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Importance of soil compaction Benching of steep slopes Filling of soils in layers Density to be achieved
Comparative study of effect of compaction on cost of pavement
PAVEMENT COURSES PAVEMENT TYPES COMPARISON OF RIGID AND FLEXIBLE PAVEMENTS Typical Design Calculations
Typical Mix Design For Concrete TYPICAL BITUMINOUS MIX DESIGN
Soil is defined, for civil engineering purposes, as a "natural aggregate of mineral grains that can be separated easily, as for example, by agitation in water". On the other hand, Rock is defined as a "natural aggregate of mineral grains connected by strong and permanent cohesive forces and these mineral grains cannot be separated easily"
Soil is the cheapest and the most widely used material in any highway construction in non-bituminous roads, either in its natural form (say gravel) or in a processed form (say stabilised soil layer). All road pavement structures rest on soil foundation. However, soils are highly heterogeneous and anisotropic in nature and occur in unlimited varieties, with widely different engineering properties which, in turn, can be influenced considerably by the presence of water in several varieties. Considering all these aspects, a thorough study of the engineering properties of soils is of vital importance in working out an appropriate design of the pavement structure which will produce an acceptable level of performance of the road over the design life under the given traffic and climatic conditions.
In any road embankment, the bulk of the material used is soil and if properly designed, should have stable slopes and should not settle to any appreciable extent. Also, the embankments require a stable foundation; if the foundation soil happens to be a soft clay, unless properly designed, excessive settlement or even ultimate failure can take place. Similarly when a road is constructed in a cutting, sound principles of soil engineering are to be employed to ensure that the slopes are stable under the climatic conditions prevailing in the area. Finally, the characteristics of the road pavement i.e., the hard crust placed on the soil formation are not only dependant on the nature of traffic but also on soil properties over which the pavement rests. Soils and their stability depends on soil strength under the given ground water and climatic conditions.
For highway engineers, a study of the compaction properties of soil is extremely important for the following reasons:
(i) Soils which are compacted to a high density have greater strength and hence a pavement constructed on such sub-grades requires lesser thickness.
(ii) Compaction of soils reduces the possibility of settlement of embankments during the life of the pavement and of slope failure.
(iii) Compacted subgrades are less susceptible to changes in moisture content. This means that swelling and shrinkage of soils, accompanying moisture changes, can be reduced.
Factors influencing compaction
The density to which soils can be compacted depends primarily on three factors:
(i) Soil type
(ii) Moisture content
(iii) Compactive effort applied.
Clearing and Grubbing
Clearing and grubbing is the first operation to be done in highway earthwork. It consists of removing trees, stumps, roots, shrubs, undergrowth, rubbish and other objectionable material from the area to be occupied by the embankment.
Stripping and storing of top-soil
In areas where the embankment soil is not conducive to the growth of turf and vegetation, the top soil should be stripped and stored. It is later laid as a blanket on the embankment slopes.
Compaction of original ground
Embankments which are less than 0.6 m height above the ground impose stresses of high magnitude on the original ground. Such low heights are generally avoided. In situations where it is unavoidable the density of the original ground should be satisfactory. If its compaction is less than 90 per cent of Proctor's density, the original ground should be brought to a compaction of at least 100 per cent of Proctor's density by loosening, watering and rolling in layers of 25 cm. This treatment should extend to a depth of 0.5 m. If the next 15 cm below this depth does not have a compaction of at least 90 per cent, it should be made up to at least 95 per cent.
When the embankment is constructed on hillside slopes, it is necessary to bench the surface of the hill slope with benches having a height of 0.5 m and width 1.5 -3.0 m, to add to the stability (Fig.). These benches should have a gentle fall towards the hill-side.
(i) Should be rectangular in shape and as near to the road boundary as possible. (ii) No borrow pit should be located within 5 m from the toe of the final embankment, making due allowance for future widening.
(iii) The depth of borrow-pits should be so regulated that they do not cut an imaginary line having a slope of 1 in 4 projected from the edge of the bank. The maximum depth should be limited to 1.5 m.
(iv) Borrow-pits should not be dug continuously. Ridges not less than 8 m width be left at intervals not exceeding 300 m. Small drains should be cut through them to facilitate drainage of water.
(v) Depth of borrow pits in temporarily acquired land should not be greater than 45 cm.
(vi) Borrow-pits should not be dug within 0.8 km of towns and villages, as water collecting in them breeds mosquitoes.
It is the general practice that soils for embankment should be deposited in layers each layer is compacted before the next one is laid. The depth of layers is generally restricted to 20 cm. Where sheep-foot rollers are used for compaction, the depth should not exceed length of the tamping feet by more than 5 cm. The soil clods should be broken to have a maximum size of 15 cm. In the top the embankment, the maximum size should be still less, say 6 cm. Earthwork is done by using excavators, scrapers, dozers, graders and dumpers. Scrapers are able to dump an in layers in one operation. But if dumpers and trucks are used, the soil is first dump then spread by a grader or a dozer.
Moisture content
Soil tests would have indicated the OMC at which the density would be max the natural soil has less moisture than indicated by these tests, additional water is sprinkled on the layers. If the natural moisture content is higher than desired, the soil is allowed aerate before rolling is done.
Highly expansive soils, such as black cotton soil if at all permitted, are compacted at a moisture content 3 to 4 per cent above the optimum to a density not exceeding 90 per cent of the standard Proctor dry density.
Compaction equipment
Mechanical equipment is needed for achieving the desired compaction. In India is standard equipment used is the three-wheel steel-tyred roller. Other equipment sheep-foot roller, pneumatic tyred roller, vibratory roller and power rammers are also used. Suitability of particular equipment depends upon the type of soil to be compacted.
Laboratory Maximum Dry Density:
The maximum dry density that can be achieved in the field with reasonable compaction efforts is determined in laboratory as described in IS 2720 Part 7 and 8. Part 7 is used for light compaction and part 8 is used where heavy compaction equipments are available. Relationship between moisture content and dry density is plotted. Thus optimum moisture content and corresponding Maximum Dry Density is achieved. This is called OMC and MDD curve, it is very useful for compaction of embankment layers and also in base and subbase of pavement.
Typical OMC and MDD Curve is shown below.
The density to be aimed at depends on the type of soil and the position of the layer. Higher densities are desired in the top 0.5 m of the embankment immediately below the pavement than in the other portions. Table below gives MOST recommendations
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Type of work
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Relative compaction as %age of Max. Laboratory Dry Density (Heavy Compaction IS 2720-8) |
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Sub-grade and earthen shoulders |
Not Less Than 97% |
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Embankment |
Not Less Than 95% |
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Expensive clays |
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a) Subgrade and 500 mm portion just below the subgrade |
Not allowed |
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b) Remaining portion of embankment |
Not Less Than 90% |
Compaction procedure and its control
The compaction equipment is selected with due regard to the type of soil to be compacted and the availability of equipment. The exact number of passes to be made on the soil needs to be determined in order to achieve in the field the compaction standards.
Comparative study of effect of compaction on cost of pavement:
The compaction of soil increases its load bearing capacity. Incase of flexible pavements strength of its supporting embankment is measured as California Bearing Ratio (CBR). CBR increases with degree of compaction. In case of Sand, silt and loams the CBR from light compaction to heavy compaction can increase CBR from 50 to 100 percent of the CBR of light compacted soil. In a typical case, the CBR of Soil was increased from 5% to 11% with heavy compaction thus the design crust thickness reduced approximately 35%.
The fig shows the relationship of CBR and Crust Thickness.
A pavement is designed to support the wheel loads imposed on it from traffic moving over it. Additional stresses are also imposed by changes in the atmospheric temperature & humidity etc.
It should be strong enough to take the stresses imposed on it and it should be thick enough to distribute the loads on the earthen subgrade.
Desirable for a Pavement:
1. It should be structurally strong enough to withstand the load imposed on it.
2. It should be sufficiently thick to distribute the loads and stresses to a safe limit to the subgrade soil.
3. It should provide a reasonably hard wearing surface, so that the abrasion action of wheels does not damage the surface.
4. It should be dust-free so that traffic safety is not impaired.
5. Its riding quality should be good. It should be smooth enough to provide comfort to the road users at the high speeds at which modern vehicles are driven.
6. The surface of the pavement should develop as low friction with the tyres as possible. This will reduce the energy consumption of the vehicles.
7. The surface of the pavement should have a texture and adequate roughness to prevent skidding of vehicles.
8. The surface should not produce excessive sound from moving vehicles.
9. The surface should be impervious so that water does not get into the lower layers of pavement.
10. should have long life and the cost of maintaining it annually should minimum.
A pavement consists of one or more layers. The simplest classification is given in Fig below
The topmost layer is the surfacing, the purpose of which is to provide a smooth, abrasion resistant, dust free, reasonably water proof and strong layer.
The base, which comes next below, is the medium through which the stresses imposed are distributed evenly to the subbase layer. Additional layers further help in distributing the loads to sub-base layer. The subgrade is the compacted natural earth immediately below the pavement layers.
The functions of the sub-base layer are:
(i) To provide additional help to the base and surface courses in distributing the loads.
(ii) To prevent intrusion of fine-grained road-bed soils into base courses.
(iii) To minimise the damaging effects of frost action.
(iv) To facilitate drainage offree water that might get accumulated below the pavement.
The functions of the base course are:
(i) To act as the structural portion of the pavement and thus distribute the loads.
(ii) If constructed directly over the sub-grade, to prevent intrusion of subgrade soils into the pavement.
The functions of the surface course are:
(i) To perform as a structural portion of the pavement.
(ii) To resist the abrasive forces of traffic.
(iii) To reduce the amount of surface water penetrating the pavement.
(iv) To provide a skid-resistant surface.
(v) To provide a smooth and uniform riding surface.
From the point of view of structural performance, pavements can be classified
(i)
Flexible
(ii ) Rigid
(iii) Semi-rigid
(iv) Composite.
A flexible pavement is essentially a layered system which has low flexural strength. Thus, the external load is largely transmitted to the subgrade by the lateral distribution with increasing depth. Because of the low flexural strength, the pavement deflects momentarily under load but rebounds to its original level on removal of load. The pavement thickness is so designed that the stresses on the subgrade soil are kept within its bearing power and the subgrade is prevented from excessive deformations. This implies that in a flexible pavement, the subgrade plays an important role as it carries the vehicle loads transmitted to it through the pavement. The strength and smoothness of the pavement surface depends to a great extent on the permanent deformation suffered by the subgrade and its resistance to such deformation. If the pavement itself is very strong, but it is constructed on loose and poor subgrade, it can fail. As a contrast, a rigid pavement derives its capacity to withstand loads from the flexural strength or beam (flexural) strength, permitting the slab to bridge over minor irregularities in the Subgrade, sub-base or base upon which it rests. This implies that the inherent strength of the slab itself is called upon to play a major role in resisting the wheel load. Minor imperfections or localised weak spots in the material below the slab can be taken care of by the slab itself. This is not to under-rate the role of the subgrade soil. In fact, a good, stable and uniform support is necessary for a rigid pavement as well. But as long as a certain minimum requirement is met with in this regard, the performance of the rigid pavement is more governed by the strength of the slab itself than by the subgrade support.
A third category of pavements has become popular during recent times. Known as semi-rigid pavement, it represents an intermediate state between the flexible and the rigid pavement. It has much lower flexural strength compared to concrete slabs, but it also derives support by the lateral distribution of loads through the pavement depth as in a flexible pavement. Typical examples of a semi-rigid pavement are the lean-concrete base, soil-cement and lime-puzzalona concrete construction.
A composite pavement is one which comprises of multiple, structurally significant layers of different-sometimes heterogeneous-composition. A typical example is the brick- sandwiched concrete pavement, which has been tried in India. It consists of top and bottom layers of cement concrete which sandwich a brick layer in the neutral axis zone. The design of composite pavements lies outside the well-established fields of flexible or rigid pavement design and is still in its infancy.
COMPARISON OF RIGID AND FLEXIBLE PAVEMENTS
Design precision
A cement concrete pavement is amenable to a much more precise structural analysis than a flexible pavement. This is because of the fact that the flexural strength of concrete, which is used as the main basis for design, is well understood. On the other hand, flexible pavement designs are mainly empirical. It may be because of the design precision associat~d with a concrete pavement that the accuracy in predicting the performance of a rigid pavement is relatively higher than for flexible pavements. Latest research in understanding the perfor- mance of bituminous materials has furthered the knowledge on their behaviour. Computer aided analysis of layered systems is making the flexible pavement design more exact than hitherto.
Life
Cement concrete slabs of a thin section (about 10 cm), constructed in the early 1940s are still in existence in India, though many of them have cracked badly and a few of them have been ripped open and rebuilt in recent times. But the fact remains that for nearly 3 or 4 decades they gave trouble-free performance, inspite of their structural inadequacy and the unprece- dented growth in traffic and increases in wheel loads. A major project in cement concrete road construction, between Agra and Mathura, completed nearly two decades ago, has provided a very satisfactory and trouble-free pavement. More recently, the Mumbai-Pune Expressway was constructed with a cement concrete pavement. It can safely be said that a well-designed concrete slab has a life of about 40 years. Compared to this, the life of a flexible pavement generally varies from 10 to 20 years. Even this shorter life can be achieyed only with extra maintenance input as discussed separately.
Maintenance
A well-designed cement concrete pavement needs very little maintenance. The only maintenance needed is in respect of joints. Continuously Reinforced Concrete Pavements (CRCP) have reduced the number of joints to be attended to. The hard surface can withstand the abrasion caused by iron-tyred vehicles (bullock carts) in India and studded tyres in the West used under snowy conditions. The surface is unaffected by spillage of oil and lubricants. Bituminous surfaces, on the other hand, need great inputs in maintenance. Sealing cracks, making good potholes, resurfacing and resealing are done very frequently. The surface is affected by spillage of oil and lubricants. The surface is also affected by natural weathering agents like air, water and temperature changes. On a good two lane road, the cost of preventive maintenance and post maintenance of a bituminous surfacing, can be anything in the range of Rs. 30,000-100,000 per km per annum. A cement concrete pavement on the other hand, needs a small amount for maintaining joints. This amount can be of the order of Rs. 5,000-10,000 per km per annum.
Initial
cost
The
argument so far used against a cement concrete slab is that it is much more
costly than a flexible pavement. When comparisons are made, they should be on an
equitable basis. For example, it is unfair to compare a 20 cm thick cement
concrete slab with a flexible pavement consisting of stabilised sub-base, a
water-bound macadam base and a thin open-textured carpet. The latter
specifications no doubt represent the rock-bottom needs of a road in India,
but these specifications can hardly provide a smooth and durable surface. A good
comparable flexible pavement is also quite expensive.
Stage
construction
Due to
extreme scarcity of resources in the country, road construction is generally
done adopting a policy of stage construction especially for low volume roads. A
new road, for example, is constructed with the barest minimum specifications,
which may involve just a thin bituminuous surfacing over a partially designed
thickness. As traffic grows, additional layers, in the form of water-bound
macadam, bitumen-bound bases and superior surfacings are added on. Initial
outlay is minimum and additional outlays are in keeping with traffic growth.
Thus, at no stage is the investment made in advance of the actual requirement.
This is a great advantage when dealing with new roads in an atmosphere of
austerity. Cement concrete slabs do not fit into such a scheme of stage
construction.
Availability
of materials
Cement,
bitumen, stone aggregates and graveVsand are the major materials involved in
pavement construction. Cement has been in serious short supply in the country
for the past many decades. The situation has eased very considerably now since
many new cement plants have been licensed. It is now certain that the Indian
highway engineers will start constructing concrete roads again after a lapse of
nearly forty years. Bitumen is also not available plentifully in India.
While some of the Indian crudes yield good quality bitumen, there are others
which do not. There is also the danger of the entire oil reserves in the world
shrinking during the next two or three decades. Bitumen is thus also a scarce
commodity, not only in India, but
also worldwide. Moreover, import of bitumen involves foreign exchange, whereas
cement is indigenously manufactured. In locations where stone aggregates are
scarce, cement concrete may have an advantage, since the total construction
thickness may be less than that for a flexible pavement.
Surface
characteristics
A good
cement concrete surface is smooth and free from rutting, potholes and
corrugations. Thus the riding quality of a cement concrete surface is always
assured. In a bituminous surface, it is only the asphaltic concrete surface that
can give comparable ride ability. Thin surfaces treatments such as premix
open-textured carpets and surface dressing are very rough. A well-constructed
cement concrete pavement surface can have a permanent non-skid surface. On the
other hand, if the design is faulty, a cement concrete surface may become very
smooth in course of time. If it does, it is extremely costly to restore the
non-skid characteristics. Grooving and etching will have to be adopted. Grooved
slabs cause noise. A bituminous surface can also be designed to have a good
skid-resistant surface. If it fattens up, a rough seal coat with a brushing of
coarse aggregates can easily restore the lost property.
Penetration
of water
A
cement concrete slab is practically impervious, ,except at joints. If joints are
sealed and well-maintained, water will not penetrate and soften the subgrade. If
joints are faulty, water easily finds its way in and serious defects such as
"mud-pumping" can follow. A bituminous surface is not impervious. Water can find
its way into the lower layers through cracks and pores. Such water can impair
the stability of the pavement.
Glare
and night visibility
Concrete
pavements have a grey colour which can cause glare under sunlight. Black
bituminous pavements are free from this defect. On other hand, bituminous roads
need more street lighting.
Traffic
dislocation during construction
A
cement concrete pavement requires 28 days before it can be thrown open
to
Environmental
considerations during construction
Overall
economy on a life-cycle basis
| Pavement Salb Design | |||||||||||||
| 1 | Design Parameters | ||||||||||||
| Location of pavment | Punjab | ||||||||||||
| Design Wheel Load, p | 5100 | Kg | |||||||||||
| Present Trffic intensity | 300 | Veh/day | |||||||||||
| Design tyre pressure p | 7.2 | Kg/Cm^2 | |||||||||||
| Foundation Strength k | 6 | Kg/Cm^2 | |||||||||||
| Concrete Flexural Strength Fr | 40 | Kg/Cm^2 | |||||||||||
| Other Conc. Parameters | |||||||||||||
| E | 300000 | Kg/Cm^2 | |||||||||||
| u | 0.15 | ||||||||||||
| 0.00001 | degree C | ||||||||||||
| 2 | Design procedure | ||||||||||||
| Joint spacing and Lanewidth | |||||||||||||
| Contraction Joint Spacing L= | 4.5 | M | |||||||||||
| LaneWidth W= | 3.5 | M | |||||||||||
| Tentative Design thickness of slab h= | 22 | Cm | |||||||||||
| i) | Temparture stress for edge region. | 13.58 | |||||||||||
| Table 2 | |||||||||||||
| IRC 58 Table 2 For Punjab | Temprature differancewrto thickness of slab | ||||||||||||
| Slab thickness = | 10 cm | 15 cm | 20 cm | 25 cm | 30 cm | ||||||||
| Temprature Variation= | 10.2 | 12.5 | 13.1 | 14.3 | 15.8 | ||||||||
| ii) | From Table 6 I= | 81.89 | |||||||||||
| Table 6 | |||||||||||||
| h cm | K=6 | K=8 | K=10 | K=15 | K=30 | ||||||||
| 15 | 61.44 | 57.18 | 54.08 | 48.86 | 41.09 | ||||||||
| 16 | 64.49 | 60.02 | 54.76 | 51.29 | 43.31 | ||||||||
| 17 | 67.49 | 62.81 | 59.40 | 53.67 | 45.14 | ||||||||
| 18 | 70.44 | 65.56 | 62.01 | 56.03 | 47.07 | ||||||||
| 19 | 73.36 | 68.28 | 54.57 | 58.35 | 49.06 | ||||||||
| 20 | 76.24 | 70.95 | 67.10 | 60.63 | 50.99 | ||||||||
| 21 | 79.08 | 73.59 | 69.60 | 63.89 | 52.89 | ||||||||
| 22 | 81.89 | 76.20 | 72.08 | 65.13 | 54.77 | ||||||||
| 23 | 84.66 | 78.80 | 74.52 | 67.33 | 56.62 | ||||||||
| 24 | 87.41 | 81.35 | 76.94 | 69.31 | 58.45 | ||||||||
| 25 | 90.13 | 83.88 | 79.32 | 71.68 | 60.28 | ||||||||
| L/I= | 5.5 | Table 8 | |||||||||||
| W/I= | 4.3 | L/I or W/I | |||||||||||
| 1 | 0 | ||||||||||||
| 2 | 0.040 | ||||||||||||
| 3 | 0.175 | ||||||||||||
| 4 | 0.440 | ||||||||||||
| 5 | 0.720 | ||||||||||||
| 6 | 0.920 | ||||||||||||
| 7 | 1.030 | ||||||||||||
| 8 | 1.075 | ||||||||||||
| 9 | 1.080 | ||||||||||||
| 10 | 1.075 | ||||||||||||
| 11 | 1.050 | ||||||||||||
| From table 8 | 12&above | 1.000 | |||||||||||
| cL | 0.82 | ||||||||||||
| wL | 0.524 | ||||||||||||
| FromFig 3 | |||||||||||||
| cl & wl wrto above e= | 16 | kg/cm^2 | |||||||||||
| Residual Strength to suport the load= | 24 | ||||||||||||
| Load Stress for edge region | |||||||||||||
| From fig 1 h= 22, k = 6 | 23.4 | ||||||||||||
Typical Mix Design For Concrete
A Gap Graded Mix Design:
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Mix Design for Pavement Quality Concrete M-35 (Flexural 40 Kg/Cm^2) |
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A |
Design Stipulations |
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Minimum flexural strength required at 28 days ( S') |
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N/mm2 |
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Maximum size of the aggregates |
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degree of workablity |
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mm extra cement v'p=0.1 m3/m3 |
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degree of quality control |
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Good |
Co efficient 10% |
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Acceptance tolrance |
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Entrapped air ve |
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B |
Test data for materials: |
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Compressive strength of cement at 7 days |
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33 |
N/mm2 |
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Specific gravity of cement |
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3.15 |
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Specific gravity of Coarse Aggregates |
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Specific gravity of Fine Aggregates |
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Water absorption |
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Coarse aggregates |
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Fine aggregates |
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Free surface moisture |
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Coarse aggregates |
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Sand |
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Bulk density of saturated surface dry |
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Coarse aggregates |
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da |
1475 |
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Sand |
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ds |
1550 |
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C |
Design strength of concrete( S ) |
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S= S'/(1-t.V/100) |
4.71 |
N/mm2 |
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D |
Selection of water cement ratio: |
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From fig 1 of IRC 59 corresponding to the 7 days strength of 330 kg/cm2 and design flexural strength |
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4.71 |
N/mm2 |
water cement ratio required = |
0.40 |
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E |
Calculation of Mix Proportion per 1 M3 of Wet Concrete |
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V'= |
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Cum |
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There fore |
V=V'-ve-v'p= |
0.89 |
Cum |
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Wa |
Weight of aggregates |
Vxdac = |
1312.75 |
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n1=(1-da/sga/1000)x100= |
44.55 |
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Ws |
Weight of sand n1xVxds/100= |
614.55 |
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Wc |
100Sc |
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1+rxSc |
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n2=(1-ds/1000/sgf)x100= |
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|
|
Wc= |
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|
|
365.07 |
|
|
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|
|
Ww= |
rxwc |
|
|
146.03 |
|
|
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|
Mix Proportion Kg/m3 |
|
|
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|
|
| |
|
|
Water |
Cement |
Sand |
C.bajri |
|
|
|
|
|
|
|
|
146.03 |
365.07 |
614.55 |
1312.75 |
|
|
|
2438.40 |
|
|
|
|
0.4 |
1 |
1.68 |
3.60 |
|
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|
F |
Actual quantiites required for the mix per 50 kg bag of cement |
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| |||||
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|
Cement |
50 |
Kg |
|
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|
|
I |
For cement =50Kg, net quantities required of materials are |
|
|
|
|
| ||||
|
|
Water |
Cement |
Sand |
C.bajri |
|
|
|
|
|
|
|
|
Kg |
Kg |
Kg |
Kg |
|
|
|
|
|
|
|
|
20.00 |
50.00 |
84.17 |
179.79 |
|
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Water to be added= |
|
|
|
|
|
20 |
Liter |
| |
|
|
Extra water to be added to cater for absorption by coarse aggregate= |
|
0.719 |
Liter |
| |||||
|
|
water to be deducted for fre moisture present in fine aggreegate (sand)= |
|
-1.683 |
Liter |
| |||||
|
|
|
|
|
|
Net water to be added |
|
19.036 |
Liter |
| |
|
|
Actual quantity of fine aggregate ( Sand ) |
84.17 |
1.683 |
|
85.85 |
Kg |
| |||
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|
|
Actual quantity of coarse aggregates |
|
179.79 |
0.719 |
|
180.51 |
Kg |
| ||
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|||||