Benefits of steel reinforced concrete slabs

Here are the benefits of steel reinforced concrete slabs:
• Steel reinforcing is simple to place.
• Steel reinforcing reduces random cracking.
• Steel reinforcing reduces and controls crack width and
helps maintain aggregate interlock.
• Displacement and curling can be minimized when steel
reinforced concrete is provided.
• Strength is increased with steel reinforced concrete—
even the smallest cross sectional area of steel reinforcement
will provide reserve strength of l 6 percent and more.
• Most importantly, steel reinforcement saves money over
the life of the slab.
• Finally, admixtures are not an alternative to steel reinforcement;
they both do different things in the concrete.

Therefore, admixtures cannot be substituted for steel reinforcement.
The steel reinforcement industry is dedicated to providing
quality steel reinforcement to the construction industry. It is
also essential that steel reinforcement be sized, spaced, and
placed properly. It is vital to have a well-graded and compacted
granular subbase.
Of course, total quality can only be achieved when well
qualified suppliers and contractors are on the construction
sites.
References
1. Lanning, Anne, “Synthetic Fibers,” Concrete Construction, July 1992.
2. Ringo, Boyd C., and Anderson, Robert B., “Designing Floor Slabs On
Grade Step-by-Step Procedures Sample Solutions and Commentary “Second
Edition, The Aberdeen Group, 1996.
3. Anderson, Robert B., “Innovative Ways to Reinforce Slabs-On-
Ground” WRI Publication TF-705, 1996.
4. “Supports for Welded Wire Reinforcements in Slabs-On-Grade,” WRI
Publication TF-702.

Design Loads for Residential Buildings .

The load combinations in Table 3.1 are recommended for use with design specifications based on allowable stress design (ASD) and load and resistance factor design (LRFD). Load combinations provide the basic set of building load conditions that should be considered by the designer. They establish the proportioning of multiple transient loads that may assume point-in-time values when the load of interest attains its extreme design value. Load combinations are intended as a guide to the designer, who should exercise judgment in any particular application. The load combinations in Table 3.1 are appropriate for use with the design loads determined in accordance with this chapter.
The principle used to proportion loads is a recognition that when one load attains its maximum life-time value, the other loads assume arbitrary point-in-
time values associated with the structure’s normal or sustained loading conditions. The advent of LRFD has drawn greater attention to this principle (Ellingwood et al., 1982; Galambos et al., 1982). The proportioning of loads in this chapter for allowable stress design (ASD) is consistent with and normalized to the proportioning of loads used in newer LRFD load combinations. However, this manner of proportioning ASD loads has seen only limited use in current code-recognized documents (AF&PA, 1996) and has yet to be explicitly recognized in design load specifications such as ASCE 7. ASD load combinations found in building codes have typically included some degree of proportioning (i.e., D + W
+ 1/2S) and have usually made allowance for a special reduction for multiple transient loads. Some earlier codes have also permitted allowable material stress increases for load combinations involving wind and earthquake loads. None of these adjustments for ASD load combinations is recommended for use with Table 3.1 since the load proportioning is considered sufficient

It should also be noted that the wind load factor of 1.5 in Table 3.1 used for load and resistant factor design is consistent with traditional wind design practice (ASD and LRFD) and has proven adequate in hurricane-prone environments when buildings are properly designed and constructed. The 1.5 factor is equivalent to the earlier use of a 1.3 wind load factor in that the newer wind load provisions of ASCE 7-98 include separate consideration of wind directionality by adjusting wind loads by an explicit wind directionality factor, KD, of 0.85. Since the wind load factor of 1.3 included this effect, it must be adjusted to 1.5 in compensation for adjusting the design wind load instead (i.e., 1.5/1.3 = 0.85). The 1.5 factor may be considered conservative relative to traditional design practice in nonhurricane-prone wind regions as indicated in the calibration of the LRFD load factors to historic ASD design practice (Ellingwood et al., 1982; Galambos et al., 1982). In addition, newer design wind speeds for hurricane-prone areas account for variation in the extreme (i.e., long return period) wind probability that occurs in hurricane hazard areas. Thus, the return period of the design wind speeds along the hurricane-prone coast varies from roughly a 70- to 100-year return period on the wind map in the 1998 edition of ASCE 7 (i.e., not a traditional 50-year return period wind speed used for the remainder of the United States). The latest wind design provisions of ASCE 7 include many advances in the state of the art, but the ASCE commentary does not clearly describe the condition mentioned above in support of an increased wind load factor of 1.6 (ASCE, 1999). Given that the new standard will likely be referenced in future building codes, the designer may eventually be required to use a higher wind load factor for LRFD than that shown in Table 3.1. The above discussion is intended to help the designer understand the recent departure from past successful design experience and remain cognizant of its potential future impact to building design.
The load combinations in Table 3.1 are simplified and tailored to specific application in residential construction and the design of typical components and systems in a home. These or similar load combinations are often used in practice as short-cuts to those load combinations that govern the design result. This guide makes effective use of the short-cuts and demonstrates them in the examples provided later in the chapter. The short-cuts are intended only for the design of residential light-frame construction.

Note:
1The load combinations and factors are intended to apply to nominal design loads defined as follows: D = estimated mean dead weight of
the construction; H = design lateral pressure for soil condition/type; L = design floor live load; Lr = maximum roof live load anticipated
from construction/maintenance; W = design wind load; S = design roof snow load; and E = design earthquake load. The design or nominal
loads should be determined in accordance with this chapter.
2Attic loads may be included in the floor live load, but a 10 psf attic load is typically used only to size ceiling joists adequately for access
purposes. However, if the attic is intended for storage, the attic live load (or some portion) should also be considered for the design of
other elements in the load path.
3The transverse wind load for stud design is based on a localized component and cladding wind pressure; D + W provides an adequate and
simple design check representative of worst-case combined axial and transverse loading. Axial forces from snow loads and roof live loads
should usually not be considered simultaneously with an extreme wind load because they are mutually exclusive on residential sloped
roofs. Further, in most areas of the United States, design winds are produced by either hurricanes or thunderstorms; therefore, these wind
events and snow are mutually exclusive because they occur at different times of the year.
4For walls supporting heavy cladding loads (such as brick veneer), an analysis of earthquake lateral loads and combined axial loads should
be considered. However, this load combination rarely governs the design of light-frame construction.
5Wu is wind uplift load from negative (i.e., suction) pressures on the roof. Wind uplift loads must be resisted by continuous load path
connections to the foundation or until offset by 0.6D.
6The 0.6 reduction factor on D is intended to apply to the calculation of net overturning stresses and forces. For wind, the analysis of
overturning should also consider roof uplift forces unless a separate load path is designed to transfer those forces.

What is Structural Systems

Over many years, engineers have observed that some structural systems perform
better in earthquakes than others. Based on these observations, the Provisions
design criteria for building structures are based on the structural system used.
Structural systems are categorized based on the material of construction (e.g.,
concrete, masonry, steel, or wood), by the way in which lateral forces induced by
earthquake shaking are resisted by the structure (e.g., by walls or frames), and by
the relative quality of seismic-resistant design and detailing provided.
The Provisions recognizes six broad categories of structural system:

• Bearing wall systems,
• Building frame systems,
• Moment-resisting frame systems,
• Dual systems,
• Cantilever column systems, and
• Systems not specifically designed for seismic resistance.

In bearing wall systems, structural walls located throughout the structure provide
the primary vertical support for the building’s weight and that of its contents as
well as the building’s lateral resistance. Bearing wall buildings are commonly
used for residential construction, warehouses, and low-rise commercial buildings
of concrete, masonry, and wood construction. Figures 21, 22, and 23 show typical
bearing wall buildings.

Building frames are a common structural system for buildings constructed of
structural steel and concrete. In building frame structures, the building’s weight
is typically carried by vertical elements called columns and horizontal elements
called beams. Lateral resistance is provided either by diagonal steel members
(termed braces) that extend between the beams and columns to provide horizontal
rigidity or by concrete, masonry, or timber shear walls that provide lateral
resistance but do not carry the structure’s weight. In some building frame
structures, the diagonal braces or walls form an inherent and evident part of the
building design as is the case for the high-rise building in San Francisco shown in
Figure 24. In most buildings, the braces or walls may be hidden behind exterior
cladding or interior partitions.
Moment-resisting frame systems are commonly used for both structural steel and
reinforced concrete construction. In this form of construction, the horizontal
beams and vertical columns provide both support for the structure’s weight and
the strength and stiffness needed to resist lateral forces. Stiffness and strength are
achieved through the use of rigid connections between the beams and columns
that prevent these elements from rotating relative to one other. Although somewhat
more expensive to construct than bearing wall and braced frame structural
systems, moment-resisting frame systems are popular because they do not
require braced frames or structural walls, therefore permitting large open spaces
and facades with many unobstructed window openings. Figure 25 shows a steel
moment-resisting frame building under construction.
Dual systems, an economical alternative to moment-resisting frames, are commonly
used for tall buildings. Dual system structures feature a combination of
moment-resisting frames and concrete, masonry, or steel walls or steel braced

frames. The moment-resisting frames provide vertical support for the structure’s
weight and a portion of the structure’s lateral resistance while most of the lateral
resistance is provided either by concrete, masonry, or steel walls or by steel braced
frames. Some dual systems are also called frame-shear wall interactive systems.
Cantilever column systems are sometimes used for single-story structures or in
the top story of multistory structures. In these structures, the columns cantilever
upward from their base where they are restrained from rotation. The columns
provide both vertical support of the building’s weight and lateral resistance to
earthquake forces. Structures using this system have performed poorly in past
earthquakes and severe restrictions are placed on its use in zones of high seismic
activity.

In regions of relatively low seismic risk, the NEHRP Recommended Seismic
Provisions permits the design and construction of structural steel buildings that
do not specifically conform to any of the above system types. These buildings are
referred to as “structures not specifically detailed for seismic resistance.”

In addition to these basic structural systems and the primary materials of construction,
the Provisions also categorizes structural systems based on the quality
and extent of seismic-resistant detailing used in a structure’s design. Systems that
employ extensive measures to provide for superior seismic resistance are termed
“special” systems while systems that do not have such extensive design features
are typically called “ordinary” systems. The Provisions also includes design rules
for structural systems intended to provide seismic resistance that is superior to
that of “ordinary” systems but not as good as that of “special” systems; these systems
are called “intermediate” systems.

What is Buildings

Generally, a building can be defined as an enclosed structure intended for human
occupancy. However, a building includes the structure itself and nonstructural
components (e.g., cladding, roofing, interior walls and ceilings, HVAC systems,
electrical systems) permanently attached to and supported by the structure. The
scope of the Provisions provides recommended seismic design criteria for all
buildings except detached one- and two-family dwellings located in zones of
relatively low seismic activity and agricultural structures (e.g., barns and storage
sheds) that are only intended to have incidental human occupancy. The Provisions
also specifies seismic design criteria for nonstructural components in buildings
that can be subjected to intense levels of ground shaking.

What is Load Combinations

As per IS 1893 (Part 1): 2002 Clause no. 6.3.1.2,
the following load cases have to be considered for
analysis:
1.5 (DL + IL)<>
1.2 (DL + IL ± EL)<>
1.5 (DL ± EL)<>
0.9 DL ± 1.5 EL<>
Earthquake load must be considered for +X, -X,
+Z and –Z directions. Moreover, accidental
eccentricity can be such that it causes clockwise
or anticlockwise moments. Thus, ±EL above
implies 8 cases, and in all, 25 cases as per Table 3
must be considered. It is possible to reduce the
load combinations to 13 instead of 25 by not
using negative torsion considering the symmetry
of the building. Since large amount of data is
difficult to handle manually, all 25-load
combinations are analysed using software.
For design of various building elements (beams or
columns), the design data may be collected from
computer output. Important design forces for
selected beams will be tabulated and shown
diagrammatically where needed. . In load
combinations involving Imposed Loads (IL), IS
1893 (Part 1): 2002 recommends 50% of the
imposed load to be considered for seismic weight
calculations. However, the authors are of the
opinion that the relaxation in the imposed load is
en conservative. This example therefore, considers
100% imposed loads in load combinations.
For above load combinations, analysis is
performed and results of deflections in each
storey and forces in various elements are
obtained.

How to Test Soil Compaction

There are many types of Soil compaction tests which are performed on soil. Some of these are :-

1) The Sand Cone Method
One of the most common test to determine the field density of soil is the sand-cone method. But it has a major limitation that this test is not suitable for saturated and soft soils

The formula used are
Volume of soil, ft³ (m³)=[weight of sand filling hole, lb (kg)] /[ Density of sand, lb/ft³ (kg/m³)]

% Moisture = 100(weight of moist soil – weight of dry soil)/weight of dry soil

Field density, lb/ft³ (kg /m³)=weight of soil, lb (kg)/volume of soil, ft³ (m³)

Dry density=field density/(1 + % moisture/100)

% Compaction=100 (dry density)/max dry density

Maximum density is found by plotting a density–moisture curve.

2) California Bearing Ratio
The California bearing ratio (CBR) is used as a determine the quality of strength of a soil under a pavement. It also measures the thickness of the pavement, its base, and other layers.
CBR = F/F_o
where
F = force per unit area required to penetrate a soil mass with a 3-in² (1935.6-mm² ) circular piston (about 2 in (50.8 mm) in diameter) at the rate of 0.05 in/min (1.27 mm/min)

F₀ = force per unit area required for corresponding penetration of a standard material.

3) Soil Permeability
Darcy’s law is applicable in determining the soil permeability. Darcy law states that
V = kiA

where
V = rate of flow, cm³ /s,
A = cross-sectional area of soil conveying flow, cm²
k = Coefficient of permeability which depends on grain-size distribution, void ratio and soil fabric. The value varies from 10 cm/s for gravel to less than 10^–7 for clays.

Compression test for concrete, should test cubes or test cylinders be adopted?

Basically, the results of compression test carried out by using cubes are higher than that by cylinders. In compression test, the failure mode is in the form of tensile splitting induced by uniaxial compression. However, since the concrete samples tend to expand laterally under compression, the friction developed at the concrete-machine interface generates forces which apparently increase the compressive strength of concrete.

 However, when the ratio of height to width of sample increases, the effect of shear on compressive strength becomes smaller. This explains why the results of compression test by cylinders are lower than that of cubes. Reference is made to Longman Scientific and Technical (1987).

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

How to Determine Particle Size Distribution Of Soil

This test is done to determine the particle size distribution of soil as per IS: 2720 (Part 4) – 1985. The appratus required to do this test :-
i) A set of fine IS Sieves of sizes – 2mm, 600µm, 425µm, 212µm and 75µm
ii) A set of coarse IS Sieves of sizes – 20mm, 10mm and 4.75mm
iii) Weighing balance, with an accuracy of 0.1% of the weight of sample
iv) Oven
v) Mechanical shaker
vi) Mortar with rubber pestle
vii) Brushes
viii) Trays

PREPARATION OF SAMPLE
i) Soil sample, as received from the field, should be dried in air or in the sun. In wet weather, the drying apparatus may be used in which case the temperature of the sample should not exceed 60^oC. The clod may be broken with wooden mallet to hasten drying. Tree roots and pieces of bark should be removed from the sample.

ii) The big clods may be broken with the help of wooden mallet. Care should be taken not to break the individual soil particles.

iii) A representative soil sample of required quantity as given below is taken and dried in the oven at 105 to 120^oC.

Procedure to determine Particle Size Distribution Of Soil
i) The dried sample is taken in a tray, soaked in water and mixed with either 2g of sodium hexametaphosphate or 1g of sodium hydroxide and 1g of sodium carbonate per litre of water, which is added as a dispersive agent. The soaking of soil is continued for 10 to 12hrs.

ii) The sample is washed through 4.75mm IS Sieve with water till substantially clean water comes out. Retained sample on 4.75mm IS Sieve should be oven-dried for 24hrs. This dried sample is sieved through 20mm and 10mm IS Sieves.

iii) The portion passing through 4.75mm IS Sieve should be oven-dried for 24hrs. This oven-dried material is riffled and about 200g taken.

iv) This sample of about 200g is washed through 75µm IS Sieve with half litre distilled water, till substantially clear water comes out.

v) The material retained on 75µm IS Sieve is collected and dried in oven at a temperature of 105 to 120^oC for 24hrs. The dried soil sample is sieved through 2mm, 600µm, 425µm
and 212µm IS Sieves. Soil retained on each sieve is weighed.

vi) If the soil passing 75µm is 10% or more, hydrometer method is used to analyse soil particle size.

HYDROMETER ANALYSIS
i) Particles passed through 75µm IS Sieve along with water are collected and put into a 1000ml jar for hydrometer analysis. More water, if required, is added to make the soil water suspension just 1000ml. The suspension in the jar is vigorously shaken horizontally by keeping the jar in-between the palms of the two hands. The jar is put on the table.

ii) A graduated hydrometer is carefully inserted into the suspension with minimum disturbance.

iii) At different time intervals, the density of the suspension at the centre of gravity of the hydrometer is noted by seeing the depth of sinking of the stem. The temperature of the suspension is noted for each recording of the hydrometer reading.

iv) Hydrometer readings are taken at a time interval of 0.5 minute, 1.0 minute, 2.0 minutes, 4.0 minutes, 15.0 minutes, 45.0 minutes, 90.0 minutes, 3hrs., 6hrs., 24hrs. and 48hrs.

v) By using the nomogram given in IS: 2720 (Part 4) – 1985, the diameter of the particles for different hydrometer readings is found out.

REPORTING OF RESULTS
After completing mechanical analysis and hydrometer analysis, the results are plotted on a semi-log graph with particle size as abscissa (log scale) and the percentage smaller than the specified diameter as ordinate

What About Hydraulic Jump

The abrupt increase in depth of rapidly flowing water is called hydraulic depth.Flow at the jump changes from a supercritical to a subcritical stage with an accompanying loss of kinetic energy. The change in depth occurs over a finite distance, known as the length of jump. The upstream surface of the jump, known as the roller, is a turbulent mass of water.

The depth before a jump is the initial depth, and the depth after a jump is the sequent depth. The specific energy for the sequent depth is less than that for the initial depth because of the energy dissipation within the jump.

F=[ d₂²- d₁²]w/2

where d₁ =depth before jump, ft (m)

d₂ =depth after jump, ft (m)

w=unit weight of water, lb/ft³ (kg/m³)

What is Sieve Analysis of Aggregates

SIEVE ANALYSIS
Sieve analysis helps to determine the particle size distribution of the coarse and fine aggregates.This is done by sieving the aggregates as per IS: 2386 (Part I) – 1963. In this we use different sieves as standardized by the IS code and then pass aggregates through them and thus collect different sized particles left over different sieves.

The apparatus used are -
i) A set of IS Sieves of sizes – 80mm, 63mm, 50mm, 40mm,31.5mm, 25mm, 20mm, 16mm, 12.5mm, 10mm, 6.3mm,4.75mm, 3.35mm, 2.36mm, 1.18mm, 600µm, 300µm, 150µm and 75µm.

ii) Balance or scale with an accuracy to measure 0.1 percent of the weight of the test sample.
The weight of sample available should not be less than the weight given below:-

The sample for sieving should be prepared from the larger sample either by quartering or by means of a sample divider.

Procedure to determine particle size distribution of Aggregates.
i) The test sample is dried to a constant weight at a temperature of 110 + 5^oC and weighed.
ii) The sample is sieved by using a set of IS Sieves.
iii) On completion of sieving, the material on each sieve is weighed.
iv) Cumulative weight passing through each sieve is calculated as a percentage of the total sample weight.
v) Fineness modulus is obtained by adding cumulative percentage of aggregates retained on each sieve and dividing the sum by 100.

Determining Water Content In Soil – Oven Drying Method

This test is done to determine the water content in soil by oven drying method as per IS: 2720 (Part II) – 1973. The water content (w) of a soil sample is equal to the mass of water divided by the mass of solids.
Apparatus required :-
i) Thermostatically controlled oven maintained at a temperature of 110 ± 5^oC
ii) Weighing balance, with an accuracy of 0.04% of the weight of the soil taken
iii) Air-tight container made of non-corrodible material with lid
iv) Tongs

PREPARATION OF SAMPLE
The soil specimen should be representative of the soil mass. The quantity of the specimen taken would depend upon the gradation and the maximum size of particles as under:

Procedure to determine Water Content In Soil By Oven Drying Method
i) Clean the container, dry it and weigh it with the lid (Weight ‘W₁‘).

ii) Take the required quantity of the wet soil specimen in the container and weigh it with the lid (Weight ‘W₂‘).

iii) Place the container, with its lid removed, in the oven till its weight becomes constant (Normally for 24hrs.).

iv) When the soil has dried, remove the container from the oven, using tongs.

v) Find the weight ‘W₃‘ of the container with the lid and the dry soil sample.

REPORTING OF RESULTS
The water content
w = [W₂-W₃] / [W₃ -W₁]*100%
An average of three determinations should be taken. A sample calculation is shown below

How to Determine The Plastic Limit Of Soil

This test is done to determine the plastic limit of soil as per IS: 2720 (Part 5) – 1985.The plastic limit of fine-grained soil is the water content of the soil below which it ceases to be plastic. It begins to crumble when rolled into threads of 3mm dia. The apparatus used:

i) Porcelain evaporating dish about 120mm dia.
ii) Spatula
iii) Container to determine moisture content
iv) Balance, with an accuracy of 0.01g
v) Oven
vi) Ground glass plate – 20cm x 15cm
vii) Rod – 3mm dia. and about 10cm long

PREPARATION OF SAMPLE
Take out 30g of air-dried soil from a thoroughly mixed sample of the soil passing through 425µm IS Sieve. Mix the soil with distilled water in an evaporating dish and leave the soil mass for naturing. This period may be upto 24hrs.

Procedure to determine The Plastic Limit Of Soil
i) Take about 8g of the soil and roll it with fingers on a glass plate. The rate of rolling should be between 80 to 90 strokes per minute to form a 3mm dia.

ii) If the dia. of the threads can be reduced to less than 3mm, without any cracks appearing, it means that the water content is more than its plastic limit. Knead the soil to reduce the water content and roll it into a thread again.

iii) Repeat the process of alternate rolling and kneading until the thread crumbles.

iv) Collect and keep the pieces of crumbled soil thread in the container used to determine the moisture content.

v) Repeat the process at least twice more with fresh samples of plastic soil each time.

REPORTING OF RESULTS
The plastic limit should be determined for at least three portions of the soil passing through 425µm IS Sieve. The average water content to the nearest whole number should be reported.

How to Determine The Specific Gravity Of Soil

This test is done to determine the specific gravity of fine-grained soil by density bottle method as per IS: 2720 (Part III/Sec 1) – 1980. Specific gravity is the ratio of the weight in air of a given volume
of a material at a standard temperature to the weight in air of an equal volume of distilled water at the same stated temperature.
The apparatus used:
i) Two density bottles of approximately 50ml capacity along with stoppers
ii) Constant temperature water bath (27.0 + 0.2^oC)
iii) Vacuum desiccator
iv) Oven, capable of maintaining a temperature of 105 to 110^oC
v) Weighing balance, with an accuracy of 0.001g
vi) Spatula

PREPARATION OF SAMPLE
The soil sample (50g) should if necessary be ground to pass through a 2mm IS Sieve. A 5 to 10g sub-sample should be obtained by riffling and oven-dried at a temperature of 105 to 110^oC.
Procedure to Determine the Specific Gravity of Fine-Grained Soil
i) The density bottle along with the stopper, should be dried at a temperature of 105 to 110^oC, cooled in the desiccator and weighed to the nearest 0.001g (W₁).
ii) The sub-sample, which had been oven-dried should be transferred to the density bottle directly from the desiccator in which it was cooled. The bottles and contents together with the stopper should be weighed to the nearest 0.001g (W₂).
iii) Cover the soil with air-free distilled water from the glass wash bottle and leave for a period of 2 to 3hrs. for soaking. Add water to fill the bottle to about half.
iv) Entrapped air can be removed by heating the density bottle on a water bath or a sand bath.
v) Keep the bottle without the stopper in a vacuum desiccator for about 1 to 2hrs. until there is no further loss of air.
vi) Gently stir the soil in the density bottle with a clean glass rod, carefully wash off the adhering particles from the rod with some drops of distilled water and see that no more soil particles are lost.
vii) Repeat the process till no more air bubbles are observed in the soil-water mixture.
viii) Observe the constant temperature in the bottle and record.
ix) Insert the stopper in the density bottle, wipe and weigh(W₃).
x) Now empty the bottle, clean thoroughly and fill the density bottle with distilled water at the same temperature. Insert the stopper in the bottle, wipe dry from the outside and weigh (W₄ ).
xi) Take at least two such observations for the same soil.

REPORTING OF RESULTS
The specific gravity G of the soil = (W₂ – W₁) / [(W₄-₁)-(W₃-W₂)]
The specific gravity should be calculated at a temperature of 27^oC and reported to the nearest 0.01. If the room temperature is different from 27^oC, the following correction should be done:-
G’ = kG
where,
G’ = Corrected specific gravity at 27^oC
k = [Relative density of water at room temperature]/ Relative density of water at 27^oC.
A sample proforma for the record of the test results is given below. Relative density of water at various temperatures is taken from table here.

When Things Go Wrong Structural Stress and Fatigue

When a structure is poorly designed or built, it may not be able to withstand all of the forces it has to face. When a structure has to face large

combinations of internal and external forces over a long period of time, the structure might weaken. This may result in structural stress. At first, signs of structural stress may disappear when the internal and external forces are reduced. For example, if you place an abnormally large book on the middle of a bookshelf, the shelf might bend. The

bend in the shelf is a sign of stress. When the book is removed,

the shelf may go back to its original shape. However, if the

shelf cannot withstand the stress, it might crack. Permanent

changes, like the bookshelf cracking, are signs of structural

How to Determine The Liquid Limit Of Soil

This test is done to determine the liquid limit of soil as per IS: 2720 (Part 5) – 1985. The liquid limit of fine-grained soil is the water content at which soil behaves practically like a liquid, but has small shear
strength. It’s flow closes the groove in just 25 blows in Casagrande’s liquid limit device. The apparatus used :-
i) Casagrande’s liquid limit device
ii) Grooving tools of both standard and ASTM types
iii) Oven
iv) Evaporating dish
v) Spatula
vi) IS Sieve of size 425µm
vii) Weighing balance, with 0.01g accuracy
viii) Wash bottle
ix) Air-tight and non-corrodible container for determination of moisture content
PREPARATION OF SAMPLE
i) Air-dry the soil sample and break the clods. Remove the organic matter like tree roots, pieces of bark, etc.
ii) About 100g of the specimen passing through 425µm IS Sieve is mixed thoroughly with distilled water in the evaporating dish and left for 24hrs. for soaking.



Procedure to Determine The Liquid Limit Of Soil
i) Place a portion of the paste in the cup of the liquid limit device.
ii) Level the mix so as to have a maximum depth of 1cm.
iii) Draw the grooving tool through the sample along the symmetrical axis of the cup, holding the tool perpendicular to the cup.
iv) For normal fine grained soil: The Casagrande’s tool is used to cut a groove 2mm wide at the bottom, 11mm wide at the top and 8mm deep.
v) For sandy soil: The ASTM tool is used to cut a groove 2mm wide at the bottom, 13.6mm wide at the top and 10mm deep.
vi) After the soil pat has been cut by a proper grooving tool, the handle is rotated at the rate of about 2 revolutions per second and the no. of blows counted, till the two parts of the soil sample come into contact for about 10mm length.
vii) Take about 10g of soil near the closed groove and determine its water content
viii) The soil of the cup is transferred to the dish containing the soil paste and mixed thoroughly after adding a little more water. Repeat the test.
ix) By altering the water content of the soil and repeating the foregoing operations, obtain at least 5 readings in the range of 15 to 35 blows. Don’t mix dry soil to change its consistency.
x) Liquid limit is determined by plotting a ‘flow curve’ on a semi-log graph, with no. of blows as abscissa (log scale) and the water content as ordinate and drawing the best straight line through the plotted points.

REPORTING OF RESULTS
Report the water content corresponding to 25 blows, read from the ‘flow curve’ as the liquid limit.
A sample ‘flow curve’ is given as