Advantages of using Portland pozzolana cement over OPC

Pozzolana combines with lime and alkali in cement when water is added and forms compounds which contribute to strength, impermeability and sulphate resistance. It also contributes to workability, reduced bleeding and controls destructive expansion from alkali-aggregate reaction. It reduces heat of hydration thereby controlling temperature differentials, which causes thermal strain and resultant cracking n mass concrete structures like dams. The colour of PPC comes from the colour of the pozzolanic material used. PPC containing fly ash as a pozzolana will invariably be slightly different colour than the OPC.One thing should be kept in mind that is the quality of cement depends upon the raw materials used and the quality control measures adopted during its manufacture, and not on the shade of the cement. The cement gets its colour from the nature and colour of raw materials used, which will be different from factory to factory, and may even differ in the different batches of cement produced in a factory. Further, the colour of the finished concrete is affected also by the colour of the aggregates, and to a lesser extent by the colour of the cement. Preference for any cement on the basis of colour alone is technically misplaced.

Settling Of Cement

When water is mixed with cement, the paste so formed remains pliable and plastic for a short time. During this period it is possible to disturb the paste and remit it without any deleterious effects. As the reaction between water and cement continues, the paste loses its plasticity. This early period in the hardening of cement is referred to as ‘setting’ of cement.

Initial and final setting time of cement

Initial set is when the cement paste loses its plasticity and stiffens considerably. Final set is the point when the paste hardens and can sustain some minor load. Both are arbitrary points and these are determined by Vicat needle penetration resistance

Slow or fast setting normally depends on the nature of cement. It could also be due to extraneous factors not related to the cement. The ambient conditions play an important role. In hot weather, the setting is faster, in cold weather, setting is delayed Some types of salts, chemicals, clay, etc if inadvertently get mixed with the sand, aggregate and water could accelerate or delay the setting of concrete.

Storage of Cement






It needs extra care or else can lead to loss not only in terms of financial loss but also in terms of loss in the quality. Following are the don’t that should be followed -

(i) Do not store bags in a building or a godown in which the walls, roof and floor are not completely weatherproof.

(ii) Do not store bags in a new warehouse until the interior has thoroughly dried out.

(iii) Do not be content with badly fitting windows and doors, make sure they fit properly and ensure that they are kept shut.

(iv) Do not stack bags against the wall. Similarly, don’t pile them on the floor unless it is a dry concrete floor. If not, bags should be stacked on wooden planks or sleepers.

(v) Do not forget to pile the bags close together

(vi) Do not pile more than 15 bags high and arrange the bags in a header-and-stretcher fashion.

(vii) Do not disturb the stored cement until it is to be taken out for use.

(viii) Do not take out bags from one tier only. Step back two or three tiers.

(ix) Do not keep dead storage. The principle of first-in first-out should be followed in removing bags.

(x) Do not stack bags on the ground for temporary storage at work site. Pile them on a raised, dry platform and cover with tarpaulin or polythene sheet.

Coarse Aggregate

Coarse aggregate for the works should be river gravel or crushed stone .It should be hard, strong, dense, durable, clean, and free from clay or loamy admixtures or quarry refuse or vegetable matter. The pieces of aggregates should be cubical, or rounded shaped and should have granular or crystalline or smooth (but not glossy) non-powdery surfaces.Aggregates should be properly screened and if necessary washed clean before use.

Coarse aggregates containing flat, elongated or flaky pieces or mica should be rejected. The grading of coarse aggregates should be as per specifications of IS-383.

After 24-hrs immersion in water, a previously dried sample of the coarse aggregate should not gain in weight more than 5%.

Aggregates should be stored in such a way as to prevent segregation of sizes and avoid contamination with fines.

Depending upon the coarse aggregate color, there quality can be determined as:

Black => very good quality

Blue => good

Whitish =>bad quality



Fine Aggregate

Aggregate which is passed through 4.75 IS Sieve is termed as fine aggregate. Fine aggregate is added to concrete to assist workability and to bring uniformity in mixture. Usually, the natural river sand is used as fine aggregate. Important thing to be considered is that fine aggregates should be free from coagulated lumps.

Grading of natural sand or crushed stone i.e. fine aggregates shall be such that not more than 5 percent shall exceed 5 mm in size, not more than 10% shall IS sieve No. 150 not less than 45% or more than 85% shall pass IS sieve No. 1.18 mm and not less than 25% or more than 60% shall pass IS sieve No. 600 micron.

BRICKWORK

Brickwork is masonry done with bricks and mortar and is generally used to build partition walls. In our site, all the external walls were of concrete and most of the internal walls were made of bricks. English bond was used and a ration of 1:4 (1 cement: 4 coarse sand) and 1:6 were used depending upon whether the wall is 4.5 inches or 9 inches. The reinforcement shall be 2 nos. M.S. round bars or as indicated. The diameter of bars was 8mm. The first layer of reinforcement was used at second course and then at every fourth course of brick work. The bars were properly anchored at their ends where the portions and or where these walls join with other walls. The in laid steel reinforcement was completely embedded in mortar.

Bricks can be of two types. These are:

1) Traditional Bricks-The dimension if traditional bricks vary from 21 cm to 25cm in length,10 to 13 cm in width and 7.5 cm in height in different parts of country .The commonly adopted normal size of traditional brick is 23 * 11.5*7.5 cm with a view to achieve uniformity in size of bricks all over country.

2) Modular Bricks- Indian standard institution has established a standard size of bricks such a brick is known as a modular brick. The normal size of brick is taken as 20*10*10 cm whereas its actual dimensions are 19*9*9 cm masonry with modular bricks workout to be cheaper there is saving in the consumption of bricks, mortar and labour as compared with masonry with traditional bricks.

Strength of brick masonry

The permissible compressive stress in brick masonry depends upon the following factors:

1. Type and strength of brick.

2. Mix of motor.

3. Size and shape of masonry construction.

The strength of brick masonry depends upon the strength of bricks used in the masonry construction. The strength of bricks depends upon the nature of soil used for making and the method adopted for molding and burning of bricks .since the nature of soil varies from region to region ,the average strength of bricks varies from as low as 30kg/sq cm to 150 kg /sq cm the basic compressive stress are different crushing strength.



There are many checks that can be applied to see the quality of bricks used on the site.Normally the bricks are tested for Compressive strength, water absorption, dimensional tolerances and efflorescence. However at small construction sites the quality of bricks can be assessed based on following, which is prevalent in many sites.

• Visual check - Bricks should be well burnt and of uniform size and color.

• Striking of two bricks together should produce a metallic ringing sound.

• It should have surface so hard that can’t be scratched by the fingernails.

• A good brick should not break if dropped in standing position from one metre above ground level.

• A good brick shouldn’t absorb moisture of more than 15-20% by weight, when soaked in water For example; a good brick of 2 kg shouldn’t weigh more than 2.3 to 2.4 kg if immersed in water for 24 hours.

Precautions to be taken in brick masonry work

• Bricks should be soaked in water for adequate period so that the water penetrates

to its full thickness. Normally 6 to 8 hours of wetting is sufficient.

• A systematic bond must be maintained throughout the brickwork. Vertical joints

shouldn’t be continuous but staggered.

• The joint thickness shouldn’t exceed 1 cm. It should be thoroughly filled with the

cement mortar 1:4 to 1:6 (Cement: Sand by volume)

• All bricks should be placed on their bed with frogs on top (depression on top of the

brick for providing bond with mortar).

• Thread, plumb bob and spirit level should be used for alignment, verticality and

horizontality of construction.

• Joints should be raked and properly finished with trowel or float, to provide good bond.

• A maximum of one metre wall height should be constructed in a day.

• Brickwork should be properly cured for at least 10 days

REINFORCEMENT

Steel reinforcements are used, generally, in the form of bars of circular cross section in concrete structure. They are like a skeleton in human body. Plain concrete without steel or any other reinforcement is strong in compression but weak in tension. Steel is one of the best forms of reinforcements, to take care of those stresses and to strengthen concrete to bear all kinds of loads

Mild steel bars conforming to IS: 432 (Part I) and Cold-worked steel high strength deformed bars conforming to IS: 1786 (grade Fe 415 and grade Fe 500, where 415 and 500 indicate yield stresses 415 N/mm2 and 500 N/mm2 respectively) are commonly used. Grade Fe 415 is being used most commonly nowadays. This has limited the use of plain mild steel bars because of higher yield stress and bond strength resulting in saving of steel quantity. Some companies have brought thermo mechanically treated (TMT) and corrosion resistant steel (CRS) bars with added features.

Bars range in diameter from 6 to 50 mm. Cold-worked steel high strength deformed bars start from 8 mm diameter. For general house constructions, bars of diameter 6 to 20 mm are used

Transverse reinforcements are very important. They not only take care of structural requirements but also help main reinforcements to remain in desired position. They play a very significant role while abrupt changes or reversal of stresses like earthquake etc.

They should be closely spaced as per the drawing and properly tied to the main/longitudinal reinforcement

Terms used in Reinforcement

Bar-bending-schedule

Bar-bending-schedule is the schedule of reinforcement bars prepared in advance before cutting and bending of rebars. This schedule contains all details of size, shape and dimension of rebars to be cut.

Lap length

Lap length is the length overlap of bars tied to extend the reinforcement length.. Lap length about 50 times the diameter of the bar is considered safe. Laps of neighboring bar lengths should be staggered and should not be provided at one level/line. At one cross section, a maximum of 50% bars should be lapped. In case, required lap length is not available at junction because of space and other constraints, bars can be joined with couplers or welded (with correct choice of method of welding).

Anchorage Length

This is the additional length of steel of one structure required to be inserted in other at the junction. For example, main bars of beam in column at beam column junction, column bars in footing etc. The length requirement is similar to the lap length mentioned in previous question or as per the design instructions

Cover block

Cover blocks are placed to prevent the steel rods from touching the shuttering plates and there by providing a minimum cover and fix the reinforcements as per the design drawings. Sometimes it is commonly seen that the cover gets misplaced during the concreting activity. To prevent this, tying of cover with steel bars using thin steel wires called binding wires (projected from cover surface and placed during making or casting of cover blocks) is recommended. Covers should be made of cement sand mortar (1:3). Ideally, cover should have strength similar to the surrounding concrete, with the least perimeter so that chances of water to penetrate through periphery will be minimized. Provision of minimum covers as per the Indian standards for durability of the whole structure should be ensured.

Shape of the cover blocks could be cubical or cylindrical. However, cover indicates thickness of the cover block. Normally, cubical cover blocks are used. As a thumb rule, minimum cover of 2” in footings, 1.5” in columns and 1” for other structures may be ensured.

Structural element	Cover to reinforcement (mm)
Footings	40
Columns	40
Slabs	15
Beams	25
Retaining wall	25 for earth face 20 for other face

Things to Note

Reinforcement should be free from loose rust, oil paints, mud etc. it should be cut, bent and fixed properly. The reinforcement shall be placed and maintained in position by providing proper cover blocks, spacers, supporting bars, laps etc. Reinforcements shall be placed and tied such that concrete placement is possible without segregation, and compaction possible by an immersion vibrator.

For any steel reinforcement bar, weight per running meter is equal to d*d/162 Kg, where d is diameter of the bar in mm. For example, 10 mm diameter bar will weigh 10×10/162 = 0.617 Kg/m

Three types of bars were used in reinforcement of a slab. These include straight bars, crank bar and an extra bar. The main steel is placed in which the straight steel is binded first, then the crank steel is placed and extra steel is placed in the end. The extra steel comes over the support while crank is encountered at distance of ¼(1-distance between the supports) from the surroundings supports.

For providing nominal cover to the steel in beam, cover blocks were used which were made of concrete and were casted with a thin steel wire in the center which projects outward. These keep the reinforcement at a distance from bottom of shuttering. For maintaining the gap between the main steel and the distribution steel, steel chairs are placed between them

SHUTTERING AND SCAFFOLDING

DEFINITION

The term ‘SHUTTERING’ or ‘FORMWORK’ includes all forms, moulds, sheeting, shuttering planks, walrus, poles, posts, standards, leizers, V-Heads, struts, and structure, ties, prights, walling steel rods, bolts, wedges, and all other temporary supports to the concrete during the process of sheeting.



FORM WORK

Forms or moulds or shutters are the receptacles in which concrete is placed, so that it will have the desired shape or outline when hardened. Once the concrete develops adequate strength, the forms are removed. Forms are generally made of the materials like timber, plywood, steel, etc.

Generally camber is provided in the formwork for horizontal members to counteract the effect of deflection caused due to the weight of reinforcement and concrete placed over that. A proper lubrication of shuttering plates is also done before the placement of reinforcement. The oil film sandwiched between concrete and formwork surface not only helps in easy removal of shuttering but also prevents loss of moisture from the concrete through absorption and evaporation.

The steel form work was designed and constructed to the shapes, lines and dimensions shown on the drawings. All forms were sufficiently water tight to prevent leakage of mortar. Forms were so constructed as to be removable in sections. One side of the column forms were left open and the open side filled in board by board successively as the concrete is placed and compacted except when vibrators are used. A key was made at the end of each casting in concrete columns of appropriate size to give proper bondings to columns and walls as per relevant IS.






CLEANING AND TREATMENT OF FORMS

All rubbish, particularly chippings, shavings and saw dust, was removed from the interior of the forms (steel) before the concrete is placed. The form work in contact with the concrete was cleaned and thoroughly wetted or treated with an approved composition to prevent adhesion between form work and concrete. Care was taken that such approved composition is kept out of contact with the reinforcement.

DESIGN

The form-work should be designed and constructed such that the concrete can be properly placed and thoroughly compacted to obtain the required shape, position, and levels subject

ERECTION OF FORMWORK

The following applies to all formwork:

a) Care should be taken that all formwork is set to plumb and true to line and level.

b) When reinforcement passes through the formwork care should be taken to ensure close

fitting joints against the steel bars so as to avoid loss of fines during the compaction of

concrete.

c) If formwork is held together by bolts or wires, these should be so fixed that no iron is

exposed on surface against which concrete is to be laid.

d) Provision is made in the shuttering for beams, columns and walls for a port hole of

convenient size so that all extraneous materials that may be collected could be

removed just prior to concreting.

e) Formwork is so arranged as to permit removal of forms without jarring the concrete.

Wedges, clamps, and bolts should be used where practicable instead of nails.

f) Surfaces of forms in contact with concrete are oiled with a mould oil of approved

quality. The use of oil, which darkens the surface of the concrete, is not allowed. Oiling

is done before reinforcement is placed and care taken that no oil comes in contact with

the reinforcement while it is placed in position. The formwork is kept thoroughly wet

during concreting and the whole time that it is left in place.

Immediately before concreting is commenced, the formwork is carefully examined to ensure the following:

a) Removal of all dirt, shavings, sawdust and other refuse by brushing and washing.

b) The tightness of joint between panels of sheathing and between these and any hardened core.

c) The correct location of tie bars bracing and spacers, and especially connections of

bracing.

d) That all wedges are secured and firm in position.

e) That provision is made for traffic on formwork not to bear directly on reinforcement

steel.

VERTICALITY OF THE STUCTURE

All the outer columns of the frame were checked for plumb by plumb-bob as the work proceeds to upper floors. Internal columns were checked by taking measurements from outer row of columns for their exact position. Jack were used to lift the supporting rods called props

STRIPPING TIME OR REMOVAL OF FORMWORK

Forms were not struck until the concrete has attained a strength at least twice the stress to which the concrete may be subjected at the time of removal of form work. The strength referred is that of concrete using the same cement and aggregates with the same proportions and cured under conditions of temperature and moisture similar to those existing on the work. Where so required, form work was left longer in normal circumstances

Form work was removed in such a manner as would not cause any shock or vibration that would damage the concrete. Before removal of props, concrete surface was exposed to ascertain that the concrete has sufficiently hardened. Where the shape of element is such that form work has re-entrant angles, the form work was removed as soon as possible after the concrete has set, to avoid shrinkage cracking occurring due to the restraint imposed. As a guideline, with temperature above 20 degree following time limits should be followed:

Structural Component	Age
Footings	1 day
Sides of beams, columns, lintels, wall	2 days
Underside of beams spanning less than 6m	14 days
Underside of beams spanning over 6m	21 days
Underside of slabs spanning less than 4m	7 days
Underside of slabs spanning more than 4m	14 days
Flat slab bottom	21 days

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