Showing posts with label concrete technology. Show all posts
Showing posts with label concrete technology. Show all posts

Tuesday, 9 January 2018

ASBESTOS

Asbestos is a general name for several varieties of fibrous minerals which are available in nature. But presently, most of the commercial asbestos produced is ‘chriotile’ [Mg6SiO11(OH)6.H2O].

Properties of Asbestos

1. It is flexible, soft and non-porous.

2. It is fireproof and acid proof material.

3. It is a good insulator of heat and electricity.

4. When it is mixed with cement and water, it retains shape firmly.

5. Its colour is brown or grey.

6. It can be cut into pieces or can be drilled.

7. It possesses high tensile strength in the direction of its fibres.

8. Its specific gravity is 3.10.

Uses of Asbestos

1. Asbestos cement sheets are the cheapest roofing materials.

2. Asbestos cement pipes are used as down take pipes of rain water from the roof.

3. With bitumen it forms good damp proof layer.

4. It is used for preparing fire proof ropes and clothes.

5. It is used as covering material for fuse and electric switch boxes.

6. It is useful for insulating boilers, furnaces etc.

 

Physical Tests on Cement

(a) Soundness Test:

It is conducted by sieve analysis. 100 gms of cement is taken and sieved through IS sieve No. 9 for fifteen minutes. Residue on the sieve is weighed. This should not exceed 10 per cent by weight of sample taken.

(b) Setting Time:

Initial setting time and final setting time are the two important physical properties of cement. Initial setting time is the time taken by the cement from adding of water to the starting of losing its plasticity. Final setting time is the time lapsed from adding of the water to complete loss of plasticity.

Vicat apparatus is used for finding the setting times Vicat apparatus consists of a movable rod to which any one of the three needles shown in figure can be attached. An indicator is attached to the movable rod. A vicat mould is associated with this apparatus which is in the form of split cylinder.

 

Before finding initial and final setting time it is necessary to determine water to be added to get standard consistency. For this 300 gms of cement is mixed with about 30% water and cement paste prepared is filled in the mould which rests on non porous plate. The plunger is attached to the movable rod of vicat apparatus and gently lowered to touch the paste in the mould. Then the plunger is allowed to move freely. If the penetration is 5 mm to 7mm from the bottom of the mould, then cement is having standard consistency. If not, experiment is repeated with different proportion of water fill water required for standard consistency is found. Then the tests for initial and final setting times can be carried out as explained below:

 

Initial Setting Time: 300 gms of cement is thoroughly mixed with 0.85 times the water for standard consistency and vicat mould is completely filled and top surface is levelled. 1 mm square needle is fixed to the rod and gently placed over the paste. Then it is freely allowed to penetrate. In the beginning the needle penetrates the paste completely.

As time lapses the paste start losing its plasticity and offers resistance to penetration. When needle can penetrate up to 5 to 7 mm above bottom of the paste experiment is stopped and time lapsed between the addition of water and end if the experiment is noted as initial setting time.

 Final Setting Time. The square needle is replaced with annular collar. Experiment is continued by allowing this needle to freely move after gently touching the surface of the paste. Time lapsed between the addition of water and the mark of needle but not of annular ring is found on the paste. This time is noted as final setting time.

 

(c) Soundness Test:

This test is conducted to find free lime in cement, which is not desirable. Le Chatelier apparatus shown in  is used for conducting this test. It consists of a split brass mould of diameter 30 mm and height 30 mm. On either side of the split, there are two indicators, with pointed ends. The ends of indicators are 165 mm from the centre of the mould.

 

Properly oiled Le Chatelier mould is placed on a glass plate and is filled completely with a cement paste having 0.78 times the water required for standard consistency. It is then covered with another glass plate and a small weight is placed over it. Then the whole assembly is kept under water for 24 hours.

 

The temperature of water should be between 24°C and 50°C. Note the distance between the indicator. Then place the mould again in the water and heat the assembly such that water reaches the boiling point in 30 minutes. Boil the water for one hour. The mould is removed from water and allowed to cool. The distance between the two pointers is measured. The difference between the two readings indicate the expansion of the cement due to the presence of unburnt lime. This value should not exceed 10 mm.

 

(d) Crushing Strength Test:

For this 200 gm of cement is mixed with 600 gm of standard sand confirming to IS 650–1966. After mixing thoroughly in dry condition for a minute distilled potable water P4+ 3 percentage is added where P is the water required for the standard consistency.

 

They are mixed with trowel for 3 to 4 minutes to get uniform mixture. The mix is placed in a cube mould of 70.6 mm size (Area 5000 mm2) kept on a steel plate and prodded with 25 mm standard steel rod 20 times within 8 seconds. Then the mould is placed on a standard vibrating table that vibrates at a speed of 12000 ± 400 vibration per minute.

 

A hopper is secured at the top and the remaining mortar is filled. The mould is vibrated for two minutes and hopper removed. The top is finished with a knife or with a trowel and levelled. After 24 ± 1 hour mould is removed and cube is placed under clean water for curing.After specified period cubes are tested in compression testing machine, keeping the specimen on its level edges.

 

 Average of three cubes is reported as crushing strength. The compressive strength at the end of 3 days should not be less than 11.5 N/mm2 and that at the end of 7 days not less than 17.5 N/mm2.

Tests on Bricks

The following laboratory tests may be conducted on the bricks to find their suitability:

(i) Crushing strength                     

(ii) Absorption

(iii) Shape and size and

(iv) Efflorescence.

(i) Crushing Strength:

The brick specimen are immersed in water for 24 hours. The frog of the

brick is filled flush with 1:3 cement mortar and the specimen is stored in damp jute bag for 24 hours and then immersed in clean water for 24 hours. The specimen is placed in compression testing machine with 6 mm plywood on top and bottom of it to get uniform load on the specimen. Then load is applied axially at a uniform rate of 14 N/mm2 . The crushing load is noted. Then the crushing strength is the ratio of crushing load to the area of brick loaded. Average of five specimen is taken as the crushing strength.

(ii) Absorption Test:

 Brick specimen are weighed dry. Then they are immersed in water for a

period of 24 hours. The specimen are taken out and wiped with cloth. The weight of each specimen in wet condition is determined. The difference in weight indicate the water absorbed. Then the percentage absorption is the ratio of water absorbed to dry weight multiplied by 100. The average of five specimen is taken. This value should not exceed 20 per cent.

(iii) Shape and Size:

 Bricks should be of standard size and edges should be truely rectangular with sharp edges. To check it, 20 bricks are selected at random and they are stacked along the length, along the width and then along the height. For the standard bricks of size 190 mm × 90 mm × 90 mm.

IS code permits the following limits:

Lengthwise: 3680 to 3920 mm

Widthwise: 1740 to 1860 mm

Heightwise: 1740 to 1860 mm.

The following field tests help in ascertaining the good quality bricks:

(i) uniformity in size

(ii) uniformity in colour

(iii) structure

(iv) hardness test

(v) sound test

(vi) strength test.

(i) Uniformity in Size:

 A good brick should have rectangular plane surface and uniform in size.

This check is made in the field by observation.

(ii) Uniformity in Colour:

 A good brick will be having uniform colour throughout. This observation may be made before purchasing the brick.

(iii) Structure:

 A few bricks may be broken in the field and their cross-section observed. The

section should be homogeneous, compact and free from defects such as holes and lumps.

(iv) Sound Test:

 If two bricks are struck with each other they should produce clear ringing sound.

The sound should not be dull.

(v) Hardness Test:

 For this a simple field test is scratch the brick with nail. If no impression is marked on the surface, the brick is sufficiently hard

(vi) Efflorescence:

The presence of alkalies in brick is not desirable because they form patches of gray powder by absorbing moisture. Hence to determine the presence of alkalies this test is performed as explained below: Place the brick specimen in a glass dish containing water to a depth of 25 mm in a well ventilated room.

 After all the water is absorbed or evaporated again add water for a depth of 25 mm. After second evaporation observe the bricks for white/grey patches. The observation is reported as ‘nil’, ‘slight’,

‘moderate’, ‘heavy’ or serious to mean

(a) Nil: No patches

(b) Slight: 10% of area covered with deposits

(c) Moderate: 10 to 50% area covered with deposit but unaccompanied by flaking of the surface.

(d) Heavy: More than 50 percent area covered with deposits but unaccompanied by flaking of

the surface.

(e) Serious: Heavy deposits of salt accompanied by flaking of the surface.

Saturday, 25 February 2017

Quality control of Ready Mix Concrete plant.


Introduction

As per Indian Standard code of practice (IS 4926) Ready Mixed Concrete (RMC) is defined as the concrete delivered in plastic condition and requiring no further treatment before being placed in position in which it is to set and harden. Instead of being batched and mixed on site, concrete is delivered for placing from central batching plant. First RMC was patented in Germany in 1903, but the transportation system it had not done properly introduced to achieve proper workability of concrete. The first delivery of RMC was made in Baltimore in 1913, and the transit mixer was born in 1926. In 1931 erected a plant at Bedfont, west of London and launched a company named as Ready Mixed Concrete Ltd.

What is need of RMC plant?

The consumer wanted his concrete delivered to the job in a ready-to-place condition. Volume batching is completely replaced by the weight batching and presently computerized weigh batchers are used in most of the batching plants. Aggregates are stored in properly installed bins and cement and flyash are stored in silos. Conveyors are used to transport the aggregates. Cement and flyash is pumped into the central mixer with pneumatic pumps. Electronic moisture meters, digital admixture dispensers are used in fully automatic batching plants. So due to such practices in RMC plant the workable concrete can produce which may be more durable, because of that there is a need of RMC plant.

Wednesday, 15 June 2016

Types of Stairs


The stairs may be built with wood, concrete masonry or with cast iron. Wooden stairs are not safe, because of the danger of fire. However they are used in buildings to access to small areas in the upper floors.

 Cast iron or steel stairs in the spiral forms were used commonly to reduce stair case area. In many residential buildings masonry stairs are also used. Reinforced concrete stairs are very commonly used in all types of buildings.

PLASTERING


Applying mortar on the surfaces of walls, columns, ceiling etc. to get smooth finish is termed as plastering.

Mortar used for plastering may be lime mortar, cement mortar or lime-cement mortar.

Lime mortar used lime to sand ratio of 1 : 3 or 1 : 4.

Cement mortar of 1 : 4 or 1 : 6 mix is very commonly used for plastering, richer mix being used for outer walls.

CELLULAR CONCRETE


It is a light weight concrete produced by introducing large voids in the concrete or mortar. Its density varies from 3 kN/m3 to 8 kN/m3. It is also known as aerated, foamed concrete.

Properties of cellular concrete
1. It has low weight.

2. It has good fire resistance.

Compaction factor test.


This is another test to  identify the workability of concrete. This test is conducted in the laboratory.

The test equipment consists of two hoppers and a cylinder fixed to a stand, the dimensions and the distances between the three vessels being standardized. Vessel A and B are having hinged bottoms whereas cylinder C is having fixed bottom.

What is uses of timber?


1. For heavy construction works like columns, trusses, piles.

2. For light construction works like doors, windows, flooring and roofing.

3. For other permanent works like for railway sleepers, fencing poles, electric poles and gates.

4. For temporary works in construction like scaffolding, centering, shoring and strutting, packing of materials.

What is CEMENT & its Chemical properties


Cement is a commonly used binding material in the construction. The cement is obtained by burning a mixture of calcarious and argillaceous material at a very high temperature and then grinding the clinker so produced to a fine powder.

It was first produced by a mason Joseph  Aspdin in England in 1924. He named it as portland cement.

#Chemical properties of ordinary portland cement.

What are the common building stones?


(i) Basalt and trap

(ii) Granite

(iii) Sand stone

(iv) Slate

(v) Quartzite.

(vi) Marble

Uses of Stones.


(i) Stone masonry is used for the construction of foundations, walls, columns and arches.

(ii) Stones are used for flooring.

(iii) Stone slabs are used as damp proof courses, lintels and even as roofing materials.

What are the Properties of Stones?


(i) Structure

(ii) Texture

(iii) Density

(iv) Appearance

What is stone ?


Stone is a ‘naturally available building material’ which has been used from the early age of civilization. It is available in the form of rocks, which is cut to required size and shape and used as building block. It has been used to construct small residential buildings to large palaces and temples all over the world.

A. Type of Stones

• Geological   • Physical    • Chemical

Monday, 6 June 2016

Self compacting concrete.


A concrete which is capable to compact itself by its own self-weight under gravity without any external efforts like vibration is called as self compacting concrete. the mix is required to have ability of passing, filling and being stable.

Following ingredient are used to prepare self compacting concrete.

1. cement :- OPC 43 or 53 grade.

2. Aggregates:- well grade rounded or cubical aggregate of size 10 to 20mm uniformly graded fine aggregates.

3. Good quality of mixing water.

4. Super plasticizer like poly-carboxylated ether to improve the workability and viscosity modifying agent.

5. Mineral admixture like fly ash, ground granulated blast furnace slag, silica flume, fiber, finely crushed lime stone, dolomite or granite

6. Under self weight , scc should level and deform itself without any compaction and external vibration. There should be not be any entrapped air in concrete.

7. SCC should be fully flowable but without segregation and bleeding. This is achieved by keeping higher viscosity of cement and mortar to ensure flowability while maintaning no sedimentation of bigger aggregates.

What do you mean by Maturity of concrete ?


It is not only the time but also the temperature during the early period of hydration that influence the rate of gain of strength of concrete. since the strength development of concrete depends on both time and temperature it can be said that strength is a function of summation of product of time and temperature.

This summation is called maturity of concrete.

Maturity = ∑ (time X temperature)

What is membrane curing?


Sometimes, concrete works are carried out in places where there is acute shorting of water. the lavish application of water for water curing is not possible for reason of economy.

It has been pointed out earlier that curing does not mean only application of water, it means also creation of condition for promotion of uninterrupted and progressive hydration.

Membrane curing is a good method of maintaining a satisfactory state of wetness in the body of concrete to promote satisfactory state of wetness in the body of concrete to promote condition hydration when original water/cement ratio used is not less that 0.5. to achieve best result, membrane is applied after one or two days of actual wet curing.

Since no replenishing of water is done after the membrane has been applied it should be ensure that the membrane is of good quality and it is applied effectively. two or three coats may be requires for effective sealing of the surface to prevent the evaporation of the water.

when water proofing paper or polyethylene paper are used care must be taken to see that these are not punctured anywhere and also whether adequate lapping is given at the junction and this is effectively sealed.

Water curing


This is by far the best method of curing as it satisfies all the requirements of curing, namely, promotion of hydration, elimination of shrinkage and absorption of the heat of hydration. it is desirable that a certain extent of water curing is done before the concrete is covered with membranes. water curing can be done in following ways:-

1. Immersion

2. Ponding

3. Spraying

4. Wet covering

the precast concrete items are normally immersed in curing tanks for a certain duration. pavement slabs, roof slabs etc. are covered under water by making small  ponds. vertical retaining wall or plastered surfaces or concrete column etc. are cured by spraying water. In some cases, wet covering such as wet gunny bags, hessian cloth, jute matting, straw etc. are wrapped to vertical surface for keeping the concrete wet. for horizontal surfaces saw dust, earth or sand are used as wet covering to keep the concrete in wet condition for longer time so that the concrete is not unduly dried to prevent hydration.

Curing


The process of maintaining a satisfactory moisture content and a favorable temperature in concrete during the period immediately following placement so that hydration of cement may continue until the desired properties are developed to a sufficient degree to meet the requirement of service is called curing of concrete.

curing methods may be divided broadly into four categories:-

1. water curing

2. membrane curing

3. Application of heat

4. Miscellaneous