Chapter 1

Introduction to Reinforced Concrete

Concrete and Reinforced Concrete

Concrete is a mixture of sand, gravel, crushed rock, or other aggregates held
together in a rocklike mass with a paste of cement and water.
Reinforced concrete is a combination of concrete and steel wherein the steel
reinforcement provides the tensile strength lacking in the concrete.

Advantages and Disadvantages of Concrete as Structural Material

Advantages:

a. It has a relatively high compressive strength.

b. It has better resistance to fire than steel.
c. It has a long service life with low maintenance cost.
d. In some types of structures, such as dams, piers, and footings, it is
the most economical structural material.
e. It can be cast to take the shape required, making it widely used in
precast structural components.
Disadvantages:

a. It has a low tensile strength of about one-tenth of its compressive

strength.
b. It needs mixing, casting, and curing, all of which affect the final
strength of concrete.
c. The cost of the forms used to cast concrete is relatively high.
d. It has a low compressive strength as compared to steel (the ratio
is about 1:10, depending on materials), which leads to large
sections in columns of multistory buildings.
e. Cracks develop in concrete due to shrinkage and the application
of live loads.
Properties of Concrete

Properties of Fresh Concrete

a. The mix should be able to produce a homogeneous fresh concrete

from ingredients under the action of mixing forces. This property is
termed mixability.

b. The mix should be stable, in that it should not segregate during

transportation and placing when it is subjected to forces during

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 handling operation of limited nature. The tendency to bleeding must
also be minimized.

c. The mix should be cohesive and sufficiently mobile to be placed in

the form around the reinforcement and cast to the required shape.
The property is termed flowability of fresh concrete.

d. The mix should be amenable to proper and thorough compaction into

a dense and compact concrete with minimum voids under the
facilities of compaction available at the sire. This property is termed
compactibility of concrete.

e. It should be able to produce a satisfactory surface finish without

honeycombing or blow holes. This capability is termed finishability.

The diverse requirements of mixability, stability, transportability, placeability,

flowability, compactibility, and finishability of fresh concrete are collectively referred
to as workability.

Tests to measure workability:

a. Slump test
b. Compacting factor test
c. Vee-bee consisitency test
d. Flow test
Properties of Hardened Concrete:

a. Compressive Strength
The compressive strength of concrete is based on 150 mm diameter by 300
mm (6 in. x 12 in.) compression specimens cured under laboratory
conditions and tested to crushing at a specified rate of loading at 28 days.
The modulus of elasticity of concrete defining the slope of the tangent to the
stress-strain diagram may be taken as

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 b. Shrinkage
The contraction in volume of concrete as it hardens. The magnitude of
shrinkage strains depends upon the composition of the concrete. The larger
the fraction of the of the total volume of concrete that is made up of the
hydrated cement paste, the greater is the shrinkage.

Ways to minimize tensile cracking due to shrinkage:

• Cure the concrete well

• Add reinforcements to limit the width of crack.
• Use construction and expansion joints to control the location of
cracks
• Minimize water content
• Limit the area or length of concrete poured at a given time

c. Creep
Under sustained compressive loads, concrete will continue to deform for
long periods of time. After the initial deformation occurs, the additional
deformation is called creep or plastic flow.

d. Tensile Strength
The tensile strength of concrete varies from about 8% to 15% of its
compressive strength. A major reason or this is the fact that concrete is filled
with fine cracks. The cracks have little effect when concrete is subjected to
compression loads because the loads cause the crack to close and permit
compression to transfer.
Tests use to measure tensile strength of concrete:

a. Split cylinder test

b. Modulus of rupture
For members subjected to flexure, the value of modulus of rupture rather than
split cylinder test is used in the design. The code specifies a value of

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If the measured average splitting tensile strength of lightweight concrete, f ct, is used
to calculate λ, laboratory test shall be conducted to establish the value of f ct and the
corresponding value of fcm and λ shall be calculated by:

Aggregates

The aggregates used in concrete occupy about three-fourths of the concrete

volume. Since they are less expensive than the cement, it is desirable to use s much
of them as possible. Both fine aggregates (usually sand) and coarse aggregates
(usually gravel or crushed stone) are used. Any aggregate that passes a No. 4 sieve
(which has wires spaced 6.25 mm on centers in each direction) is said to be fine
aggregate. Material of a larger size is coarse aggregate.

Depending upon their size, the aggregates are classified as:

A. Fine Aggregates.
When the aggregate is sieved through 4.75mm sieve, the aggregate
passed through it called as fine aggregate. Natural sand is generally
used as fine aggregate, silt and clay are also come under this category.
The soft deposit consisting of sand, silt and clay is termed as loam. The
purpose of the fine aggregate is to fill the voids in the coarse aggregate
and to act as a workability agent

B. Coarse Aggregates
When the aggregate is sieved through 4.75mm sieve, the aggregate
retained is called coarse aggregate. Gravel, cobble and boulders come
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 under this category. The maximum size aggregate used may be
dependent upon some conditions. Coarse aggregates usually obtained
crashing granite, gneiss, crystalline lime stone and good variety of
sandstone etc.In general, 40mm size aggregate used for normal
strengths and 20mm size is used for high strength concrete. The size
range of various coarse aggregates given below.

Unless field practice verifies that concrete with a certain size aggregate can be
successfully placed and compacted to prevent honeycomb or voids, the maximum
size of coarse aggregate not exceed:

1. On-fifth of the smallest dimensions of the form

2. One-third the depth of slabs
3. Three-fourths of the minimum clear spacing between reinforcement
Reinforcing Steel Bars
Reinforcement used for concrete structures maybe in the form of bars or
welded wire fabric. Reinforcing bars are referred to as plain or deformed bars. The
deformed bars, which have ribbed projections rolled onto their surfaces to provide
better bonding between the concrete and the steel, are used for almost all
applications. Plane bars are not used very often except for wrapping around
longitudinal bars, primarily in columns.

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 Philippine Standard Reinforcing Bars
Nominal
Philippine Standard Unit
Near ASTM Sectional
Designation Weight
Designation Area
(mm) (kg/m)
(mm2)
6 #2 28.27 0.222
10 #3 78.54 0.616
12 #4 113.10 0.888
16 #5 201.10 1.579
20 #6 314.20 2.466
25 #8 491.90 3.854
28 #9 615.75 4.833
32 #10 804.25 6.313
36 #11 1017.90 7.991

ASTM Standard Reinforcing Bars

Nominal Bar Size Nominal Nominal Area Mass (kg/m)
Diameter (mm2)
(mm)
#10 11.3 100 0.785
#15 16.0 200 1.570
#20 19.5 300 2.355
#25 25.2 500 3.925
#30 29.9 700 6.495
#35 35.7 1000 7.850
#45 43.7 1500 11.775
#55 66.4 2500 19.625

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Grade, Types, and Sizes of Reinforcing Bars

Loads and Load Combinations

One of the most important and most difficult task faced by the structural engineer is the
accurate estimation of the loads that ma be applied to a structure during its life. No loads
that may reasonably be expected to occur may be overlooked. After loads are estimated,
the next problem is to decide the worst possible combinations of these loads that might
occur at one time. Loads are classified as being dead, live, or environmental.
Dead Loads: are loads of constant magnitude that remain in one position. They
include the weight of the structure under consideration as well as any fixture that are
permanently attached to it. For a reinforced concrete building, some dead loads are frames,
walls, floors, ceilings, stairways, roofs, and plumbing. Unit weight of concrete = 23.54
kN/m3.

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 Live Loads: are loads than can change in magnitude and position. They include
occupancy loads, warehouse materials, construction loads, overhead service cranes,
equipment operating loads, and many others. In general, they are induced by gravity.
Environmental Loads: are loads caused by the environment where the structure is
located. For buildings, they are caused by rain, wind, temperature change, and earthquake.
In fact, these are also live loads, but they are the results of the environment where the
structure is located. Although they do vary with time, they are not all caused by gravity or
operating conditions, as is typical with other loads.

 Load Combinations (ASD/WSD)

1. D + F
2. D + H + F + L + T
3. D + H + F + (Lr or R)
4. D + H + F + 0.75[L + T(Lr or R)]
5. D + H + F (0.6W or E/1.4)

 Load Combinations (LRFD/USD)

1. 1.4(D + F)
2. 1.2(D + F + T) + 1.6(L + H) + 0.5(Lr or R)
3. 1.2D + 1.6(Lr or R) + ((0.5 or 1.0)*L or 0.5W)
4. 1.2D + 1.0W + (0.5 or 1.0)*L + 0.5(Lr or R)
5. 1.2D +1.0E + (0.5 or 1.0)*L
6. 0.9D + 1.0W +1.6H
7. 0.9D + 1.0E + 1.6H
Where:
D = dead load
E = earthquake load
F = load due to fluids with well-defined pressures and maximum heights
H = load due to lateral pressure of soil and water in soil
L = live load, except roof live load, including any permitted live load
reduction
Lr = roof live load, including any permitted
P = ponding load
R = rain load on the undeflected roof
T = self-straining force and effects arising from contraction or expansion
resulting from temperature change, shrinkage, moisture change, creep
in component materials, movement due to differential settlement, or
combination therof
W = load due to wind pressure

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  You will notice that the large load factor found in the LRFD/USD load combinations are
absent from the ASD/WSD load combination equations.
 Also, the predictability of the loads is not considered. For example both dead load
and live load have the same load factor in equations where there are both likely to
occur at full value simultaneously.
 The probability associated with accurate load determination is not considered at all
in the ASD method.

The following are the most common load combinations for most applications:

1. 1.4D
2. 1.2D 1.6L
3. 1.2D + 1.0L + 1.0W
4. 1.2D + 1.0L + 1.0E
5. 0.9D + 1.0W
6. 0.9D + 1.0E

 MINIMUM SPACING OF REINFORCEMENTS (NSCP 425.2)

 For parallel non-prestressed reinforcement in a horizontal layer, clear spacing
shall be at least the greatest of 25 mm, db, and (4/3)dagg.
 For parallel non-prestressed reinforcement in two or more horizontal layers,
reinforcement in the upper layers shall be placed directly above
reinforcements in the bottom layer with a clear spacing of 25 mm.
 For longitudinal reinforcements in columns, and boundary elements in walls,
clear spacing between bars shall be at least the greatest of 40 mm, 1.5db, and
(4/3)dagg.

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  Bundled Reinforcements (NSCP 425.6)
 Non-prestressed Reinforcement
 Groups of parallel reinforcing bars bundled in contact to act as a unit shall be limited to four in
any one bundle.
 Bundled bars shall be enclosed within transverse reinforcement. Bundled bars in compression
members shall be enclosed by transverse reinforcement at least 12 mm  in size.
 Bars larger than a 36 mm  shall not be bundled in beams.
 Individual bars within a bundle terminated within the span of flexural members shall terminate
at different points with at least 40db stagger.
 Development length for individual bars within a bundle, in tension or compression, shall be that
of the individual bar, increased 20 percent for three-bar bundle, and 33 percent for a four-bar
bundle.
 A unit bundled bars shall be treated as a single bar with an area equivalent to that of the bundle
and centroid coinciding with that of the bundle. The diameter of equivalent bar shall be used for
db in (a) through (e):
a. Spacing limitations based on db;
b. Cover requirements based on db;
c. Spacing and cover values in Section 425.4.2.2;
d. Confinement terms in Section 425.4.2.3;
e. e factor in Section 425.4.4.

 Bundled-bar Arrangement

 Bundled-bar Equivalent Diameter

 SPECIFIED CONCRETE COVER REQUIREMENTS (NSCP 420.6.1.3)

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