Week 3; uses of building materials
Unit 10 Construction Materials — Types and Uses
131
sand, coarse and fine aggregates, admixtures,
and water. When first mixed, it is
plastic
(able to
flow and be shaped) and can be cast to take the
shape of the formwork provided.
Hardening of the concrete is caused by a
chemical reaction between the cement and water
called
hydration.
Most mixtures of concrete set
within 4–12 hours, depending on the tempera-
ture, the volume of the pour, type of cement, and
admixtures. When the temperature is below 70°F
(20°C), the reaction slows. Very little chemical
reaction takes place below 40°F (4°C), and
almost none occurs at 32°F (0°C). The rule of
thumb is: if you are comfortable, the concrete is
comfortable. Concrete continues to harden for
months after the initial set, but most placements
reach their compressive or design strength within
28 days. Forms can be removed after one to
several days or when the concrete can support
itself. This should be determined by an engineer.
Types of Cement
Cement
binds the concrete mix together.
There are a number of types of cement. The
most common, used for general construction, is
called
Type I Normal Portland cement.
Another
variation used in construction is
white Portland
cement.
It is light-colored and used chiefly for
architectural effects. White Portland cement is
made from carefully selected raw materials and
develops the same strength as the normal gray-
colored Portland cement.
Types of cement include:
•
Type I, Normal Cement (most common)
•
Type II, Moderate Sulfate Resistance
(slow-reacting)
•
Type III, High Early Strength (fast-setting)
•
Type IV, Low Heat of Hydration (low heat
generation)
•
Type V, High Sulfate Resistance
These other types of cements, along with
aggregates and admixtures, are available to
produce special types of concrete. Type IV is low
heat generation for large construction building
foundation projects, such as dams. Others have
high early strength to produce concrete that sets
faster than normal, permitting earlier form removal
and thus speeding construction. Still others are
more resistant to deterioration caused by sulfates
and alkalis in the soil.
D
B
Gravel
Head wall
A
Rip-
rap
C
Sand
Pea gravel
Figure 10-2.
Samples of drawing symbols for gravel. A—Plan view
of a headwall with rip-rap used to prevent erosion from
drainage water outflow. This type of structure would be
found on a site plan. B—Plan view of a gravel
driveway. C—Section view of sand layer above pea
gravel. D—Section view of crushed gravel.
132
Section 3 Specifications and Materials
Concrete Mixes
A concrete mix should be designed to
produce the desired result. Characteristics and
properties of concrete depend on the materials,
and their proportions, that make up the mixture.
This will determine the workability, strength, dura-
bility, economy, volume stability, and appearance
of the finished hardened concrete. Enough water
is added to make the mix plastic, so that it will
flow into the forms. Too much water, however, will
reduce the strength and durability of the
concrete, so the contractor needs to be careful. A
typical mix would consist of 10% cement, 15%
water, 25% fine aggregates, 45% coarse aggre-
gates, and 1% to 5% entrained or entrapped air.
Any material added to the concrete mix —
other than cement, sand, aggregate, and water
— is known as an
admixture.
Admixtures are
used to make the mix more workable, retard or
speed up hardening, increase freeze resistance,
or increase chemical resistance. Common admix-
tures to concrete include
air-entrainment,
used to
improve durability in freeze/thaw environments;
retarders,
used to slow down the initial set of
fresh concrete, especially in hot weather;
accel-
erators,
used to speed up the initial set of fresh
concrete in cold weather;
water reducers
, used to
reduce the amount of water required for a desired
workability and water-to-cement ratio for strength,
and
coloring agents
, used for altering the color of
the concrete mixture. Concrete is typically trans-
ported to the jobsite in a ready-mix truck.
Reinforced Concrete
Concrete has great compressive strength, but
very little tensile (pulling) strength. To overcome this
weakness, concrete is cast around steel
rein-
forcing bars
. These bars (commonly referred to as
“rebar”) have high tensile strength. As the concrete
hardens, it grips the steel to form a bond. The size
of the bar is indicated by the bar number, which is
a multiple of 1/8
′′
. For example, a #4 bar is 1/2
′′
in
diameter (4
×
1/8
′′
= 1/2
′′
). See
Figure 10-3.
Refer
to Unit 12 for more information.
Reinforcing bars are round in shape, with
projections (called
deformations
) formed in the
rolling process to strength bonding with the
concrete. Bars are placed after the forms are
constructed,
Figure 10-4.
The concrete is then
cast around the bars.
Figure 10-3.
Reinforcing steel bars (“rebar”). Bar diameters are identified by a number that is a multiple of 1/8
′′
. From left, bars
shown are #18, #14, #11, #9, #8, #7, #6, #5, #4, #3. The #18 bar is 2 1/4
′′
in diameter; the #3 bar is 3/8
Division 06 – Wood
and Plastics
Wood continues to be one of the chief
building materials,
Figure 10-20
. It is used for
structural framing (rough carpentry), trim, floors,
walls, and cabinetry (finish carpentry and archi-
tectural woodwork). Relative to its weight, wood
has high strength in compression, tension, and
bending. It also has excellent impact resistance.
While steps have been taken to substitute other
materials, wood remains a valuable and widely-
used residential construction material.
Figure 10-18.
This steel frame is designed to support the entire
weight of the building.
Figure 10-19.
Open-web steel joists combine strength with light
weight.
Wood Classification
Woods are broadly classified as either hard-
woods or softwoods. There are many varieties
used for construction. These classifications are
not an exact measure of hardness or softness
(because this varies) but a general classifica-
tion based on type of tree. In addition to hard-
ness or softness, woods vary in strength,
weight, texture, workability, and cost. Building
specifications usually indicate the type and
grade of lumber to be used in different parts of
the construction.
Assignment
- State 5 uses of building materials
- What is the function of cement in building