Building Plinth

Building Plinth

This is the portion of structure between the surface of the surrounding ground and surface of the floor, immediately above the ground. As per Byelaws, the plinth should not be less than 45cm. The basic requirements of plinth area

1.                       To transmit the load of the super-structure to the foundation.

2.                       To act as a retaining wall so as to keep the filling portion below the raised floor or the building.

3.                       To protect the building from damp or moisture penetration into it.

4.                       It enhances the architectural appearance of the building.

Uses of Concrete

Uses of Concrete

·     For heavy loaded R.C.C columns and R.C.C arches of long spans

·     For small pre cast members of concrete like fencing poles, telegraph poles etc. watertight construction.

·     For water tanks, bridges, sewers etc.

·     For foot path, concrete roads.

·     For general work of RCC such as stairs, beams, columns, slabs, etc.

·     For mass concrete for heavy walls, foundation footings etc.

Types of Cement

Types of Cement

In addition to ordinary cement, the following are the other varieties of cement.

·     Acid Resistance Cement : This is consists of acid resistance aggregates such as quartz, quartzite’s, etc, additive such as sodium fluro silicate (Na2SiO6) and aqueous solution of sodium silicate. This is used for acid-resistant and heat resistant coating of installations of chemical Industry. By adding 0.5 percent of unseed oil or2 percent of ceresil, its resistance to water is increased and known as acid water resistant cement.

·     Blast Furnace Cement: For this cement slag as obtained from blast furnace in the manufacture of pig iron and it contains basic elements of cement, namely alumina, lime and silica. The properties of this cement are more or less the same as those of ordinary cement and prove to be economical as the slag, which is waste product, is used in its manufacture.

·     Colored Cement: Cement of desired color may be obtained by intimately mixing mineral pigments with ordinary cement. The amount of coloring may vary from 5 to 10 percent and strength of cement if it is exceeds 10 percent. Chromium oxide gives brown, red or yellow for different proportions. Colored cements are used for finishing of floors, external surfaces, artificial marble, windows.

·     Expanding Cement : This type of cement is produced by adding an expanding medium like sulpho – aluminate and a stabilizing agent to ordinary cement. Hence this cement expands where as other cement shrinks. Expanding cement is used for the construction of water retaining structures and also for repairing the damaged concrete surfaces.

·     High alumina Cement: This cement is produced by grinding clinkers formed by calcining bauxite and lime. The total content should not be less than 32 percent and the ratio by weight of alumina to lime should be between 0.85 and 1.30.

Functions of Ingredients in Cement

Functions of Ingredients in Cement

1.Lime : Lime is the important ingredient of cement and its proportion is to be maintained carefully. Lime in excess makes the cement unsound and causes the cement to expand and disintegrate. On the other hand, if lime is in deficiency the strength of the cement is decreased and it causes cement to set quickly.

2. Silica : This also an important ingredient of cement and it gives or imparts quick setting property to imparts strength to cement.

3. Alumina : This ingredient imparts quick setting properly to cement. Express alumina weakens the cement.

4. Calcium Sulphate : This ingredient is in the form of gypsum and its function is to increase the initial setting time of cement.

5. Magnesia : The small amount of this ingredient imparts hardness and color to cement.

6. Sulphur : A very small amount of sulphur is useful in making sound cement. If it is in excess, it causes the cement to become unsound.

7. Alkalies : Most of the alkalies present in raw material are carried away by the flue gases during heating and only small quantity will be left. If they are in excess in cement, efflorescence is caused.

About Cement

About Cement

Cement in its broadest term means any substance which acts as a binding agent for materials natural cement (Roman Cement) is obtained by burning and crushing the stones containing clay, carbonates of lime and some amount of carbonate of magnesia. The clay content in such stones is about 20 to 40 percent. Natural cement resembles very closely eminent hydraulic lime. It is not strong as artificial cement, so it has limited use in practice.

Artificial cement is obtained by burning at very high temperature a mixture of calcareous and argillaceous materials in correct proportion. Calcined product is known as clinker. A small quantity of gypsum is added to clinker and it is then pulverized into very fine powder is known as cement. Cement was invented by a mason Joseph Aspdin of leeds in England in 1824. The common variety of artificial cement is known as normal setting cement or ordinary cement or Portland cement.

Manufacture of bricks

Manufacture of bricks

To minimize breakages in cold weather, increase the cement content of the mix or the curing period before moving blocks.

Ordering and stockpiling materials

Aggregates and cement should be ordered in good time. Stocks should be sufficient to prevent stoppages due to lack of material.

As a rough guide, using an aggregate: cement ratio of 8:1 by loose volumes, three and a half bags of cement and a cubic meter of aggregate will be enough to make about 400 bricks. The number of blocks produced from the same quantity of material will depend on block size and whether they are solid or hollow.

Aggregates must be stockpiled in such a way that contamination is prevented and mixing of different types is prevented.

Cement must be stored in such a manner that it is kept dry. Cement in bags should be used within one month of being delivered.

Batching

Cement, if supplied in bags, should preferably be batched by the full bag. Cement supplied in bulk may be weighed (preferable) or batched by loose volume (not recommended).

It is important to batch all materials accurately. Batching containers, e.g. wheelbarrows, buckets, drums and wooden boxes, should be loosely filled to the brim and struck off flush with it. To avoid errors, there should be enough containers for a full batch to be made without using any container more than once. Dented or broken containers must not be used. The amount of water to be added to the mix is judged by eye and by doing some simple tests

Water content

Water content is critical. The mixture must be wet enough to bind together when compacted, but it should not be so wet that the blocks slump (sag) when the mould is removed. A common mistake is the use of mixes that are too dry, resulting in incomplete compaction. The moisture content should be as high as possible as this allows better compaction and thus gives the best strength.

Mixing

Hand mixing with the use of shovels should be done on a level concrete slab or steel plate.

First spread the sand out 50 to 100 mm thick. Then distribute the cement, and stone if any, evenly over the sand. Mix aggregate and cement until the color is uniform. Spread the mixture out,

sprinkle water over the surface and mix. Continue with this process until the right amount of water has been mixed in.

Molding

Hand operated machines should be used as instructed by the manufacturer.

The mould of a powered machine should be filled until approximately six to eight cycles of compaction are required to bring the compacting head to its stops. Too little or poor compaction should be avoided as it results in greatly reduced strengths.

Curing

Curing is the process of maintaining a satisfactory moisture content and a favorable temperature in the blocks to ensure hydration of the cement and development of optimum strength.

Bricks

Bricks

Bricks are obtained by moulding clay in rectangular blocks of uniform size and then by drying and burning these blocks. As bricks are of uniform size, they can be properly arranged, light in weight and hence bricks replace stones.

Composition Manufacture Process.

Composition – Following are the constituents of good brick earth.

Alumina : - It is the chief constituent of every kind of clay. A good brick earth should contain 20 to 30 percent of alumina. This constituent imparts plasticity to earth so that it can be molded. If alumina is present in excess, raw bricks shrink and warp during drying and burning.

Silica -A good brick earth should contain about 50 to 60 percent of silica. Silica exists in clay either as free or combined form. As free sand, it is mechanically mixed with clay and in combined form; it exists in chemical composition with alumina. Presence of silica prevents crackers shrinking and warping of raw bricks. It thus imparts uniform shape to the bricks. Durability of bricks depends on the proper proportion of silica in brick earth. Excess of silica destroys the cohesion between particles and bricks become brittle.

Lime– A small quantity of lime is desirable in finely powdered state to prevents shrinkage of raw bricks. Excess of lime causes the brick to melt and hence, its shape is last due to the splitting of bricks.

Oxide of iron - A small quantity of oxide of Iron to the extent of 5 to 6 percent is desirable in good brick to imparts red colour to bricks. Excess of oxide of iron makes the bricks dark blue or blackish.

Magnesia - A small quantity of magnesia in brick earth imparts yellow tint to bricks, and decreases shrinkage. But excess of magnesia decreases shrink leads to the decay of bricks.

The ingredients like, lime, iron pyrites, alkalies, pebbles, organic matter should not present in good brick earth

Aggregates as Building & Construction Materials

Aggregates as Building & Construction Materials

Aggregates - Grading: Aggregates is derived from igneous, sedimentary and metamorphic rocks or is manufacture from clays, slag etc. The properties of concrete are directly related to those of its constituents and should be hard, strong, durable, and free from clay, loam, vegetables and other such foreign matters. The presence of clay or dirt coating prevents the adhesion of cement on the surface of aggregates and ultimately retards the setting and hardening of cement and reduces the strength, durability and soundness of concrete. Depending upon their size, the aggregates are classified as (i)Fine Aggregative (ii) coarse aggregates.

Fine Aggregates: The material, most of when passes through 4.75mm I.S. sieve size, is termed as fine aggregates. It should not contain more than 1 to 8% of fine particles, which may be obtained from sea, river, lake or pit may be used as fine aggregates but care should be taken all its impurities must be removed.

Coarse Aggregates: The material whose particles are of such size as are retained on 4.75mm, I.S sieve are called coarse aggregates. The size of the coarse aggregates used depends upon the nature of work. The maximum size may be 23mm for mass concrete such as dams etc. and 63mm for plain concrete. Crushed hard stone and gravel is the common materials used as coarse aggregates for structural concretes. Coarse aggregates usually obtained by crashing granite, gneiss, crystalline lime stone and good variety of sandstone etc.

Qualities of a good building stone

The following are the qualities or requirements of a good building stone.

1. Crushing strength: For a good building stone, the crushing strength should be greater than l000kg per cm2.

2. Appearance: Good building stone should be a uniform color, and free from clay holes, spots of other color bands etc capable of preserving the color for longtime.

3. Durability: A good building stone should be durable. The factors like heat and cold alternative wet and dry, dissolved gases in rain, high wind velocity etc affect the durability.

4. Fracture: For good building stone its fracture should be sharp, even and clear.

5. Hardness: The hardness greater than 17, treated as hard used in road works. It is between 14 to 17, medium hardness, less 14 said be poor hardness.

6. Percentage wear: For a good building stone, the percentage wear should be equal to or less than 3 percent.

7. Resistance to fire: A good building stone be fire proof. Sandstone, Argillaceous stone resists fire quite well.

8. Specific gravity: For a good building stone the specific gravity should be greater than 8.7 or so.

9. Texture: A good building stone should have compact fine crystalline structure should be free from cavities, cracks or patches of stuff or loose material.

10. Water absorption: For a good building stone, the percentage absorption by weight after 24 hours should not exceed 0.60.

11. Seasoning: Stones should be well seasoned before putting into use. A period of about 6 to 12 months is considered to be sufficient for proper seasoning.

12. Toughness Index: Impact test, the value of toughness less than 13 – Not tough, between 13 and 19 – Moderate, greater than 19- high

Central Mixed Concrete

Central Mixed Concrete

Central-mixing concrete batch plants include a stationary, plant-mounted mixer that mixes the concrete before it is discharged into a truck mixer. Central-mix plants are sometimes referred to as wet batch or pre-mix plants. The truck mixer is used primarily as an agitating haul unit at a central mix operation. Dump trucks or other non-agitating units are sometimes be used for low slump and mass concrete pours supplied by central mix plants. About 20% of the concrete plants in the use a central mixer. Principal advantages include:

·     Faster production capability than a transit-mix plant

·     Improved concrete quality control and consistency and

·     Reduced wear on the truck mixer drums.

There are several types of plant mixers, including:

·     Tilt drum mixer

·     Horizontal shaft paddle mixer

·     Dual shaft paddle mixer

·     Pan mixer

·     Slurry mixer

The tilting drum mixer is the most common American central mixing unit. Many central-mix drums can accommodate up to 12 yd3 and can mix in excess of 200 yd3per hour. They are fast and efficient, but can be maintenance-intensive since they include several moving parts that are subjected to a heavy load.

Horizontal shaft mixers have a stationary shell and rotating central shaft with blades or paddles. They have either one or two mixing shafts that impart significantly higher horsepower in mixing than the typical drum mixer.

Pan mixers are generally lower capacity mixers at about 4 to 5 yd3 and are used at precast concrete plants.

Slurry Mixing The slurry mixer is a relative newcomer to concrete mixing technology. It can be added onto a dry-batch plant and works by mixing cement and water that is then loaded as slurry into a truck mixer along with the aggregates. It is reported to benefit from high-energy mixing. Another advantage is that the slurry mixer reduces the amount of cement dust that escapes into the air.

Check by Consumer Before Ordering the Ready Mix Concrete

Check by Consumer Before Ordering the Ready Mix Concrete

The following need to be looked into by the consumer:

·     Reliability of the plant and transit mixers for consistent and continuous concrete supply as per requirement.

·     Calibrations of all measuring devices and their accuracy.

·     Mode of operation of plant should preferably be fully automatic and not manual.

·     Quality of materials proposed to be used.

·     Adequacy of quantity of materials proposed to be used.

·     Compliance of concrete specifications based on the mix parameters specified.

·     Adequacy of testing facilities

·     Time likely to be taken by transit mixers from plant to site and back.

Objective of Ready Mix Concrete

Objective of Ready Mix Concrete

The main objective to choose this topic is that an engineer should know the advantages of Ready Mix Concrete and disadvantages of Site mixed concrete. As Ready Mix Concrete are widely using in bigger and medium size of projects today, engineer should be aware the technicality of the Ready Mix Concrete and the operational work to ensure the quality of work to be maintain. Site engineer to be know that what are the steps to be taken to check the concrete in Ready Mix Concrete, what is required to be specified for Ready Mix Concrete, what is the information required to be supplied by the Ready Mix Concrete supplier, what checks are necessary by the consumer before ordering Ready Mix Concrete, what are the checks needed at site prior and after to receipt of Ready Mix Concrete.

Use of Ready Mix and Concrete

Use of Ready Mix and Concrete

RMC is generally looked upon as a costly product rather than a facility to get an appropriate quality product on site as and when required.The first cost of RMC may seem higher . However, there are several hidden advantages which can cause considerable reduction in cost to the owner. Since they cannot be accurately determined, they are ignored while evaluating the cost of RMC over site mixed produced concrete.

The following points answer the above question:

·     Generally speaking, the quality of concrete will be superior than site mixed concrete. However ,it will greatly depend on the controls and checks exercised at site and at RMC producer's plant.

·     There is a considerable wastage of materials on site due to poor storage conditions and repeated shifting of the mixer location. This is prevented if RMC is used.

·     In most cities, the plot area is barely sufficient to store reinforcement steel, formwork, concrete and other construction materials. Using RMC can cause less congestion and better housekeeping on the site resulting in efficient working environment.

·     Obtaining RMC at site can reduce supervision and labor costs which would otherwise be required for batching and mixing of concrete at site.

·     Many sites in cities, house their work force on the site itself to reduce the time and cost of daily travel. This creates unsafe and unhygienic conditions on the site as well as for the surrounding areas. This will reduce to a certain extent if RMC is utilized.

·     Fluctuation of raw material prices and their availability has always caused delays and problems of inventory and storage for site producers of concrete. This is totally avoided when RMC is used.

·     RMC plants have proper facilities to store and accurately batch concrete admixtures (chemical and mineral). To improve properties of concrete both in plastic and in hardened stage this accuracy is useful.

·     In general, RMC plants have superior and accurate batching arrangements than the weigh batchers used on site.

·     RMC plants have superior mixers than the rotating drum mixers generally used for mixing concrete materials at site.

·     RMC plants have efficient batching and mixing, facilities which improve both quality and speed of concrete production.

·     Temperature control of concrete in extreme weather conditions can be exercised in a much better manner than done at site.

·     RMC helps encourage" mechanization and new technologies like pumped concrete bulk transportation of cement production of self-compacting concrete and high strength high performance concrete.

·     New materials like micro silica and fibers can be safely used in RMC which in conventional concrete may pose problems.

·     Introduction of RMC improves the rate of supply of concrete in the formwork and thereby automatically improves quality of formwork, layout of reinforcement steel and its detailing and safety / strength of scaffolding and staging.

Test Methods of Sprayed Concrete

Test Methods of Sprayed Concrete

Determination of early strengths

To determine the very early strengths (in the range 0 to 1 N/mm²), the Proctor or Penetrating needle is used.

The following methods are well established for compressive strength testing between 2 and 10 N/mm²:

·     Kaindl/Meyco: Determination by the pull-out force of bolts.

·     HILTI (Dr. Kusterle): Determination of the impression depth (I) and pull-out force (P) of nails shot with a HILTI DX 450L shot bolt machine (load and nail size are standard).

·     Simplified HILTI method (Dr. G. Bracher, Sika):Determination of the impression depth (I) of nails shot with a HILTI DX 450L shot bolt machine (load and nail size are standard). Determining the required early strength by this method should be accurate to ±2 N/mm².

Concrete Flexural Strength

Concrete Flexural Strength

Concrete is basically used under compressive stress and the tensile forces are absorbed by reinforcement bars. Concrete itself has some tensile and flexural strength, which is strongly dependent on the mix. The critical factor is the bond between aggregate and hydrated cement. Concrete has a flexural strength of approximately 2 N/mm²  to 7 N/mm²

Influences on flexural strength

Flexural strength increases

·     As the standard cement compressive strength increases (CEM 32.5; CEM 42.5; CEM 52.5)

·     As the water/cement ratio falls

·     By the use of angular and broken aggregate

Applications

·     Steel reinforced fiber concrete

·     Runway concrete

·     Shell structure concrete

Test methods

Principle:- A bending moment is exerted on prism test specimens by load transmission through upper and lower rollers.

·     Prism dimensions:

Width = height = d

Length > 3.5 d

Two test methods are used:

·     2-point load application

Load transfer

above through 2 rollers at a distance d (each one ½ d from centre of prism).

The reference method is 2-point load application.

·     1-point load application (central)

Load transfer above through 1 roller, in centre of prism.

In both methods the lower rollers are at a distance of 3 d (each one 1½ d from centre of prism).

Concrete as Construction Materials

Concrete as Construction Materials  

Concrete is the most widely used man made construction material in the world, and is second only water as the most utilized substance on planet. It is obtained by mixing cementitious materials, water and aggregates (and sometimes admixtures) in required proportions. The mixture when placed in forms and allowed to cure, hardens into a rock-like mass know as concrete. The hardening is caused by chemical reaction between water and cement and it continues for a long time, and consequently concrete grows stronger with age. The hardened concrete may also be considered  as an artificial stone in which the voids of larger particles (coarse aggregate) are filled by the smaller particles ( fine aggregate) and the voids of fine aggregate are filled with cement. In a concrete mix the cementitious material and water form a paste called cement-water paste which in addition to filling the voids of fine aggregates , coast the surface of fine and coarse aggregates and binds them together as it cures, thereby cementing the particles of the aggregates together in a compact mass.

The strength, durability and other characteristics of concrete depend upon the properties of its ingredients, on proportion of mix, the method of compaction and other control during placing, compaction and curing. The popularity of concrete is due to the fact that from the common ingredients, it is possible to tailor the properties of concrete to meet demands of any particular situation. The advance in concrete technology  have paved the way  to make the best use of local availability materials by judicious mix proportioning and proper workmanship, so as the produce concrete satisfying performance requirement.

The key to producing a strong , durable and uniform concrete, i.e. high performance concrete  lies in the careful control of its basic and process components .

Frost and Freeze/Thaw Resistance

Frost and Freeze/Thaw Resistance

Frost stress

Damage to concrete structures due to frost can generally be expected when they have been penetrated by moisture and are exposed to frequent freeze/thaw cycles in that condition. The damage to the concrete occurs due to the cyclic freezing and thawing of the water which has been absorbed due to capillary suction. Destruction follows due to the increase in volume of the water [ice] in the outer concrete layers.

Essentials for high frost resistance

·     Frost proof aggregates

·     Impermeable concrete structure and/or

·     Concrete enriched with microspores

·     Thorough and careful curing

·     Degree of hydration of the concrete as high as possible (i.e. it is not a good idea to place concrete immediately before periods of frost)

Test methods

·     Frost resistance

This can be estimated by comparing the fillable and non-fillable voids.

Freeze/thaw resistance

Given the extensive use of deicing salts (generally sodium chloride NaCl, intended to lower the freezing point of the water on roads and prevent ice formation etc.), the concrete surface cools abruptly due to heat extraction from the concrete. These interactions between frozen and unfrozen layers cause structural breakdown in the concrete.

Conditions for freeze/thaw resistance

·     Frostproof aggregates

·     Concrete with an impermeable structure enriched with microspores

·     Thorough and careful curing

·     Avoid too much fine mortar enrichment of the surface area

·     Concreting as long as possible before the first freeze/thaw stress so that the concrete can dry out.

Determination of Fresh Concrete Density

Determination of Fresh Concrete Density

Principle: The fresh concrete is compacted in a rigid, watertight container and then weighed.

The minimum dimensions of the container must be at least four times the maximum nominal size of the coarse aggregate in the concrete, but must not be less than 150 mm. The capacity of the container must be at least 5 liters. The top edge and base must be parallel.

(Air void test pots with a capacity of 8 liters have also proved very suitable.)

The concrete is compacted mechanically with a poker or table vibrator or manually with a bar or tamper.

Fresh Concrete Temperature

Fresh Concrete Temperature

The fresh concrete temperature should not be too low, so that the concrete gains sufficient strength fast enough and does not suffer damage from frost at an early age.

·     The fresh concrete temperature should not drop below +5°C during placement and installation.

·     The freshly placed concrete should be protected from frost. Freezing resistance is reached at a compressive strength of approximately 10N/mm².

·     On the other hand too high concrete temperatures can result in (cause) placement problems and decline of certain hardened concrete properties. To avoid this, the fresh concrete temperature should not go above 30°C during placement and installation.

Precautions at low temperatures

The concrete should be protected from rain and frost during handling. Concreting is only possible in freezing temperatures if special protective measures are taken. They must be in place from the start of concrete production to the end of curing.

They depend on the outside temperature, air humidity, wind conditions, fresh concrete temperature, heat development and dissipation and the dimensions of the concrete pour.

The fresh concrete must not be colder than +5°C during placement and installation without additional protective measures. The mixing water and aggregates should be preheated if necessary.

Precautions at high temperatures

The concrete should be protected from drying out during handling.

Concreting is only possible at high temperatures if special protective measures are provided. These must be in place from the start of concrete production to the end of curing. They are dependent on the outside temperature, air humidity, wind conditions, fresh concrete temperature, heat development and dissipation and the dimensions of the pour.

The fresh concrete must not be hotter than +30°C during placement and installation without these protective measures.

Fresh Concrete Density

Fresh Concrete Density

Fresh concrete density means the mass in kg per m³ of fresh, normally compacted concrete, including its remaining voids.

Given the same quantity of cement and aggregate, a lower fresh concrete density indicates a lower concrete strength because the density falls as the water and void content increases.

The fresh concrete density falls

• as the water content increases
• as the void content increases

The fresh concrete density increases

• as the cement content rises
• as the water/cement ratio decreases
• as the void content decreases