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