Q. What is portland cement?
Portland cement is composed of calcium silicates and aluminate and aluminoferrite It is obtained by blending predetermined proportions limestone clay and other minerals in small quantities which is pulverized and heated at high temperature – around 1500 deg centigrade to produce ‘clinker’. The clinker is then ground with small quantities of gypsum to produce a fine powder called Ordinary Portland Cement (OPC). When mixed with water, sand and stone, it combines slowly with the water to form a hard mass called concrete.
Q. Is there any shelf life of cement?
Cement is a hygroscopic material meaning that it absorbs moisture In presence of moisture it undergoes chemical reaction termed as hydration. Therefore cement remains in good condition as long as it does not come in contact with moisture. If cement is more than three months old then it should be tested for its strength before being taken into use.
Q. Does fineness of cement affect strength gain?
Fineness defines the surface area of cement particles present in per unit weight, which implies that more fineness means more particles in unit weight. This enhances the reaction rate which in turn will result in faster gain of strength at earlier stages as well as liberates higher heat, therefore proper curing in initial days is very essential.
Q. What is 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.
Q. What are the reasons for slow or fast setting of concrete or mortar?
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.
Q. What are the different grades of Ordinary Portland Cement (OPC)?
The Bureau of Indian Standards (BIS) has classified OPC in three different grades The classification is mainly based on the compressive strength of cement-sand mortar cubes of face area 50 cm2 composed of 1 part of cement to 3 parts of standard sand by weight with a water-cement ratio arrived at by a specified procedure.
The grades are:
(i) 33 grade
(ii) 43 grade
(iii) 53 grade
The grade number indicates the minimum compressive strength of cement sand mortar in N/mm2 at 28 days, as tested by above mentioned procedure.
Q. What is Portland Pozzolana Cement?
Portland Pozzolana Cement (PPC) is obtained by either intergrinding a pozzolanic material with clinker and gypsum, or by blending ground pozzolana with Portland cement. Nowadays good quality fly ash is available from Thermal Power Plants, which are processed and used in manufacturing of PPC.
Q. What are the 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.
Q. Does the shade of cement affect quality?
No. 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.
Q. What is the effect of long storage periods on cement?
Cement which is in the form of a fine powder has a tendency to absorb moisture present in the atmosphere. When it absorbs moisture it hydrates, and when subsequently used does not contribute to the strength development. Jute bags (gunny bags) in which cement is bagged are neither airtight nor damp-proof and do not prevent absorption of moisture. Cement deteriorates in quality on long storage. Cement bagged in woven polythene bags or paper bags are not likely to deteriorate to the extent mentioned above. The loss of strength also depends on the condition of the godown. It is advisable to use cement within three months of its bagging, or to test the cement for its strength if stored for longer periods. Hence cement bought first should be used first.
Q. How should cement be stored?
Precautions that must be taken in the storage of Portland cement are given below in a series of DON’Ts.
(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 windowsand 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.
Q. How to identify the time for which the cement was stored before use?
On the cement bag, week number, month and year of manufacturing are being mentioned and this can be checked before use.
Q. What is concrete?
Concrete is a mixture of cement, sand, stone aggregates and water.
Q. What is RCC?
If a concrete mix is placed in and around a cage of steel rods, it is called Reinforced Cement Concrete (RCC).
Q. What is mortar?
Mortar is a mix of cement, sand and water, to be used for brick works/block works and plaster.
Q. How much water should be added in a concrete mix of one bag of cement for normal construction work?
Normally the amount of water that is required per bag of cement is 25 -28 liters only.
Q. How does color affect the quality of cement & its concrete?
Quality of cement has nothing to do with its color.
Q. What precautions should one take for water to be used in concrete construction?
It is good to use potable quality of water. It should be free from impurities and harmful ingredients. Seawater isn’t recommended. The water fit for mixing is fit for curing too. Use of minimum quantity of mixing water, consistent with the degree of workability required to enable easy placing and compaction of concrete, is advisable Ensure that water is measured and added Low water to cement ratio is essential for good performance of the structure in the long run.
Q. What is the minimum recommended concrete mix proportion for RCC works?
1:1.5:3, where 1 part of cement is to be mixed with 1.5 parts of sand and 3 parts of coarse aggregates. Water requirement for the
mix would be less than 25 liters per bag of cement.
Q. What are the common mistakes, which affect the quality of concrete?
The ways in which concrete maybe spoilt are many, most common of them being: Use of too much or too little water for mixing, or
water carelessly added during mixing. Incomplete mixing of aggregate with cement Improper grading of aggregates resulting in
segregation or bleeding of concrete. Inadequate compaction of concrete Using concrete which has already begun to set. Placing of
concrete on a dry foundation without properly wetting it with water. Use of dirty aggregate or water containing earthy matter, clay or
lime. Too much troweling of the concrete surface. Leaving the finished concrete surface exposed to sun and wind during the first ten
days after placing without protecting it and keeping it damp by proper methods of curing.
Q. What are the factors responsible for governing the compressive strength of concrete?
The compressive strength is governed by the following factors:
(i) w/c ratio
(ii) characteristics of cement
(iii) characteristics of aggregates
(iv) time of mixing
(v) degree of compaction
(vi) temperature and period of curing
(vii) age of concrete
(viii) air entertainment
(ix) conditions of testing
Q. Within how much time the freshly prepared mortar / concrete should be consumed?
The mortar / concrete should be consumed as early as possible after addition of water to it. The hydration of cement starts the moment water is added to it. As the hydration progresses the cement paste starts stiffening and loses its plasticity. The concrete should not be disturbed after this. Normally, this is about 45 – 50 minutes.
Q. Why is compaction essential?
Green concrete has all the three phases – solids, water air. In order to make the concrete impervious & attain its maximum strength it is required to remove the entrapped air from the concrete mass when it is still in plastic state. If the air is not removed completely, the concrete loses strength considerably. It has been that 5% voids reduce the strength by about 30% and 10% voids reduce the strength by over 50%. Compaction eliminates air bubbles and brings enough fine material both to the surface and against the forms to produce the desired finish. One can use such hand tools as steel rods, paddling sticks, or tampers, but mechanical vibrators are best. Any compacting device must reach the bottom of the form and be small enough to pass between reinforcing bars. Since the strength of the concrete member depends on proper reinforcement location, be careful not to displace the reinforcing steel.
Q. What is ‘curing’ and why is it so important?
The term ‘curing’ is used to include maintenance of a favorable environment for the continuation of chemical reactions, i.e. retention of moisture within, or supplying moisture to the concrete from an external and protection against extremes of temperature.
Q. What is the correct method of curing?
If a concrete mix is placed in and around a cage of steel rods, it is called Reinforced Cement Concrete (RCC).
Walls: Water should be sprinkled from the top such that it covers the whole area of the wall and it should be remain wet.
Slab: Ponding should be done on the slab by constructing bunds of mortar of approximately lmxim and water should be
Beams and columns: The beams and columns can be maintained wet by tying gunny bags around the periphery and by maintaining
it wet always.
Q. What methods are commonly employed to ensure sufficient moisture for curing?
Ponding, continuous sprinkling, covering with wet cloth, cotton mats or similar materials, covering with specially prepared paper, polyethylene, sealing coat applied as a liquid commonly known as ‘curing compound’ which hardens to form a thin protective membrane, are some of the methods by which concrete is cured.
Q. When should curing be started and when is it complete?
Curing should be started just after the surfaces begin to dry. Normally 7 to 14 days curing is considered adequate.
Q. What is steel reinforcement? Why is it required in a concrete structure?
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 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.
Q. What is 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.
Q. What are the different types of steel reinforcements being used in a reinforced concrete structure?
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.
Q. Why cover blocks are required to be placed before concreting? What are their sizes?
Cover blocks are placed to prevent the steel rods from getting exposed to the atmosphere, and to place and fix the reinforcements as per the design drawings. Once the steel is exposed to the atmosphere, corrosion starts. 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.
Q. How important are transverse reinforcements like links and stirrups? What precautions should be taken while tying
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.
Q. What is a lap or development length? Where and how should they be provided?
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).
Q. What is 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.
Q. What is the checklist for steel reinforcement before the placement/pour of concrete?
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.
Q. For a given diameter, how do we calculate the weight of steel per meter length of the steel bar and vice versa?
For any steel reinforcement bar, weight per running meter is equal to d2/162 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.
Q. Can we receive the quality test certificate of steel from the supplier?
Yes. We can receive the test certificate of the batch of materials supplied at our site. The certificate will confirm the compliance of quality of supplied materials as per the requirement of relevant Indian standard code.
Q. Why plastering is required?
Plaster protects structure from temperature variations; external attacks of sulphates, chlorides, etc. Plaster also provides smooth &
aesthetic surface on RCC & Brickwork surface.
Q. What precautions should be taken during plaster works?
Preferably use cement which releases low heat of hydration. A blended Cement is a good choice.
a) Use optimum water at the time of mixing.
b) Do not use dry cement on the plaster surface.
c) At the junction of Brickwork &. RCC, chicken mesh or fiber mesh may be used.
d) Wet the surface before plastering.
e) Cure the surface for at least 10 to 12 days.
Q. What are carpet area, built-up area & super built-up area?
Carpet Area: This is the area of the apartment/building, which does not include the area covered by the walls. Built up Area: The carpet area plus the area of the walls. Super Built up Area: This includes the built up area along with the area under common spaces such as the lobby, lifts, stairs, etc.
Q. What are the things one should keep in mind before finalizing a land deal?
Some points which one must pay attention to prior to land deal –
a) House plot should not be under any acquisition proceedings of any government bodies .
b) Verify from the planning authorities whether the land was designated for residential use.
c) The survey number of land is critical.
d) Certificate obtained from registrar’s office should confirm that there is no encumbrance on the property.
e) The title deed of the land should be clear and an advocate should be consulted for this.
f) Verify the rates after contacting some property consultants and some people who have purchased their plots recently. You
should also refer guidelines and the market rates published in the government manual of your city/town.
g) Stamp duty at the time of registration is mandatory.
Q. What are the important considerations before selecting a piece of land?
While selecting the land one should give importance to following issues –
a) Required size of plot and construction area
b) Existing rates area wise
c) Distances from Railway Station, Bus Stand and Air Port.
d) Distance from your work place.
e) Distances from hospital and medical clinics.
f) Available infrastructure like water supply, drainage, road etc.
Q. What are the points, one should pay attention to while awarding the contracts for house construction?
Whatever the type of contract, one needs to pay close attention to the following aspects –
a) Type of materials
b) Quality of work
c) Advance payment, if any
d) Progress &. time schedule
e) Future measurement &. payment conditions
Q. What is the ratio of material, labor and other expenditures for house construction?