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SITE INVESTIGATION AND SOIL EXPLORATION
BEARING CAPACITY OF FOUNDATIONS
Before starting Construction one should have the clear idea about the estimated cost of the project. Component wise break up is very important aspect. If one don't have sufficient funds available to complete the project in one go, it can be spitted in sub components. In this page efforts are made to show a typical picture of various components of a Building Project, % age cost involved and Time Required for completion of different components.
Cost Components
1. Land: Cost of land is ideally verified from the actual site and its market price.
2. Upto D.P.C. (Damp Proof Level) in other words Plinth Level (without Basement) = 8%
3. Upto Roof Level of Ground Floor (without Door and Window Frames) = 7%
4. Complete Structure Work Upto Roof Level = 13%
5 Door and Window Frames = 8%
6 Floorings = 20%
7 Doors and Windows = 12%
8 Electrification = 10%
9 Sanitary Installations = 15%
10 Finishing = 7%
For Further Details about Economy In Construction Please Click Economy Tips
Graphical View For a Typical Multi Story Residential Building Expenditures
If the construction is being carried out through contractor this job is on the contractor other wise one have to Plan For Men and Material in well advance so that the speed of work is not hampered. Some time due to non availability of proper material and equipment labour becomes redundant and becomes a liablity.
In this age when the prices increase day by day the completion of project well in time is great achievement. Timely completion benefits of utilization of building and early return of the investment. Delay in project completion some times give other competitors advantage of first launch. Now a days most of the buildings are being constructed with loaned money so the interest in also a matter of concern. For Timely completion we have tested tools such as
Bar Chart
Critical Path Method (CPM)
and Program Evaluation and Review Technique (PERT)
All these are Planning Tools with the help of these tools we can plan our work what activity follows what, without the completion of which activity which activity can not be started.
one need thorough knowledge of the construction activities and time required for completion of each activity, only then a meaningful planning can be made.
Typical Bar Chart for Single Story Building is given below
INTRODUCTION
Subsurface conditions at any given site must be adequately explored to obtain information required in design and construction of foundations. The investigations may range in scope from a simple examination of the surface soils with a few shallow trial pits to a detailed study of the soil and ground-water conditions to a considerable depth below the surface by means of boreholes, and in situ tests on the soils encountered. The extent of the work depends on. the importance and. foundation arrangement of the structure, the complexity of the soil condition and the Information which may be available on the behaviour of existing foundations on similar soils. A detailed site investigation involving deep bore holes and laboratory testing of soils is always necessary for heavy structures like bridges, multistory buildings and industrial plants.
SOIL EXPLORATION
The purpose of soil exploration is to obtain information based on which the following points can be determined.
For new structures:
(i) The type and depth of foundation can be selected.
(ii) The bearing capacity of the selected foundation can be determined.
(iii) The settlement can be predicted.
(iv) The ground-water level can be established.
(v) The earth pressure against walls and abutments can be evaluated.
(vi) Adequate provisions can be made against possible constructional difficulties.
For existing structures:
(i) The safety of the structure can be investigated.
(ii) Further settlement can be predicted.
(iii) Remedial measures can be suggested if the structure is unsafe or likely to suffer detrimental settlement.
Soil samples are of two types: disturbed and undisturbed.
(i) A disturbed sample is that in which the natural structure of the soil gets partly or fully modified and destroyed by the method of sampling, Natural water component may be preserved by suitable precautions. The sample is suitable for mechanical analysis, determination of index properties of soil classification and for carrying out compaction tests.
(ii) An undisturbed sample is that in which the structure and properties of the material are preserved. This may be obtained by careful protection and packing and by the use of a correctly designed sampler. Only undisturbed samples are suitable for tests on shear strength, consolidation and permeability.
Sampling Tools
To take undisturbed samples from bore holes, properly designed sampling tools are required. The sampling tube when forced into the ground should cause as little remoulding and disturbance as possible. The degree of disturbance is controlled by the following features of the sampler:
(i) Cutting edge
(ii) Inside wall friction and Cutting Edge
Wall Friction
The wall friction can be reduced by:
(i) providing suitable inside clearance,
(ii) giving a smooth finish to the sample tube, and
(iii) applying oil on the walls of the sampler.
depth of exploration required, depends on the type of the proposed structure, its total weight, the size, shape and disposition of the loaded area, the physical properties of the soil that constitute the different strata e site. Exploration, in general, should be carried out to a depth up to which the increase in pressure due to structural loading is likely to cause
foundation failure. Such a minimum depth is known as critical depth or significant depth. The net loading intensity at any level below the foundation be obtained by approximately assuming a spread of load of, two vertical to one horizontal, from all sides of the foundation. Due allowance should be made for the overlapping effects of the load from closely spaced footings. It is rally safe to assume the significant depth up to a level at whjch the net increase in vertical pressure becomes less than 10% of the initial overburden pressure.
The following guide rules may also be followed to decide the depth of exploration.
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For isolated spread footing or raft |
1.5 times the width.
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Adjacent footings with clear spacing less than twice the width |
1.5 times the length.
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For pile foundations |
10 to 30 m and more or at least 1.5 times the width of the structure |
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Base of retaining walls |
One and a half times the base width or the exposed height, whichever is greater. |
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For black cotton areas, from the consideration of weathering. the exploration should be carried to a minimum depth of 4 m.
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2.1 INTRODUCTION
Any foundation should be designed so as to satisfy two essential requirements:
(i) It must have a certain specified safety against ultimate failure.
(ii) The settlements under working loads should not exceed the allowable limits for the superstructure in question.
One of these two criteria will determine the dimensions of the foundations.
Ultimate Bearing Capacity
The ultimate bearing capacity is defined as the minimum gross pressure at the base of the foundation at which the soil mass fails in shear.
Gross pressure is the total pressure at the base of the foundation due to the weight of the superstructure, self-weight of the foundation and the weight of earth fill, if any.
SAFE BEARING CAPACITY
The safe bearing capacity is the ultimate bearing capacity divided by factor of safety.
Factor of Safety
The factor of safety to be selected depends on how accurately the soil conditions are known, the type of loading and the hazard imposed by a complete foundation failure.
For most structures, where no possibility of soil failure can be tolerated and when there is a reasonably accurate soil and loading information available, a safety factor of 2.5 is employed.
If there is a large component of live load that is likely to develop, a safety factor of 2 may be employed.
When the soil conditions are not well established, a factor of 3 should be used. For temporary structures,
where some risk of bearing failure can be tolerated a safety factor of 1.5 may be used.
METHODS OF ESTIMATING BEARING CAPACITY
The various methods of obtaining the values of bearing capacity may be classified as:
(i) Analytical methods involving the use of soil parameters.
(ij) Field tests.
(iii) Presumptive bearing capacity values from codes (not recommended nowadays)
(iv) Experimentation with foundation models.
Proper Soil Investigation and Calculation of Bearing Capacity along with well defined structure system can save lot of construction cost in any project. Where as reverse can make over safe or under safe structures. Some buildings could not get the required building safety certificates due to poor design or non technical construction reasons.
INTRODUCTION
The invention of cement as a hydraulic lime, prepared by the combination of clay with lime is attributed to an English bricklayer, Joseph Aspadin 1824. Cement is used in different forms to suit various needs and requirements. Cement, though it constitutes only about 13 % by volume in concrete, the proportioning influences workability, strength, durability and economy to a very large extent.
CONSTITUENTS OF CEMENT
Two parts of calcareous materials (mixture of clay and calcium carbonate) and one part of argillaceous materials (silicates of alumina) are mixed and burnt at temperatures ranging from 1400°C to 1500°C, and 4 to 5% by weight gypsum (CaS04, iH2O) is added during final grinding to get ordinary portland cement.
The chief chemical constituents of cement are
lime (CaO) 60 to 67%,
Silica (Si02) 17 to 25%
Alumina (Al2O3) 3 to 8%
Iron oxide (Fe203) 0.5 to 6%
and traces of magnesia. The lime content provides the strength and silica increases the setting time. Iron oxide gives the grey colour and free lime and magnesia reduce the strength of cement.
Fineness-It is a measure of the size of the particles of cement. Now it is measured as specific surface area as given below
Soundness-This is expressed as expansion after setting, due to the presence of free oxides of calcium and magnesium (CaO,MgO), It is measured by Le Chatlier's apparatus.
Setting time- Loss of plasticity is measured by the initial setting time and gain of particular hardness is measured by the final setting time.
Compressive strength-Cubes of cement mortar 1 : 3 having an area of 50cm2 are tested for three days and then seven days strength and compared with a standard test value available to ascertain the quality of cement.
OPC cements 33,43 and 53 grade cements should have 28 days strength of 33,43 & 53 N/Sq.mm respected should also develop 50% strength at 3 days and 75% at 7 days.
By slightly changing the chemical composition, it is possible to obtain cement exhibiting the desired properties. Ordinary portland cement is used for all construction works except where sulphates are present in the soil ground water leading to expansion and breaking of concrete, known as term "sulphate action".
The rate of setting is the same as for O.P.C. Fine grading and increase in calcium silicate content ensure seven days strength of O.P.C. to be obtained
in three days. Where formwork is to be removed early, for winter concreting and urgent repair works, R.H.P.C. is used. It should not to be used
mass concreting due to the high rate of heat evolution.
By intergrinding 2% by weight of calcium chloride with R.H.P .C., high early
strength (25% more than R.H.P.C) and quick setting (5 to 30 minutes)
be achieved. This cement is used for cold weather concreting and for under
water concreting works.
When granulated blast furnace slag (a mixture of lime, silica and alumina)
obtained as a waste product in the manufacture of pig iron, is inter-ground
with portland cement clinker and mixed to a proportion of 2 : 1, this special
cement is manufactured. The rate of hardening is slower and hence heat of hydration is less. It is resistant to sulphate action and is used mass concrete and in sea water construction.
The rise of temperature in the interior of mass concrete will lead to serious cracking. Lowering of the heat of hydration of cement can be obtained increasing the proportion of dicalcium silicate and reducing tricalcium aluminate and restricting tricalcium silicate. The development of strength is slow but the ultimate strength is the same as that of portland cement.
Two per cent of sulphate in soil or 0.5 % in groundwater reacts with cement leading to the formation of supho-aluminates which have expansive properties and cause disintegration of concrete. By restricting tricalcium silica to 5% and grinding to a fineness higher than O.P.C., high resistance sulphate action is produced. Early strength is low but the ultimate strength is quite high. Carbonisation In air gives a hard surface to the concrete, is expensive and is used in sea water construction and for structures in sulphatic soils.
Restricting the iron oxide to less than 1 % is useful in avoiding the characteristic grey colour of cement. By adopting an oil fuel for burning and using chalk with a low iron content, white clay or "snowcrete" is manufactured. This forms the base for coloured cements used in the manufacture and joining of mosaic tiles, terracotta. etc. Coloured portland cements are got by adding strong pigments up to 10% to O.P.C. Light colours can be got by adding pigments to white portland cement.
Naturally occurring cement rocks may be burnt and used as natural cement but the properties may vary and be quite different from O.P.C.
This form of cement is made from well granulated slag (80 to 85%), calcium sulphate (10 to 15%) and O.P.C. 1 to 20% It sets quickly, the heat of hydration is low and it is highly resistant to chemical attack. It can be used for all purposes without mixing with other cements or admixtures.
Composed of portland cement clinker, limestone, gypsum and an air entraining agent, masonry cement has the advantages of fattiness, high workability and retention of the mixed water from the sucking action of bricks. This is used extensively in masonry works.
Concrete shrinks when it sets. For repair work, this is a disadvantage. By the addition of an expansive agent calcium sulpho-aluminate up to 5% expansion up to 12 to 15 mm/m can be got. This can be used for prestressing and for setting right the damage due to settlement, etc.
For lining of oil wells where cement is to harden quickly and possess pumping quality for three hours working at a pressure of 1400 kgf/cm2 at a temperature of 150°C, special oil well cement, making very little use of tricalcium silicate and retarders is used. By adding agents like stearic acid, cleic acid and boric acid to O.P .C. during grinding, hydrophobic cement is produced which does not lump during storage. Waterproof cement is produced by adding substances like calcium stearate or aluminium stearate to OPC cement. High Alumina Cement By fusing limestone and bauxite (hydrated alumina and iron oxide) at 1600°C and grinding the clinker, high alumina cement is produced. Eighty per cent of the ultimate strength is developed in 24 h. Due to the large amount of heat evolved, its use is restricted to thin sections and lifts, restricted to 30 cm at a time. Twice the amount of water is required for O.P.C., is necessary for full hydration of high alumina cement and the workability is quite high. The resistance to chemical attack and heat being high, this cement is used in sea water construction and refractory works. It should not be mixed
with O.P .C. to avoid flash set.
If more than 2% of water or moisture is absorbed by the cement, the hardening time is retarded and the strength is reduced. If the absorption exceeds 5%. the cement is ruined. best method of storing for over a year is to keep it in bulk in bins 2 metre deep. Bagged cement can be kept for months if stored in a weather proof sheds with a dry floor well above the ground level and the surrounding area well drained, There is no use of moving and restacking to reduce warehouse pack as this procedure exposes fresh cement to air. Cement in woven bags should be used in three months and if it has been kept for longer
the setting and strength properties should be tested before use.
INTRODUCTION
Whether an agent is inter-ground with cement as an additive or is added to the gauge water as admixture during the manufacture of concrete, the end results are the same. They consist chiefly of those which accelerate and those which retard hydration or setting of cement. These are finely divided materials for improving workability, of water-proofers, pigments, wetting, dispersing and air entraining agents or puzzolanas.
Addition of calcium chloride, 2% by weight of cement makes the concrete set early and acquire strength soon so that form work can be removed early to bring the structure into service. Final setting time is reduced to 2hrs and one day strength is doubled. Sulphates of sodium and potassium {Na2S04,K2SO4} and sodium chloride (NaCI) are also used as accelerating agents. These are ideal for concreting in cold weather. Excessive use of these agents increases drying shrinkage.
Delayed setting is required in tropical conditions, pumping of concrete, tunnels, oil well construction, soil cement construction, etc. This is brought about by adding plaster of paris or gypsum (CaSO4, 0.5H20). Initial setting time is increased from 30 min to 10 hrs. 0.20% by weight of sugar increases the final set to 72 h, Milk powder, ammonium chloride, sodium bi-carbonate are all useful as retarders. But their effects on the strength and permeability must be studied before using them as retarders.
The two functions of waterproof concrete are: to be impervious to water under pressure; and to resist absorption of water. A concrete having proper mix, design, low water/ cement ratio and good sound aggregate is impervious and needs no additives. Hence, waterproofers are added more to resist absorption of water. Silicates of sodium, aluminum and zinc sulphates and calcium chloride accelerate setting and make the concrete impervious at an early age. Chalk, talc and fuller's earth act as workability aids and increase the density due to better compaction. Lime, calcium chloride (chemically active) and resin, vegetable oil or cold tar residue and bitumen (inert) are water-repellant agents and are used in damp-proof course and roofing works.
Finely ground mineral powder increases the paste content and hence the cohesiveness and workability of the mix. If added in large quantities, water requirements will be increased and this results in a loss of strength. Lime is useful in increasing the workability of mortar for brickwork.
Air contained unintentionally impairs the strength of concrete as continuous channels may be formed by these voids. Air, in the form of 0.05 mm dia. Air bubbles in billions, constituting up to 3 to 6% by absolute volume, increase workability and resistance to weather and frost. The increased cohesiveness makes concrete less liable for segregation and bleeding or formation of a layer of water on the surface after compaction.
(i) foaming agents such as aluminum powder, which generate gases by chemical action,
(ii) wetting agents or surface active agents like soaps, fats, oils, detergents, and
(iii) dispersion agents which prevent coagulation of cement rendering the particles mutually repellant.
As air entraining agents are required in small quantities like 0.005% to 0.05% by weight of cement. As the workability is increased, considerable reduction in water content is possible, thereby compensating for the loss in streogtl1 due to air entrainment. Unless the site control is good, air entraining should not be attempted. Pumping of concrete, transporting without segregation are all possible by this technique. Air entraining has very little effect on the permeability of concrete but increases the durability in cold countries and resistance to chemical attack.
A puzzolana is a silicious material, which by itself possesses no cementitious properties. However, it will, either in processed or unprocessed form, and in finelv divided form, react in the presence of water with lime, to form compounds. of low solubility having cementitious properties. Clay and shales calcined to become active, volcanic tuff and pumicite are naturally occurring puzzolanas, whereas good blast furnace slag and flyash are the artificial varieties. Each of them contains from 50 to 60% of pure silica. The optimum replacement advocated is 10 to 30% by weight of cement. Now a days Portland Pozzolana Cements are available in market having trade names similar to OPC cement that already contain flyash up to 30%. Before further adding pozzolana material it should be consulted according to specific condition in general it is not recommended to add pozzolana more than 30% by weight of cement.
The principal advantages of this admixture are:
(i) Economy .
(ii) Improvement of workability
(iii) Reduction of bleeding and segregation
(iv) Greater impermeability and reduced attack from sulphates
(v) Low heat of hydration
The undesirable effects are the slower rate of development of strength and increase of shrinkage.
For mass structures like dams, harbour works, surkhi which is a natural puzzolana, is used as a substitute for a part of cement up to 20% by weight.
It has all the advantages of economy and workability and the slower gain of strength is of little consequence.
Flyash
It is the residue obtained from the combustion of pulverized coal collected from the flue gases or power plants. Usually flyash is made up of 30 to 60% silica and is finer than cement. Flyash is puzzolanic, as its silica over a long period combines with the lime liberated during the hydration of cement. Curing at high pressure and temperature (above 100°) in an autoclave promotes the reaction between lime and silica in flyash. it is used in the preparation of lightweight concrete blocks used as a substitute for bricks in building construction.