The History of Concrete:

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1

The History of Concrete:

The importance of concrete in modern society cannot be underestimated. Look around
you and you will find concrete structures everywhere such as buildings, roads, bridges,
and dams. There is no escaping the impact concrete makes on
your everyday life.


Hoover Dam (4,360,000 cubic yards of concrete)





What is in Concrete?


Concrete is a composite material which is made up
of a filler and a binder. Typical
concrete is a mixture of fine aggregate (sand), coarse aggregate (rock), cement, and
water.


2

Under no circumstances should the word "cement" be used to refer to the
composite product "concrete".


Portland cement, so named
for its color similarity with limestone near Portland England,
is composed primarily of four complex compounds: tricalcium silicate, dicalcium silicate,
tricalcium aluminate, and tetracalcium aluminoferrite.

Water

is the key ingredient, which when mixed wi
th cement, forms a paste that binds the
aggregate together. The water causes the hardening of concrete through a process called
hydration. Hydration is a chemical reaction in which the major compounds in cement
form chemical bonds with water molecules and
become hydrates or hydration products.
Details of the hydration process are explored in the next section. The water needs to be
pure, typically drinkable, in order to prevent side reactions from occurring which may
weaken the concrete or otherwise interfer
e with the hydration process. The role of water
is important because the water to cement ratio is the most critical factor in the production
of "perfect" concrete. Too much water reduces concrete strength, while too little will
make the concrete unworkable
. Concrete needs to be
workable
so that it may be
consolidated and shaped into different forms (i.e.. walls, domes, etc.). Because concrete
must be both strong and workable, a

careful balance of the cement to water ratio is
required when making concrete.

Aggregates

are chemically inert, solid bodies held together by the cement. Aggregates
come in various shapes, sizes, and materials ranging from fine particles of sand to large
,
coarse rocks. Because cement is the most expensive ingredient in making concrete, it is
desirable to minimize the amount of cement used. 70 to 80% of the volume of concrete is
aggregate in order to keep the cost of the concrete low. The selection of an a
ggregate is
determined, in part, by the desired characteristics of the concrete. For example, the
density of concrete is determined by the density of the aggregate. Soft, porous aggregates
can result in weak concrete with low wear resistance, while using h
ard aggregates can
make strong concrete with a high resistance to abrasion.

Aggregates should be clean, hard, and strong. The aggregate is usually washed to remove
any dust, silt, clay, organic matter, or other impurities that would interfere with the
bon
ding reaction with the cement paste. It is then separated into various sizes by passing
the material through a series of screens with different size openings. The final properties
of the concrete will depend on the cement characteristics, the type and amou
nt of
aggregate, the water
-
cement ratio, and the completeness of the reaction (subject to time,
humidity, and temperature).

Properties of Concrete


Concrete has many properties that make it a popular construction material. The correct
proportion of ingred
ients, placement, and curing are needed in order for these properties
to be optimal.


3

Good
-
quality concrete has many advantages that add to its popularity. First, it is
economical when ingredients are readily available. Concrete's long life and relatively
low
maintenance requirements increase its economic benefits. Concrete is not as likely to rot,
corrode, or decay as other building materials. Concrete has the ability to be molded or
cast into almost any desired shape. Building of the molds and casting can

occur on the
work
-
site which reduces costs.

Concrete is a non
-
combustible material which makes it fire
-
safe and able withstand high
temperatures. It is resistant to wind, water, rodents, and insects. Hence, concrete is often
used for storm shelters.

Con
crete does have some limitations despite its numerous advantages. Concrete has a
relatively low tensile strength (compared to other building materials), low ductility, low
strength
-
to
-
weight ratio, and is susceptible to cracking. Concrete remains the mater
ial of
choice for many applications regardless of these limitations.

The compressive strength of concrete is usually at least ten times its tensile strength, and
five to six times its flexural strength. The principal factors governing compressive
strength

are given below:



Water
-
cement ratio is by far the most important factor.



The age of the cured concrete is also important. Concrete gradually builds
strength after mixing due to the chemical interaction between the cement and the
water. It is normally tes
ted for its 28 day strength, but the strength of the concrete
may continue to increase for a year after mixing.



Character of the cement, curing conditions, moisture, and temperature. The
greater the period of moist storage (100% humidity) and the higher t
he
temperature, the greater the strength at any given age.



Air entrainment, the introduction of very small air voids into the concrete mix,
serves to greatly increase the final product's resistance to cracking from freezing
-
thawing cycles. Most outdoor st
ructures today employ this technique.



4

The History of Concrete:

A Timeline


Cement has been around for at least 12 million years. When the earth itself was
undergoing intense geologic changes natural, cement was being created. It was this
natural cement
that humans first put to use. Eventually, they discovered how to make
cement from other materials.

12,000,000 BC

Reactions between
limestone
and oil shale during spontaneo
us
combustion occurred in palestine

to form a natural deposit of
cement
compounds.

3000 BC

Egyptians

Used mud mixed with straw to bind dried bricks. They also used
gypsum
mortars and mortars of lime in the pyramids.

Chinese

Used cementitious materials to hold bamboo together in their boats
and in the Great Wall.

800 BC

Greeks, Crete
& Cyprus

Use
d lime mortars which were much harder than later Roman
mortars.

300 BC

Babylonians
& As Syrians

Used bitumen to bind stones and bricks.

300 BC
-

476
AD

Romans

Used pozzolana cement from Pozzuoli, Italy near Mt. Vesuvius to
build the Appian Way, Roman
baths, the Coliseum and Pantheon in
Rome, and the Pont du Gard aqueduct in south France. They used
lime as a cementitious material. Pliny reported a mortar mixture of
1 part lime to 4 parts sand. Vitruvius reported a 2 parts pozzolana
to 1 part lime. Anima
l fat, milk, and blood were used as admixtures
(substances added to cement to increase the properties.)
These
structures still exist today!


1200
-

1500

The Middle
Ages

The quality of cementing materials deteriorated. The use of burning
lime and
pozzolan

(admixture) was lost, but reintroduced in the
1300's.

1678

Joseph Moxon wrote about a hidden fire in heated lime that appears
upon the addition of water.

1779

Bry Higgins

was issued a patent for hydraulic cement (stucco) for
exterior plastering use.

1780

Bry Higgins published "Experiments and Observations Made With
the View of Improving the Art of Composing and Applying
Calcereous Cements and of Preparing Quicklime."

1
793

John Smeaton found that the calcination of limestone containing

5

clay gave a lime which hardened under water (hydraulic lime). He
used hydraulic lime to rebuild Eddystone Lighthouse in Cornwall,
England which he had been commissioned to build in 1756, b
ut had
to first invent a material that would not be affected by water. He
wrote a book about his work.

1796

James Parker from England patented a natural hydraulic cement by
calcining nodules of impure limestone containing clay, called
Parker's Cement or
Roman Cement.

1802

In France, a similar Roman Cement process was used.

1810

Edgar Dobbs received a patent for hydraulic mortars, stucco, and
plaster, although they were of poor quality due to lack of kiln
precautions.

1812
-
1813

Louis Vicat of France

prepared artificial hydraulic lime by calcining
synthetic mixtures of limestone and clay.

1818

Maurice St. Leger was issued patents for hydraulic cement. Natural
Cement was produced in the USA. Natural cement is limestone that
naturally has the appropri
ate amounts of clay to make the same
type of concrete as John Smeaton discovered.

1820
-

1821

John Tickell and Abraham Chambers were issued more hydraulic
cement patents.

1822

James Frost of England prepared artificial hydraulic lime like
Vicat's and c
alled it British Cement.

1824

Joseph Aspdin of England invented
portland cement

by burning
finely ground chalk with finely divided clay in a lime
kiln
until
carbon dioxide was driven off. The sintered product was then
ground and he called it portland cement named after the high
quality building stones quarried at Portland, England.

1828

I. K.
Brunel is credited with the first engineering application of
portland cement, which was used to fill a breach in the Thames
Tunnel.

1830

The first production of lime and hydraulic cement took place in
Canada.

1836

The first systematic tests of tensile
and compressive strength took
place in Germany.

1843

J. M. Mauder, Son & Co. were licensed to produce patented
portland cement.

1845

Isaac Johnson claims to have burned the raw materials of portland
cement to
clinkering

temperatures.

1849

Pettenkofer & Fuches performed the first accurate chemical
analysis of portland cement.

1860

The beginning of the era of portland cements of modern

6

composition.

1862

Blake Stonebrea
ker of England introduced the jaw breakers to
crush clinkers.

1867

Joseph Monier of France reinforced William Wand's (USA) flower
pots with wire ushering in the idea of iron reinforcing bars (re
-
bar).

1871

David Saylor was issued the first American pat
ent for portland
cement. He showed the importance of true clinkering.

1880

J. Grant of England show the importance of using the hardest and
densest portions of the clinker. Key ingredients were being
chemically analyzed.

1886

The first rotary kiln was
introduced in England to replace the
vertical shaft kilns.

1887

Henri Le Chatelier of France established oxide ratios to prepare the
proper amount of lime to produce portland cement. He named the
components: Alite (tricalcium silicate), Belite (dicalcium

silicate),
and Celite (tetracalcium aluminoferrite). He proposed that
hardening is caused by the formation of crystalline products of the
reaction between cement and water.

1889

The first concrete reinforced bridge is built.

1890

The addition of gypsu
m when grinding clinker to act as a
retardant

to the
setting

of concrete was introduced in th
e USA. Vertical shaft
kilns were replaced with rotary kilns and ball mills were used for
grinding cement.

1891

George Bartholomew placed the first concrete street in the USA in
Bellefontaine, OH.
It still exists today!


1893

William Michaelis claimed th
at hydrated metasilicates form a
gelatinous mass (gel) that dehydrates over time to harden.

1900

Basic cement tests were standardized.

1903

The first concrete high rise was built in Cincinnati, OH.

1908

Thomas Edison built cheap, cozy concrete houses

in Union, NJ.
They
still exist today!


1909

Thomas Edison was issued a patent for rotary kilns.

1929

Dr. Linus Pauling of the USA formulated a set of principles for the
structures of complex silicates.

1930

Air entraining agents were introduced to im
prove concrete's
resistance to freeze/thaw damage.

1936

The first major concrete dams, Hoover Dam and Grand Coulee
Dam, were built.
They still exist today!


1956

U.S. Congress annexed the Federal Interstate Highway Act.

1967

First concrete domed sport

structure, the Assembly Hall, was

7

constructed at The University of Illinois, at Urbana
-
Champaign.

1970's

Fiber reinforcement in concrete was introduced.

1975

CN Tower in Toronto, Canada, the tallest slip
-
form building, was
constructed.

Water Tower Pl
ace in Chicago, Illinois, the tallest building was
constructed.

1980's

Superplasticizers were introduced as admixtures.

1985

Silica fume was introduced as a pozzolanic additive.

The "highest strength" concrete was used in building the Union
Plaza cons
tructed in Seattle, Washington.

1992

The tallest reinforced concrete building in the world was
constructed at 311 S. Wacker Dr., Chicago, Illinois.

Concrete
-

Materials

Table 1.
Classes of Aggregates

class

examples of
aggregates used

uses

ultra
-
light
weight

vermiculite

ceramic spheres

perlite

lightweight concrete which can be sawed or
nailed, also for its insulating properties

lightweight

expanded clay

shale or slate

crushed brick

used primarily for making lightweight concrete
for structures, also u
sed for its insulating
properties.

normal
weight

crushed limestone

sand

river gravel

crushed recycled
concrete

used for normal concrete projects

heavyweight

steel or iron shot

steel or iron
pellets

used for making high density concrete for
shielding aga
inst nuclear radiation


Examples of classes of concrete aggregate are shown in Table 1. The choice of aggregate
is determined by the proposed use of the concrete. Normally sand, gravel, and crushed
stone are used as aggregates to make concrete. The aggreg
ate should be well
-
graded to
improve packing efficiency and minimize the amount of
cement paste

needed. Also, this
makes the concrete more workable.


8

Hydration of Portland Cem
ent


Concrete is prepared by mixing cement, water, and aggregate together to make a
workable paste. It is molded or placed as desired, consolidated, and then left to harden.
Concrete does not need to dry out in order to harden as commonly thought.

The con
crete (or specifically, the cement in it) needs moisture to hydrate and
cure

(harden). When concrete dries, it actually stops getting stronger. Concrete with too little
water
may be dry but is not fully reacted. The properties of such a concrete would be less
than that of a wet concrete. The reaction of water with the cement in concrete is
extremely important to its properties and reactions may continue for many years. This
ver
y important reaction will be discussed in detail in this section.

Portland cement consists of five major compounds and a few minor compounds. The
composition of a typical portland cement is listed by weight percentage in Table 2.


Table 2.
Composition of

portland cement with chemical composition and weight percent.

Cement Compound

Weight Percentage

Chemical Formula

Tricalcium silicate

50 %

Ca
3
SiO
5

or 3CaO
.
SiO
2

Dicalcium silicate

25 %

Ca
2
SiO
4

or 2CaO
.
SiO
2

Tricalcium aluminate

10 %

Ca
3
Al
2
O
6

or 3CaO

.
Al
2
O
3

Tetracalcium aluminoferrite

10 %

Ca
4
Al
2
Fe
10

or 4CaO
.
Al
2
O
3
.
Fe
2
O
3

Gypsum

5 %

CaSO
4
.
2H
2
O

When water is added to cement, each of the compounds undergoes hydration and
contributes to the final concrete product. Only the calcium silicates contribute to str
ength.
Tricalcium silicate is responsible for most of the early strength (first 7 days). Dicalcium
silicate, which reacts more slowly, contributes only to the strength at later times.
Tricalcium silicate will be discussed in the greatest detail.

The equat
ion for the hydration of tricalcium silicate is given by

Tricalcium silicate + Water
---
>Calcium silicate hydrate+Calcium hydroxide + heat


2 Ca
3
SiO
5

+ 7 H
2
O
---
> 3 CaO
.
2SiO
2
.
4H
2
O + 3 Ca(OH)
2

+ 173.6kJ


Upon the addition of water, tricalcium silicate rapid
ly reacts to release calcium ions,
hydroxide ions, and a large amount of heat. The pH quickly rises to over 12 because of
the release of alkaline hydroxide (OH
-
) ions. This initial hydrolysis slows down quickly
after it starts resulting in a decrease in he
at evolved.


9

The reaction slowly continues producing calcium and hydroxide ions until the system
becomes saturated. Once this occurs, the calcium hydroxide starts to crystallize.
Simultaneously, calcium silicate hydrate begins to form. Ions precipitate out

of solution
accelerating the reaction of tricalcium silicate to calcium and hydroxide ions. (Le
Chatlier's principle). The evolution of heat is then dramatically increased.

The formation of the calcium hydroxide and calcium silicate hydrate crystals prov
ide
"seeds" upon which more calcium silicate hydrate can form. The calcium silicate hydrate
crystals grow thicker making it more difficult for water molecules to reach the
unhydrated tricalcium silicate. The speed of the reaction is now controlled by the r
ate at
which water molecules diffuse through the calcium silicate hydrate coating. This coating
thickens over time causing the production of calcium silicate hydrate to become slower
and slower.



Figure 1.
Schematic illustration of the pores in calcium silicate through different stages
of hydration.

The diagrams shown in Figure 1 represent the formation of pores as calcium silicate
hydrate is formed. No
te in diagram (a) that hydration has not yet occurred and the pores
(empty spaces between grains) are filled with water. Diagram (b) represents the
beginning of hydration. In diagram (c), the hydration continues. Although empty spaces
still exist, they are

filled with water and calcium hydroxide. Diagram (d) shows nearly
hardened cement paste. Note that the majority of space is filled with calcium silicate
hydrate. That which is not filled with the hardened hydrate is primarily calcium
hydroxide solution. T
he hydration will continue as long as water is present and there are
still unhydrated compounds in the cement paste.

Dicalcium silicate also affects the strength of concrete through its hydration. Dicalcium
silicate reacts with water in a similar manner c
ompared to tricalcium silicate, but much
more slowly. The heat released is less than that by the hydration of tricalcium silicate
because the dicalcium silicate is much less reactive. The products from the hydration of
dicalcium silicate are the same as th
ose for tricalcium silicate

Dicalcium silicate + Water
---
>Calcium silicate hydrate + Calcium hydroxide +heat



10

2 Ca
2
SiO
4

+ 5 H
2
O
---
> 3 CaO
.
2SiO
2
.
4H
2
O + Ca(OH)
2

+ 58.6 kJ


The other major components of portland cement, tricalcium aluminate and tetracalcium
aluminoferrite also react with water. Their hydration chemistry is more complicated as
they involve reactions with the gypsum as well. Because these reactions do not contribute
significantly to strength, they will be neglected in this discussion. Although
we have
treated the hydration of each cement compound independently, this is not completely
accurate. The rate of hydration of a compound may be affected by varying the
concentration of another. In general, the rates of hydration during the first few days
ranked from fastest to slowest are

tricalcium aluminate > tricalcium silicate > tetracalcium aluminoferrite > dicalcium
silicate.

Heat is evolved with cement hydration. This is due to the breaking and making of
chemical bonds during hydration. The heat g
enerated is shown below as a function of
time.



Figure 2.
Rate of heat evolution during the hydration of portland cement

As shown in Figure 2, the sta
ge I hydrolysis of the cement compounds occurs rapidly
with a temperature increase of several degrees. Stage II is known as the
dormancy period.

The evolution of heat slows dr
amatically in this stage. The dormancy period can last from
one to three hours. During this period, the concrete is in a plastic state which allows the
concrete to be transported and placed without any major difficulty. This is particularly
important for t
he construction trade who must transport concrete to the job site. It is at the
end of this stage that initial setting begins. In stages III and IV, the concrete starts to
harden and the heat evolution increases due primarily to the hydration of tricalcium

silicate. Stage V is reached after 36 hours. The slow formation of hydrate products occurs
and continues as long as water and unhydrated silicates are present.


Strength of Concrete



11

The strength of concrete is very much dependent upon the hydration reac
tion just
discussed. Water plays a critical role, particularly the amount used. The strength of
concrete increases when less water is used to make concrete. The hydration reaction itself
consumes a specific amount of water. Concrete is actually mixed with
more water than is
needed for the hydration reactions. This extra water is added to give concrete sufficient
workability. Flowing concrete is desired to achieve proper filling and composition of the
forms

. The water not consumed in the hydration reaction will remain in the
microstructure pore space. These pores make the concrete weaker due to the lack of
strength
-
forming calcium silicate hydrate bonds. Some pores will remain n
o matter how
well the concrete has been compacted. The relationship between the water/cement ratio
and porosity is illustrated in Figure 3.



Figure 3.
Schematic drawings to demonstrate the relationship between the water/cement
ratio and porosity.

The empty space (porosity) is determined by the water to cement ratio. The relationship
between the water to cement ratio and strength is shown in Figure 4.


12



Figure 4.
A plot of concrete strength as a function of the water to cement ratio.

Low water to cement ratio leads to high strength but low workability.

High water to
cement ratio leads to low strength, but good workability.

The physical characteristics of aggregates are shape, texture, and size. These can
indirectly affect strength because they affect the workability of the concrete. If the
aggregate ma
kes the concrete unworkable, the contractor is likely to add more water
which will weaken the concrete by increasing the water to cement mass ratio.

Time is also an important factor in determining concrete strength. Concrete hardens as
time passes. Why? R
emember the hydration reactions get slower and slower as the
tricalcium silicate hydrate forms. It takes a great deal of time (even years!) for all of the
bonds to form which determine concrete's strength. It is common to use a 28
-
day test to
determine the

relative strength of concrete.

Concrete's strength may also be affected by the addition of admixtures. Admixtures are
substances other than the key ingredients or reinforcements which are added during the
mixing process. Some admixtures add fluidity to c
oncrete while requiring less water to be
used. An example of an admixture which affects strength is superplasticizer. This makes
concrete more workable or fluid without adding excess water. A list of some other
admixtures and their functions is given below
. Note that not all admixtures increase
concrete strength. The selection and use of an admixture are based on the need of the
concrete user, as shown in Table 3.



Table 3.
A table of admixtures and their functions.


13

TYPE

FUNCTION

AIR ENTRAINING

improves

durability, workability, reduces
bleeding
,
reduces freezing/thawing problems (e.g. special
detergents)

SUPERPLASTICIZERS

increase strength by decreasing water needed for
wor
kable concrete (e.g. special polymers)

RETARDING

delays setting time, more long term strength, offsets
adverse high temp. weather (e.g. sugar )

ACCELERATING

speeds setting time, more early strength, offsets adverse
low temp. weather (e.g. calcium chlorid
e)

MINERAL
ADMIXTURES

improves workability, plasticity, strength (e.g. fly ash)

PIGMENT

adds color (e.g. metal oxides)

Durability is a very important concern in using concrete for a given application. Concrete
provides good performance through the servi
ce life of the structure when concrete is
mixed properly and care is taken in curing it. Good concrete can have an infinite life span
under the right conditions. Water, although important for concrete hydration and
hardening, can also play a role in decrea
sed durability once the structure is built. This is
because water can transport harmful chemicals to the interior of the concrete leading to
various forms of deterioration. Such deterioration ultimately adds costs due to
maintenance and repair of the concr
ete structure. The contractor should be able to
account for environmental factors and produce a durable concrete structure if these
factors are considered when building concrete structures.


Mix Design Procedure

In order to determine the proper amounts o
f water, cement, coarse aggregate, and fine
aggregate that is to be used for a concrete mix, factors such as strength requirements, the
type of structure, the environmental exposure of the structure, and the workability must
be considered. This is demonstr
ated in the procedure outlined below.



Concrete Mix Design Procedure




The structural and environmental design requirements of the project must be met.
(Information related to these requirements is presented in Tables 1 and 2 and Figure 1
below and in the
ENGR 314 Laboratory Manual.

Take the specified safety factor, if any
,

14

into account by increasing the minimum strength value by the specified amount, when
determining the necessary compressive strength.



If a water
-
to
-
cement ratio between the values given in the table is desired, linear
interpolation should be used to de
termine the appropriate amount of each constituent
material required for the mix. For example, if a .43 water
-
to
-
cement ratio is desired,
interpolate between the data for the .40 and .45 water
-
to
-
cement ratios.



The weight of each component required to p
repare the desired volume of concrete must
be determined. The weight of each component in pounds that is required to produce 1
cubic yard of concrete is shown in Table 2. If a volume other than 1 cubic yard of
concrete is required, the appropriate conversi
on factor must be used with the data from
Table 2 to calculate the required amount of each constituent.



If the sand is known to absorb water, be sure to take this into account and increase the
amount of water used by an appropriate amount so that the appropriate amount of water
will be available for the hydration reaction.



Concrete Mix Design Example


A retaining wall is to be built in Jackson, Mississippi. Pouring conditions call for a 4 inch
slump, and Type I Portland cement with a sack weight of 94 lb/cu ft is to be used. Sand
available in the area may be used, but it has been dried so that it will a
bsorb 0.25% of its
weight in water. Coarse aggregate with a maximum size of 3/4 inch is available in the
area and will be used for the project. The 28 day strength requirement for this project is
5000 psi. Initially, a trial mix of 2 cubic yards is to be p
roduced. Determine the amounts
of water, cement, fine aggregate, and coarse aggregate that are needed for this trial mix.



Based on Table 1, the maximum water
-
to
-
cement ratio that should be used for the
retaining wall in an area with mild temperatures s
hould be selected on the basis of
strength and workability requirements, but minimum cement content should not be less
than 470 lb. per cubic yard.



Based on Figure 1, the maximum water
-
to
-
cement ratio that will meet the compressive
strength requirements

of 5750 psi (5000 psi + 15%) is 0.50.



Table 2 is used to determine the amount of each component required. Since the sand
will absorb 0.25% of its weight in water, this amount of additional water must be added
to the mix in order to have the necessary a
mount of water available for the hydration
reaction.



Note that Table 2 gives the amount of each component that is needed for 1 cubic yard
of concrete. The amounts must be adjusted according to the amount of concrete needed.
For example, 2 cubic yards ar
e needed in this example, so the amounts in Table 2 must be
multiplied by 2.



Table 1


15

ACI Recommended Maximum Permissible Water
-
Cement Ratios

For Different Types of Structures and Degrees of Exposure.



Type of Structure

Exposure Condition


Severe T
emperatures

Freeze and Thawing

(Air
-
entrained only)

Mild Temperatures

Rainy, or arid




Air

Fresh

Water

Salt

Water



Air

Fresh

Water

Salt

Water

1. This section such as reinforced
piles and pipe

0.49

0.44

0.40

0.53

0.49

0.40

2. Bridge decks

0.44

0
.44

0.40

0.49

0.49

0.44

3. Very thin section such as curbs,
architectural and all sections with
less than 1 inch of concrete over
reinforcement

0.49

-

-

0.53

0.49

-

4. Moderate sections such as
retaining wall, pier, beam

0.53

0.49

0.44

*

0.53

0.44

5. Co
ncrete slab laid on ground

0.53

-

-

*

-

-

6. Pavement

0.49

-

-

0.53

-

-



* Water
-
cement ratio should be selected on basis of strength and workability
requirements, but minimum cement content should not be less than 470 lb. per cubic
yard.


See Recommen
ded Practice for Selecting Proportions for Concrete (ACI 613
-
54) for
more details.


Table 2

Suggested Trial Mixes for Non
-
air
-
entrained Concrete

of Medium Consistency ( 3
-
4 inch slump).


16



Water
-
Cement

Ratio lb/lb



Max Size

Aggregate



Air Content

Entrapped %



Water

lb/cu yd

of concrete



Cement

lb/cu yd

of concrete



Sand

% of

total

Sand

lb/cu yd

of concrete

Rock

lb/cu yd

of
concrete

0.40

3/8

1/2

3/4

1

1 1/2

3

2.5

2

1.5

1

385

365

340

325

300

965

915

850

815

750

54

47

39

36

33

1350

1220

1080

1020

1000

1150

1400

1680

1830

1990

0.45

3/8

1/2

3/4

1

1 1/2

3

2.5

2

1.5

1

385

365

340

325

300

855

810

755

720

665

56

48

41

38

35

1440

1300

1160

1100

1080

1150

1400

1680

1830

1990

0.50

3/8

1/2


3/4

1

1 1/2

3

2.5

2

1.5

1

385

365

340

325

300

770

730

680

650

600

57

49

42

39

38

1570

1430

1270

1210

1180

1150

1400

1680

1830

1990

0.55

3/8

1/2

3/4

1

1 1/2

3

2.5

2

1.5

1

385

365

340

325

300

700

665

620

590

545

5
8

51

43

40

37

1570

1430

1270

1210

1180

1150

1400

1680

1830

1990


17


Figure 1. Water
-
to
-
cement Ratio vs. Compressive Strength (psi) Data.