3. Properties of Concrete
3.1 Properties of
Concrete
Concrete is an artificial conglomerate stone made essentially of Portland cement,
water, and aggregates. When first mixed the water and cement constitute a paste which
surrounds all the individual pieces of aggregate to make a plastic mixture. A chemical
reaction called hydration takes place between the water and cement, and concrete normally
changes from a plastic to a solid state in about 2 hours. Thereafter the concrete
continues to gain strength as it cures. A typical strength-gain curve is shown in Figure
1. The industry has adopted the 28-day strength as a reference point, and specifications
often refer to compression tests of cylinders of concrete which are crushed 28 days after
they are made. The resulting strength is given the designation f'c
During the first week to 10 days of curing it is important that the concrete not be
permitted to freeze or dry out because either of these, occurrences would be very
detrimental to the strength development of the concrete. Theoretically, if kept in a moist
environment, concrete will gain strength forever, however, in practical terms, about 90%
of its strength is gained in the first 28 days.
Concrete has almost no tensile strength (usually measured to be about 10 to 15% of its
compressive strength), and for this reason it is almost never used without some form of
reinforcing. Its compressive strength depends upon many factors, including the quality and
proportions of the ingredients and the curing environment. The single most important
indicator of strength is the ratio of the water used compared to the amount of cement.
Basically, the lower this ratio is, the higher the final concrete strength will be. (This
concept was developed by Duff Abrams of The Portland Cement Association in the early 1920s
and is in worldwide use today.) 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). In practical terms, typical values are in
the 0.4 to 0.6 range in order to achieve a workable consistency so that fresh concrete can
be placed in the forms and around closely spaced reinforcing bars.
Typical stress-strain curves for various concrete strengths are shown in Figure 2. Most
structural concretes have f'c values in the 3000 to 5000 psi range. However, lower-story
columns of high-rise buildings will sometimes utilize concretes of 12,000 or 15,000 psi to
reduce the column dimensions which would otherwise be inordinately large. Even though
Figure 2 indicates that the maximum
strain that concrete can sustain before it crushes varies inversely with strength, a value
of 0.003 is usually taken (as a simplifying measure) for use in the development of design
equations.
Because concrete has no linear portion to its stress-strain curve, it is difficult to
measure a proper modulus of elasticity value. For concretes up to about 6000 psi it can be
approximated as
(1)
where w is the unit weight (pcf), f'c is the cylinder strength (psi).
(It is important that the units of f'c be expressed in psi and not ksi whenever the square
root is taken). The weight density of reinforced concrete using normal sand and stone
aggregates is about 150 pcf. If 5 pcf of this is allowed for the steel and w is taken as
145 in Equation (1), then
(2)
E values thus computed have proven to be acceptable for use in deflection
calculations.
As concrete cures it shrinks because the water not used for hydration gradually evaporates
from the hardened mix. For large continuous elements such shrinkage can result in the
development of excess tensile stress, particularly if a high water content brings about a
large shrinkage. Concrete, like all materials, also undergoes volume changes due to
thermal effects, and in hot weather the heat from the exothermic hydration process adds to
this problem. Since concrete is weak in tension, it will often develop cracks due to such
shrinkage and temperature changes. For example, when a freshly placed concrete
slab-on-grade expands due to temperature change, it develops internal compressive stresses
as it overcomes the friction between it and the ground surface. Later when the concrete
cools land shrinks as it hardens) and tries to contract, it is not strong enough in
tension to resist the same frictional forces. For this reason contraction joints are often
used to control the location of cracks that inevitably occur and so-called temperature and
shrinkage reinforcement is placed in directions where reinforcing has not already been
specified for other reasons. The purpose of this reinforcing is to accommodate the
resulting tensile stresses and to minimize the width of cracks that do develop.
In addition to strains caused by shrinkage and thermal effects, concrete also deforms
due to creep. Creep is Increasing deformation that takes place when a material sustains a
high stress level over a long time period. Whenever constantly applied loads (such as dead
loads) cause significant compressive stresses to occur, creep will result. In a beam, for
example, the additional longterm deflection due to creep can be as much as two times the
initial elastic deflection The way to avoid this increased deformation is to keep the
stresses due to sustained loads at a low level. This is usually done by adding compression
steel.
3.2 Mix Proportions
The ingredients of concrete can be proportioned by weight or volume. The goal is to
provide the desired strength and workability at minimum expense. Sometimes there are
special requirements such as abrasion resistance, durability in harsh climates, or water
impermeability, but these properties are usually related to strength. Sometimes concretes
of higher strength are specified even though a lower f'c value would have met all
structural requirements.
As mentioned previously, a low water-to-cement ratio is needed to achieve strong
concrete. It would seem therefore that by merely keeping the cement content high one could
use enough water for good workability and still have a low w/c ratio. The problem is that
cement is the most costly of the basic ingredients. The dilemma is easily seen in the
schematic graphs of Figure 3.
Since larger aggregate sizes have relatively smaller surface areas (for the cement
paste to coat) and since less water means less cement, it is often said that one should
use the largest practical aggregate size and the stiffest practical mix. (Most building
elements are constructed with a maximum aggregate size of 3/4 to 1 in, larger sizes being
prohibited by the closeness of the reinforcing bars.)
A good indication of the water content of a mix land thus the workability) can be had
from a standard slump test. In this test a metal cone 12 in tall is filled with fresh
concrete in a specified manner. When the cone is lifted, the mass of concrete
"slumps" downward (Figure 4) and the vertical drop is referred to as the slump.
Most concrete mixes have slumps in the 2- to 5-in range.
3.3 Portland Cement
The raw ingredients of Portland cement are iron ore, lime, alumina and silica, which
are used in various proportions depending upon the type of cement being made. These are
ground up and fired in a kiln to produce a clinker. After cooling, the clinker is very
finery ground (to about the texture of talcum powder) and a small amount of gypsum is
added to retard the initial setting time. There are five basic types of Portland cement in
use today:
-
Type I
- General purpose
-
Type II
- Sulfate resisting, concrete in contact with high sulfate soils
-
Type III
- High early strength, which gains strength faster than Type I, enabling forms to be removed sooner
-
Type IV
- Low heat of hydration, for use in massive construction
-
Type V
- Severe sulfate resisting
Type I is the least expensive and is used for the majority of concrete structures. Type
III is also frequently employed because it enables forms to be reused quickly, allowing
construction time to be reduced. It is important to note that while Type III gains strength
faster than Type I, it does not take its initial set any sooner).
3.4 Aggregates
Fine aggregate (sand) is made up of particles which can pass through a 3/8 in sieve;
coarse aggregates are larger than 3/8 inch in size. Aggregates should be clean, hard, and
well-graded, without natural cleavage planes such as those that occur in slate or shale.
The quality of aggregates is very important since they make up about 60 to 75% of the
volume of the concrete; it is impossible to make good concrete with poor aggregates. The
grading of both fine and coarse aggregate is very significant because having a full range
of sizes reduces the amount of cement paste needed. Well-graded aggregates tend to make
the mix more workable as well.
Normal concrete is made using sand and stones, but lightweight concrete can be made
using industrial by-products such as expanded slag or clay as lightweight aggregates. This
concrete weighs only 90 to 125 pcf and high strengths are more difficult to achieve
because of the weaker aggregates. However, considerable savings can be realized in terms
of the building self-weight, which may be very important when building on certain types of
soil. Insulating concrete is made using perlite and vermiculite, it weighs only about 15
to 40 pcf and has no structural value.
3.5 Admixtures
Admixtures are chemicals which are added to the mix to achieve special purposes or to
meet certain construction conditions. There are basically four types: air-entraining
agents, workability agents, retarding agents, and accelerating agents.
In climates where the concrete will be exposed to freeze-thaw cycles air is
deliberately mixed in with the concrete in the form of billions of tiny air bubbles about
0.004 in in diameter. The bubbles provide interconnected pathways so that water near the
surface can escape as it expands due to freezing temperatures. Without air-entraining, the
surface of concrete will almost always spall off when subjected to repeated freezing and
thawing. (Air-entraining also has the very beneficial side effect of increasing
workability without an increase in the water content.) Entrained air is not to be confused
with entrapped air, which creates much larger voids and is caused by improper placement
and consolidation of the concrete. Entrapped air, unlike entrained air, is never
beneficial.
Workability agents, which include water-reducing agents and plasticizers, serve to
reduce the tendency of cement particles to bind together in flocs and thus escape complete
hydration. Fly ash, a by-product of the burning of coal that has some cementitious
properties, is often used to accomplish a similar purpose. Superplasticizers are
relatively new admixtures which when added to a mixture serve to increase the slump
greatly, making the mixture very soupy for a short time and enabling a low-water-content
or otherwise very stiff) concrete to be easily placed. Superplasticizers are responsible
for the recent development of very high strength concretes, some in excess of 15,000 psi
because they greatly reduce the need for excess water for workability.
Retarders are used to slow the set of concrete when large masses must be placed and the
concrete must remain plastic for a long period of time to prevent the formation of
"cold joints" between one batch of concrete and the next batch. Accelerators
serve to increase the rate of strength gain and to decrease the initial setting time. This
can be beneficial when concrete must be placed on a steep slope with a single form or when
it is desirable to reduce the time period in which concrete must be protected from
freezing. The best known accelerator is calcium chloride, which acts to increase the heat
of hydration, thereby causing the concrete to set up faster.
Other types of chemical additives are available for a wide range of purposes. Some of
these can have deleterious side effects on strength gain, shrinkage, and other
characteristics of concrete, and test batches are advisable if there is any doubt
concerning the use of a particular admixture.
3.6 The ACI Code
The American Concrete Institute (ACI), based in Detroit, Michigan, is an organization
of design professionals, researchers, producers, and constructors. One of its functions is
to promote the safe and efficient design and construction of concrete structures. The ACI
has numerous publications to assist designers and builders; the most important one in
terms of building structures is entitled Building Code Requirements for Reinforced
Concrete and Commentary. It is produced by Committee 318 of the American Concrete
Institute and contains the basic guidelines for building code officials, architects,
engineers, and builders regarding the use of reinforced concrete for building structures.
Information is presented concerning materials and construction practices, standard tests,
analysis and design, and structural systems. This document has been adopted by most
building code authorities in the United States as a standard reference. It provides all
rules regarding reinforcing sizes, fabrication, and placement and is an invaluable
resource for both the designer and the detailer.
Periodic updates occur (1956, 1963, 1971, 1977, 1983, and 1989), and this text makes
constant reference to the 1989 edition, calling it the ACI Code or merely the Code.
Documents and officials also refer to it by its numerical designation, ACI 318-89.
3.7 References
Boethius, A. and Ward1-Perkins, J. B. (1970). Etruscan and roman
Architecture, Penguin Books, Middlesex, England.
Cassie, W. F. (1965). "The First Structural Reinforced
Concrete," Structural Concrete, 2(10).
Collins, P. (1959). Concrete, The Vision of a New Architecture,
Faber and Faber, London.
Condit, C. W. (1968). American Building, Materials and Techniques
from the First Colonial Settlements to the Present, University of Chicago Press.
Drexler, A. (1960). Ludwig Miles van der Rohe, George Braziller,
New York.
Farebrother, J. E. C. (1962). "Concrete - Past, Present, and
Future," The structural Engineer, October.
Mainstone, R, J. (1975). Developments in Structural Form, The MIT
Press, Cambridge.
