CIVL 1101 Concrete Introduction 1/9

Properties of Concrete Properties of Concrete

 Concrete is an artificial conglomerate stone made  While cement in one form or another has been around for
essentially of Portland cement, water, and aggregates. centuries, the type we use was invented in 1824 in Britain.
 It was named Portland cement
because it looked like the
stone quarried on the Isle of
Portland.

Properties of Concrete Properties of Concrete

 Joseph Aspdin (1779-1835) patented the clay and limestone cement  Joseph's son, William Aspdin’s (1815 – 1864) kiln used to make the
known as Portland cement in 1824. first genuine Portland cement.

Properties of Concrete Properties of Concrete

 Portland cement is produced by mixing ground limestone,
 Portland cement was first used in the civil
engineering project by Isambard Kingdom clay or shale, sand and iron ore.
Brunel (1806-1859).
 This mixture is heated in a rotary kiln to temperatures as
 Brunel worked for several years as assistant high as 1,600 degrees Celsius.
engineer on the project to create a tunnel
under London's River Thames
 The heating process causes the materials to break down
and recombine into new compounds that can react with
water in a crystallization process called hydration.

Isambard Kingdom Brunel

CIVL 1101 Concrete Introduction 2/9

Portland Cement Portland Cement

 The raw ingredients of

Portland cement are iron
ore, lime, alumina and
silica.

 These are ground up and

fired in a kiln to produce a
clinker.

 After cooling, the clinker is

very finery ground.

Properties of Concrete Properties of Concrete

 When first mixed the water and cement constitute a paste  Tricalcium silicate – C3S
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.
 Dicalcium silicate – C2S
 Concrete continues to gain strength as it cures.

 Heat of hydration - is the heat given off during the

chemical reaction as the cement hydrates.

Properties of Concrete Properties of Concrete

 Tricalcium aluminate – C3A  Tricalcium aluminate – C3A

 Tetracalcium aluminoferrite
CIVL 1101 Concrete Introduction 3/9

Properties of Concrete Properties of Concrete

 Scanning-electron micrographs of hardened cement paste  Hydration of cement paste

Properties of Concrete Properties of Concrete

 Image shown is a two-dimensional  Stages of hydration:

slice from a three-dimensional
spherical computational volume
 Unhydrated cement cores are dark
blue,
 Inner C-S-H product is red,
 Outer C-S-H project is yellow, and
 Water-filled space is light blue

Properties of Concrete Properties of Concrete

 Range in proportions of materials used in concrete, by absolute volume.
 Stages of hydration:

 Bars 1 and 3 represent rich mixes with small size aggregates.

 Bars 2 and 4 represent lean mixes with large size aggregates.
CIVL 1101 Concrete Introduction 4/9

Water/Cement Ratio Water/Cement Ratio

 The single most important indicator of strength is the ratio
of the water used compared to the amount of cement
(w/c ratio)

 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.

Water/Cement Ratio Water/Cement Ratio

 A minimum w/c ratio (water-to-cement ratio) of about 0.3 Advantages of low water/cement ratio:
by weight is necessary to ensure that the water comes
into contact with all cement particles (thus assuring  Increased strength
complete hydration).
 Lower permeability
 Typical values are  Increased resistance to weathering
in the 0.4 to 0.6  Better bond between concrete and reinforcement
 Reduced drying shrinkage and cracking
 Less volume change from wetting and drying

Concrete Curing Concrete Curing

 Curing - maintenance of a satisfactory moisture content
and temperature in concrete for a suitable period of time
immediately following placing & finishing so that the
desired properties may develop.

 Factors that effect curing:

Time

Temperature

Moisture
CIVL 1101 Concrete Introduction 5/9

Concrete Curing Concrete Curing

Concrete strength gain versus time for concrete exposed to
outdoor conditions. Concrete continues to gain strength for
many years when moisture is provided by rainfall and other
environmental sources.

Compressive Strength Compressive Strength

Compressive Strength - is defined as the measured Compressive Strength - is defined as the measured
maximum resistance of a concrete or mortar specimen to an maximum resistance of a concrete or mortar specimen to an
axial load, usually expressed in psi (pounds per square axial load, usually expressed in psi (pounds per square
inch) at an age of 28-days. inch) at an age of 28-days.

Properties of Concrete Properties of Concrete

 During the first week to 10 days of curing it is important
1.00 that the concrete not be permitted to freeze or dry out

0.75
 In practical terms, about 90% of its strength is gained in
the first 28 days.

0.50  Concrete compressive strength

depends upon many factors:
0.25  quality and proportions
of the ingredients
 the curing environment.
3 7 14 28
Age (days)
CIVL 1101 Concrete Introduction 6/9

Stress–Strain Diagram Concrete Material Properties

 Most structural concrete have f’c values in the 3,000 to

σ
5,000 psi range.
12
 High-rise buildings sometimes utilize concrete of 12,000
or 15,000 psi
8

 Concrete has no linear portion to its stress-strain curve,

4 therefore it is difficult to measure the modulus of elasticity

Strain 

Concrete Material Properties Concrete Material Properties

 For concretes up to about 6,000 psi it can be  The weight density of reinforced concrete using normal
approximated as: aggregates is about 150 lb/ft3 (pcf).

 If 5 pcf of this is allowed for the steel and w is taken as

E  33w 1.5 f' c 145 pcf then:

 where w is the unit weight (pcf), f’c is the cylinder

strength (psi). E  57,000 f 'c

Concrete Material Properties Freeze-Thaw Resistance

 Effect of voids in concrete on modulus of elasticity,  Concrete used in structures and pavements is expected
compressive strength, and flexural strength to have long life and low maintenance.
 It must have good durability to resist anticipated exposure
conditions.
 The most potentially destructive weathering factor is
freezing and thawing while the concrete is wet,
particularly in the presence of deicing chemicals.
 Deterioration is caused by the freezing of water and
subsequent expansion in the paste, the aggregate
particles, or both.
CIVL 1101 Concrete Introduction 7/9

Specimens Subjected to 150 Cycles of

Freezing and Thawing Freeze-Thaw Resistance

 Non-air-entrained
 High water-cement ratio

 Air-entrained
 Low water-cement ratio Type I cement

Freeze-Thaw Resistance Concrete Shrinkage

 As concrete cures it shrinks because the water not used
for hydration gradually evaporates from the hardened mix

 Concrete, like all materials, also undergoes volume

changes due to thermal effects.

 The heat from the exothermic hydration process adds to

this problem.
Type I cement

Concrete Shrinkage Mix Proportions

 Since concrete is weak in tension, it will often develop  The ingredients of concrete can be proportioned by
cracks due to such shrinkage and temperature changes. weight or volume.

 Consider a freshly placed  The goal is to provide the desired strength and workability
concrete slab-on-grade at minimum expense.

 A low w/c ratio is used to achieve strong concrete.

 Could you increased the cement content and use enough

water for good workability and still have a low w/c ratio?
CIVL 1101 Concrete Introduction 8/9

Concrete Mix Design Relationships Aggregate Size and Shape

 Larger aggregate sizes have relatively smaller surface
Strength

areas (for the cement paste to coat)

 Use the largest practical aggregate size and the stiffest

practical mix.
w/c
Cost

cement
Workability

water

Workability Slump Test

 Workability - that property of freshly mixed concrete that  A good indication of the water content of a mix and thus
determines its working characteristics, i.e. the ease with the workability) can be had from a standard slump test.
which it can be mixed, placed, compacted and finished.

 Factors effecting workability:

 Method and duration of transportation
 Quantity and characteristics of cementing materials
 Concrete consistency (slump)
 Aggregate grading, shape & surface texture
 % entrained air
 Water content
 Concrete & ambient air temperature  Most concrete mixes have slumps in the 2- to 5-in. range.
 Admixtures

Slump Test Slump Test

CIVL 1101 Concrete Introduction 9/9

Consolidation Curing of Concrete

 Good consolidation (left) is needed to achieve a dense
and durable concrete. Why cure concrete? Curing serves two main
 Poor consolidation (right can result in early corrosion of purposes:
reinforcing steel and low compressive strength.
 it retains moisture in the slab so that the concrete
continues to gain strength

 it delays drying shrinkage until the concrete is strong

enough to resist shrinkage cracking

Types of Portland Cement Aggregates

 There are five basic types of Portland cement in use
today:  Coarse aggregates are larger than 3/8 inch in diameter

Type I General purpose

 Fine aggregate (sand) is made up of particles which are
Type II Sulfate resisting, concrete in contact with high sulfate smaller than 3/8 ” in diameter
soils
Type III High early strength, which gains strength faster than  The quality of aggregates is very important since they
Type I, Enabling forms to be removed sooner make up about 60 to 75% of the volume of the concrete
Type IV Low heat of hydration, for use in massive construction
Type V Severe sulfate resisting  Normal and lightweight concrete

Admixtures Properties of Concrete

 Admixtures are chemicals which are added to the mix to

achieve special purposes

 There are basically four types:

The End
 air-entraining agents,
 workability agents,
 retarding agents, and
 accelerating agents

 Also test batches of concrete is investigate the effects of

concrete performance