam tested in flexure. The end parts of beam are left intact after failure in flexure and, because the beam is usually of square cross section, this part of the beam could be used to find out the compressive strength.

The cube specimen is of the size 15 x 15 x 15 cm. If the largest nominal size of the aggregate does not exceed 20 mm, 10 cm size cubes may also be used as an alternative. Cylindrical test specimens have a length equal to twice the diameter. They are 15 cm in diameter and 30 cm long. Smaller test specimens may be used but a ratio of the diameter of the specimen to maximum size of aggregate, not less than 3 to 1 is maintained.

Specimen (Cube and Cubical) under Universal Compression Testing Machine

Moulds

Metal moulds, preferably steel or cast iron, thick enough to prevent the distortion are required. They are made in such a manner as to facilitate the removal of the moulded specimen without damage. The angle between the adjacent faces and top and bottom planes of the mould is 90°. Each mould is provided with a metal base plate of 6.5 mm thick. Square mould should have internal distance 15 cm (+- 0.02 mm) between adjacent faces.

During assembling the mould, it is coated with thiny layer of oil. When assembled ready for use; the mean internal diameter of the cylindrical mould should be 15 cm (+- 0.02 mm). The height maintained is 30 cm (+- 0.1 mm). A steel bar 16 mm in diameter and 0.6 m long; bullet pointed at the lower end serves as a tamping bar.

Compacting by Hand

Tamping bar is used for compacting by hands. Concrete should be compacted by giving the uniform number of strokes per layer. For square mould; 35 strokes for 15 cm layer and 25 strokes for 10 cm layer of concrete in cubes. For cylindrical specimen, should have minimum 30 strokes per layer. Where voids are left by tamping bar, the sides of the mould are tapped to close the voids.

Compacting by Vibration

In this each layer is compacted by vibration. This is done by the means of an electric or pneumatic hammer or vibrator or vibrating table. If care is not taken care of then severe segregation takes place in the mould. Segregation results in low strength when cubes are crushed.

Curing

The test specimens are stored on the site at a place free from vibrations. It is stored in damp / moist place for 24 hours at 27°C (+- 2°C). After this period, the specimen is removed from mould. Then it is cured in fresh water for next 24 hours at 27°C (+- 2°C).

Making and Curing test specimen in the field

The test specimens are stored under damp matting, sacks or other similar material for 24 hours from the time of addition of water to the other ingredients. The temperature of the place of storage should be within the range of 22° to 32°C. After the period of 24 hours, they should be marked for later identification removed from the mould. Unless required for testing within 24 hours, stored in clean water at a temperature of 24° to 30°C until they are transported to the testing laboratory.

They should be sent to the testing laboratory well packed in damp sand, damp sacks, or other suitable material. So as to arrive there in a damp condition not less than 24 hours before the time of test. On arrival at the testing laboratory, the specimens are stored in water at a temperature of 27° (+- 2°C). Records of the daily maximum and minimum temperature should be kept both during the period the specimens remain on the site and in the laboratory particularly in cold weather regions.

Comparison between Cube and Cylinder Strength

It is difficult to say whether cube test gives more realistic strength properties of concrete or cylinder gives a better picture about the strength of concrete. However, it can be said that the cylinder is less affected by the end restrains caused by platens and hence it seems to give more uniform results than cube. Therefore, the use of cylinder is becoming more popular,
particularly in the research laboratories.

Cylinders are cast and tested in the same position, whereas cubes are cast in one direction and tested from the other direction. In actual structures in the field, the casting and loading is similar to that of the cylinder and not like the cube. As such, cylinder simulates the condition of the actual structural member in the field in respect of direction of load.

The points in favour of the cube specimen are that the shape of the cube resembles the shape of the structural members often met with on the ground. The cube does not require capping, whereas cylinder requires capping. The capping material used in case cylinder may influence to some extent the strength of the cylinder.

Flexural Strength of Concrete

Concrete as we know is relatively strong in compression and weak in tension. In reinforced concrete members, little dependence is placed on the tensile strength of concrete since steel reinforcing bars are provided to resist all tensile forces. However, tensile stresses are likely to develop in concrete due to drying shrinkage, rusting of steel reinforcement, temperature gradients and many other reasons. Therefore, the knowledge of tensile strength of concrete is of importance.

A concrete road slab is called upon to resist tensile stresses from two principal sources- wheel loads and volume change in the concrete. Wheel loads may cause high tensile stresses due to bending, when there is an inadequate subgrade support. Volume changes, resulting from changes in temperature and moisture, may produce tensile stresses, due to warping and due to the movement of the slab along the subgrade.

Stresses due to volume changes alone may be high. The longitudinal tensile stress in the bottom of the pavement, caused by restraint and temperature warping, frequently amounts to as much as 2.5 MPa at certain periods of the year and the corresponding stress in the transverse direction is approximately 0.9 MPa. These stresses are additive to those produced by wheel loads on unsupported portions of the slab.

Determination of Tensile Strength

Direct measurement of tensile strength of concrete is difficult. Neither specimens nor testing apparatus have been designed which assure uniform distribution of the “pull” applied to concrete. After number of investigations beam tests are found to be dependable to measure flexural strength of concrete.

The mould should be metallic i.e. steel or cast iron. The standard size of the specimen is 15 x 15 x 70 cm. Alternatively, if the largest nominal size of the aggregate does not exceed 20 mm; the size of the specimen 10 x 10 x 50 cm may be used. The tamping bar should be of steel, weighing 2 kg. It has length of 40 cm, ramming face 25 mm square.

Procedure

Test specimens are stored in water at a temperature of 24° to 30°C for 48 hours before testing. They are tested immediately on removal from the water whilst they are still in a wet condition. The dimensions of each specimen should be noted before testing. No preparation of the surfaces is required.

Specimen is placed in the machine in such a manner that the load is applied to the uppermost surface. Axis of specimen is carefully aligned with the axis of loading device. Load is applied without any shock and increasing continuously. It is applied at the rate of 400 kg/min for 15 cm specimen; and 180 kg/min for 10 cm specimen. Load is kept on increasing until the specimen fails. Maximum applied load is recorded.

Indirect Tension Test Methods

1. Cylinder Splitting Tension Test

This is also known as “Brazilian Test”. This test was developed in Brazil in 1943. This test is carried out by placing a cylindrical specimen horizontally between the machine. Load is applied until the failure of the cylinder, along the vertical diameter.

The main advantage of this method is that the same type of specimen and the same testing machine as are used for the compression test can be employed for this test. That is why this test is gaining popularity. Splitting strength gives about 5 to 12% higher value than the direct tensile strength.

2. Ring Tension Test

Another test for finding out the tensile strength of concrete is known as “Ring Tension test”. Briefly, a hydro-static pressure is applied radially against the inside periphery of 15 cm diameter, 4 mm thick and 4 mm high concrete ring specimen.

3. Double Punch Test

Yet another test to find out the indirect tensile strength of concrete is known as “double punch test”. In this test, a concrete cylinder is placed vertically between the loading plates of the compression test machine. And is compressed by the steel punches located concentrically on the top and bottom surfaces of the cylinder.

Test Core

The test specimen, cube or cylinder is made from the representative sample of concrete used for a particular member, the strength of which we are interested. As the member can not be in fact tested, we test the parallel concrete by making cubes or cylinders. It is to be understood that the strength of the cube specimen cannot be same as that of the member because of the differences with respect to the degree of compaction, curing standard, uniformity of concrete, evaporation, loss of mixing water etc. At best the result of cube or cylinder can give only a rough estimate of the real strength of the member.

core test of concrete — Core test of concrete

To arrive at a better picture of the strength of the actual member; attempts are made to cut cores from the parent concrete and test the cores for strength. Perhaps this will give a better picture about the strength of actual concrete in the member.

Core can be drilled at the suspected part of the structure; to detect segregation or honey combing; to check the bond at construction joint or to verify the thickness of pavement.

Strength of Cores

The reduction in strength of cores appear to be greater in stronger concrete. The reduction in the strength can be as high as 15 per cent for 40 MPa concrete. Generally, a reduction of 5 to 7 per cent is considered reasonable. It has been reported by man investigators that in situ concrete gains very little strength after 28 days. Tests on high strength concrete show that, although the strength of cores increase with age, the core strength, even up to the age of 1 year, remains lower than the strength of standard 28 day cylinders.

Non-Destructive Testing Methods

Non-destructive methods have been in use for about four decades. In this period, the development has taken place to such an extent that it is now considered as a powerful method for evaluating existing concrete structures, also with regard to their strength and durability apart form assessment and control of quality of hardened concrete when testing. In certain cases, the investigation of crack depth, micro-cracks, and progressive deterioration are also studied by this method.

Though non-destructive testing methods are relatively simple to perform, the analysis and interpretation of test results are not so easy. Therefore, special knowledge is required to analyses the testing of hardened properties of concrete. In the non-destructive methods of testing, the specimen are not loaded to failure. And as such the strength inferred or estimated cannot be expected to yield absolute values of strength. These methods, therefore, attempt to measure some other properties of concrete from which an estimate of its strength, durability and elastic parameters are obtained.

Based upon the above, various non-destructive methods of testing concrete have been developed:

1. Surface hardness tests:

These are of indentation type, include the Williams testing pistol and impact hammers, and are used only for estimation of concrete strength.

2. Rebound test:

The rebound hammer test measures the elastic rebound of concrete and therefore, is primarily used for estimation of concrete strength and for comparative investigations.

3. Penetration and Pull out techniques:

These include the use of the Simbi hammer, Spit pins, the Windsor probe, and the pullout test. These measure the penetration and pullout resistance of concrete and are used for strength estimations, but they can also be used for comparative studies.

4. Dynamic or vibration tests:

These include resonant frequency and mechanical sonic and ultrasonic pulse velocity methods. These are thus used to evaluate durability and uniformity of concrete and to estimate its strength and elastic properties.

5. Combined methods:

The combined methods involving ultrasonic pulse velocity and rebound hammer have been used to estimate strength of concrete.

6. Radioactive and nuclear methods:

These include the X-ray and Gamma-ray penetration tests for measurement of density and thickness of concrete. Also, the neutron scattering and neutron activation methods are used for moisture and cement content determination.

7. Magnetic and electrical methods:

The magnetic methods are primarily concerned with determining cover of reinforcement in concrete, whereas the electrical methods, including microwave absorption techniques, have been used to measure moisture content and thickness of concrete.

8. Acoustic emission techniques:

These have been used to study the initiation and growth of cracks in concrete.

9. Surfaces Hardness Methods:

The fact that concrete hardens with increase in age, the measure of hardness of surface may indicate the strength of concrete. Various methods and equipments are devised to measure hardness of concrete surface. William testing pistol, Frank spring hammer, and Einbeck pendulum hammer are thus some of the devices for measuring surface hardness.

Schmidt’s Rebound Hammer

Schmidt’s rebound hammer developed in 1948 is one of the commonly adopted equipments for measuring the surface hardness.

It consist of a spring control hammer that slides on a plunger within a tubular housing. When the plunger is pressed against the surface of the concrete, the mass rebound from the plunger. It retracts against the force of the spring. The hammer impacts against the concrete and the spring control mass rebounds, therefore taking the rider with it along the guide scale. By pushing a button, the rider can be held in position to allow the reading to be taken. The distance travelled by the mass, is therefore called the rebound number. It is indicated by the rider moving along a graduated scale.

Each hammer varies considerably in performance and needs calibration for use on concrete made with the aggregates from specific source. The test can be conducted horizontally, as well as vertically-upwards or onwards or at any intermediate angle. At each angle the rebound number will be different for the same concrete and thus will require separate calibration or correction chart.

Schmidt's Rebound Hammer — Schmidt’s Rebound Hammer

Limitation:

Although, rebound hammer provides a quick inexpensive means of checking uniformity of concrete, it has thus serious limitations and these must be recognized. The results are thus affected by:

Smoothness of surface under test.
Size, shape and rigidity of the specimen.
Age of specimen.
Type of cement.
Surface and internal moisture condition of the concrete.
Type of coarse aggregate.
Type of Mould.
Carbonation of concrete surface.

Following are more tests related to above, we’re not going to discuss all these in detail. So find the names of the relevant tests as follows –

Pullout Test
Dynamic or Vibration Methods
Resonant Frequency Method
Pulse Velocity Method (Mechanical sonic PVM and Ultrasonic PVM)
Combined Methods
Radioactive Methods
Nuclear Methods
Magnetic Methods
Electrical Methods

Testing on Composition of Hardened Concrete

Determination of cement content
Determination of the original water / cement Ratio
Physical Methods
Accelerated Curing Methods

14Jun, 2020

Top Down Construction

When the excavation its come from major cities, particularly in densely develop urban areas. In most common construction methods, buildings with underground parts are built by the bottom-up method where sub-structure and super-structure floors are constructed sequentially from the lowest level of basement to the top of the building. Though this conventional method, also called a bottom-up method, is simple in both design and construction, it is not feasible for the projects with limited construction time or with site constraints (size of location, legal issues, etc). The Top-down construction method is the solution to these problems.

Why Top Down Construction Require ?

Distance between Boundary wall of Adjacent Existing building and Building line for New building is too close. The depth of excavation is more than 6 meters. The possibility of collapse the soil is higher during excavation.
Using Top Down Construction more building and basement area can use. Suitable for two or more basement.
The first base slab level slab will act in such area as a strut member. The retaining wall panels will not allow collapse of soil.
Water table is too high. Difficult to cast Retaining wall, Diaphragm wall or Continuous Pile can use as a Retaining wall.

Methodology

Installing of Diaphragm Walls.
Installations of Barrette Piles.
Excavation of soil just below the roof slab.
Provide structural strut to support to provide Diaphragm wall.
Expose coupler Bars/Dowel bars from Diaphragm.
R.C.C. slab to be casting with suitable size opening for further construction.
After casting slab, excavation to continue through opening.
Excavation to be continue till base slab level.

Procedure of Top Down Construction

Setting out for alignment of bore cast in situ of pile.
Bore cast of in situ piles to be casting and dowels bar to be keep for retaining wall panels. The centre of piles and retaining wall panels centre line to be keep in same line. The Diameter of piles prefer 600 mm, distance between two piles centre to centre prefer 3.33 meters ( Suit for c/c distance of columns ). Depth of piles to be confirm by structure consultants.
Touching piles can use as a Retaining wall with supporting arrangement during excavation.
If area of basement is more and Span is less. Then, Diaphragm wall can be use as a Retaining wall.
Excavation to be do around 2 meter depth and Retaining wall panel to be casting in stages with dowels bars with adjacent piles and dowel bars for next retaining wall panel to be keep. If soil between piles collapse soil nails with wire mesh to be keep on earth side to retain the same. Retaining wall panels to be casting with dowels up to the first base slab bottom.

Dressing and compaction of soil to be do for PCC bed for casting of first base slab. 50 mm PCC (M15 Grade) to be do, the top levels of PCC to be keep the base slab level bottom. For beams excavation to be do in same alignment. Dimensions as require and PCC or brickwork to be do as per beam dimensions in vertical face.

After PCC 3 mm ply to be lay on PCC surface. PVC sheet to be nail with PCC surface so surface of PCC and Surface of slab concrete will separate.
Reinforcement to be complete as per drawings, for beams dowel bar to be use from Cast in Situ Piles. Dowel bars to be keep for adjacent slabs for Bottom Up Construction.
Completing of slab and beam to be complete.
After 14 days of concreting, PCC below slab to be break/remove excavation to be continue and retaining wall panels to be cat. Till final level of excavation.
Waterproofing to be do and base slab to be casting with dowels from Cast in Situ Piles.
Jacketing of columns to be do if require.

Advantages

The shorten construction period due to simultaneous construction.
More operational space gain from advance construction of floor slabs.
It is highly suitable for construction for tall buildings with deep basements to be construct in Urban areas.
These can be install in close proximity to existing structure with minimal loss of support to existing foundations.

Disadvantages

Higher cost.
The lateral displacement of retaining wall or ground settlement may possible.
The construction quality may influence.
It requires higher skill supervision and labour force.

How to test the strength of hardened concrete?

7Jun, 2020

Workability of Concrete

The quality of concrete satisfying the above requirements is termed as workable concrete. The word “workability” or workable concrete signifies much wider and deeper meaning than the other terminology “consistency” often used loosely for workability. Consistency is a general term to indicate the degree of fluidity or the degree of mobility. A concrete which has high consistency and which is more mobile, need not be of right workability for a particular job. Every job requires a particular workability. A concrete which is considered workable for mass concrete foundation is not workable for concrete to be used in roof construction, or even in roof construction, concrete considered workable when vibrator is used, is not workable when concrete is to be compacted by hand. Similarly a concrete considered workable when used in thick section is not workable when required to be used in thin sections.

A theoretical water/cement ratio calculated from the considerations discussed above is not going to give an ideal situation for maximum strength. Hundred per cent compaction of concrete is an important parameter for contributing to the maximum strength. Lack of compaction will result in air voids whose demaging effect on strength and durability is equally or more predominant than the presence of capillary cavities.

Factors Affecting Workability

Workable concrete is the one which exhibits very little internal friction between particle and particle or which overcomes the frictional resistance offered by the formwork surface or reinforcement contained in the concrete with just the amount of compacting efforts forthcoming. The factors helping concrete to have more lubricating effect to reduce internal friction for helping easy compaction are given below –

Cement content of concrete
Water Content
Mix Proportions
Shape of Aggregates
Size of Aggregates
Surface Texture of Aggregate
Grading of Aggregate
Use of Admixtures
Use of supplementary cementitious materials

Measurement of Workability

It is discussed earlier that workability of concrete is a complex property. Just as it eludes all precise definition, it also eludes precise measurements. Numerous attempts have been made by many research workers to quantitatively measure this important and vital property of concrete. But none of these methods are satisfactory for precisely measuring or expressing this property to bring out its full meaning. Some of the tests, measure the parameters very close to workability and provide useful information.

The following tests are commonly employed to measure workability –

(a) Slump Test
(b) Compacting factor Test
(c) Flow Test
(d) Kelly Ball Test
(e) Vee Bee Consistometer Test

(a) Slump Test

Slump test is the most commonly used method of measuring consistency of concrete which can be employed either in laboratory or at site of work. It is not a suitable method for very wet or very dry concrete. It does not measure all factors contributing to workability, nor is it always representative of the placability of the concrete. However, it is used conveniently as a control test and gives an indication of the uniformity of concrete from batch to batch.

Repeated batches of the same mix, brought to the same slump, will have the same water content and water cement ratio, provided the weights of aggregate, cement and admixtures are uniform and aggregate grading is within acceptable limits. Additional information on workability and quality of concrete can be obtained by observing the manner in which concrete slumps. Quality of concrete can also be further assessed by giving a few tappings or blows by tamping rod to the base plate. The deformation shows the characteristics of concrete with respect to tendency for segregation.

The appartus for conducting the slump test essentially consists of a metallic mould in the form of a frustum of a cone having the internal dimensions as under –

Bottom diameter : 20 cm
Top diameter : 10 cm
Height : 30 cm

The thickness of the metallic sheet for the mould should not be thinner than 1.6 mm. Sometimes the mould is provided with suitable guides for lifting vertically up. For tamping the concrete, a steel tamping rod 16 mm dia, 0.6 meter along with bullet end is used.

Details of the slump cone apparatus

The internal surface of the mould is thoroughly cleaned and freed from superfluous moisture and adherence of any old set concrete before commencing the test. The mould is placed on a smooth, horizontal, rigid and non-absorbant surface The mould is then filled in four layers, each approximately 1/4 of the height of the mould. Each layer is tamped 25 times by the tamping rod taking care to distribute the strokes evenly over the cross section. After the top layer has been rodded, the concrete is struck off level with a trowel and tamping rod.

The mould is removed from the concrete immediately by raising it slowly and carefully in a vertical direction. This allows the concrete to subside. This subsidence is referred as SLUMP of concrete. The difference in level between the height of the mould and that of the highest point of the subsided concrete is measured. This difference in height in mm. is taken as Slump of Concrete. ASTM measure the centre of the slumped concrete as the difference in height. ASTM also specifies 3 layers.

The pattern of slump indicates the characteristic of concrete in addition to the slump value, If the concrete slumps evenly it is called(true slamp lf one half of the cone slides down, it is called shear slump In case of a shear slump, the slump value is measured as the difference in height between the height of the mould and the average value of the subsidence. Shear slump also indicates that the concrete is non-cohesive and shows the characteristic of segregation.

(b) Compacting Factor Test

The compacting factor test is designed primarily for use in the laboratory but it can also be used in the field. It is more precise and sensitive than the slump test and is particularly useful for concrete mixes of very low workability as are normally used when concrete is to be compacted by vibration. Such dry concrete are insensitive to slump test. The diagram of the apparatus is shown. The essential dimensions of the hoppers and mould and the distance between them are shown.

Dimensions of compacting factor apparatus

The compacting factor test has been developed at the Road Research Laboratory U.K. and it is claimed that it is one of the most efficient tests for measuring the workability of concrete. This test works on the principle of determining the degree of compaction achieved by a standard amount of work done by allowing the concrete to fall through a standard height. The degree of compaction, called the compacting factor is measured by the density ratio.

The ratio of the density actually achieved in the test to density of same concrete fully compacted.

The sample of concrete to be tested is placed in the upper hopper up to the brim. The trap-door is opened so that the concrete falls into the lower hopper. Then the trap-door of the lower hopper is opened and the concrete is allowed to fall into the cylinder. In the case a dry-mix, it is likely that the concrete may not fall on opening the trap-door. In such a case slight poking by a rod may be required to set the concrete in motion. The excess concrete remaining above the top level of the cylinder is then cut off with the help of plane blades supplied with the apparatus. The outside of the cylinder is wiped clean. The concrete is filled up exactly upto the top level of the cylinder.

It is weighed to the nearest 10 grams. This weight is known as “Weight of partially compacted concrete”. The cylinder is emptied and then refilled with the concrete from the same sample in layers approximately 5 cm deep. The layers are heavily rammed or preferably vibrated so as to obtain full compaction. The top surface of the fully compacted concrete is then carefully struck off level with the top of the cylinder and weighed to the nearest 10 gm. This weight is known as “Weight of fully compacted concrete”.

The Compacting Factor = Weight of partially compacted concrete / Weight of fully compacted concrete

The weight of fully compacted concrete can also be calculated by knowing the proportion of materials, their respective specific gravities, and the volume of the cylinder. It is seen from experience, that it makes very little difference in compacting factor value, whether the weight of fully compacted concrete is calculated theoretically or found out actually after
100 per cent compaction. It can be realised that the compacting factor test measures the inherent characteristics of the concrete which relates very close to the workability requirements of concrete and as such it is one of the good tests to depict the workability of concrete.

(c) Flow Test

This is a laboratory test, which gives an indication of the quality of concrete with respect to consistency, cohesiveness and the proneness to segregation. In this test, a standard mass of concrete is subjected to jolting. The spread or the flow of the concrete is measured and this flow is related to workability.

It can be seen that the apparatus consists of flow table, about 76 cm. in diameter over which concentric circles are marked. A mould made from smooth metal casting in the form of a frustum of a cone is used with the following internal dimensions. The base is 25 cm. in diameter, upper surface 17 cm. in diameter, and height of the cone is 12 cm.

The table top is cleaned of all gritty material and is wetted. The mould is kept on the centre of the table, firmly held and is filled in two layers. Each layer is rodded 25 times with a tamping rod 1.6 cm in diameter and 61 cm long rounded at the lower tamping end. After the top layer is rodded evenly, thus the excess of concrete which has overflowed the mould is removed. The mould is lifted vertically upward and the concrete also stands on its own without support. The table is then raised and dropped 12:5 mm 15 times in about 15 seconds. The diameter of the spread concrete is measured in about 6 directions to the nearest 5 mm and the average spread is noted. The flow of concrete is the percentage increase in the average diameter of the spread concrete over the base diameter of the mould.

Flow percent = (Spread diameter in cm – 25) x 100 / 25

The value could range anything from 0 to 150 percent.

A close look at the pattern of spread of concrete can also give a good indication of the characteristics of concrete such as tendency for segregation.

Apparatus

1. Flow table

Flow table top is constructed from flat metal of minimum thickness 1.5 mm. However the top is in plan 700 mm x 700 mm. There is circle marked at center having 200 mm diameter. The front of top is thus provided with lifting handle. Total mass of flow table is about 16+-1 kg. It contains also hinges, two rigid stops, base frame etc.

2. Mould

The mould is made of metal readily not attacked by cement paste or liable to rust and of minimum thickness 1.5 mm. The interior of the mould is smooth and free from projections, such as protruding rivets, and is free from dents. The mould shall be in the form of a hollow frustum of a cone having the internal dimensions. The base and the top is open and parallel to each other and at right angles to the axis of the cone. The mould is provided with two metal foot pieces at the bottom and two handles above them.

3. Tamping Bar

The tamping bar is made of a suitable hardwood and having dimensions as shown in diagram.

Sampling

The sample of freshly mixed concrete is obtained.

Procedure

The table is made level and properly supported. Before commencing the test, the table-top and inner surface of the mould is wiped with a damp cloth. However the slump cone is placed centrally on the table. The slump cone is filled with concrete in two equal layers, however each layer tamped lightly 10 times with the wooden tamping bar. After filling the mould, the concrete is struck off flush with the upper edge of the slump cone and thus the free area of the table-top cleaned off.

Half a minute after striking off the concrete, the cone is therefore slowly raised vertically by the handles. After that, the table-top raised by the handle and allowed to fall 15 times in 15 seconds. However the concrete spreads itself out. The diameter of the concrete spread shall then be measured in two directions, parallel to the table edges. The arithmetic mean of the two diameters thus shall be the measurement of flow in mm.

(d) Kelly Ball Test

This is a simple field test consisting of the measurement of the indentation made by 15 cm diameter metal hemisphere weighing 13.6 kg. when freely placed on fresh concrete. The test has been devised by Kelly and hence known as Kelly Ball Test. This has not been covered by Indian Standards Specification. The advantages of this test is that it can be performed on the concrete placed in site and it is claimed that this test can be performed faster with a greater precision than slump test. The disadvantages are that it requires a large sample of concrete and it cannot be used when the concrete is placed in thin section. The minimum depth of concrete must be at least 20 cm and the minimum distance from the centre of the ball to nearest edge of the concrete 23 cm.

Concrete mould for kelly ball test — Concrete mould

Tamping bar for kelly ball test — Tamping bar

The surface of the concrete is struck off level, avoiding excess working, the ball is lowered gradually on the surface of the concrete. The depth of penetration is read immediately on the stem to the nearest 6 mm. The test can be performed in about 15 seconds and it gives much more consistent results than Slump Test. Diagram shows the Kelly Ball apparatus.

(e) Vee Bee Consistometer Test

This is a good laboratory test to measure indirectly the workability of concrete. This test consists of a vibrating table, a metal pot, a sheet metal cone, also a standard iron rod. Therefore apparatus is shown in the following Figure. Slump test as described earlier is performed, placing the slump cone inside the sheet metal cylindrical pot of the consistometer. The glass disc attached to the swivel arm is turned and however placed on the top of the concrete in the pot. The electrical vibrator is then switched on and simultaneously a stop watch started. The vibration is continued till such a time as the conical shape of the concrete disappears and the concrete assumes a cylindrical shape. This can be thus judged by observing the glass disc from the top for disappearance of transparency.

Immediately when the concrete fully assumes a cylindrical shape, the stop watch is switched off. Therefore, the time required for the shape of concrete to change from slump cone shape to cylindrical shape in seconds is known as Vee Bee Degree. This method is very suitable for very dry concrete whose slump value cannot be measured by Slump Test, but the vibration is too vigorous for concrete with a slump greater than about 50 mm.

20Apr, 2020

Construction of Water Bound Macadam ( WBM ) Roads

The term ‘ Macadam ‘ in the present day means, the pavement base course made of crush or broken aggregate mechanically interlock by rolling and the voids fill with screening and binding material with the assistance of water. WBM roads may also be use as a sub base, base course or even surfacing course of low volume roads. Thus thickness of each compact layer of WBM layer depends on the size and gradation of the aggregates use. Thus, the number of layers and total thickness of WBM construction depends on the design details of the pavement.

Material use in Water Bound Macadam Base ( WBM roads )

The materials use in WBM construction thus consists of the following constituents –

Coarse aggregate
Screenings
Binding material

Coarse aggregates

The coarse aggregates use in WBM roads generally consists of hard varieties of crush aggregates or broken stones. However crush able materials or soft aggregates ( such as over burnt or Jharna brick, kankar and laterite ) may be use only in the lower layers of the pavement. Crush slag obtain from blast furnace having density more than 112 kg/m^3 may also be use in the lower layers.

Courses	Los Angeles abrasion value	Aggregate impact value	Flakiness index
Sub base course	Less than 50%	Less than 40%	——————
Base course	Less than 40%	Less than 30%	——————
Surfacing course	Less than 40%	Less than 30%	Less than 15%

Screening

The screenings are use to fill up the voids in the compact layer of coarse aggregates. The screenings consists of the same material as the coarse aggregates, but of smaller size. There is no need to use screenings in case of crush able type of coarse aggregates.

Binding Material

Binding Material consisting of fine grain material passing 0.425 mm sieve is use in WBM construction thus to prevent raveling of the stones. Kankar nodules or lime stone dust may also be utilize, if locally available. The binding material with plasticity index value 4 to 8 is therefore use in WBM surface course construction. The plasticity index of binding material should be less than 6.0 in the case of WBM layers use as base course or sub base course or when the WBM is provide with bituminous surfacing. If the screenings use consists of crush able material like moorum or soft gravel, there is no need to apply binding material, unless the plasticity index value is less than 4.0.

Quantity of materials

However, Approximate loose quantities of materials require for 100 mm thick WBM grading -1 for sub base course layer in m^3 per 10 m^2 area :-

Type	Size range, mm	Sieve size, mm
coarse aggregate	90 to 45 mm	1.21 to 1.43
stone screening type-A	13.2 mm	0.27 to 0.30

Therefore approximate loose quantities of materials require for 75 mm thick WBM grading -2 for base course layer in m^3 per 10 m^2 area :-

Types of Agg.	Size range, mm	Sieve size, mm
Coarse aggregate	63 to 45 mm	0.91 to 1.07
Stone screening type-A	13.2 mm	0.12 to 0.15 for base coarse 0.10 to 0.12 for surface course

Thus, Approximate loose quantities of materials require for 75 mm thick WBM grading -2 for base course layer in m^3 per 10 m^2 area :-

Type	Size range, mm	Sieve size, mm
Coarse aggregate	63 to 45 mm	0.91 to 1.07
Stone screening type-B	11.2 mm	0.20 to 0.22 for base coarse 0.16 to 0.18 for surface course

The approximate loose quantities of materials require thus 75 mm thick WBM grading -3 for base course layer in m^3 per 10 m^2 area :-

Types of Agg.	Size range, mm	Sieve size, mm
Coarse aggregate	53 to 22.4 mm	0.91 to 1.07
Stone screening type-B	11.2 mm	0.18 to 0.21 for base coarse 0.14 to 0.17 for surface course

Construction Steps of WBM roads

Step 1

The foundation for receiving the new layer of WBM roads may be either the sub grade or sub base or base course. This layer is thus prepare by scarifying and filling the depressions and patching the pot holes, the receiving surface is brought to the required grade and camber.

Step 2

The dust and other loose materials are clean.

Step 3

Lateral confinement is to be provide before starting WBM roads construction. This may be do by constructing the shoulders in advance to a thickness equal to that of the compact WBM roads layer to be lay and therefore by trimming the inner sides of the shoulder vertically.

Step 4

In order to it, coarse aggregates spread-ed uniformly to proper profile to uniform thickness upon the prepare foundation and check by templates.

Step 5

The layer is compact by a three wheel power roller of 8 to 10 tonnes capacity or also by an equivalent vibratory roller. Rolling is starting from the edges, the roller being run forward and backward until the edges are compact. The run of the roller is then gradually shift towards the center line of the road, uniformly overlapping each preceding rear wheel track by one half width. This process is repeat by rolling from either edge towards the center line until adequate compaction is achieve. On super elevated portions of the road or on divide roads, thus rolling is commence from the inner or lower edge and progress gradually towards the outer or upper edge of the pavement. If require, slight sprinkling of water may be do during rolling.

Step 6

Afterwards the coarse aggregates are roll adequately, the dry screenings are apply gradually over the surface to fill the interstices in three or more applications, dry rolling is continued as the screenings are being spread and broom carried out.

Step 7

Afterwards the application of screenings, the surface is sprinkle with water, sweep and roll, wet screenings are sweep into the voids using hand brooms. Additional screenings are apply and roll till the coarse aggregates are well bond and firmly set.

Step 8

Binding material is apply at a uniform and slow rate at two or more successive thin layers, after each application of binding material, the surface is copiously sprinkle with water and thus wet slurry sweep with brooms to fill the voids. Thus this is follow by rolling with a 8 to 10 tonnes roller and water is apply to the wheels to wash down the binding materials that sticks to the roller.

Step 9

After final compaction, then the layer allowed to dry over night. On the next day the “hungry” spots are locate and therefore filled with screenings or binding materials, lightly sprinkle with water if necessary and roll. No traffic is allow till the WBM roads layer sets and dries out. In the case of WBM roads base course, the layer is allow to dry completely without permitting traffic to ply and then the bituminous surfacing is lay. Limit construction traffic may also be permit to ply over the WBM roads layer taking proper care not to damage the layer.

Step 10

After the WBM roads layer is fully dry, steps are thus taken to lay the next layer, in the case of WBM surface course the road is open to traffic.

Rectification of defects

The surface evenness of longitudinal direction generally checked by 3.0 m straight edge along the middle of each traffic lane. The cross profile therefore checked using camber templates at intervals of 10 m. Where the surface undulations exceed the specify limits, the defective area with a minimum of 10 m^2 area around the spot is scarify, reshape with additional aggregate/screening material as require and re-compact properly as specify earlier.

Quality control checks during the construction of WBM roads

The Los Angeles abrasion value or aggregate impact value and flakiness index tests are therefore to be conduct on the coarse aggregates at a rate of one test per 200 m^3 to check whether the specify requirements are fulfill.
Grading of the coarse aggregates and screenings however to be check at a rate of one test per 100 m^3.
The surface levels of compact WBM layer therefore checked, allowable tolerance is (+15 mm and -15 mm ).

Category Archives: Building construction

Why Does Concrete Need Reinforcement ?

Characteristics of a Reinforcing Material

SUITABILITY OF STEEL AS A REINFORCING MATERIAL

FORMS OF STEEL REINFORCEMENT

Mild Steel Plain Bars.

High Yield Strength Deformed Bars (HYSD Bars).

HYSD Bars of Different Grades

Advantages of HYSD Bars.

Disadvantages of HYSD bars

Testing of Hardened Concrete

Compression Test

Moulds

Compacting by Hand

Compacting by Vibration

Curing

Making and Curing test specimen in the field

Comparison between Cube and Cylinder Strength

Flexural Strength of Concrete

Determination of Tensile Strength

Procedure

Indirect Tension Test Methods

1. Cylinder Splitting Tension Test

2. Ring Tension Test

3. Double Punch Test

Test Core

Strength of Cores

Non-Destructive Testing Methods

1. Surface hardness tests:

2. Rebound test:

3. Penetration and Pull out techniques:

4. Dynamic or vibration tests:

5. Combined methods:

6. Radioactive and nuclear methods:

7. Magnetic and electrical methods:

8. Acoustic emission techniques:

9. Surfaces Hardness Methods:

Schmidt’s Rebound Hammer

Limitation:

Following are more tests related to above, we’re not going to discuss all these in detail. So find the names of the relevant tests as follows –

Testing on Composition of Hardened Concrete

Top Down Construction

Why Top Down Construction Require ?

Methodology

Procedure of Top Down Construction

Advantages

Disadvantages

Workability of Concrete

Factors Affecting Workability

Measurement of Workability

The following tests are commonly employed to measure workability –

(a) Slump Test

Details of the slump cone apparatus

(b) Compacting Factor Test

The ratio of the density actually achieved in the test to density of same concrete fully compacted.

The Compacting Factor = Weight of partially compacted concrete / Weight of fully compacted concrete

(c) Flow Test

Flow percent = (Spread diameter in cm – 25) x 100 / 25

Apparatus

1. Flow table

2. Mould

3. Tamping Bar

Sampling

Procedure

(d) Kelly Ball Test

(e) Vee Bee Consistometer Test

Construction of Water Bound Macadam ( WBM ) Roads

Material use in Water Bound Macadam Base ( WBM roads )

Coarse aggregates

Screening

Binding Material

Quantity of materials

Construction Steps of WBM roads

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Step 7