Concrete and construction techenolohy

Cement:

Cement is a finely pulversied material which by itself is not a binder but develpos the binding property as a result of hydeation. (chemical reaction between cement, minerals and water)

A cement is called a hydraulic when the hyderation products are stable in an aqueous environment. (hyderation cement is a portland cement)

The most commonly used hydeaulic cement formaking concrete is portland cement which consist essentially of a hydeaulic calcium silicates.

Calcium silicate hydrates formed on the hyderation of portland cement are primarily responciable for its adhesive characteristics and are stable in aqueous environment.

Concrete as a mixture of hydraulic cement, aggregrate and water.

Admixture : materials other than aggrregates cement water which added to the concrete batch immediately before mixing.

Chemical admixture can modify the setting and hardening characteristics of the cement paste.

Water reducing admixture can plasticize fresh concrete mixture by reducing the surface tension of water.

Air entraining admixture can improve the durability of water exposed to cold weather.

Mineral admixtures such as pozzolans can reduce thermal mash cracking in mass concrete.

High strength cement

cement having much higher strength than ordinary portland cement.

Cement used in pre-stressed concrete and air field work and used in high strength concrete.

Concrete based on unit weight it is classified as,

1. Normal weight concrete(stractural purpose -2400 kg/m3 )2. Light weight concrete (1800kg/m3)3. Heavy weight concrete (3200kg/m3)

Concrete base on compressive strength

1. Low sterngth concrete(<20mpa)2. Moderate strength concrete(20-40mpa)3. Hight strength concrete(>40mpa)

Highly active puzzling (rice busk ash and condensed silica fumes) are capable of producing high strength concrete at both early & late stages.

Use of mineral admixtures increases the strength of concrete at both early & later stages.

Use of flash in concrete can produce compressive strength of about 59mpa to 56 days.

It is used to structural purpose.

Test for concrete

Fresh concrete or plastic concrete is a freshly mixed material which can be measured to any shape.

SLUMP TEST

It is the most commonly used method of measuring the consistence of concrete.

Mould should be placed on a smooth horizontal rigid and non-absorbent surface.

Mould is filled in four layer (1/4) Th of the high of mould.

Each layer is tamed 25 times by tamping lord.

Mould is removed from the concrete immediately by raising it slowly & carefully in a vertical direction.

Concrete will subside and subsidence is refueled as slump of concrete.

Difference in level between the high of the mould and that of highest point of the subsided concrete is measured.

Pattern of slumps shows the characteristics of concrete in addition to the slump value.

Concrete slumps evenly it is called true slump.

Shear slump indicate concrete is non-cohesive & segregation.

Very low : 0-25mm

Low : 25-50mm

Medium : 50-100mm

High : 100-175mm

COMPACTING FACTOR TEST

The degree of compacting called the compacting factor is measured by the density ratio.

Concrete to be tested is placed in the rapper hopper of the brim.

Trap door is opened so that the concrete falls into the lower hopper.

Trap door of the conical hopper is opened and concrete is allowed to fall into the cylinder.

Compacting factor = height of partially concrete / weight of fully compacted concrete.

FLOW TABLE TEST

Flow test which gives an indication of the quality of concrete with respect to consistency, cohesiveness and the proneness to segregation.

Spreader or flow of the concrete is measured.

Flow is related to workability

Mould is kept of the centre of the table filled in two layers.

Each layer is lidded 25 times with a tapping lord 1.6 cm in φ and 61 cm long rounded at lower tamping.

Mould is lifted up vertically concrete stand on its own support.

Then table is raised and dropped 12.5mm 15 times in about 15 sec.

Diameter of the spread is measured in dimensions and average is noted

Flow % =

VEE BEE TEST

It consist of a vibrating table, a cylindrical pan, a slump cone and a glass or plastic disk attached to the free moving red.

Cone is placed in the pan filled with concrete and removed.

Disc is brought into position on the top of concrete cone.

Vibrating table is set in motion.

Time required removing the concrete from the conical to cylindrical shape until the disk is completely covered with the concrete.

Measure of consistency is reported as the no of vee bee seconds.

Manufacturing of cement:-

Lime stone

Raw material

Clay

Dry – modern tech

1. Mixing

Wet – modern tech

2. Burning3. Grinding4. Paving and distribution

NON – DESTRUCTIVE TESTING:

Estimate strength

Durability

Elastic parameters

Hardness

Resistance to penetration

Rebound number

Resonant frequency

Electrical property

VARIOUS TESTS

Rebound test – Strength

Pull out techniques – strength

Surface hardness method – strength

Dynamic and vibration method – elastic property

Radioactive and nuclear method

Magnetic& electric method

SURFACE HARDNESS TEST

REBOUND HAMMER

Distance travelled by rebound number.

LIMITATIONS

Smoothness of surface

Size & shape

Age

Surface & internal moist content

Course aggregate

Cement

Mould

Carbonation of conc. Surface

PENETRATION TECHNIQUE

Hammer – samba affected by coarse aggregate arraignment

Spit pins

25 % accuracy

Windsor probe – gum herded alloy probes lauded cartridges

28 days

Comp. strength

Exposed probe length

PULL OUT TECHINIQUES

Force – pull out the concern

More – stronger the concrete

91 days

Comp strength

Pull out strength

DYNAMIC AND VIBRATION METHOD

Resonant frequency – maxi amplitude monometer

Pulse velocity

Longitudinal

Mechanical sonic pulse

Transverse / longitudinal

Transverse

Ultrasonic pulse velocity

Surface waves

Relationship between velocities of sound

Through specimen resonant modules of elasticity

Frequency

Transducers

Receive

Detritions of concrete

Determination of time of removed of formwork

RADIOACTIVE METHOD

Incident gamma rays

Emerging and rays

Density of structural member

NUCLEAR METHOD

Neutron scattering concrete content

Neutron activation

MAGNETIC METHODS:

Cover meter – Reinforcement position

PULSE VELOCITY MEASUREMENT

Direct transmission

Indirect transmission

Surface transmission

Affected by:

Smoothness

Temperature

Reinforcing steel

Moisture content of cone

Uniformity

Strength of cone

Deformation of conc.

Time of removal and form work

ULTRASONIC PULSE VELOCITY METHOD

Pulse velocity is a measurement of transits time of an ultrasonic pulse between a transmitter and a receiver.

If the distance between the transmitter and the receiver is known the velocity of the pulse can be determined.

To test concrete, contact between the concrete and the transmitter and receiver is made with a coupling agent such as a petroleum jelly.

Velocity of sound waves through the concrete is reduced by the presence of voids or cracks.

Non destructive elevation that been filled with epoxy. - In this case, the transmitter and receiver are fixed at a given distance apart and assembly is placed perpendicular to the crack.

Where plus velocity measurements are taken.

Reading of repair crack is compiled to the un cracked section.

A property repaired crack will displaced a transit velocity equal to that of the un cracked section.

PULL OFF TESTING

It measures a bond between two layers.

There are 3 types of possible tensile failure

Failure in the substructure layer

Separation at the interface between the substrate the surface layer.

Failure in the surface layer.

Testing device can also record the force required to cause failure which divide by the surface area of the specimen, will result in the tensile strength.

It is the greatly influenced by aggregate size, alignment of the device to the surface.

REBOUND AND PENETRATION METHODS:

Rebound and penetration method are used to measure the surface hardness of concrete.

Surface hardness is proportional to concrete compressive strength of concrete.

Rebound hammer methods utilize a spring loaded plunger that impact the surface – Causing the mechanism to rebound.

Rebound is measured and compared to the initial extension of the spring, yield a rebound number.

Rebound can be affect by the angle of test, surface smoothness, type of aggregates, carbonation of concrete and moisture content.

Penetrate methods is similar to rebound methods but use more powerful energy to device a probe into the concrete surface.

NON – DESTRUCTIVE TESTING:

It provides relevant means for periodic inspections in order to maintain the quality of constructed facilities.

Non – destructive techniques which are less time consuming and relatively inexpensive can be useful for the following purpose.

Test on actual structures later than on companion samples.

Test at several locations.

Test at various ages.

Quality control of actual structure.

Assessment of uniformity of concrete.

Location and assessment of the extent of cracks, voids and honey combs.

OBJECTIVES:

To obtain an estimate of properties of materials by measuring certain qualities which are empirically related to it.

Following test:

Rebound hammer

(1) When the plunger of rebound hammer is pressed against the surface of concrete.(2) A spring controlled mass, rebounds and the extent of rebound depends upon the

surface hardness of the concrete.(3) Rebound distance is measured on a graduated scale and designed as rebound no.(4) Distance depends on the value of K.E in the hammer.(5) Pact of the energy is absorbed by concrete which enables one to use the rebound

number as an indicator of concrete properties.(6) Energy absorbed by the concrete depends up on the stress and strain relationship of the

concrete.

(7) Low strength, low stiffness concrete will absorb more energy.(8) High strength, high stiffness concrete will absorb less energy. The various factors which

affect the rebound number are (a) Type of cement(b) Type of causes aggregate(c) Surface and internal moisture condition,(d) Causing and age of test specimens.(e) Surface texture(f) Orientation of instrument

Advantages:

Simple in operation speedy and inexpensive.

Applications:

Determination of concrete uniformity

Strength development monitoring

Assessment testing in precast concrete industry.

Accessing the relative quality structural members.

PULL OFF TEST:

In this method a circular steel disc, glued to the surface of the concrete with a poly-ester resin adhesives.

Force required to pull this from the surface together with an attached layer of concrete is measured.

A simple mechanical hand operator loading equipment is used.

A 50mm ф disc is generally used

It has good relation between the compressive strength & pull off strength.

ADVANTAGE & LIMITATIONS

Directly measures a strength related property.

Requires only one exposed surface

Suitable for small sections

APPLICATIONS

Used in quality control long term moistening and in site strength assessment.

BREAK OFF TEST

The break off test determines directly the flexural tensile strength in plane parallel to the concrete surface at a specified distance below the surface.

It also crenellates with compressive strength.

Used on both fresh & hardened concrete.

A hand operated pump is used to supply the hydraulic fluid to the gawk which applied a horizontal force to top of the core.

Reaction to horizontal force is provided by a ring which breaks against the counter bore.

ADVANTAGE & APPLICATION

It is handy, easy to operates results are little affected by local shrinkage & temperature forces

Used in quality control of concrete pavement and estimation of in place compressive strength

Used to evaluate bond strength between concrete & over lay materials.

ULTRASONIC PULSE VELOCITY

Determines the propagation velocity of the pulse of vibration energy through a concrete member.

Transducer contact with the surface of the concrete is made with Greece or petroleum jelly to avoid any entrapped air.

Time of travel between the initial onset and reception of the pulse is measured in different ways electronically.

Spreading a thin layer of jelly on the smooth surface of the two probes and coupling them with a slight pressure.

FACTORS AFFECTING THE PULSE VELOCITY

Surface condition & moisture content

Temperature of concrete

Micro cracks in concrete

Age of concrete

Presence of steel reinforcement

Aggregate type, content & size.

PULSE – ECHO METHOD

It is used with success in detecting very large structural cracks in concrete dams, piles, caissons, piers.To locate honey combed concrete through metal ducts.

The method is based on the principle that internal defects and boundaries of the objects will reflect incident stress waves and the reflected waves will travel back to the surface

At transient stress pull is introduced into a test object by a point impact.

Opposite face get reflected & propagate back in test object.

GRADES OF ORDINARY PORTLAND CEMENT

The bureau of Indian standards as classified ordinary Portland cement into 3 grades for producing different grades of concrete to meet the demands of the construction industry. The classification is made on the basis of compressive strength at 28 days as

35 grades – ordinary P C—IS 269:1989

43 grades - ordinary P C –IS 8112:1989

53 grade – ordinary P C –IS 12269:1987

The grade indicates compressive strength of the cement in N/mm2 at 28 days. Since, higher grades of concrete necessitate the use of higher strength of cement at 28 days. Use of 33 grade cement has drop during the last decade. Both 43 & 53 grade cement can be used for producing higher grades of concrete.

LOW ALKALI PORTLAND CEMENT

There is no separate standard for low alkali Portland cement but in regions where the aggregate have reactive silica or carbonates. Alkalis in the cement are restricted to 0.8% of soda equivalent (Na2o +0.658 K2o); with a view to avoid the occurrence of alkali aggregate reaction leading to possible cracking and destruction of concrete in such cases, instead of low alkali Portland cement can be used with added advantage.

BULKING OF SAND

Sand as delivered sometimes contains moisture which causes an elm of water on the surface of the particles fluffing their apart. This is called Bulking. This will have to be taken into account while batching the mix.

CHEMICAL ADMIXTURES

Chemical admixtures are sometimes called the fifth ingredient of concrete other than cement, coarse, and fine aggregate &water. they are inorganic (or)organic materials solid (or)liquid which when added to the normal components of mix (either concrete (or)mortar(or)paste).interact with the cementations system through chemical, physical(or)physic-chemical means modifying one (or)more properties of the mix in the fresh, setting ,hardening (or)hardened state. a number of advantages can be delved the use of admixtures, eg; in the fresh state of concrete depending on the type of admixture used they can increase the workability, without increasing the water content reduce (or)prevent settlement ,modify the rate and capacity of bleeding reduce segregation & reduce slump loss created (or)accelerate the time of initial (or)final setting. Aside from altering the properties of the fresh mix. They can reduce hat evolution during early hardening accelerate the rate of strength development at early ages increase the compressive strength of the concrete .improve durability, control alkali aggregate reactivity, produce aerated concrete improve bond between old &new concrete, inhibit corrosion of reinforcement

produce colored concrete or mortar etc…chemical admixtures can lie classified according to the type of materials consisting them.

Commonly used admixtures are,

Water reducing or pasteurizing admixtures. Set controlling admixtures or retarders Air entraining admixtures Accelerating admixtures High range water reducing or super plasticizing admixture.

Besides the above other type of admixtures are also used .they include, Grouting admixtures Pumping acids. Bounding admixtures. Expansion producing admixtures. Functional admixtures. Germicidal admixtures Insecticidal admixtures etc…

Commercially available admixtures may contain materials that separately belong to one or more groups eg; water reducing admixture &soon.

The effectiveness of an admixture depended upon such factor as type, brand amount of cement, water, aggregate shape, grading proportion, mixing time, slump temperature of concrete& air.

The physical requirements for the main type admixtures s given in IS9103:1999.

Trial mix should be made with the admixture and the job material at yet temperature & humidity anticipated on the job. In this way the compatibility of the admixture with other job materials ;as well as the effect of admixture of the properties of the fresh & hardened concrete ;can be observed .the amount of admixture recommended by the manufacture or the optimum amount determined by laboratory test should be used .

Since ,admixtures are generally supplied in bulk it is necessary to carry out uniformity test .all the lot of admixtures as per the requirements of IS9103;1999.Despite the verity and combination of admixture used, it should be borne in mind that no admixture at any time or amount can be considered as a sub title for good concreting practice.

DURABILITY;

Durable concrete can be defined as i.e. designed constructed and maintained to perform satisfactoricty in the expected environment for the specified life of the structure without undue maintained .the materials and mix proportions chosen should be such as to maintain the integrity of the concrete and to protect the embedded reinforcement.

IS 456:2000 IDENTIFIES VARIOUS FACTORS INFLUENCING DURABILITY AS.

1. Environment2. Cover to the embedded steel-type & quality of constituent

MATERIALS

Cement content & water cement ratio.

Workmanship to obtain full combination and efficient curing

Shape & size of members

CONCRETE MIX DESIGN

Different ingredients of concrete need to lie mixed in appropriate proportions during the production of concrete. This can be done either by volume or by weight the latter being more precise & scientific. It is essential that concrete mixes we ‘designed’ for a particular set of given ingredients to produce specific properties of concrete in the most economical way. This is a ‘must’ for large scale where quantity is an important yardstick.

Rational proportioning of the ingredients of concrete generally referred as mix proportioning or mix designing. It is a process by which one can arrive at the right combination of cement, aggregate, water & admixtures for producing concrete to satisfy given specifications. The purpose of mix proportioning into obtain a product that will perform to certain predetermined requirements. The objective of mix design is to ensure that the concrete.

(1) Compiles with the compressive strength as laid down in the specifications.(2) Conforms to the specified durability requirements to resist the environment in which the structure

will be serviceable during its design life.(3) It has adequate workability.(4) It is capable of being mixed transported laid down & compacted as efficiently as possible.(5) As economical as possible to achieve an optimum mix proportion to fulfill the above parameters it is

a challenging risk. The work of mix designing is a trial & error exercise which needed to be carryout by an experienced person in a laboratory. The concrete mix needs to be designed to produce the grade of concrete having characteristics strength not less than the appropriate values. The mix also needs to be designed for adequate workability. So that it could be being mixed, transported laid down and compacted as efficiently as possible. Depending upon, the placing conditions I.S.456 as recommended different ranges of workability. In addition the concrete has to satisfy the durability requirements namely the minimum cement content, maximum water content ratio & minimum grade of concrete as specified by IS 456.It will be noted that the code has specified the minimum grade of concrete to be not less than M20 for reinforcement concrete constructions to achieve the responsibility.

The responsibility for carrying out the mix design rest with the contractor or expert agency employed by the contractor. The mix so designed needs to be approved by the employers or client. The contractor or expert agency can use any mix design method in designing the mix. It would be appropriate to obtain the standard deviation based on the test strength of the samples at a project site. In case such test results are not available, assumed values of std. Deviation may be used for design of the mix in the first instants as soon as the results of the sample are available actual calculated standard deviation shall be used & the mix design properly.