The document provides information on concrete foundations, including the key ingredients of concrete and their functions. It discusses different grades of concrete based on their compressive strength. Formwork, mixing, placing, compaction and curing of concrete are explained. Different types of foundations are described, including shallow foundations like spread footings, strap footings and mat foundations. Deep foundations such as pile foundations and pier foundations are also summarized. Load bearing and framed structures are compared in terms of their suitability for different types of buildings and soils.

1. UNIT – 2
BUILDING CONSTRUCTION
CONCRETE FOUNDATION

2. TOPICS TO BE COVERED :
• CONCRETE - Ingredients of Cement Concrete, Grades of Concrete,
proportions for Nominal mix concrete, Workability & Compressive
Strength of Concrete, Curing of Concrete.
• FOUNDATION - Necessity of foundations, Definitions of Safe bearing
capacity, Ultimate bearing capacity and factor of safety, Difference
between Load Bearing & Framed Construction.

3. CONCRETE
• Plain concrete, commonly known as concrete, is an intimate
mixture of binding material, fine aggregate, coarse aggregate
and water.
• This can be moulded into desired shape and size before it
losses plasticity and hardens.
• Plain concrete is strong in compression but very week in
tension.

4. INGREDIENTS OF CEMENT CONCRETE :
• Major ingredients of concrete are:
1. Binding material (like cement, lime, polymer)
2. Fine aggregate (sand)
3. Coarse aggregates (crushed stone, jelly)
4. Water.
• A small quantity of admixtures like air entraining agents, water proofing agents, workability
agents etc. may also be added to impart special properties to the plain concrete mixture.
Depending upon the proportion of ingredient, strength of concrete varies. It is possible to
determine the proportion of the ingredients for a particular strength by mix design procedure. In
the absence of mix design the ingredients are proportioned as 1:1:2, 1:1.5:3, 1:2:4, 1:3:6 and
1:4:8, which is the ratio of weights of cement to sand to coarse aggregate.

5. PROPORTION OF CEMENT, SAND AND COARSE
AGGREGATES IN CONCRETE
• In proportioning of concrete it is kept in mind that voids in coarse aggregates
are filled with sand and the voids in sand are filled with cement paste.
Proportion of ingredients usually adopted for various works are shown in
Table below.

6. GRADES OF CONCRETE
Grade of concrete Strength in N/mm2 Proportion
M5 5 1:5:10
M7.5 7.5 1:4:8
M10 10 1:3:6
M15 15 1:2:4
M20 20 1:1.5:3
M25 (Design Mix) 25 1:1:2
Grade of concrete is defined as the minimum strength the concrete must posses after
28 days of construction with proper quality control.
Grade of concrete is denoted by prefixing M to the desired strength in MPa. ... For example, for
M20 concrete, mix proportion will be 1:1.5:3 for cement : sand: coarse aggregates.

8. • A minimum w/c ratio (water-to-cement ratio) of about
0.3 by weight is necessary to ensure that the water
comes into contact with all cement particles (thus
assuring complete hydration).
• Typical values are in the 0.4 to 0.6
Advantages of low
water/cement ratio:
•Increased strength
•Lower permeability
•Increased resistance to weathering
•Better bond between concrete and
reinforcement
•Reduced drying shrinkage and
cracking
•Less volume change from wetting
and drying

9. FUNCTIONS OF VARIOUS INGREDIENTS
• Cement is the binding material. After addition of water it hydrates
and binds aggregates and the surrounding surfaces like stone and
bricks. Generally richer mix (with more cement) gives more
strength.
• Setting time starts after 30 minutes and ends after 10 hours. Hence
concrete should be laid in its mould before 30 minutes of mixing of
water and should not be subjected to any external forces till final
setting takes place.

10. • Coarse aggregate consists of crushed stones. It should
be well graded and the stones should be of igneous origin.
They should be clean, sharp, angular and hard. They give
mass to the concrete and prevent shrinkage of cement.
• Fine aggregate consists of river sand. It prevents
shrinkage of cement.
• When surrounded by cement it gains mobility enters the
voids in coarse aggregates and binding of ingredients
takes place. It adds density to concrete, since it fills the
voids. Denser the concrete higher is its strength.

11. • Water used for making concrete should be clean. It activates the
hydration of cement and forms plastic mass. As it sets completely
concrete becomes hard mass.
• Water gives workability to concrete which means water makes it
possible to mix the concrete with ease and place it in final position.
More the water better is the workability. However excess water
reduces the strength of concrete. Figure shows the variation of
strength of concrete with water cement ratio.
• To achieve required workability and at the same time good strength a
water cement ratio of 0.4 to 0.45 is used, in case of machine mixing
and water cement ratio of 0.5 to 0.6 is used for hand mixing.

12. COMPRESSIVE STRENGTH OF CONCRETE
• Compressive Strength - is defined as the measured maximum resistance
of a concrete specimen to an axial load, usually expressed in N/mm2 at an
age of 28 days

13. In practical terms, about 90% of its strength is gained in the
first 28 days.

14. • During the first week to 10 days of curing it is
important that the concrete not be permitted to
freeze or dry out
• Concrete compressive strength depends upon many
factors:
• Quality and proportions of the ingredients
• The curing environment

15. • Workability - that property of freshly mixed concrete
that determines its working characteristics, i.e. the ease
with which it can be mixed, placed, compacted and
finished.
• Factors effecting workability:
• Method and duration of transportation
• Quantity and characteristics of cementing materials
• Concrete consistency (slump)
• Aggregate grading, shape & surface texture
• % entrained air
• Water content
• Concrete & ambient air temperature
• Admixtures
WORKABILITY OF CONCRETE

16. SLUMP CONE TEST
• Concrete slump test or slump cone test is
to determine the workability or consistency of concrete
mix prepared at the laboratory or the construction site
during the progress of the work.
• Concrete slump test is carried out from batch to batch
to check the uniform quality of concrete during
construction.
• A good indication of the water content of a mix and thus
the workability can be DETERMINED from a standard
slump test.

19. SLUMP CONE
• The slump cone consists of a metallic mould in the form of a
frustum of a cone having internal dimensions as
• Bottom diameter : 20 cm
• Top diameter : 10 cm
• Height : 30 cm
• Thickness : ≤ 16 mm

21. CURING OF CONCRETE
• Curing - maintenance of a satisfactory moisture
content and temperature in concrete for a suitable
period of time immediately following placing &
finishing so that the desired properties may develop.
• Factors that effect curing:
• Time
• Temperature
• Moisture

22. • Curing of Concrete is a method by which the concrete is protected
against loss of moisture required for hydration and kept within the
recommended temperature range. Curing will increase the strength
and decrease the permeability of hardened concrete.
Curing of Concrete : -
• Shading concrete work
• Covering concrete surfaces with hessian or gunny bags
• Sprinkling of water
• Ponding method
• Membrane curing
• Steam curing

23. FOUNDATIONS
• FOUNDATION : In engineering,
a foundation is the element of
a structure which connects it to the
ground, and transfers loads from the
structure to the ground.
• Foundations are generally considered
either shallow or deep.
• Foundation engineering is the application
of soil mechanics and rock
mechanics (Geotechnical engineering) in
the design of foundation elements of
structures.

26.  It should be able to bear live and dead load
 It should be taken sufficiently deep so as to reach hard strata.
 Base of the foundation should be rigid so that differential settlements are minimized.
 Foundation should transfer the load of the super structure to the base soil in such a
way that there is no settlement.
 The load acting on the sub soil per unit area should always be less than the safe
bearing capacity (SBC) of the soil .
 It must distribute the load of the structure on sufficiently large area.
 Area of the footing should be such large that the intensity of the load at its base
doesn't exceed the SBC of the sub soil.
 They should be taken sufficiently deep to guard the building against damage or distress caused
by swelling or shrinkage of the sub soil.

28. Bearing capacity of the soil ≈ supporting power of the soil

30. Load bearing Structure Framed structure
A load bearing masonry structure has load bearing
walls which receive the load and transmit the same
to the ground through their foundations
A framed structure has columns erected which in
turn are braced together by beams and slab.
These load bearing walls supports the entire load
including their self weight.
Space between column and beams are filled by
panel walls.
Foundations for load bearing walls may be of two
types:
i) Simple strip footing
ii) Strip footing with masonry offsets (Stepped
footing).
Foundations for framed structures are of two types:
i) Isolated footing
spread footing
slopped footing
stepped footing
ii) Pad footing
simple pad footing
stepped pad footing
Suitable up to three floors Suitable for any number of floors
Cost is less Cost is more
Less space More space
Not suitable for all types of soil Suitable for any type of soil.

31. Advantages of framed structure
 They are meant for multistoried buildings
 Framed structures provide greater floor area. The walls and partition walls which are thin resulting
increase of floor area.
 Additions and alterations can be more easily done in the case of framed structures.
 Construction time is less in framed constructions
 They can resist earth quake shocks better than a load bearing structure.

33. Types of Foundation
Broadly Classified to two types:
1) Shallow foundation : Depth ≤ Breadth
2) Deep foundation : Depth > Breadth
Shallow foundations located just below the lowest part of the
superstructure they support; deep foundations extend
considerably deeper in to earth.

35. Spread footing
It is the type of footing which supports either one wall or one column spread the
superimposed load of wall or column over a large area.
Spread footing for single column: It contains only one column.
It is of three types
Single footing Stepped footing Slopped footing

37. Spread footing for walls

41. STRAP Footing or CANTILEVER Footing
Independent footings connected by a beam
Provided when the distance between the
columns are large

46. Mat foundation:
It is a combined footing that covers the entire area beneath a structure and supports all
the walls and columns.
It is suitable when the load coming on the soil is practically uniform or soil is of
yielding nature or where soil tends to cause differential settlement or spread footing
would cover more than half the area of the foundation.

47. Pile Foundation:
It is the type of foundation in which load is
transmitted to the lower level by means of
vertical members known as piles.
Concrete piles are made up of
concrete.
30 to 50 cm in Φ and 20 m
length
Made of seasoned timber.
20 to 50 cm in Φ and length 20
times Φ
Bottom is sharpened and
provided with iron shoe.
It can be of I or hollow pipe section.
Composition of wood and concrete.
Wooden portion will be at the bottom
portion and concrete at the upper.

50. Pier Foundation
Pier foundation is generally shallower
in depth that the pile foundation
It is preferred when there exists hard
strata at considerable depths