6.

11 Admixtures for Concrete Internal stresses reduce the

durability of hardened concrete,
Admixtures are ingredients other especially when cycles of freeze
than portland cement, water, and and thaw are repeated many
aggregates that may be added to times. The impact of each of these
concrete to impart a specific quality to mechanisms is mitigated by providing
either the plastic (fresh) mix or the a network of tiny air voids in the
hardened concrete (ASTM C494). hardened concrete using air
Some admixtures are charged into the entrainers. In the late 1930s, the
mix as solutions. In such cases, the introduction of air entrainment in
liquid should be considered part of the concrete represented a major advance
mixing water. If admixtures cannot be in concrete technology. Air
added in solution, they are either entrainment is recommended for all
weighed or measured by volume as concrete exposed to freezing.
recommended by the manufacturer.
Admixtures are classified by the In addition to improving durability,
following chemical and functional air entrainment provides other
physical characteristics: important benefits to both freshly
1.air entrainers mixed and hardened concrete. Air
2.water reducers entrainment improves concrete’s
3.retarders resistance to several destructive
4.hydration controller admixtures factors, including freeze–thaw
5.accelerators cycles, deicers and salts, sulfates, and
6.specialty admixtures alkali–silica reactivity. Air entrainment
also increases the workability of fresh
6.11.1 Air Entrainers concrete. Air entrainment decreases
Air entrainers produce tiny air the strength of concrete, however,
bubbles in the hardened concrete to this effect can be reduced for
provide space for water to expand moderate-strength concrete by
upon freezing (Figure 6.9). As lowering the water–cement ratio and
moisture within the concrete pore increasing the cement factor. High
structure freezes, three mechanisms strength is difficult to attain with air-
contribute to the development of entrained.
internal stresses in the concrete: 6.11.2 Water Reducers
1.Critical saturation—Upon freezing,
water expands in volume by 9%. If Water Reducers mechanism-
the percent saturation exceeds 91.7%, Cement grains develop static
the volume increase generates stress electric charge on their surface as
in the concrete. a result of the cement-grinding
2.Hydraulic pressure—Freezing process. Unlike charges attract,
water draws unfrozen water to it. The causing the cement grains to cluster
unfrozenwater moving throughout the or “flocculate”, which in turn limits the
concrete pores generates stress, workability. The chemicals in the
depending on length of flow path, rate water-reducing admixtures reduce the
of freezing, permeability, and static attraction among cement
concentration of salt in pores. particles. The molecules of water-
3.Osmotic pressure—Water moves reducing admixtures have both
from the gel to capillaries to satisfy positive and negative charges at one
thermo-dynamic equilibrium and end, and a single charge (usually
equalize alkali concentrations. Voids negative) on the other end.
permit water to flow from the
interlayer hydration space and Water reducing admixtures can be
capillaries into the air voids, where it used indirectly to gain strength.
has room to freeze without damaging Since the water-reducing admixture
the parts. increases workability, we can take
advantage of this phenomenon to
decrease the mixing water, which
in turn reduces the water–cement 4.placing cement underwater
ratio and increases strength. 5.placing concrete by pumping
Hewlett (1988) demonstrated that 6.consolidating the concrete is difficult
water reducers can actually be used
to accomplish three different When superplasticizers are used,
objectives. the fresh concrete stays workable
for a short time, 30–60 minutes,
1. Adding a water reducer without and is followed by rapid loss in
altering the other quantities in the mix workability. Superplasticizers are
increases the slump, which is a usually added at the plant to ensure
measure of concrete consistency and consistency of the concrete. In critical
an indica-tor of workability, as situations, they can be added at the
discussed in Chapter 7. jobsite, but the concrete should be
thoroughly mixed following the
2. The strength of the mix can be addition of the admixture. The setting
increased by using the water reducer time varies with the type of agents,
by lower-ing the quantity of water and the amount used, and the interactions
keeping the cement content constant. with other admixtures used in the
3. The cost of the mix, which is concrete.
primarily determined by the amount of 6.11.13 Retarders
cement,can be reduced. In this case,
the water reducer allows a decrease in Some construction conditions require
the amount of water. The amount of that the time between mixing and
cement is then reduced to keep the placing or finishing the concrete be
water–cement ratio equal to the increased. In such cases, retarders
original mix. Thus, the quality of the can be used to delay the initial set
mix, as measured by compressive of concrete. Retarders are used for
strength, is kept constant, although several reasons, such as the following:
the amount of cement is decreased. 1.offsetting the effect of hot weather
2.allowing for unusual placement or
The name “water reducer” may long haul distances
imply the admixture reduces the 3.providing time for special finishes
water in the concrete mix; this is (e.g., exposed aggregate)
not the case. A water reducer allows
the use of a lower amount of mixing Retarders can reduce the strength
water while maintaining the same of concrete at early ages (e.g., 1
workability level. Used in this manner, to 3 days). In addition, some
the water reducer allows a lower retarders entrain air and improve
water–cement ratio and therefore workability. Other retarders increase
increases the strength and other the time required for the initial set but
desirable properties of the concrete. reduce the time between the initial
and final set. The properties of
Superplasticizers retarders vary with the materials used
Superplasticizers (plasticizers), or in the mix and with job conditions.
high-range water reducers,can either Thus, the use and effect of retarders
greatly increase the flow of the fresh must be evaluated experimentally
concrete or reduce the amount of during the mix design process.
water required for a given consistency.

Superplasticizers can beused when: 6.11.4 Hydration-Control

1.a low water–cement ratio is Admixtures
beneficial (e.g., high-strength
concrete, early strength gain, and These admixtures have the ability
reduced porosity) to stop and reactivate the
2.placing thin sections hydration process of concrete. They
3.placing concrete around tightly consist of two parts: a stabilizer and
spaced reinforcing steel an activator. Adding the stabilizer
completely stops the hydration of the 3.concrete is subjected to alkali–
cementing materials for up to 72 aggregate reaction
hours, while adding the activator to 4.concrete is in contact with water or
the stabilized concrete reestablishes soils containing sulfates
normal hydration and setting. These 5.concrete is placed during hot
admixtures are very useful in weather
extending the use of ready-mixed 6.mass applications of concrete
concrete when the work at the jobsite
is stopped for various reasons. They The American Concrete Institute
are also useful when concrete is being (ACI) recommends the following
hauled for a long time. limits to water-soluble chloride ion
content based on percent weight of
6.11.5 Accelerators cement (American Concrete
Institute, 1999)
Accelerators are used to develop
early strength of concrete at a faster Prestressed concrete - 0.06
rate than that developed in normal Reinforced concrete subjected to
concrete. The ultimate strength, chloride in service - 0.15
however, of high early strength Reinforced concrete protected from
concrete is about the same as that of moisture - 1.00
normal concrete. Accelerators are Other reinforced concrete - 0.30
used to:
1.increase rate of strength gain Several alternatives to the use of
2.reduce the amount of time before calcium chloride are available. These
finishing operations begin include the following:
3.reduce curing time 1.using high early strength (Type III)
4.plug leaks under hydraulic pressure cement
efficiently 2.increasing cement content
3.curing at higher temperatures
Calcium chloride, CaCl2, is the most 4.using non–calcium chloride
widely used accelerator (ASTM D98). accelerators such as triethanolamine,
Both initial and final set times are sodium thiocyanate, calcium formate,
reduced with calcium chloride. The or calcium nitrate
initial set time of 3 hours for a typical
concrete can be reduced to 1.5 hours 6.11.6 Specialty Admixtures
by adding an amount of calcium The civil engineer should be aware
chloride equal to 1% of the cement of these admixtures but will need to
weight; 2% reduces the initial set time study their application in detail, as
to 1 hour. Typical final set times are 6 well as their cost, before using them.
hours, 3 hours, and 2 hours for 0%, Examples of specialty admixtures
1%, and 2% calcium chloride. Figure include:
6.12 shows that strength development • workability retaining
is also affected by CaCl2 for plain • corrosion inhibitors
portland cement concrete (PCC) • damp-proofing agents
and portland cement concrete with • permeability-reducing agents
2% calcium chloride. Concrete with • fungicidal, germicidal, and
CaCl2 develops higher early insecticidal admixtures
strength compared with plain • pumping aids
concrete cured at the same • bonding agents
temperature (Hewlett, 1988). • grouting agents
The PCA recommends against using • gas-forming agents
calcium chloride under the following • coloring agents
conditions: • shrinkage reducing
1.concrete is prestressed 6.12 Supplementary Cementitious
2.concrete contains embedded Materials
aluminum such as conduits, especially
if the aluminum is in contact with steel
Several by-products of other of silicates and aluminosilicates of
industries have been used in concrete calcium, which is developed in a
as supplementary cementitious molten condition simultaneously with
materials, SCM, since the 1970s, iron in a blast fur-nace. The molten
especially in North America. These slag is rapidly chilled by quenching
materials have been used to improve in water to form a glassy, sandlike
some properties of concrete and granulated material. The material is
reduce the problem of discarding then ground to less than 45 mm. The
them. Most of today’s high- specific gravity of slag cement is in the
performance concrete mix-tures have range of 2.85 to 2.95.The rough and
one or more SCMs to enhance angular-shaped ground slag in the
workability, durability, strength, etc. presence of water and an acti-vator,
(CJSI, 2015a). Since these materials NaOH or CaOH, both supplied by
are cementitious, they can be used in portland cement, hydrates and sets
addition to or as a partial replacement in a manner similar to portland
for portland cement. In fact, two or cement.
more of these supplemen-tary
cementitious additives have been used Silica Fume
together to enhance concrete Silica fume is a by-product of the
properties. These supplementary production of silicon metal or ferrosil-
cementitious materials include fly icon alloys. One of the most beneficial
ash, ground granulated blast uses for silica fume is as a mineral
furnace slag, silica fume, and natural admixture in concrete. Because of its
pozzolans. chemical and physical properties, it is
Fly ash a very reactive pozzolan. Concrete
containing silica fume can have very
Fly ash is the most commonly used high strength and can be very durable.
pozzolan in civil engineering Silica fume is available from suppliers
structures. Fly ash is a by-product of of concrete admixtures and, when
the coal-fired electricity production. specified, is simply added during
Combusting pulverized coal in an concrete production either in wet
electric power plant burns off the or dry forms. Placing, finishing, and
carbon and most volatile materials. curing silica fume concrete require
special attention on the part of the
Fly ash is primarily a silica glass concrete contractor.
composed of silica (SiO2), alumina
(Al2O3), iron oxide (Fe2O3), and lime Natural Pozzolan
(CaO). Fly ash is classified (ASTM
C618) as follows: A pozzolan is a siliceous and
aluminous material which, in itself,
Class N—Raw or calcined natural possesses little or no cementitious
pozzolans, including diatomaceous value but will, in finely divided form
earths, opaline cherts and shales, ruffs and in the presence of moisture, react
and volcanic ashes or pumicites, and chemically with calcium hydroxide at
some calcined clays and shales ordinary tempera-tures to form
Class F—Fly ash with pozzolan compounds possessing cementitious
properties properties (ASTM C595). Naturally
Class C—Fly ash with pozzolan and occurring pozzolans, such as fine
cementitious properties volcanic ash, combined with burned
Class F fly ash usually has less than lime, were used about 2000 years
5% CaO but may contain up to 10%. ago for building construction, and
Class C fly ash has 15% to 30% CaO. pozzolan continues to be used today.
As shown in Table 6.2, calcium
Slag Cement hydroxide is one of the products
Slag cement is made from iron blast generated by the hydration of C3S and
furnace slag. It is a nonmetallic C2S. In fact, up to 15% of the weight
hydraulic cement consisting basically of port-land cement is hydrated lime.
Adding a pozzolan to portland cement from the bottom. In this case, the
generates an opportunity to convert concrete slab may curl due to the
this free lime into a cementitious relative difference in shrinkage.
material.
Approaches
7.3 Curing Concrete
1. Maintaining the presence of
Curing is the process of maintaining water in the concrete during
satisfactory moisture content and early ages.
temperature in the concrete for a 2. Preventing loss of mixing water
definite period of time. Hydration of from the concrete by sealing
cement is a long-term process and the surface
requires water and proper
3. Accelerating the strength gain
temperature. Therefore, curing allows
by supplying heat and
continued hydration and,
additional moisture to the
consequently, continued gains in
concrete
concrete strength. In fact, once curing
stops, the concrete dries out, and the 7.3.1 Ponding or Immersion
strength gain stops.
Ponding involves covering the
If concrete is cured for only 3 days, it exposed surface of the concrete
will reach about 60% of the strength structure with water. Ponding can be
of continuously cured concrete; if it is achieved by forming earth dikes
cured for 7 days, it will reach 80% of around the concrete surface to retain
the strength of continuously cured water. This method is suitable for flat
concrete. If curing stops for some time surfaces such as floors and
and then resumes again, the strength pavements, especially for small jobs.
gain will also stop and reactivate. The method requires intensive labor
and supervision. Immersion is used
Increasing temperature increases the
to cure test specimens in the
rate of hydration and, consequently,
laboratory, as well as other concrete
the rate of strength development.
members, as appropriate.
Temperatures below 10°C (50°F) are
unfavorable for hydration and should 7.3.2 Spraying or Fogging
be avoided, if possible, especially at (Spraying Method)
early ages.
A system of nozzles or sprayers can
Proper curing not only increases be used to provide continuous
strength but also provides other spraying or fogging. This method
desirable properties such as durability, requires a large amount of water and
water tightness, abrasion resistance, could be expensive. It is most suitable
volume stability, resistance to freeze in high temperature and low humidity
and thaw, and resistance to deicing environments. Commercial test
chemicals. laboratories generally have a
controlled temperature and humidity
Curing should start after the final set
booth for curing specimens.
of the cement. If concrete is not cured
after setting, concrete will shrink, 7.3.3 Wet Coverings
causing cracks. Drying shrinkage
Moisture-retaining fabric coverings
can be prevented if ample water is
saturated with water, such as burlap,
provided for a long period of time. An
cotton mats, and rugs, are used in
example of improper curing would be
many applications The fabric can be
a concrete floor built directly over the
kept wet, either by periodic watering
subgrade, not cured at the surface,
or covering the fabric with
with the moisture in the soil curing it
polyethylene film to retain moisture.
7.3.4 Impervious Papers or Plastic additional heat is needed during cold
Sheets weather. Steam curing can be attained
either with or without pressure. Steam
Evaporation of moisture from concrete
at atmospheric pressure is used for
can be reduced using impervious
enclosed cast-in-place structures and
papers, such as kraft papers, or plastic
large precast members. High-pressure
sheets, such as polyethylene film.
steam in autoclaves can be used at
Impervious papers are suitable for
small manufactured plants.
horizontal surfaces and simply shaped
concrete structures, while plastic 7.3.8 Insulating Blankets or
sheets are effective and easily applied Covers
to various shapes. Periodic watering is
When the temperature falls below
not required when impervious papers
freezing, concrete should be insulated
or plastic sheets are used.
using layers of dry, porous material
Discoloration, however, can occur on
such as hay or straw. Insulating
the concrete surface.
blankets manufactured of fiberglass,
7.3.5 Membrane-Forming cellulose fibers, sponge rubber,
Compounds mineral wool, vinyl foam, or open-cell
polyurethane foam can be used to
Various types of liquid membrane-
insulate formwork. Moisture proof
forming compounds can be applied to
commercial blankets can also be used.
the concrete surface to reduce or
retard moisture loss. These can be 7.3.9 Electrical, Hot Oil, and
used to cure fresh concrete, as well as Infrared Curing
hardened concrete, after removal of
Precast concrete sections can be cured
forms or after moist curing. Curing
using electrical, oil, or infrared curing
compounds can be applied by hand or
techniques. Electrical curing includes
by using spray equipment. Either one
electrically heated steel forms, and
coat or two coats (applied
electrically heated blankets.
perpendicular to each other) are used.
Reinforcing steel can be used as a
Normally, the concrete surface should
heating element, and concrete can be
be damp when the curing compound is
used as the electrical conductor. Steel
applied. Curing compounds should not
forms can also be heated by
be used when subsequent concrete
circulating hot oil around the outside
layers are to be placed, since the
of the structure. Infrared rays have
compound hinders the bond between
been used for concrete curing on a
successive layers. Also, some
limited basis.
compounds affect the bond between
the concrete surface and paint. 7.3.10 Curing Period
7.3.6 Forms Left in Place The curing period should be as long
as is practical. The minimum time
Loss of moisture can be reduced by
depends on several factors, such as
leaving the forms in place as long as
type of cement, mixture proportions,
practical, provided that the top
required strength, ambient weather,
concrete exposed surface is kept wet.
size and shape of the structure, future
If wood forms are used, the forms
exposure conditions, and method of
should also be kept wet. After
curing. For most concrete structures,
removing the forms, another curing
the curing period at temperatures
method can be used.
above 5°C (40°F) should be a
7.3.7 Steam Curing minimum of 7 days or until 70% of
specified compressive or flexure
Steam curing is used when early
strength is attained. The curing period
strength gain in concrete is required or
can be reduced to 3 days if high early Also, nonuniform shrinkage could
strength concrete is used and the happen due to the nonuniform loss of
temperature is above 10°C (50°F). water. This may happen in mass
concrete structures, where more water
PROPERTIES OF HARDENED
is lost at the surface than at the
CONCRETE
interior. In cases such as this, cracks
7.4.1 Early Volume Change may develop at the surface. In other
cases, curling might develop due to
When the cement paste is still plastic, the nonuniform curing throughout the
it undergoes a slight decrease in structure and, consequently,
volume of about 1%. This shrinkage is nonuniform shrinkage
known as plastic shrinkage and is
due to the loss of water from the 7.4.2 Creep Properties
cement paste, either from evaporation
Creep is defined as the gradual
or from suction by dry concrete below
increase in strain, with time, under
the fresh concrete.
sustained load. Creep of concrete is a
Plastic shrinkage may cause long-term process, and it takes place
cracking; it can be prevented or over many years. Although the
reduced by controlling water loss. In amount of creep in concrete is
addition to the possible decrease in relatively small, it could affect the
volume when the concrete is still performance of structures. The effect
plastic, another form of volume of creep varies with the type of
change may occur after setting, structure. In simply supported
especially at early ages. reinforced concrete beams, creep
increases the deflection and,
If concrete is not properly cured and is therefore, increases the stress in the
allowed to dry, it will shrink. This steel. In reinforced concrete columns,
shrinkage is referred to as drying creep results in a gradual transfer of
shrinkage, and it also causes cracks. load from the concrete to the steel.
Shrinkage takes place over a long Creep also could result in losing some
period of time, although the rate of of the prestress in prestressed
shrinkage is high early and then concrete structures, although the use
decreases rapidly with time. In fact, of high-tensile stress steel reduces
about 15% to 30% of the shrinkage this effect. Rheological models,
occurs in the first 2 weeks, while 65% discussed in Chapter 1, have been
to 85% occurs in the first year. used to analyze the creep response of
Shrinkage and shrinkage-induced concrete (Neville, 1996).
cracking are increased by several
factors, including lack of curing, high 7.4.3 Permeability
water–cement ratio, high cement
Permeability is an important factor
content, low coarse aggregate
that largely affects the durability of
content, existence of steel
hardened concrete. Permeable
reinforcement, and aging. On the
concrete allows water and chemicals
other hand, if concrete is cured
to penetrate, which, in turn, reduces
continuously in water after setting,
the resistance of the concrete
concrete will swell very slightly due to
structure to frost, alkali–silica
the absorption of water. Since
reactivity, and other chemical attacks.
swelling, if it happens, is very small, it
Water that permeates into reinforced
does not cause significant p problems.
concrete causes corrosion of steel
Swelling is accompanied by a slight
rebars. Furthermore, impervious
increase in weight (Neville, 1996).
concrete is a prerequisite in watertight
structures, such as tanks and dams.
Typically, the air voids in the cement The modulus of elasticity of
paste and aggregates are small and do concrete is commonly used in
not affect permeability. However, the designing concrete structures. Since
air voids that do affect permeability of the stress–strain relationship is not
hardened concrete are obtained from exactly linear, the classic definition of
two main sources: incomplete modulus of elasticity (Young’s
consolidation of fresh concrete and modulus) is not applicable. A chord
voids resulting from evaporation of modulus in compression is more
mixing water that is not used for commonly used to represent the
hydration of cement. modulus of elasticity of concrete. It is
determined according to ASTM C469.
Therefore, increasing the water–
cement ratio in fresh concrete has a Poisson’s ratio is used in advanced
severe effect on permeability. structural analysis of shell roofs, flat-
plate roofs, and mat foundations.
7.4.4 Stress–Strain Relationship
Poisson’s ratio of concrete varies
It can be seen that increasing the between 0.11 and 0.21, depending on
water–cement ratio decreases both aggregate type, moisture content,
strength and stiffness of the concrete. concrete age, and compressive
The stress–strain behavior is close to strength. A value of 0.15 to 0.20 is
linear at low stress levels, then commonly used.
becomes nonlinear as stress increases.
The modulus of elasticity of concrete
With a water–cement ratio of 0.50 or
increases when the compressive
less and a strain of up to 0.0015, the
strength increases.
stress–strain behavior is almost linear.
With higher water–cement ratios, the
stress–strain behavior becomes
nonlinear at smaller strains. The
curves also show that high-strength
concrete has sharp peaks and sudden TESTING OF HARDENED
failure characteristics when compared CONCRETE
to low-strength concrete. 7.5.1 Compressive Strength Test

The compressive strength test is

the test most commonly performed on
hardened concrete. Compressive
strength is one of the main structural
design requirements to ensure that
the structure will be able to carry the
intended load. As indicated earlier,
compressive strength increases as the
water–cement ratio decreases. Since
the water–cement ratio is directly
related to the concrete quality,
compressive strength is also used as a
The elastic limit can be defined as
measure of quality, such as durability
the largest stress that does not cause
and resistance to weathering.
a measurable permanent strain. When
the concrete is loaded slightly beyond
the elastic range and then unloaded, a
small amount of strain might remain
initially, but it may recover eventually
due to creep.
7.5.2 Split-Tension Test Maturity of a concrete mixture is
defined as the degree of cement
The split-tension test (ASTM
hydration, which varies as a function
C496) measures the tensile strength
of both time and temperature.
of concrete.
Therefore, it is assumed that, for a
particular concrete mixture, strength
is a function of maturity. Maturity
meters have been developed to
provide an estimate of concrete
7.5.3 Flexure Strength Test strength by monitoring the
temperature of concrete with time.
The flexure strength test (ASTM This test (ASTM C1074) is performed
C78) is important for design and on fresh concrete and continued for
construction of road and airport several days. The maturity meter
concrete pavements. must be calibrated for each concrete
mix.

7.6 Alternatives to Conventional

Concrete

7.5.4 Rebound Hammer Test Several alternatives increase the

flexibility and applications of concrete
The rebound hammer test, also
known as the Schmidt hammer test, Some of these alternatives include the
is a nondestructive test performed on following:
hardened concrete to determine the • self-consolidating concrete
hardness of the surface. The larger the • flowable fill
rebound, the harder is the concrete •shotcrete
surface and, therefore, the greater is •lightweight concrete
the strength. • heavyweight concrete
•high-strength concrete
7.5.5 Penetration Resistance Test
•shrinkage-compensating concrete
The penetration resistance test, •polymers and concrete fiber-
also known as the Windsor Probe reinforced concrete
test, is standardized by ASTM C803. •roller-compacted concrete
The instrument is a gunlike device that •high-performance concrete
shoots probes into the concrete •pervious concrete
surface in order to determine its
7.6.1 Self-Consolidating Concrete
strength. The amount of penetration
of the probe in the concrete is Self-consolidating concrete (SCC),
inversely related to the strength of also known as self-compacting
concrete. concrete is a highly flowable,
nonsegregating concrete that can
7.5.6 Ultrasonic Pulse Velocity
spread into place, fill the formwork,
Test
and encapsulate the reinforcement,
The ultrasonic pulse velocity test without any mechanical consolidation
(ASTM C597) measures the velocity (NRMCA).
of an ultrasonic wave passing through
Two important properties specific to
the concrete In this test, the path
SCC in its plastic state are its
length between transducers is divided
flowability and stability. The high
by the travel time to determine the
flowability of SCC is achieved by
average velocity of wave propagation.
using high-range waterreducing
7.5.7 Maturity Test admixtures without adding extra
mixing water. The stability, or concrete,” is a relatively dry mixture
resistance to segregation, is that is consolidated by the impact
attained by increasing the amount of force and can be placed on vertical or
fines and/or by using admixtures that horizontal surfaces without sagging.
modify the viscosity of the mixture.
Supplementary cementitious
Fines could be either cementitious
materials, such as fly ash and silica
materials or mineral fines.
fume, can be used in shotcrete to
7.6.2 Flowable Fill improve workability, chemical
resistance, and durability.
Flowable fill is a self-leveling and
self-compacting, cementitious material 7.6.4 Lightweight Concrete
with an unconfined compressive
The use of lightweight concrete in a
strength of 8.3 MPa (1200 psi) or less.
structure is usually predicated on the
Flowable fill is primarily used as a
overall cost of the structure; the
backfill material in lieu of compacted
concrete may cost more, but the
granular fill.
reduced dead weight can reduce
One of the unique properties of structural and foundation costs.
flowable fill that makes it
7.6.5 Heavyweight Concrete
advantageous compared with
compacted granular fill is its Biological shielding used for nuclear
flowability. High flowability and self- power plants, medical units, and
leveling characteristics allow flowable atomic research and test facilities
fill to eliminate voids and access requires massive walls to contain
spaces that prove to be difficult or radiation. Concrete is an excellent
impossible with compacted granular shielding material. For biological
fill. shields, the mass of the concrete can
be increased with the use of
Flowable fill has several advantages
heavyweight aggregates.
over compacted granular backfill.
Flowable fill does not require 7.6.6 High-Strength Concrete
compaction, which is a main concern
with granular backfill. Flowable fill can Concrete made with normal-weight
also reach inaccessible locations, such aggregate and having compressive
as places around pipes, which are hard strengths greater than 40 MPa (6000
to reach with granular backfill. psi) is considered to be high-strength
Flowable fill also has a greater bearing concrete. Producing a concrete with
capacity than compacted granular fill more than 40 MPa compressive
and no noticeable settlement. It can strength requires care in the
even be placed in standing water. proportioning of the components and
These advantages result in reduced in- in quality control during construction.
place costs for labor and equipment,
In particular, the porosity of the
as well as time saving during
cement paste and the transition zone
construction.
between the cement paste and the
Flowable fill is typically used as backfill aggregate are the controlling factors
for utility trenches, retaining walls, for developing high strength. High-
pipe bedding, and building excavation. strength concrete has excellent
durability due to its tight pore
7.6.3 Shotcrete structure.
Shotcrete is mortar or small- 7.6.7 Shrinkage-Compensating
aggregate concrete that is sprayed at Concrete
high velocity onto a surface Shotcrete,
also known as “gunite” or “sprayed
Normal concrete shrinks at early ages,
especially if it is not properly cured.
The addition of alumina powders to
the cement can cause the concrete to 7.6.10 Roller-Compacted Concrete
expand at early ages.
Based on the unique requirements for
Shrinkage-compensating cement is mass concrete used for dam
marketed as Type K cement. construction, roller-compacted
Expansive properties can be used to concrete (RCC) was developed. This
advantage by restraining the concrete, material uses a relatively low cement
either by reinforcing or by other factor, relaxed gradation
means, at early ages. As the requirements, and a water content
restrained concrete tries to expand, selected for construction
compressive stresses are developed. considerations rather than strength.
These compressive stresses reduce
the tensile stresses developed by 7.6.11 High-Performance
drying shrinkage, and the chance of Concrete
the concrete cracking due to drying
The American Concrete Institute (ACI)
shrinkage is reduced.
defines HPC as concrete that meets
7.6.8 Polymers and Concrete special performance and uniformity
requirements, which cannot always be
Polymers can be used in several obtained using conventional
ways in the production of concrete. ingredients normal mixing procedures,
The polymer can be used as the sole and typical curing practices.
binding agent to produce polymer
concrete. Polymers can be mixed with 7.6.12 Pervious Concrete
the plastic concrete to produce
Concrete is generally designed to have
polymer–portland cement concrete.
limited amount of air voids in order to
Polymers can be applied to hardened
increase its strength and durability,
concrete to produce polymer-
which is required in most concrete
impregnated concrete.
structures. However, in response to
Polymer concrete is a mixture of sustainability considerations,
aggregates and a polymer binder. pervious concrete has been
developed that is specifically designed
7.6.9 Fiber-Reinforced Concrete to allow water to pass through it.
The brittle nature of concrete is due to The pervious concrete may be used
the rapid propagation of microcracking as the surface layer for light-traffic
under applied stress. However, with pavement designed to capture and
fiber-reinforced concrete, failure store rainfall and runoff, which is then
takes place due to fiber pull-out or allowed to percolate into the subgrade
debonding. Unlike plain concrete, soil.
fiber-reinforced concrete can sustain
load after initial cracking. This P
effectively improves the toughness of σ=
the material. A
The addition of fibers to concrete
reduces the workability. Since the
addition of fibers does not greatly
increase the strength of concrete, its
use in structural members is limited.