Properties of Concrete

Properties of Concrete

What Are The Properties of Concrete?

Most people commonly use the words cement and concrete interchangeably. It's important to note that cement is only one component of three concrete properties and the term "concrete" is a mixture of the three.

Concrete properties consist of three basic components: water, aggregate (rocks and sand), and portland cement. Cement, usually in powder form, acts as a binding agent when mixed with water and aggregates.

This combination or mixture, when poured, hardens into this durable material called concrete that we are all familiar with.

The Three Basic Components of Concrete

1. Portland Cement - The cement and water form a paste that coats the aggregates and sand in the mix. The paste hardens and binds the aggregates and sand together.

2. Water - Water is needed to form a chemical reaction with the cement (hydration) to form heat. This starts the drying and hardening process. Water is also used to provide workability with the concrete.

The amount of water in the mix in pounds compared with the amount of cement is called the water/cement ratio. The lower the w/c ratio the stronger the concrete.

3. Aggregates - Sand is the fine aggregate and crushed stone and gravel are the coarse aggregate. These two materials help strengthen the concrete.

Some Desired Properties of Good Concrete

1. A workable mix that is easy to place and consolidate.

2. A high quality concrete using the lowest water-cement ratio possible, using the proper size coarse aggregate practical for the job you're doing, using the optimum ratio of fine to coarse aggregate.

3. For hardened concrete resistance to freezing and thawing, deicing chemicals, wear resistance, strength, and low permeability (water tightness).

These are the basics of concrete properties to produce a concrete mix. There are other admixtures that are added to improve workability, durability, and set times.

Masonry

Masonry

Masonry is defined as the art of construction in which building units, such as clay bricks, sand-lime, bricks, stones, Pre-cast hallow concrete blocks, concrete slabs, glass bricks, combination of some of these building units etc are arranged systematically and bonded together to form a homogeneous mass in such a manner that they can with stand point to other loads and transmit then through the mass without fail or disintegration.

Masonry can be classified into the following categories.

1. Stone masonry

2. Brick masonry

3. Hallow block concrete masonry

4. Reinforced masonry

5. Composite masonry

These can be further sub-divided into varies types depending upon workmanship and type of materials used.

Objects of foundations

Objects of foundations

Every structure consists of two parts. (1) Foundation and (2) Super structure. The lowest artificially prepared parts of the structure which are in direct contact with the ground and which transmit the loads of the structure to the ground are known as Foundation or Substructure. The solid ground on which the foundation rest is called the “foundation bed” or foundation soil and it ultimately bears the load and interacts with the foundations of buildings.

Objects of foundations

Foundations are provided for the following purposes.

1.                        To distribute the total load coming on the structure on large area.

2.                        To support the structure.

3.                        To give enough stability to the structures against various distributing forces such as wind, rain etc.

4.                        To prepare a level surface for concreting and masonry work. The general inspection of site of work serves as a good for determine the type of foundation, to be adopted for the proposed work and in addition, it helps in getting the data w.r.to the following items.

    i.            Behavior of ground due to variations in depth of water table.

ii.            Disposal of storm water at site.

iii.            Nature of soil by visual examination.

iv.            Movement of ground due to any reason etc.

Concrete Compaction - Methods

Concrete Compaction - Methods

Concrete should be placed and compacted immediately after mixing. The concrete should be placed within 30 to 40 minutes to prevent the danger of concrete getting its initial set, before laying the concrete, the shuttering should be cleaned of all of dust or debris. Crude oil or grease etc is usually applied to the shuttering before concreting to prevent the shuttering absorbing the water from the concrete or getting stuck to it. In placing the concrete, care should be taken to see that it should not be thrown from heights. Concrete should be laid in layers 15 to 30 cm (6” to 12”) in thickness and each layer should be properly compacted before laying the next one.

Compaction of concrete should be proceed immediately after placing. The function of compaction of concrete is to expel the air bubbles in the mass and make it impermeable in addition to its securing the desired strength. The concrete mass should be consolidated or compacted till the cream of the cement starts appearing on the surface. Over compaction may lead to segregation of concrete while-under-compaction may leave air voids in concrete and results in honey combing. Compaction may be done by hand or mechanical device.

Hand compaction : The hand compaction may be done by rodding, tamping or hammering. Tamping is usually adopted for compacting concrete for slabs or other such surfaces. Rodding is done for thin vertical members. Hammering is done for massive plain concrete works and for compacting an almost dry concrete the surface is beaten with heavy flat bottom rammers till the thin film of mortar start appearing on the surface.

Mechanical compaction: Mechanical compaction is done by the use of vibrators. Vibrators are of three types 1. Internal 2. External 3. Surface. Internal vibrators are commonly used in large works for flat surface compaction. In this the vibrator is immersed in the full depth of concrete layer. The vibrator should be kept in one position for about 3 minutes and then removed and placed another position. External vibrators are placed against the form work and are only adopted for thin section of members or in places where internal vibrators cannot be used with ease. Surface vibrators are generally employed in concrete road construction. Compaction of concrete by use of vibrators permits the use of stiff concrete mix of high strength and ensure better compaction than that obtained by the method of hand compaction

Preparation of concrete mix

Preparation of concrete mix

There are two types of concrete mixing

 (i)Hand mixing

(ii)Machine mixing

Hand Mixing: This method of mixing concrete is resorted to when the quantity if concrete to be used in a work is insufficient to warrant the necessity of machine. This is used with advantage in places where machinery cannot be used on account of their non-availability or in works near a hospital where the noise of machine is not desirable. Hand mixing is done on a clean, hard and impermeable surface. Cement and sand are first mixed dry with the help of shovels until the mixture attains uniform color. Aggregative are then added to this mixture and the whole mixture is then turned by shovels until the stone pieces uniformly spread throughout. After this, desired are quantity of water is poured into the heap from a can fitted with a rose. The mass is then turned until a workable mixture is obtained. It is advised to add 10% extra cement to guard against the possibility of inadequate mixing by this method.

Machine Mixing: The machine used for mixing concrete is termed as concrete mixer. Two types of concrete mixers are in common are

1. Continuous mixers

2.Batch mixers

Continuous mixers are employed in massive construction where large and continuous flow of concrete is desired. The process of feeding the mixing is more or less automatic. The machine requires careful supervision so as to obtain the concrete mix of desired consistency.

In batch type of concrete mixer. The desired proportion of materials are fed into the hopper of a drum in which the materials get mixed by the series of blades or baffles inside the mixer. Batch mixers are further two types 1. Tilting drum type 2. closed drump type. In the first type, components are fed in the revolving drum in a tilted position and after sometime the concrete mix is discharged by tilting the drums in the opposite direction. In the latter type the drum remains rotating in one direction and emptied by means of hopper which tilts to receive the discharge.

While using the mixer, coarse aggregates should be fed first, sand and cement should be put afterwards. In this revolving state, the components get mixed while water is poured with the help of can. The concrete should be for atleast 2 minutes, the time being measured after all the ingredients including water have been fed into the drum.

Bulking of Sand

Bulking of Sand

The presence of moisture in sand increases the volume of sand. This is due to fact that moisture causes film of water around the sand particles which result in the increase of volume of sand. For a moisture content of 5 to 8 percent, the increase in volume may be about 5 to 8 percent, depending upon the grading of sand. The finer the material, the more will be the increase in volume for a given moisture content. This phenomenon is known as bulking of sand.

When moisture content is increased by adding more water, sand particles pack near each other and the amount of bulking of sand is decreased. Thus the dry sand and the sand completely flooded with water have practically the same volume.

For finding the bulking of sand, a test is carried out with following procedure.

·     A container is taken and it is filled two third with the sample of sand to be tested.

·     The height is measured, say 20cm.

·     Sand is taken out of container.

·     The container is filled with water.

·     Sand is then slowly dropped in the container and it is thoroughly stirred by means of a rod.

·     The height of sand is measured say 16cm, then bulking of sand =

                         20-16                4

              =   ----------------  =  --------     or 25%

                       16                       16

Uses of Cement

Uses of Cement

1. Cement mortar for masonry work, plaster, pointing etc.

2. Concreter for laying floors, roofs and constructing lintels, beams, weather sheds, stairs, pillars etc.

3. Construction of important engineering structure such as bridges, culverts, dams, tunnels storage reservoirs, light houses, deckles etc.

4. Construction of water tanks, wells, tennis courts, septic tanks, lampposts, roads, telephone cabins etc.

5. Making joints for drains, pipes etc.

6. Manufacture of pre cast pipes, piles, garden seats, artificially designed urns, flowerpots, etc dustbins, fencing posts etc.
7. Preparation of foundations, watertight floors, footpaths etc.

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

Grading of Aggregates

Grading of Aggregates

Grading of aggregates consists of proportioning the fine and coarse aggregates in such a ratio, so as to get strongest and densest mix with the least amount of cement. Grading the aggregates is so graded as to have minimum voids when mixed with all ingredients, and water should render a concrete mass of easy workability.

The grading of aggregates are done by the following methods

·     By trail – In this method, proportioning of aggregates as to give heaviest weight for same volume, yield the densest concrete

·     By finesse modules method (sieve analysis method): in this method, the samples of both coarse and fine aggregates are passed through a set of nine standard sieve and the percentage of sample retained on each of the said sieves is determined. The total of these percentages divided by 100 gives the finesses modulus of sample

·     By minimum voids method: This method is based on the fact, that so obtain dense concrete the quantity of cement should also be slightly in excess of voids more that the fine aggregates. In this method the voids in the fine and coarse aggregates are separately found out with the help of graduated cylinder and water. The percentage of voids I aggregate, “X” given by the equation.

X = (V1 – V2) x 100

                                                                                       V2

Where v1, volume of water filled

Where v2, volume of aggregates.

·     By arbitrary standards: It is a commonly adopted method of propitiating the aggregates in a concrete mix for small works of moderate importance. This method is not recommended for large works or important works in this method, the volume of cement, sand and coarse aggregates are taken in the proportion of 1:n:2n respectively. The quantity of water to be used a varied suit the workability descried.

Ex:                           1:1:2 M250 rich mix for columns, beams

                                   1:1:3 – M200 Water retaining structures etc

                                  1:3:6 – M150 slab’s columns roads etc

                                  1:3:6 – M100 – foundations,

                                   1:4:8 - For mass concrete.

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.

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.

Equipment Required in Ready Mix Concrete

Equipment Required in Ready Mix Concrete

Following are the equipments required in Ready Mix Concrete

1.Batching plant

2.Transit mixer

BATCHING

Batching plants are classified as

1.Manual

2.Semiautomatic

3.Fully automatic

STORAGE

Storage of the raw materials is done by following methods: -

INLINE BINS Inert raw materials like fine & coarse aggregates are stored in bins called as

“Inline Bins” where the trucks carrying fine & coarse aggregate can dump the material easily.

The aggregates required are fed by the means of aggregate belt conveyer. On the aggregate belt conveyer the aggregates are weighed automatically by means of computer form the computer room presents on the plant.

SILOS

Cement & Flash are stored in airtight container called as “Silos”. The required quantity of cement & fly ash is extracted by the silos. There are two cement silos and one silo of fly ash.

Information to be Supplied by the Producer

Information to be Supplied by the Producer

The Ready Mix Concrete supplier must provide the following information to the consumer if and when requested:

·     Nature and source of each constituent material including the name of the manufacturer in case of branded products like cement, admixtures etc.

·     Proportion of quantity of each constituent per CuM of fresh concrete.

·     Generic type of the active constituent of the chemical admixture and its solid content.

·     Chloride content in all constituent materials.

·     Compatibility of cement and chemical/mineral admixtures.

·     Compatibility of admixtures with one another when more than two types of admixtures are proposed.

·     Initial and final setting time of concrete when admixture is used.

·     Details of plant and machinery (capacity CuM/hr), storage (CuM) availability, type of facilities to dose admixtures, type of moisture measurement arrangement, type of mixer, rated capacity (CuM/min.) of the mixer.

·     Availability of number of transit mixers and their capacities.

·     Details of last calibrations done on various weighing /dosing equipments

·     Testing facilities available at RMC plant

·     Capacity and type of concrete pump and placing equipment available (if required).

Need to be Specified for Ready Mix Concrete

Need to be Specified for Ready Mix Concrete

The following need to be specified very clearly:

·     Characteristic strength or grade (N/mm2)

·     Target workability or slump in mm required at site

·     Exposure conditions for durability requirements

·     Maximum water to cement ratio

·     Minimum cement content

·     Maximum aggregate size

·     Type of cement

·     Mineral admixture and its proportion (Kg/m3)

·     Maximum aggregate size

·     Rate of gain of strength (for formwork removal or prestressing etc.)

·     Maximum temperature of concrete at the time of placing (in extreme climatic conditions or incase of massive concrete pours)

·     Type of surface finish desired

·     Method of placing

·     Rate of supply desired to match the placing and compaction speed planned at site.

·     Quantity of concrete required.

·     Lift and lead of concrete transportation and placement at site.

·     Frequency of concrete testing

·     Details of materials and their required tests.

·     Permeability tests required (if any)

·     Placing of concrete in formwork to be under scope of RMC supplier (if required)

·     Permissible wastage

·     Mode of measurement.

Admixtures

Admixtures

Ready Mix Concrete is generally transported to different construction sites and delivered with the help of revolving type transits mixers. These sites are located at long distances and the concrete delivered is workable, plastic and flowable. Experience shows that slump loss takes place with certain types of cement and to prevent this superpplasticiser are used.

Ready Mix Concrete's often use admixtures formulated for special purposes lik e:

(a) Improvement of screeds, renders, mortars and concrete for increased water resistance and less risk - from aggressive materials,

( b) Integral water proofers,

 (c) Foamed - for light weight insulation. void filling and semi structural support,

(d) Water repelling for semi-dry block concrete,

(e) Microsillica for use in high performance concrete. The introduction of microsillica was observed to improve the pore solution chemistry of HPC. For higher replacement of cement by silica fumes, the concentration of both K+ and OH-ions are substantially removed. However, up to 20% replacement will not cause a drop in pH below that of saturated CaOH solution, which is approx.12.5. Even at 30%

replacement,  pH does not drop below 11.5, which is considered to be a threshold value for maintaining a good passivity of embedded steel.

Batching and Mixing Plants

Batching and Mixing Plants

The principal functional elements of every stationary concrete production Plant comprises of the following:

·     Storage of materials - Silos, containers and bins

·     Batching arrangement

·     Measuring and recording equipment

·     Mixing equipment

·     Control systems

·     Electrical, hydraulic and pneumatic drives
·     Conveying systems (belt / screw conveyors)

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.