concrete

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CONCRETE

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INTRODUCTION OF CONCRETE

Concrete is a mixture of sand, gravel, crushed rock or other aggregate held together by a hardened paste of cement and water.

This mixture, when properly proportioned, is at first a plastic mass that can be cast or molded into a predetermined size and shape.

Upon hydration of the cement by the water, concrete becomes stone like in strength, hardness and durability.

Cement

(+ Admixture) → Cement paste

+ Water + → mortar

fine aggregate + → concrete

coarse aggregate


Different between cement and concrete

Cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates and paste. The

aggregates are sand and gravel or crushed stone; the paste is water and Portland cement.

Concrete gets stronger as it gets older. Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver.

Cement comprises from 10 to 15 percent of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass.

This hardening process continues for years meaning that concrete gets stronger as it gets older.

So, there is no such thing as a cement sidewalk, or a cement mixer; the proper terms are concrete sidewalk and concrete mixer.


Classifications of concrete

Based on unit weight Ultra light concrete <1,200 kg/m3 Lightweight concrete 1200- 1,800 kg/m3 Normal-weight concrete ~ 2,400 kg/m3

Heavyweight concrete > 3,200 kg/m3


Classifications of concrete (Cont’d)

Based on strength (of cylindrical sample) Low-strength concrete < 20 MPa compressive

strength Moderate-strength concrete 20 -50 MPa compressive

strength High-strength concrete 50 - 200 MPa compressive

strength Ultra high-strength concrete > 200 MPa compressive

strength


Classifications of concrete (Cont’d)

Based on additives: Normal concrete Fiber reinforced concrete Polymer concrete


Materials used in concrete:

Cement Water Aggregates Admixture


Cement

A mixture of compounds made by burning limestone and clay together at very high temperature ranging from 1400 to 1500°C. the production of Portland cement begins with the quarrying of limestone, CaCO3. Then mixed with clay (or shale), sand and iron ore and ground together to form a homogenous powder.


Water

It is the key ingredient. When mixed with cement, forms a paste that binds the

aggregates together Water causes the hardening of concrete through process

call hydration. The water needs to be pure in order to prevent side

reaction from occurring which may weaken the concrete or otherwise interfere with hydration process.

The ratio of cement and water is the most critical factor in the production of ‘perfect’ concrete.

Too much water can reduces concrete strength but high workability

Too little water will make the concrete unworkable but high strength


Aggregates

Chemically inert, solid bodies, held together by the cement.

Come in various shapes, sizes and materials ranging from fine particles of sand to large, coarse rock.

Soft, porous aggregates can result in weak concrete with low wear resistance.

Hard aggregates can make strong concrete with high resistance to abrasion

Should be clean, hard and strong. Usually washed to remove any dust silt, clay, organic matter.


Admixture

A material, other than aggregates, cement, or water, added in small quantities to the mix in order to produce some desired modifications, either to the physical or chemical properties of the mix or of the hardened product.

The most common admixtures affect plasticity, air entrainment and curing time.


PROPERTIES OF CONCRETE

Grades of concrete Generally graded according to its compressive strength at

28 days Concrete hardens and gains strength as it hydrates. The

hydration process continues over a long period of time. It happens rapidly at first and slows down as time goes by. To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected by specification writing authorities as the age that all concrete should be tested. At this age, a substantial percentage of the hydration has taken place.

The various grades of concrete as stipulated in codes of Practice BS8110 grouped the grade in nine categories which is best known based on their characteristic strength in N/mm2


Grades of concrete proposed by Code Practice BS8110

Grade Characteristic strength (N/mm2)

Lowest grade suitable for specific purposes

7

10

7.0

10.0

Mass concrete

15 15.0 Reinforced concrete using light weight aggregates

20

25

20.0

25.0

Reinforced concrete using heavy weight aggregates

30 30.0 Prestressed post-tensioned concrete

40

50

60

40.0

50.0

60.0

Prestressed pre-tensioned concrete


Workability of concrete

workability implies the ease with which a concrete mix can handled from the mixer to its finally compacted shape.

Factors affecting workability: Water cement ratio Aggregates (shape, texture, size) Fineness of cement Time and temperature Admixture

Measurement of workability Slump test Compacting factor test Flow test Kelly ball test Vee Bee consistometer test


Slump Test

This test method covers the determination of slump of concrete, both in the laboratory and in the field.

This test determines slump of plastic hydraulic cement concretes

Apparatus : Mold - in the form of the lateral surface of the

frustum of a cone with base 200mm in diameter, the top 100mm in diameter and the height 300mm inches.

Tamping rod - round, straight steel rod 16mm inches in diameter and 600mm in length.


PROCEDURE : Dampen the mold and place it on

a flat, moist, non absorbent surface.

Rod each layer with 25 strokes of the tamping rod. Rod the top, second and bottom layer throughout its depth.

In filling and rodding the top layer, heap the concentrate above the mold before rodding is started. Remove the mold immediately from the concrete by raising it carefully in a vertical direction.

Immediately measure the slump by determining the vertical difference between the top of the mold and the displaced original center of the top surface of the specimen.


(a) True slump

(b) Shear slump

(c) Collapse slump

Conventionally, when shear or collapse slump occur, the test is considered invalid.


Compacting Factor Test

The compacting factor test gives the behavior of fresh concrete under the action of external forces, i.e to measure the degree of compaction obtained by doing a standard amount of work on the concrete. The method of determining the compacting factor test is described in BS 1881: Part 103 : 1983.


Compacting Factor Test Equipment

The compacting factor equipment consists of two conical hoppers mounted vertically above a cylinder.

Each of the conical hoppers comprise of a hinged flange and a quick release mechanism to allow the concrete sample to flow freely into the cylinder.

The hoppers and cylinder is mounted on a steel rigid frame and are easily removed for cleaning. The apparatus is protested against corrosion.


In making the test, the top hopper is filled with a representative sample of the concrete.

When completely filled, a hinged door at the bottom is released and the concrete allowed to fall into the second hopper.

The filling of the second hopper is thus affected by a standard method. The concrete is similarly released from the second hopper and falls into the cylindrical container.

Surplus concrete is struck off by simultaneously working two steel floats from the outside to the center. The contents of the cylinder are then weighed to the nearest 10 grams giving the weight of partially compacted concrete.

The cylinder is then refilled from the same sample in layer approximately 50mm deep, the layers being rammed to obtain full compaction.

The top surface is gain struck off level with the top of the cylinder and the weight the concrete container again determined which is known as the weight of fully compacted concrete.


The compacting factor is the ratio of the weight of partially compacted concrete to the weight of fully compacted concrete. The difference in the two weights is due to air voids, and the closer the values, the less the air voids and the higher the compacting factor. The workability is therefore increase as the compacting factor approaches unity.

Compacting factor = weight of partially compacted

Fully compacted


Mixing concrete

Concrete can be mixed on site or brought to site as ready mix from works where it is mixed in large quantities and distributed to sites.

Mixing directly on site will only happen for small jobs or those which are so large, as in the case of civil engineering contracts for bridges, reservoirs or motorways, that large-scale mixing is the only solution.

Mixing directly on site can be manual and use the machine (drum concrete mixer)

All machines used for mixing concrete have to be cleaned everyday, usually with water and loose aggregates

The ingredients (cement, aggregates, water) can be count by weight or volume.


DRUM MIXER & READYMIX CONCRETR TRUCK


Transporting concrete

The various methods used to move the concrete from the mixer or truck to the forms depend largely upon the job conditions.

On small jobs, wheelbarrows are the usual means of transportation.

However, concrete can be handled and transported by many methods, including the use of chutes, buggies operated over runways, buckets handled by cranes or cable ways, small rail cars, trucks, pumps to force the concrete through pipelines, and equipment to force the concrete through hoses pneumatically.


Transporting concrete


Placing Concrete All concrete forms must be clean, tight, adequately braced,

and constructed of materials that will impart the desired texture to the finished concrete.

Sawdust, nails, and other debris should be removed from the forms before the concrete is placed.

Wood forms should be moistened before the concrete is placed, otherwise they will absorb water from the concrete and swell.

In addition, the forms should be oiled or lacquered to make form removal easier.

Reinforcing steel should be clean and free of loose rust or mill scale at the time the concrete is placed. Any coatings of hardened mortar should be removed from the steel.


The concrete should be placed between the forms or screeds as close as possible to its final position.

To consolidate the concrete, it should be mechanically vibrated or spaded as it goes into the form.

Then the concrete is thoroughly spaded next to the forms to eliminate voids or honeycombing at the sides.

In inaccessible areas, the forms can be tapped lightly with a hammer to achieve consolidation.

This operation makes a dense concrete surface by forcing the coarse aggregate away from the form or face.

The concrete should not be overworked while it is still plastic. Overworking will cause too much water and fine material to be brought to the surface. This may later lead to scaling or dusting.


Segregation (separation)

Segregation is when the aggregates separate from the rest of the concrete. This causes weakening and excessive curling and shrinkage. Some of the ways to avoid segregation include: Placing the concrete as close as possible to its final

position. Do not drop from higher that 2-3 feet. Avoid high slumps. Do not move the concrete with a vibrator.


Bleeding (water concentration)

Bleeding means the concentration of water at certain portions of the concrete.

The locations with increased water concentration are concrete surface, bottom of large aggregate and bottom of reinforcing steel.

Bleed water trapped under aggregates or steel lead to the formation of weak and porous zones, within which micro cracks can easily form and propagate.


Compacting concrete After placing the concrete it has to be compacted by

removing voids. This can be achieved by overfilling and physically tamping

the concrete into place, or by using mechanical vibration. Poker vibrators are used which allow air bubbles to rise to

the surface with a cement-rich thin film. When this activity stops the poker can be moved along

usually at intervals of between 300 and 500mm. When pre-cast elements are made, the concrete is poured

into forms which are vibrated as a whole on tables. Surface vibrators are only used for concrete which has a

maximum depth of 150mm for floors or roads. There is an approximate loss of strength of 5% for every

1% of air in the mix. For a concrete mix to be durable it must be dense.


Curing Concrete

Concrete hardens because of hydration, the chemical reaction between Portland cement and water.

As long as the temperatures are favorable and moisture is present to hydrate the cement, the following properties of concrete improve with age: durability (resistance to freezing and thawing), strength, watertightness, wear resistance, and volume stability.


Effect of Curing

All of the desirable properties of concrete are improved by the proper curing process.

Soon after the concrete is placed, the increase in strength is very rapid (for a period of 3 to 7 days). The strengthening then continues slowly for an indefinite period.

Concrete which is moist cured for 7 days is about 50 percent stronger than that which is exposed to dry air for the same period.

If the concrete is kept damp for one month, the strength is about double that of concrete cured in dry air.


FACTORS AFFECTING THE PERFORMANCE OF CONCRETE

FRESH CONCRETE CEMENT

Composition Quantity

AGGREGATES Size Shape Grading Quantity Moisture

WATER Quantity

MIXING TRANSPORTING PLACING COMPACTING

HARDENED CONRETE CURING

Assignment 1

Why is Concrete Important? need to create a list of the importance of concrete

and explain how it affects your lives.

Applications of Concrete Need to create a list of the past, present, and future

applications of concrete.

Describe the process of curing.

Factors affecting concrete strength

Concrete porosity: voids in concrete can be filled with air or with water. Air voids are an obvious and easily-visible example of pores in concrete. Broadly speaking, the more porous the concrete, the weaker it will be. Probably the most important source of porosity in concrete is the ratio of water to cement in the mix, known as the 'water to cement' ratio. This parameter is so important it will be discussed separately below.

Water/cement ratio: this is defined as the mass of water divided by the mass of cement in a mix. For example, a concrete mix containing 400 kg cement and 240 litres (=240 kg) of water will have a water/cement ratio of 240/400=0.6. The water/cement ratio may be abbreviated to 'w/c ratio' or just 'w/c'. In mixes where the w/c is greater than approximately 0.4, all the cement can, in theory, react with water to form cement hydration products. At higher w/c ratios it follows that the space occupied by the additional water above w/c=0.4 will remain as pore space filled with water, or with air if the concrete dries out.

Consequently, as the w/c ratio increases, the porosity of the cement paste in the concrete also increases. As the porosity increases, the compressive strength of the concrete will decrease.

Soundness of aggregate: it will be obvious that if the aggregate in concrete is weak, the concrete will also be weak. Rocks with low intrinsic strength, such as chalk, are clearly unsuitable for use as aggregate.

Aggregate-paste bond: the integrity of the bond between the paste and the aggregate is critical. If there is no bond, the aggregate effectively represents a void; as discussed above, voids are a source of weakness in concrete.

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