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.

Curing of Concrete

Curing of Concrete

Curing may be defined as the process of maintaining satisfactory moisture and temperature conditions for freshly placed concrete for some specified time for proper hardening of concrete. Curing in the early ages of concrete is more important. Curing for 14 days is very important. Better to continue it for 7 to 14 days more. If curing is not done properly, the strength of concrete reduces. Cracks develop due shrinkage. The durability of concrete structure reduces.

The following curing methods are employed:

(a)  Spraying of water

(b)  Covering the surface with wet gunny bags, straw etc.

(c)  Ponding

(d)  Steam curing and

(e)  Application of curing compounds.

(a) Spraying of water: Walls, columns, plastered surfaces are cured by sprinkling water.

(b) Wet covering the surface: Columns and other vertical surfaces may be cured by covering the surfaces with wet gunny bags or straw.

(c) Ponding: The horizontal surfaces like slab and floors are cured by stagnating the water to aheight of 25 to 50 mm by providing temporary small hunds with mortar.

(d) Steam curing: In the manufacture of pre-fabricated concrete units steam is passed over the units kept in closed chambers. It accelerates curing process, resulting into the reduction of curing period.

(e) Application of curing compounds: Compounds like calcium chloride may be applied on the curing surface. The compound shows affinity to the moisture and retains it on the surface. It keeps the concrete surface wet for a long time.

Preparing and Placing of Concrete

Preparing and Placing of Concrete

The following steps are involved in the concreting:

1.Batching

2.Mixing

3.Transporting and placing and

4.Compacting.

1. Batching: The measurement of materials for making concrete is known as batching. The following two methods of batching is practiced:

(a) Volume batching

(b) Weight batching

(a) Volume Batching: In this method cement, sand and concrete are batched by volume. A gauge box is made with wooden plates, its volume being equal to that of one bag of cement. One bag of cement has volume of 35 litres. The required amount of sand and coarse aggregate is added by measuring on to the gauge box. The quantity of water required for making concrete is found after deciding water cement ratio. For example, if water cement ratio is 0.5, for one bag of cement (50 kg), water required is 0.5 × 50= 25 kg, which is equal to 25 litres. Suitable measure is used to select required quantity of water.

(b) Weight Batching: This is the recommended method of batching. A weighing platform is used in the field to pick up correct proportion of sand and coarse aggregates. Large weigh batching plants have automatic weighing equipments.

2.Mixing: To produce uniform and good concrete, it is necessary to mix cement, sand and coarse aggregate, first in dry condition and then in wet condition after adding water. The following methods are practiced:

(a) Hand Mixing

(b) Machine Mixing.

(a)  Hand Mixing: Required amount of coarse aggregate for a batch is weighed and is spread on an impervious platform. Then the sand required for the batch is spread over coarse aggregate. They are mixed in dry condition by overturning the mix with shovels. Then the cement required for the batch is spread over the dry mix and mixed by shovels. After uniform texture is observed water is added gradually and mixing is continued.

(b) Machine Mixing: In large and important works machine mixing is preferred. A typical concrete mixer. Required quantities if sand and coarse aggregates are placed in the drum of the mixer. 4 to 5 rotations are made for dry mixing and then required quantity of cement is added and dry mixing is made with another 4 to 5 rotations. Water is gradually added and drum is rotated for 2 to 3 minutes during which period it makes about 50 rotations. At this stage uniform and homogeneous mix is obtained.

3. Transporting and Placing of Concrete. After mixing concrete should be transported to the final position. In small works it is transported in iron pans from hand to hand of a set of workers. Wheel barrow and hand carts also may be employed. In large scale concreting chutes and belt conveyors or pipes with pumps are employed. In transporting care should be taken to see that seggregation of aggregate from matrix of cement do not take place.

Concrete is placed on form works. The form works should be cleaned and properly oiled. If concrete is to be placed for foundation, the soil bed should be compacted well and is made free fromloose soil.

Functions of Various Ingredients

Functions of Various Ingredients

Cement is the binding material. After addition of water it hydrates and binds aggregates and the surrounding surfaces like stone and bricks. Generally richer mix (with more cement) gives more strength. Setting time starts after 30 minutes and ends after 6 hours. Hence concrete should be laid in its mould before 30 minutes of mixing of water and should not be subjected to any external forces till final setting takes place.

Coarse aggregate consists of crushed stones. It should be well graded and the stones should be of igneous origin. They should be clean, sharp, angular and hard. They give mass to the concrete and prevent shrinkage of cement. Fine aggregate consists of river sand. It prevents shrinkage of cement. When surrounded by cement it gains mobility enters the voids in coarse aggregates and binding of ingredients takes place. It adds density to concrete, since it fills the voids. Denser the concrete higher is its strength.

Water used for making concrete should be clean. It activates the hydration of cement and forms plastic mass. As it sets completely concrete becomes hard mass. Water gives workability to concrete which means water makes it possible to mix the concrete with ease and place it in final position. More the water better is the workability. However excess water reduces the strength of concrete. The variation of strength of concrete with water cement ratio. To achieve required workability and at the same time good strength a water cement ratio of 0.4 to 0.45 is used, in case of machine mixingand water cement ratio of 0.5 to 0.6 is used for hand mixing.

Concrete

Concrete

 Plain concrete, commonly known as concrete, is an intimate mixture of binding material, fine aggregate, coarse aggregate and water. This can be easily moulded to desired shape and size before it loses plasticity and hardens. Plain concrete is strong in compression but very weak in tension. The tensile property is introduced in concrete by inducting different materials and this attempt has given rise to RCC, RBC, PSC, FRC, cellular concrete and Ferro cement.

Plain Concrete

Major ingredients of concrete are:

1.Binding material (like cement, lime, polymer)

2.Fine aggregate (sand)

3.Coarse aggregates (crushed stone, jelly)

4.Water.

A small quantity of admixtures like air entraining agents, water proofing agents, workability agents etc. may also be added to impart special properties to the plain concrete mixture.

Depending upon the proportion of ingredient, strength of concrete varies. It is possible to determine the proportion of the ingredients for a particular strength by mix design procedure. In the

absence of mix design the ingredients are proportioned as 1:1:2, 1:11/2 :3, 1:2:4, 1:3:6 and 1:4:8, which is the ratio of weights of cement to sand to coarse aggregate.

In proportioning of concrete it is kept in mind that voids in coarse aggregates are filled with sand and the voids in sand are filled with cement paste. Proportion of ingredients usually adopted for various works

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.

Building Foundation

Building Foundation

The foundation is the most critical part of any structure and most of the failure is probably due to faulty foundations rather than any other cause. The purpose of foundation is to transmit the anticipated loads safety to the soil.

Basic requirements:

·     To distribute the total load coming on the structure over a large bearing area so as to prevent it from any movement.

·     To load the bearing surface or area at a uniform rate so as to prevent any unequal or relative settlement.

·     To prevent the lateral movement of the structure.

·     To secure a level or firm natural bed, upon which to lay the courses of masonry and also support the structure.

·     To increase the suitability of the structure as a whole, so as to prevent it from overturning or sliding against such as wind, rain, frost etc.

Curing of concrete

Curing of concrete

Curing of concrete is one of the essential requirement of process of concreting. Curing is process of keep the set concrete damp for some days in order to enable the concrete gain more strength.

Purposes:

·     Curing protects concrete surfaces from sun and wind

·     Presence of water is essential to cause the chemical action which a companies the setting of concrete

The Many Uses of Ready-Mix Concrete

The Many Uses of Ready-Mix Concrete

Concrete is a popular building material. You can see it in homebuilding sites, commercial building sites and government projects such as bridges and highways. It is easier to complete big projects like these by using ready-mix concrete. Completing projects on a tight deadline is possible and achieved in less than half the time it would when not using this product.

Ready-mix concrete comes from manufacturers and plants ready for delivery and mixing at work or project sites. With the use of a concrete mixer, one can get a precise mixture of concrete for the project. Using this type of cement material eliminates too much confusion at the work site. It also saves considerable amount of time because it requires less time to prepare than concrete prepared from scratch.

Cements, sands, aggregates (gravels) and water are the main contents of the mixture. It has the same ingredients as the ones that are not except that instead of carrying and mixing the raw materials at the site, the ready-mix arrives at the site on mixer trucks already pre-mixed and ready to use. Adding different additives and aggregates offsite and then delivering onsite for a different project based upon the specification of the customer is another feature of the ready-mix concrete. Different textures, finishes and colors are also available in ready-mix form.

Using ready-mix concrete eliminates having to carry and mixing the materials on site, which is a painstaking process. It eliminates errors that come with wrong measurement of water and the concrete materials. Using ready-mix saves time and effort. Big projects take less time to complete after pouring in the mixture using the transit mixer. The quality of the product is also much better than those that come in a non-ready mixed form.

Ready-mix concrete has a big potential in a lot of building projects. Aside from using it on big infrastructure projects such as building bridges, highways and huge buildings, we see this type of concrete as the choice of homeowners when building driveways, walkways made of concrete. Some homeowners choose concrete for their kitchen and bathroom countertops and as floorings. Stained concrete floors and counters provide a rustic look and patina adding more character to the interior of the home.

When using ready-mix concrete one must work with caution. One must ensure that there is enough space for the transit mixer. In addition, the location should be strong enough to handle the weight of the transit mixer and the concrete. The workers practice caution when working with the concrete mixer, by avoiding standing near the way of the mixer, especially when operating and pouring concrete. Accidents can and do happen which can endanger the well-being of the workers.

Characteristics of sand

Characteristics of sand

·     It should be chemically inert

·     It should be clean and coarse. It should be free from organic matter.

·     It should contain sharp, angular and durable grains.

·     It should not contain salts, which attract the moisture from atmosphere.

·  It should be well graded (i.e.) should contain particles of various sizes in suitable proportions.

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.

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.

Advantages of artificial stones

Advantages of artificial stones

1.Cavities may be kept in artificial stones to convey pipes, electric wires etc.

2.Grooves can be kept in artificial stone while it is being cast which are useful for fixing various fittings.

3.It can cast in desired shape

4.It can be made in a single piece and hence trouble of getting large blocks of stone for lintels, beams etc is avoided.

5.It can be made stronger than natural stone

6.It is cheap and economical

7.It is more durable than natural stone

8.Natural bed is absent in artificial stones and hence, the question of taking precautions with respect to the natural bed of stones does not arise.

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.

Material Required for Ready Mix Concrete

Material Required for Ready Mix Concrete

Admixture: A substance added to the basic concrete mixture to alter one or more properties of the concrete; ie fibrous materials for reinforcing, water repellent treatments, and coloring compounds.

·     Air-entraining admixtures (mainly used in concrete exposed to freezing and thawing cycles)

·     Water-reducing admixtures, plasticizers (reduce the dosage of water while maintaining the workability)

·     Retarding admixtures (mainly used in hot weather to retard the reaction of hydration)

·     Accelerating admixtures (mainly used in cold weather to accelerate the reaction of hydration)

·     Super plasticizer or high range water-reducer (significantly reduce the dosage of water while maintaining the workability)

·     Miscellaneous admixtures such as corrosion inhibiting, shrinkage reducing, coloring, pumping etc.

Aggregate: Inert particles (i.e. gravel, sand, and stone) added to cement and water to form concrete.

Cement: Dry powder that reacts chemically with water to bind the particles of aggregate, forming concrete. Portland cement is typically used in concrete production.

Fly ash: Fly ash is a by-product from coal-fired electricity generating power plants. The coal used in these power plants is mainly composed of combustible elements such as carbon, hydrogen and oxygen (nitrogen and sulfur being minor elements), and non-combustible impurities (10 to 40%) usually present in the form of clay, shale, quartz, feldspar and limestone.

Scope of Ready Mix Concrete

Scope of Ready Mix Concrete

Long, Long years ago, their where simple houses but in 21st century we can see houses constructed in R.C.C. Therefore concrete got more importance than any other construction material. So the use of concrete is increasing day by day.

For construction most of the contractors and builders have to collect the raw materials required for the construction before starting actual works. These materials should be stored at the site properly. This technique can be possible when there will be more empty space at the construction site which is not possible in congested areas. At this time there is one solution to overcome all these problems that is nothing “READYMIX CONCRETE”.

By using R.M.C we can save the time and money required for the labors. In following places ready mix concrete can be used:-

·     Major concerting projects like dams, roads, bridges, tunnels, canals etc.

·     For concreting in congested areas where storage of materials is not possible.

·     Sites where intensity of traffic makes problems.

·     When supervisor and labor staff is less.

·     To reduce the time required for construction etc.

·     Huge industrial and residential projects.

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).

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.

Testing of Concrete

Testing of Concrete

The results of concrete tests are used as the basis for deciding whether the delivered concrete is in accordance with the specification, if the reported results are below the compliance loyal, doubt is cast on the quality of the supplied concrete and the long term durability of the structure. If the investigation indicates that the sampling, preparation and or storage of the test specimens have not been in accordance with the required standard procedures, then the result will be invalid. Further valuable management time, better devoted to other management functions, will have been wasted.

It is, therefore, in everyone's interest that sampling and testing is done correctly so that the results provide a valid basis for logical decision making. Where the contractor is required to perform on-site concrete testing, it is imperative that suitable test facilities are available, that the appropriate equipment is calibrated and that the staff has been fully trained in the relevant test techniques, one can look the factors that affect the price compliance test, (i.e.) the compressive strength test.

As  a result is the average of the compressive test specimens made for the same sample of concrete. Individual variation should not be more than 15 Percent of the average. If more. The results of the average. If more, the results of the sample are invalid. This is because differences of this magnitude indicate poor sampling, cube making, curing or crushing. It is, therefore essential that correct test facilities are provided and the people with responsibility for sampling and testing concrete are suitably trained.

Slump test for predicting the workability of concrete is done at batching plant and working site. The main problem in the production of ready mixed concrete is maintaining the workability of the mix right to the time of placing. Concrete stiffens with time and the stiffening may also be aggravated by prolonged mixing and by high temperature. With the use of retarding admixtures, the time limit can be extended to 3 or even 4 hours, The United States Bureau of Reclamation provides for an extension of 3 to 6 hours in the time of contact between cement and wet aggregate in transport prior to mixing.  The general requirement is that concrete shall be discharged from the truck-mixer within 2 h of the time of loading. However, a longer period may be permitted if retarding admixtures are used or in cool humid weather or when chilled concrete is produced.