An Najah National UniversityFaculty of Engineering
Eng. Mohammed Abu Neamah
Construction Materials Course
PORTLAND CEMENT
CONCRETE
High Strength Concrete
Many advances have been made in concrete technology. One
of the most significant improvements is the production of very
high strength concrete, using supplementary cementing
materials (SCM) and water reducing admixtures
Concrete can be classified based on its strength as follows:
Conventional concrete: strength < 50 Mpa and W/C > 0.45
High strength concrete: 50 - 100 Mpa and 0.3< W/C <
0.45
Very high strength concrete: 100 - 150 Mpa and W/C
between 0.25 – 0.30
Ultra High strength concrete: strength >150 Mpa and W/C
is less than 0.25
High Strength Concrete
High strength concrete, mixed with slower reacting
supplementary cementing materials, gains a considerable
amount of strength after the standard 28 day period. Therefore
the design strength be met at 56 or 91 days of age rather than
at 28 days.
Materials, production, and placing high strength concrete must
be controlled very closely.
Mixing, Placing & Curing
There are three type of mixing procedures depending on the
type of plant and trucks are:
Central mixed: mixed in a stationary mixer at the plant and
delivered to the site in a truck with a rotating drum.
Shrink mixed: concrete is partially mixed at the plant, with
mixing completed in the truck.
Truck mixed: concrete is mixed completely in the truck,
after it has been loaded at the plant.
Mixing, Placing & Curing
Specifications for mixing:
Mixing require 70 -100 revolutions of the drum at a
mixing speed of 6 – 18 rpm for most trucks, followed by
agitating at a lower rate (2 – 6 rpm) until the concrete is
placed.
Minimum mixing time is one minutes for the first cubic
meter, and 20 seconds for each additional cubic meter.
Concrete must discharged from ready mixed trucks within
2 hours or 300 drum revolutions (whichever comes first)
according to ATSM requirement.
Mixing, Placing & Curing
Placing Concrete:
Concrete should poured closer to its final position and not
allowed to fall freely for too great distance to avoid
segregation.
Buckets, pumps, and belt conveyors are used to move the
concrete to forms
In forms, the concrete should be deposited in layers 200 –
500 mm thick
Vibration is required to consolidate the concrete and fill all
voids.
Mixing, Placing & Curing
Placing Concrete:
Over vibration (more than 15 seconds) can cause
segregation or reduce the amount of air entrained in the
concrete.
Mixing, Placing & Curing
Curing: is one of the most important aspects of concrete
construction.
Poor curing and addition extra water contributes to poor
quality concrete.
Suitable curing requires water and favorable temperature.
Concrete will gain strength indefinitely (although at slower
rate) if kept moist.
Excessive evaporation of water = surface shrinkage =
cracks.
Low temperature slow the rate of hydration
Mixing, Placing & Curing
Methods of curing used to ensure the moisture is present are:
Ponding
Sprinkling or fogging
Wet covering (burlap)
Water proof paper
Plastic film
Curing compounds that form a membrane when sprayed
on the surface
Steam curing
Mixing, Placing & Curing
Ponding and continuous sprinkling are fairly expensive
methods and are used mainly in small projects.
Wet covering are effective but must be kept continuously
damp
Water proof paper consist of two sheets of paper with an
asphalt adhesive, or plastic sheets are effective in preventing
evaporation of water.
Curing compounds sprayed on the surface are the most
common methods used for curing pavement slabs. A
membrane forming compounds prevents evaporation if the
surface is covered completely with sufficient quantity.
Mixing, Placing & Curing
The duration of the curing period varies according to the type
of project and type of concrete.
A period of 7 days is often specified
The use of accelerator reduce the time required for curing.
The use of high early strength cement (Type III) allow the
period of curing to reduced to three days or less. This concrete
will usually reach over 80% of design strength in three days.
Joints
Joints must be used in concrete construction to prevent the
concrete from cracking.
Contraction (control) joints are placed in slabs to control
random cracking. To allow for drying shrinkage.
It should be spaced at intervals of not over 30 time’s
the slab’s thickness in both directions.
Joints
Controlled of shrinkage cracking:
Unreinforced concrete: contraction joints @ 4-7 m spacing
Lightly reinforced concrete, contraction joints @ 12-30 m
spacing
Heavily reinforced concrete, no contraction joints
Joints
Isolation/Expansion Joints: Isolation joints are designed to
separate slabs from such structures as column bases and walls,
etc.
For example, if a slab were placed between two buildings, an
expansion joint should be placed adjacent to the face of at
least one of the buildings
Joints
Construction joints are stopping places in the process of
construction. Are usually located at the end of one day’s pour
or between lanes in pavement slabs. They are designed to
allow load transfer with dowels.
Special Precautions
Special precautions and construction practices are required
during very hot or cold weather.
High air temperature + high concrete temperature +low
humidity + high wind velocity = may cause surface cracking
During hot weather , ice may be used to improve conditions of
placing.
Cold weather may require heating of materials
ASTM require for minimum temperature of the concrete at
placing is 13 oC for sections up to 0.3 m thick. And 10 oC for
sections (0.3 – 0.9 m) thick
Concrete Pavement construction
Rigid pavement
Portland cement used for highway pavements
Slump < 3 cm
Whitetopping: refer to the growing practice of using concrete
overlays over failing asphalt surfaces.
Inspection & Quality Control
The quality of concrete and construction operations should be
monitored to ensure that final properties are as specified.
The plant inspector is responsible for ensuring that the
materials and mixing procedures comply with the
specifications and the mix design.
Specifications usually require that the material be measured in
batches, within the following degree of accuracy:
Cement (± 1%), Water (± 2%), Aggregate (± 1%),
Admixtures (± 3%),
Inspection & Quality Control
The quality of concrete and construction operations should be
monitored to ensure that final properties are as specified.
The plant inspector is responsible for ensuring that the
materials and mixing procedures comply with the
specifications and the mix design.
Specifications usually require that the material be measured in
batches, within the following degree of accuracy:
Cement (± 1%), Water (± 2%), Aggregate (± 1%),
Admixtures (± 3%),
Inspection & Quality Control
Strength test (ASTM standard) for ready mix concrete:
Specification requires one strength test, consisting of two
cylinders, for each 100 m3 of concrete produced. With a
minimum of one test for each class of material each day.
The strength is the average of the test results from the two
cylinders
For strength in structures designed using Ultimate strength
method, number of tested samples that have values less
than required strength should be less than 10% of tested
samples & the average of any three consecutive tests
should be equal or greater than specified strength.
Inspection & Quality Control
Strength test (ASTM standard) for ready mixed concrete:
For Other concrete: number of tested samples that have
values less than required strength should be less than 20%
of tested samples & the average of any six consecutive
tests must be as specified strength.
Inspection & Quality Control
The standard deviation value is used in quality control and
designating a design strength to produce concrete that will
adequately meet the specified strength it is calculated as
follows:
Where:
σ : is the standard deviation
X: is the test strength
X: is the mean strength
n: is the number of tests.
Inspection & Quality Control
Example: the results of 12 strength tests are 23.8, 24.3, 20.7,
26.2, 24.1, 23.4, 26.8, 22.7, 19.4, 23.4, 21.5, and 19.5 Mpa.
Find standard deviation
Average = 275.8 / 12 = 22.98 Mpa
= 2.36 Mpa
Values of standard deviation up
to 3.5 Mpa are considered very good
Inspection & Quality Control
Quality of hardened concrete
Core test: destructive test
Rebound hammer: non destructive test
Special Concretes
Portland-pozzolan cements: use pozzolanic materials as
admixtures that contains silica to reacts with calcium
hydroxide to form compounds with cementitious properties
(such as fly ash)
Advantages:
Increase workability
Used to replace up to 35% of the Portland cement
without serious strength or other problems.
Fly ash and ground slag are used with cement type I to
produce cement with low heat or sulfate resistance. Replacing
type (II, IV, or V) at more economical cost
Special Concretes
Light weight concrete can be divided into two categories :
Structural light weight concrete have densities of 1400 –
1850 kg/m3 and strength that are usually lower than normal
practice
Insulating concrete: have low strength with densities as
low as 240 kg/m3
Lightweight aggregate, expanded shale and certain types of
volcanic rock are used.
Special Concretes
Heavy weight concretes are used for shielding purposes in the
construction of nuclear reactors at power plants
Heavy aggregate: Iron-ore particles are used
Density as high as 3200 kg/m3
Architectural concrete usually produced in precast plants
Forms that produce special textures and rough surfaces are
used
Special Concretes
Fiber reinforced concretes contains various types of fibers –
steel, glass, added during the mixing operations.
Fibers can assist in preventing surface cracks and improve
toughness and flexural strength in concrete. Also increase
tensile strength of concrete
Rollcrete: Used in dam construction with lower w/c ratio and
can be placed using large economical earth moving equipment
Concrete is compacted by rollers resulting in much higher
strength than could be obtained with a similar mix without this
type of compaction
Special Concretes
Unshrinkable fill concretes used to solve depression
problems occurred after some years of trenching conducted in
urban and rural roads.
This backfill material don’t require compaction and is strong
enough to resist all settlement stresses, and can be easily
excavated in the future if necessary.
Special Concretes
Polymer concretes contains organic materials that combine
and grow into polymers , filling small pore space in concrete.
These concretes have very high strength & remarkable
durability
Polymers has been used to protect corrosion of reinforcing
steel.
Special Concretes
Alumina Cement: is manufactured in the same manner as
Portland cement.
The main product is Calcium aluminates rather than calcium
silicate
This cement hydrate rapidly reaching high strength in one day
or less
Used in many construction or repair situation where time is
important and in very cold climates
Alumina cement is more resistant to most types of chemical
attack than ordinary Portland cement