Chapter 1: Introduction Flashcards
It is a mixture of sand, gravel, crushed rock, or other aggregates held together in a rocklike mass with a paste of cement and water.
Concrete
Sometimes one or more ______ are added to change certain characteristics of the concrete such as its workability, durability, and time of hardening.
Admixtures
The term _____ indicates all types of concrete used in structural applications.
Structural Concrete
Structural Concrete may be ___, ___, ___, or ____
Plain, reinforced, pre-stressed, or partially pre-stressed
What are the five types of cement?
I - Normal Cement
II - Lower Heat of Hydration (withstand sulfate attacks)
III - High Early Strength cement (higher heat of hydration)
IV - Low Heat (for larger structures)
V - High Sulfate Resistance (to be exposed to high sulfate concentrations)
What are the types of admixtures?
- Air-entraining admixture
- Accelerating admixture
- Retarding admixture
- Superplasticizers
- Waterproofing
It is a combination of concrete and steel wherein the steel reinforcement provides the tensile strength lacking in the concrete.
Reinforced concrete
It is capable of resisting compression forces and is used in columns as well as in other situations, which are described later.
Steel reinforcing
This may be the most important material available for construction. It is used in one form or another for almost all structures, great or small—buildings, bridges, pavements,
dams, retaining walls, tunnels, drainage and irrigation facilities, tanks, and so on.
Reinforced concrete
Advantages of Reinforced Concrete as a Structural Material
- It has considerable compressive strength per unit cost compared with most other materials
- It has great resistance to the actions of fire and water.
- Reinforced concrete structures are very rigid.
- It is a low-maintenance material.
- It has a very long service life.
- It is usually the only economical material available for footings, floor slabs, basement
walls, piers, and similar applications. - Its ability to be cast into an extraordinary variety of
shapes from simple slabs, beams, and columns to great arches and shells. - Concrete takes advantage of inexpensive local materials (sand, gravel, and water) and requires relatively small amounts of cement and reinforcing steel.
- A lower grade of skilled labor is required for erection as compared with other materials
such as structural steel.
Disadvantages of Reinforced Concrete as a Structural Material
- Concrete has a very low tensile strength, requiring the use of tensile reinforcing
- Forms are required to hold the concrete in place until it hardens sufficiently.
- The low strength per unit of weight of concrete leads to heavy members.
- The low strength per unit of volume of concrete means members will be relatively large, an important consideration for tall buildings and long-span structures.
- The properties of concrete vary widely because of variations in its proportioning and mixing.
Properties of Concrete
- Compressive Strength
- Static Modulus of Elasticity
- Dynamic Modulus of Elasticity
- Poisson’s Ratio
- Shrinkage
- Creep
- Tensile Strength
- Shear Strength
It is determined by testing to failure 28-day-old 6-in. diameter by 12-in. concrete cylinders at a specified rate of loading (4-in. diameter by 8-in. cylinders were first permitted in the 2008 code in lieu of the larger cylinders).
Compressive Strength
Its value varies with different concrete
strengths, concrete age, type of loading, and the characteristics and proportions of the cement
and aggregates. Furthermore, there are several different definitions of the modulus:
(a) The initial modulus is the slope of the stress–strain diagram at the origin of the curve.
(b) The tangent modulus is the slope of a tangent to the curve at some point along the
curve—for instance, at 50% of the ultimate strength of the concrete.
(c) The slope of a line drawn from the origin to a point on the curve somewhere between
25% and 50% of its ultimate compressive strength is referred to as a secant modulus.
(d) Another modulus, called the apparent modulus or the long-term modulus, is determined
by using the stresses and strains obtained after the load has been applied for a certain
length of time
Static Modulus of Elasticity
The _____ which corresponds to very small instantaneous strains, is
usually obtained by sonic tests. It is generally 20% to 40% higher than the static modulus and
is approximately equal to the initial modulus. When structures are being analyzed for seismic
or impact loads, the use of the dynamic modulus seems appropriate.
Dynamic Modulus of Elasticity
As a concrete cylinder is subjected to compressive loads, it not only shortens in length but also
expands laterally. The ratio of this lateral expansion to the longitudinal shortening is referred
to as ______
Poisson’s Ratio
When the materials for concrete are mixed, the paste consisting of cement and water fills
the voids between the aggregate and bonds the aggregate together. This mixture needs to be
sufficiently workable or fluid so that it can be made to flow in between the reinforcing bars and
all through the forms. To achieve this desired workability, considerably more water (perhaps
twice as much) is used than is necessary for the cement and water to react (called hydration).
Shrinkage
Under sustained compressive loads, concrete will continue to deform for long periods of time.
After the initial deformation occurs, the additional deformation is called ____, or plastic flow.
Creep
The______ of concrete varies from about 8% to 15% of its compressive strength. A
major reason for this small strength 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 cracks to close and permit compression transfer. Obviously, this is not the case for
tensile loads.
Tensile strength
It is extremely difficult in laboratory testing to obtain pure shear failures unaffected by other
stresses. As a result, the tests of concrete shearing strengths through the years have yielded values all the way from one-third to four-fifths of the ultimate compressive strengths.
Shear Strength
It is granular material, such
as sand, gravel, crushed stone and
iron blast-furnace slag, and when
used with a cementing medium
forms a hydraulic cement concrete
or mortar
Aggregates
Classification of Aggregates by Density
- Lightweight
- Normal-Weight
- Heavyweight
It is more porous than their
heavier counterparts.
They are also used in
mixes for concrete blocks
and pavements, as well as
insulation.
Lightweight
