Elements of lateral load resisting masonry system are:

• Shear wall: A structural panel that can resist lateral force acting on it.
• Gable band: Building having slopped roof i.e. truss construction.
• Roof band: Used in building with roof made of flat timber or CGI sheet.
• Lintel band: Horizontal member placed at top of the opening like door and window to support unsupported wall above it.
• Sill band: Horizontal member placed at bottom of the opening to support load of opening frame.
• Tie beam: Horizontal beam that ties two structural member from separating.

1.2 In – plane and out of plane behavior of masonry wall

• Inplane and out of plane behavior of masonry wall are also called the failure mechanism of walls.

In – plane failure:

• Occurs when masonry element get pushed in the direction of the plane of wall.

Modes:

a. Sliding shear:

• Occurs when structure has low shear resistance than flexural strength.

b. Shear failure:

• An internal splitting force caused by two force acting in opposite direction.

c. Bending failure:

• Occur when tensile strength is equal or greater than ultimate strength of structure.

Out plane failure:

• Occurs when masonry element get pushed in the direction perpendicular to the wall.

Mode:

a. Topping failure:

• Forward rotation or movement of mass of structure due to vertical cracks.

b. Over turning failure:

• Occurs when movement equilibrium is not satisfied.

1.3 Failure behavior of masonry wall in lateral loads

• During lateral loading some portion of masonry walls undergo in tension and ultimately fails at tension zone.
• The failure modes depends upon the type of construction, amount and size of opening.
• The failure in unreinforced masonry building is due to structural defects such as lack of proper joint and large unsupported wall length.

The various failure in masonry structure:

1. Diagonal shear cracks.
2. Horizontal shear cracks.
3. Bending cracks at lintels and feet.
4. Bending cracks at corners.
5. Bending crack at gable.
6. Peeling of plaster.

1.4 Analysis for stresses on masonry wall under lateral loads

• When wall is subjected to lateral load such as wind, earthquake etc. bending will occur depending on support condition.

Consider a wall subjected to uniformly distributed vertical load P at centre line in one axis as shown in figure. The force is caused by self weight of wall, other external loading and applied uniform compressive stress across the section.

Compressive stress due to load P = P/A ———(i)

Again,

Consider lateral load ‘w’ KN/m² applied to the section of wall and corresponding stress is given by

     σ_b / y = M / I

or,  σ_b = M*y / I

or,  σ_b = M / Z [∴ I/y = Z]

where,

σ_b = Bending stress

M = Maximum bending moment

I = M.O.I about length of wall

Z = Section modulus

y = Distance of considered layer from NA

1.5 Ductile behavior of reinforced and unreinforced masonry structure

• Capacity of an element of structure to undergo large deformation without failure.
• Masonry is brittle in nature.
• Unreinforced masonry cannot withstand tension so cracks develops.
• Inplane and out plane failure is also due to ductility of masonry.
• To improve ductility reinforcing bars are embedded in the masonry to resist seismic force.

1.6 Compressive strength of masonry unit and masonry walls

Compressive strength of masonry units:

Apparatus required: Universal testing machine

Specimen: At least three number of masonry unit

Procedure:

• Place the specimen in flat face between the plates of testing machine.
• Applying axial load at a uniform rate of 140 kg/cm² per minute till failure occurs.
• Maximum failure is noted.

σ_c = Maximum load of failure/ Average contact area

Compressive strength of masonry walls:

Apparatus required: Universal testing machine

Specimen: At least three number of brick with certain bond type

Procedure:

• Place the specimen in flat face between the plates of testing machine.
• Applying axial load at a uniform rate of 140 kg/cm² per minute till failure occurs.
• Maximum failure is noted.

σ_c = Maximum load of failure/ Average contact area

1.7 Diagonal shear test

• In load bearing masonry wall shear wall carry vertical load and resist lateral load when combined loading creates tensile stress the tensile strength of masonry is exceeded leading tensile cracking.
• To evaluate masonry shear strength and shear elastic modulus diagonal test is performed on rectangular masonry specimen.
• In diagonal shear test a 1.2 * 1.2 m square section of wall is placed between the compressor to determine shear strength and shear modulus.
• Diagonal shear stress can be calculated by

τ = P / A * 2^1/2

where,

P = Applied load

A = Average cross section area of specimen

1.8 Non – destructive test

1. Elastic wave tomography:

• Used for locating shallow cracks and voids.
• Based on the principle of heat transfer i.e. conduction and radiation.
• Allows rapid mapping of internal condition.
• Material with no voids, gaps or cracks are more thermally conductive than material with moisture content.

2. Flat – jack test:

• Used to analyze and evaluate existing masonry structure.
• Uses small thin hydraulic jack to apply a force to a section of an existing masonry wall and measuring device determine the resulting displacement of masonry.
• Measure shear strength of structure.
• Measure compressive strength.

Types of flat jack test:

a. Single flat test or insitu shear test

b. Double flat jack test or insitu deformability test

3. Push shear test:

• Firstly, the brick, mortar are removed from test location. Also joint on opposite side also removed.
• Mortar joint above and below are not disturbed.
• Now, hydraulic ram is inserted along with pressure gauge to measure actual strength.
• A block is placed in order to distribute load uniformly and loading is done till cracking or movement is observed.
• Force at cracking or movement is noted to develop force deflection plot.

References:

• Dayaratnam, P. Brick and reinforced brick structure.
• Neville, A.M. Properties of Concrete. England: Pearson Education Limited.
• Hendry, A.W., Sinha, B.P. & Davies, S.R. Design of Masonry Structure. London: E & FN Spon.

The post Masonry Walls Under Lateral Loads and Testing of Masonry Elements appeared first on OnlineEngineeringNotes.

• A masonry structure is one made by joining (bonding) multiple number of building blocks like brick, stone, block etc. usually with cohesive material called mortar. The mortar may be mud mortar, lime mortar, cement mortar etc.

Merits:

• They are non – combustible so improves fine protective of building.
• Environment friendly.
• Economic.
• Use of locally available material.
• Most of the wall does not require painting.

Demerits:

• Extreme weather causes degradation of masonry wall.
• Low tensile strength.
• Problem in large opening.
• Due to heavy weight large foundation is required. Also cracking and settlement may occur.

Types of masonry unit:

1. Brick masonry:

• Highly durable form of construction.
• Build by placing bricks in mortar in a systematic manner to construct solid mass which can withstand external load.

2. Stone masonry:

• Use of stone and mortar.
• Used for building foundation, floor and retaining wall etc.

3. Concrete block:

• Hollow building unit of concrete.

4. Glass block:

• Fixed together to form complete wall by several method.

5. AAC (Autoclaved aerated concrete) block:

• Light weight building block ensure better durability and improves sound and heat insulation.

Types of brick wall:

a. Stretcher bond:

• Formed when stretcher of bricks are laid along the face of wall.
• Used in partion wall.

b. Header bond:

• Formed when head of bricks are laid along the face of wall.
• ¾ bat is provided in each alternative course in corner.
• Unsuitable for load bearing structure.

c. English bond:

• Consists of alternative course of headers and stretchers.
• Queen closer after first header in each heading course.

d. Flemish bond:

• Each course is comprised of alternative headers and stretchers.
• Every alternative course starts with header at corner.

e. Zigzag  bond:

• Laid in zigzag fashion.
• Used in brick flooring.

1.2 Types of masonry structure

1. Load bearing and non – load bearing masonry.

Load bearing masonry:

• Oldest and mostly used technique.
• Every wall elements carries load to the foundation which is transferred to the ground.
• Mainly used to construct residential building.

Merits:

• High fire resistant.
• Fabrication is not required.
• Aesthetically attractive.

Demerits:

• Performs bad during earthquake.
• More manpower required during construction.
• Construction is slow.

Types of load bearing masonry:

• Precast concrete wall
• Retaining wall
• Brick wall
• Stone wall

Non – load bearing masonry:

• Walls carry only their own weight.
• Used for partion wall.
• Built lighter to reduce the dead  load of structure.
• Can easily be removed without endangering the safety of the building.

Types of non – load bearing masonry:

• Hollow concrete brick wall
• Façade brick wall
• Hollow brick wall
• Brick wall

2. Reinforced and unreinforced masonry

Reinforced masonry:

• A construction system where steel bars, reinforced concrete bands, bamboo, timber etc are embedded in the mortar or placed in the holes and filled with concrete.
• Increases tensile as well as compressive strength.

Classification:

a. Reinforced hollow unit masonry:

• Vertical reinforcement is inserted inside brick or block and filled with concrete or grout.

b. Reinforced grouted cavity masonry:

• Consist of two leaves of masonry units separated by the cavity into which the vertical and horizontal reinforcement is placed and grouted with either concrete infill or grout.

c. Reinforced packet type wall:

• Pockets containing vertical bars are filled with grout.

Unreinforced masonry:

• Reinforcing bars is not placed.
• Likely to be damaged during earthquake.
• Mortar holding masonry is weak.

1.3 Properties and strength of cement mortar

• Cement mortar is a homogenous mixture of sand, cement and water in suitable proportion.

Properties:

• Should be easily workable.
• Should be durable.
• Should not affect durability of other materials.
• Should retain mixed water for longer period of time.
• Should have sufficient strength to handle the load of structure.

Strength:

• Compressive strength of cement mortar after curing 28 days is 33 N/mm² to 53 N/mm².

1.4 Introduction to codal provision (NBC 109) and guideline (NBC 202)

• In the past there was no code for designing masonry construction and at that time design was based on thumb rule. The thickness of wall were found to be very big and uneconomical.

Code design of masonry are:

• IS 1905: Code for practice for structural use of unreinforced masonry.
• NBC 202:2015: Guideline on load bearing masonry.
• NBC 109:1994: Code for use of unreinforced masonry.

1.5 Bonding elements in masonry

• Bonding is the systematic arrangement of bricks or other building units composing a wall or structure in such a way as to ensure stability and strength.

Types of bonding elements in masonry are:

a. Bond – stone:

• Large stone which passes through a wall from one side to the other to lock the layers of the wall together and reinforce the structure.

b. Bands:

• Provided to hold a masonry building as a single unit by typing all the wall together.

c. Dowels:

• Provided to transfer load without restricting horizontal joint movement due to temperature and moisture expansion and contraction in the concrete slab.

References:

• Dayaratnam, P. Brick and reinforced brick structure.
• Neville, A.M. Properties of Concrete. England: Pearson Education Limited.
• Hendry, A.W., Sinha, B.P. & Davies, S.R. Design of Masonry Structure. London: E & FN Spon.

The post Introduction to Masonry Structure and Design of Masonry Walls for Gravity Loads appeared first on OnlineEngineeringNotes.

1. Tensile strength:

• Due to brittle nature of concrete it is very weak in tension.
• Concrete develops cracks when subjected to tensile force. So it important to determine the tensile strength of concrete.

Tensile strength (F_ct) = 0.35 (F_ck)^1/2

Where,

F_ck = Characteristics compressive strength

2. Compressive strength:

• Concrete has high compressive strength.
• Most of the concrete structure are designed by adopting the value of compressive strength.

3. Shear strength:

• Occurs to the combination of tensile and compressive stress or due to the application of torsion.
• Shear strength of concrete is about 12% of compressive strength of concrete.

4. Bond strength:

• The resistance to slip of the steel reinforcement bars which are embedded in concrete.
• Higher the grade of concrete higher the bond strength.

1.2 Compressive, tensile and bond strength tests

Compressive strength tests:

1. Cube test:

• Specimen of 150 mm * 150 mm * 150 mm cube is used.
• The cube filled in three layers and well compacted.
• The top is made smooth and the finished surface is left undisturbed for 24 hours at room temperature.
• Then the mould is removed and the specimen is stored in water for curing usually up to 28 days.
• The testing of specimen is taken at 1,3,7,14,28,90 and 365 days.
• The test is performed by standard uniaxial compression test and at each time, minimum three sample are tested.

Compressive strength of concrete = Force at failure / Area

2. Cylinder test:

• The test is carried out using concrete cylinder of diameter 150 mm and 300mm height.
• The cylinder is filled in three layer and well compacted.
• Cube strength is higher than cylinder strength.

Tensile strength test:

• It is very complex to apply uniaxial tension to a concrete specimen so the tensile strength of concrete is determined by two test.

The test which are performed for tensile strength test are as follows:

1. Flexural test:

• Evaluates the tensile strength of concrete indirectly.
• Test the ability of unreinforced concrete beam or slab to withstand failure in bending.

2. Splitting test:

• Method of determining tensile strength of concrete using a cylinder which splits across vertical diameter.
• Concrete cylinder is placed horizontally.

Horizontal tensile strength (F_st) = 2P/ πLD

Where,

P = Applied load

D = Diameter of cylinder

L = Length of cylinder

Bond strength test:

• Bond test can be done by pullout test.

Pullout test:

• Destructive technique that involves drilling a hole into the hardened concrete and inserting oversized screw into that hole.

1.3 Non – destructive tests

• Test which are performed without destruction of the sample are known as non – destructive test.

Objective:

• To determine the strength of concrete.
• To determine cracks in concrete.
• To determine permeability of concrete.
• To determine the thickness of concrete layer.

Merits:

• Less wastage of material.
• Accident prevention.

Demerits:

• Expensive.
• Dangerous (Radiation hazards)

Method of non – destructive test are:

1. Rebound hammer test:

• Done to estimate the compressive strength of concrete.
• Also known as Schmidt hammer test.
• It is a device used to measure the elastic properties or strength of concrete.

Merits:

• Easy to carry out.
• Cheap.
• Skilled manpower not required.

Demerits:

• Interpretation of data is difficult.
• Only measure strength of concrete close to the surface.

2. Ultrasonic pulse velocity method:

• Used to determine the integrity and quality of concrete by measuring the speed and attenuation of ultrasonic wave passing through the element being tested.
• There are three ways of measuring pulse velocity through concrete. They are as follows:

a. Direct transmission

b. Indirect transmission

c. Surface transmission

1.4 Variability of concrete strength and acceptance criteria.

Variability of concrete strength:

• Strength concrete varies from batch to batch over a period of time.

Factors affecting variation of concrete strength:

• Quality of raw material.
• W/C ratio.
• Age of concrete.
• Compaction of concrete.
• Curing.

Acceptance criteria:

a. Compressive strength:

• The mean strength determined from any group of four non – overlapping consecutive test results, complied with the appropriate limits col 2 of table.
• Any individual test results complies with the appropriate limits in col 3 of table.

b. Flexural strength:

• Mean compressive strength of four consecutive test results must be more than equal to characteristic strength of concrete +3 N/mm².
• Any test results is not less than specified characteristic strength -0.3 N/mm².

1.5 Quality control and quality assurance

Quality control:

• Quality means degree of excellence.
• Good design is not sufficient but its actual implementation is required for fully quality work and good performance of structure.

Quality can be controlled by:

• Appropriate mixing.
• Proper compaction.
• Correct placing.
• Adequate curing.

Quality assurance:

• Systematic process of determining whether the concrete structure meets specified requirement is called quality assurance.

References:

• Dayaratnam, P. Brick and reinforced brick structure.
• Neville, A.M. Properties of Concrete. England: Pearson Education Limited.
• Hendry, A.W., Sinha, B.P. & Davies, S.R. Design of Masonry Structure. London: E & FN Spon.

The post Testing of concrete and quality control: Variability of concrete strength and acceptance criteria and Non – destructive tests appeared first on OnlineEngineeringNotes.

• Mixture made with light weight coarse aggregate such as shale, clay or silt giving low density.
• The density of light weight aggregate is less than 1200 kg/m³.

Merits:

• Minimize the dead load of the building.
• Simple to handle which decrease the cost of transportation.
• Enhances workability.
• Thermal conductivity decreases.
• Stronger and more durable.

Demerits:

• Sensitive to amount of water used in mix.
• Due to porosity and angular aggregate placing and finishing is difficult.

1.2 Aerated concrete

• Light weight concrete prepared by introducing large volume of air into the slurry of cement (with or without lime), fine sand and water.
• No coarse aggregate is used.
• Also known as gas/foam/ cellular concrete.

Merits:

• Low density of 800 kg/m³.
• Low thermal conductivity.
• Ensure sound proofing ten times better brick thickness.
• Fire resistant.
• Environmentally friendly.

Demerits:

• Production cost is high.
• Extra care during manufacture.
• Strength reduce in wet condition.

1.3 No – fines concrete

• Obtained by eliminating fine aggregate in mix concrete.
• There is no voids in coarse aggregate during making of no fines concrete.

Merits:

• Density is low.
• No seggregation is found.
• Shrinkage is lower than normal concrete.
• Formwork can be removed earlier.
• Gives better and attractive appearance.

Demerits:

• Low strength.
• Cannot be used in reinforced concrete structure.
• Consistency cannot be measured.

1.4 High density concrete

• Concrete with high density greater than 3360 kg/m³.
• Used for radiation shielding.

Merits:

• Easy to construct.
• Good chemical resistance.
• Cheap.
• Offers more strength.

Demerits:

• More space required.
• Weight of concrete is high.

1.5 Fiber reinforced concrete

• Produce by uniformly dispersing suitable type of discontinous and discrete fibre in mixture of concrete.

Merits:

• Improves impact strength of concrete.
• Resistance to plastic shrinkage.
• Reduce seggregation.

Demerits:

• Lack of tensile strength.
• Not environmentally friendly.

1.6 Self compacting concrete

• Ability to flow under its own weight to fill the required space or formwork completely and produce dense and homogenous material without need of vibrating compaction.

Merits:

• Faster construction and requires less manpower.
• No need to use vibrator.
• Economic.
• No seggregation and bleeding.

Demerits:

• Highly skilled worker required.
• More costly than other conventional concrete.

1.7 Shotcrete

• Concrete with small sized coarse aggregate is conveyed through a nose and pneumatically projected at high velocity onto a surface as a construction technique.
• Used in curved surface.
• Best for construction of tunnel, domes etc.

Merits:

• Reduce manpower.
• Economical.
• Easily control w/c ratio.
• Maintence cost is less.

Demerits:

• Material wastage high in compare to conventional concrete method.
• High skill manpower required.

1.8 Introduction to nominal mix and design mix

Nominal  mix:

• Nominal mix specifies the fix proportion of cement, sand and aggregate.
• Ingredient of concrete are mixed on the basis of volume.
• Used for M20 or less strength of concrete.
• Nominal mix, proportion and use are given below:

Design mix:

• Process to prepare concrete by testing all necessary properties.
• Used for M25 & above grade of cement.

1.9 Probabilistic concept in mix design approach

• The strength of concrete depends upon the quality of material and the constructional methods being used.
• Variability in quality or construction method cause variation in strength.
• The strength of concrete is determined through cube specimen varies with the size of the cube.
• The strength of specimen increases with decrease in size.
• If large number of cube strength test result are plotted on histrogram the result are found to follow bell shaped curve known as normal distribution curve.

From the curve,

Mean strength (F_m) = ΣX/N

Standard deviation (S) = [Σ(x – x̄)² / (n  – 1)]^1/2

Where,

N or n = Number of test of specimen

Characteristics strength:

• Defined as the level of strength below which a specified proportion of all valid test results is expected to fail.

Characteristics strength (F_ck)

F_ck = F_m – 1.65S

Coefficeint of variation (V):

V = (S/F_m) * 100

References:

• Dayaratnam, P. Brick and reinforced brick structure.
• Neville, A.M. Properties of Concrete. England: Pearson Education Limited.
• Hendry, A.W., Sinha, B.P. & Davies, S.R. Design of Masonry Structure. London: E & FN Spon.

The post Introduction to Special Type of Concretes and Mix Design of Concrete: Probabilistic concept in mix design approach appeared first on OnlineEngineeringNotes.

 Deformation of hardened concrete:

Following strains are responsible for the deformation of concrete:

a. Elastic strain:

• Instantaneous deformation that occurs when an external stress is first applied.

b. Shrinkage strain:

• Deformation that occurs due to the loss of moisture from concrete.

c. Creep strain:

• Deformation that occurs in the direction of force applied.

d. Thermal strain:

• Deformation due to change in temperature.

Modulus of elasticity:

• Defined as the slope of the relationship between stress and strain.
• It is a measure of stiffness or resistance to deformation of a material.
• Higher modulus indicates stiffer concrete with lesser deformation.

Types of modulus of elasticity:

1. Static modulus of elasticity:

• It is the value of modulus of elasticity determined by actual loading of concrete.

Types of static modulus of elasticity:

a. Initial tangent modulus:

• Given by the slope of a line drawn tangent to the stress- strain curve at the origin.

       b. Tangent modulus:

• Given by the slope of a line drawn tangent to the stress – strain curve at the origin.

       c. Secant modulus:

• Given by the slope of a secant joining origin and a fixed point on curve.

d. Chord modulus:

• Given by the slope of a line drawn between two point on the stress strain curve.

2. Dynamic modulus of elasticity:

• Elasticity of concrete under dynamic loads such as longitudinal and flexural vibration.

3. Flexural modulus of elasticity:

• Ability of a material to bend.

1.2 Creep, shrinkage and thermal expansion

Creep:

• Defined as gradual increase in strain (deformation) under sustained stress (or loads).
• Dependent on quality of cement paste and age of concrete.
• The amount of creep is expressed in terms of creep coefficient.

Creep coefficient (C_c) = Creep strain / Elastic strain

Factors affecting creep:

• Higher w/c ratio higher the creep.
• The stiffer the aggregate the smaller the creep.
• More aggregate fraction less is the creep.
• Increase in temperature leads to increase in creep.

Shrinkage:

• Reduction in the volume of a freshly hardened concrete under the condition of ambient temperature and humidity.
• It is caused by loss of water through evaporation or by hydration of cement.

Types:

1. Plastic shrinkage:

• Occurs due to the loss of water by evaporation from freshly prepared concrete while cement paste is plastic.
• At larger cement content of mix, plastic shrinkage of concrete is higher.

2. Drying shrinkage:

• Occurs due to the loss of water by evaporation from freshly hardened concrete exposed to air.

3. Autogenous shrinkage:

• Occurs due to loss of water by self –  desiccation of concrete during hydration.

4. Carbonate shrinkage:

• Occurs when the concrete is exposed to air containing carbon dioxide.

5. Thermal shrinkage:

• Occurs due to excessive fall in temperature.

Factors affecting shrinkage:

• Higher the w/c ratio larger the shrinkage.
• Low relative humidity more the shrinkage.
• The lighter the aggregate more the shrinkage.

Thermal expansion:

• Concrete expands on increasing the temperature.
• The coefficient of thermal expansion depends upon nature of cement, aggregate, relative humidity and size of concrete. In normal condition the expansion mainly depends upon aggregate.

1.3 Fatigue, impact and cyclic loading

Fatigue:

• Failure of a material by fracture when it is subjected to a cyclic stress.
• Fatigue is the result of repeated application of stress to the concrete.

Two types of failure in fatigue takes place in concrete are:

a. Failure occurs in sustained load (slowly increasing load)

b. Failure occurs under cyclic or repeated loading.

• For constant load of alternating stress, the fatigue strength decreases as the number of cycle increases.

Log N = (1 – F_max/F_c)/ β(1 – F_min/F_c)

Where,

F_max = Maximum stress in a cycle

F_min =  Minimum stress in a cycle

F_c = Strength under static loading

Β = 0.085

Impact and cyclic loading:

• The load that is applied suddenly at short time interval is called impact load. The strength that shows impact load is called impact strength.
• Impact strength increases with:

a. Increase in rate of loading.

b. Increase in tensile strength.

c. Decrease in aggregate strength.

• The CEB – FIP mode code (1990) recommends following relation for impact strength.

 F_c,_imp = f_c * (σ_s / σ_o) ^α
where,

F_c,_imp = Impact compressive strength

f_c = Compressive strength of concrete

σ_s = Impact strength rate

σ_o = 1 MPa

α = 1 / [5 + 9 * (f_c / f_co)]

1.4 Effect of porosity, water – cement ratio and aggregate size

Effect of porosity:

• Primary factor that governs the strength of concrete.
• Strength of concrete decreases with the increase in porosity.
• Permeability of concrete increase with increase in porosity of concrete.
• Factors like w/c ratio, degree of hydration, air content, mineral admixture, aggregates affects concrete porosity.
• Increase in w/c ratio increase the porosity.

Effect of water content cement ratio:

• W/C ratio is an index of strength of concrete. The strength of cement paste increases with increase in cement concrete and decrease in water content and air content.
• High w/c ratio increase the porosity and permeability of concrete and reduce its durability.
• From Abram’s law it is found that the cement requires about 1/5 to 1/4  of its weight of water to hydrate completely.

Mathematically,

C = A /B^x

Where,

C = Compressive strength of concrete

A, B = Empirical constant

x = w/c ratio

Effects of aggregate size:

• Aggregate is an important factor affecting the concrete strength. The most important properties of concrete are shape, texture and size of aggregate.
• Large size aggregate reduce specific surface area of aggregate which leads to reduction in bond.
• If constant workability is maintained, strength will increase with cement content.
• High strength or rich mix concrete is adversely affected by the use of large size aggregate. When large aggregate are used due to internal bleeding in transion zone it will become much weak by developing micro cracks with lower compressive strength.

1.5 Durability of concrete

• The ability of concrete to resist weathering action, chemical attack and abrasion while maintaing its desired engineering properties.

Factors that affect durability of concrete:

• W/C ratio
• Compaction
• Permeability
• Temperature
• Aggregate
• Porosity

References:

• Dayaratnam, P. Brick and reinforced brick structure.
• Neville, A.M. Properties of Concrete. England: Pearson Education Limited.
• Hendry, A.W., Sinha, B.P. & Davies, S.R. Design of Masonry Structure. London: E & FN Spon.

The post Properties of Hardened Concrete: Deformation of hardened concrete, moduli of elasticity & Fatigue, impact and cyclic loading appeared first on OnlineEngineeringNotes.

• Reaction of cement with water.
• When cement comes in contact with water chemical reaction occur which results in formation of new compound which is responsible for gain of strength and hardening of cement concrete.

1.2 W/C ratio

• Amount of water required is directly related to amount of cement used in concrete.
• W/C ratio is the ratio of amount of water to amount of cement.
• Important for hydration and quality of cement.
• More w/c ratio more cement hydrated, more is the compactness of paste and more is strength. But, too much water results in unoccupied void with low strength.
• w/c ratio = (0.45 – 0.6)

1.3 Process of manufacture of cement

1. Mixing:

• Important for uniform and homogeneous concrete.
• Method of mixing are:

a. Hand mixing:

• Mixing is done with the help of tool but with power of hand of worker.
• Suitable for small work.
• Addition 10% cement is required to compensate loss of strength than machine mixing.

b. Machine mixing:

• Power of machine is used mixing concrete.
• Economical for large work.
• Suitable for medium to large work.
• Gives more homogenous and uniform mix at low w/c.

2. Handling:

• After mixing, concrete should transported at the site immediately without drying and should remain in cohesion form.
• For conveying concrete from mixer to site following method are used:

           i. Pan method

           ii. Wheel barrow

           iii. Bucket

           iv. Conveyor belt

           v. Truck or lorries

3. Placing:

• After handling or transportation the concrete shall be placed to right shape and size.
• Depending upon the site different thing are to be considered during placing:

a. For foundation

• Vegetation or organic matter are removed.
• Loose earth should be removed or compacted.

b. For slab:

• Concrete should be dumped not dragged.
• During compaction of layered concrete, vibrator needle shall be penetrant to pervious layer.

c. For RCC:

• Mould releasing agent shall be applied on surface of formwork.

d. For column:

• Special formwork with opening on one side at different height shall be used.

4. Compaction:

• Concrete should be compacted after placing.
• Main objective is to remove air voids which might trapped during mixing, handling or placing of concrete.
• Method of compaction:

a. Hand compaction:

• Done for concrete of up to 15 – 20 cm layer.
• Concrete of high consistency is required for hand compaction.
• It can be done in three ways:

i. Rodding

ii. Ramming

iii. Tamping

b. Compaction by vibration:

• Suitable in general purpose concreting.
• It can compact concrete of even lower w/c ratio.
• Types of vibrator:

i. Needle vibrator

ii. Table vibrator

iii. Formwork vibrator

iv. Surface vibrator

5. Curing:

• Providing favorable environment for concrete to gain strength is desired time without harmful change.
• Strength of concrete is achieved by hydration of cement.
• Curing is done to keep the concrete moist and warm enough so that the hydration of cement continuous at optimum level for best gain of strength.

1.4 Workability and its test

• The amount of mechanical energy or work required to fully compact concrete without segregation.
• Increasing water content increases workability.

   Factors affecting workability:

• Water content
• Size of aggregate
• Shape of aggregate
• Use of admixture
• Time and temperature

Workability test:

1. Slump test:

• Most common and widely used method.
• Not suitable for very dry or very wet condition.
• Measures the consistency property of concrete.

Apparatus:

• Metallic mould in shape of truncated circular pyramid with top diameter 10 cm and bottom diameter 20 cm. Height of mould of 30 cm.

Procedure:

• The base is placed on a smooth surface and the container is filled is 3-4 layer. Each layer is tamped 25 times with help of tamping rod. ( 16 mm diameter and 60 cm long rod)
• After tamping the top layer the top surface is made smooth by troweling.
• Then, The cone is lifted slowly in vertical direction and the unsupported concrete will now subside.
• The new height is measured and the height of subsidence is calculated.
• The subsidence height in mm is taken as slump loss or slump value.
• Different shape of slump are:

a. True slump:

• Reduction in height of mass without any breaking up.

b. Shear slump:

• Indicates lack of cohesion.
• Indicates not suitable for placing.

c. Collapse slump:

• Indicates w/c ratio is high i.e. concrete is too wet.

2. Compaction factor test:

• More precise and sensitive than slump test.
• Useful for concrete mix of very low workability.
• It is performed to measure the level of compaction in concrete on application of fixed amount of energy for compaction by free fall.

Apparatus:

Procedure:

• Fill the first mould without compaction and closing the bottom door.
• Open the bottom door to make the concrete free fall and collect on the second mould.
• Again, open the bottom of second mould allow free fall of concrete and collect on cylinder.
• Cut the extra concrete in the cylinder to level.
• Determine the weight of partially compacted concrete due to free fall.
• Remove the concrete from cylinder and refill the cylinder in 5 cm layer each layer being compacted with tamping rod or by vibrator.
• Then compaction factor is calculated as:

CF = Weight of partially compacted concrete / Weight of fully compacted concrete

3. Flow test:

• Method to determine workability of concrete in which concrete is allowed to flow or spread by subjecting to jolting.

Apparatus:

• Mould in shape of frustusm of cone with top diameter 17 cm and bottom 25 cm.

Procedure:

• Put the concrete in the cone in two layer each layer tamped 10 times by tamping rod.
• The cone is lifted, allowing the concrete to flow.
• The flow table is then lifted up about 4 cm and then dropped 15 times in 15 second.
• After this the diameter of concrete is measured.
• The flow is determined as:

Flow% = [Diameter of flow (cm) – 25] * 100 /25

1.5 Segregation and bleeding:

Segregation:

• Separation of the components of fresh concrete, resulting in non – uniform mix.

Segregation can be categorized into three types:

• Coarse aggregate separating out from rest of the concrete.
• Paste separating out from coarse aggregate.
• Water separating out from rest of the concrete.

Cause:

• Insufficient mixing.
• Excess water.
• Primary cause of segregation is difference is specific gravity and size of constituents of concrete.

Factors affecting segregation:

• High quantity of large size particle and gap grading.
• Low cement content.
• Too dry or too wet concrete.

Effect:

• Decrease strength of concrete.
• Doesn’t provide homogenous mass throughout the concrete.

Prevention:

• The concrete should be placed as soon as possible.
• When transporting the mix, loading should be done carefully.
• Correctly proportioning the mix.
• Use of certain workability agent.

Bleeding:

• Particular form of segregation in which the appearance of water on the surface of concrete after consolidating the concrete.
• Happens due to excessive quantity of water mix or excessive compaction.

Cause of bleeding:

• Lack of finesse.
• Too much water content in mix.
• Too much finishing.
• High temperature.

Effects of bleeding:

• Loss in homogeneity.
• Reduce bond between the aggregate and cement paste and hence reduce strength of concrete.

Prevention of bleeding:

• Addition of pozzolana.
• Increase the finess of cement.
• Use of air entrainment.
• Decrease the w/c ratio.

1.6 Concrete in extreme temperature:

• In places which experience extreme weather condition different problems are encountered in preparation, placing and curing of concrete.

1. Hot weather concreting:

• Operation of concreting done at temperature above 40^oC.

Effect:

• High temperature of fresh concrete results in a more rapid hydration and leads to accelerated setting.
• Reduce the handling time of concrete.
• Difficulty in air – entrainment and curing.
• Rapid evaporation and loss of water.

      Precaution of hot weathering:

• Use of retarding admixture.
• Use of cold water.
• Use of low heat cement.
• Curing of concrete should be cured with cold water.
• Covering the concrete with moist material.

2. Cold weathering concreting:

• Concreting operation done at temperature below 5^oC.

Effects:

• Rate of hydration decrease with decrease in temperature.
• Delay in removal of formwork.
• Ordinary method of curing becomes difficult.
• During transportation concrete becomes hard due to ice formation.

Precaution of cold weathering:

• Concrete work should be done on day time.
• Warm water should be added for mixing of ingredients of concrete.
• Use of accelerating admixture.
• Use of high heat cement and RHC.
• Protective cover should be used for concrete.

1.7 Quality control of site:

• Quality control of site objective is to reduce the different variation and carry out the process mixing, handling, compacting and curing with utmost care so that a concrete of high workability, strength and durability is obtained.

Advantage:

• Reduce maintenance cost.
• Minimize the risk of over estimation of budget.
• Helps to obtain desired characteristics of concrete.

References:

• Dayaratnam, P. Brick and reinforced brick structure.
• Neville, A.M. Properties of Concrete. England: Pearson Education Limited.
• Hendry, A.W., Sinha, B.P. & Davies, S.R. Design of Masonry Structure. London: E & FN Spon.

The post Properties of Fresh Concrete: Concreting in extreme temperature and Quality control at site appeared first on OnlineEngineeringNotes.

Concrete:

• Composite mixture of binder, aggregate, water and sometimes admixture.
• Commonly used building material.

Merit:

• Ability to cast in any shape
• Economic
• Durable
• Fire and water resistant
• High strength

Demerit:

• Low tensile strength
• Low toughness
• Low curing time

Uses:

• Resistance to weather and fire resistant
• Strength increases with time
• Economical and energy efficient
• Durable

1.2 Constituent of concrete:

1. Coarse and Fine aggregate:

• Chemically inactive material i.e. inert material.
• Obtain from disintegration of strong rock. Example : gravel, pebble, sand etc.
• Contributes 70 to 80 % of volume of hardened concrete.

Properties of good aggregate:

• Must be clean, hard, strong, dense, durable, properly shaped, well graded.
• Inertness and soundness.
• High strength and toughness.
• High bond strength.
• Angular shape aggregate are good due to good interlocking.

2. Cement:

• Chemically active ingredient of concrete which shows binding properties after reaction with water.
• Manufactured by dry and wet process.
• Components used for making cement are:

i. Limestone, clay, chalk-lime (60%)

ii. Silica (20%)

iii. Aluminium (10%)

Iv. Others: Iron oxide, carbon dioxide etc.

Bougie compound:

Bougie compound are formed during hydration of cement.

1. Tricalcium silicate: Responsible for initial setting and early strength.

2. Dicalcium silicate: Provide good ultimate strength.

3. Tricalcium aluminate: First compound to react with water.

4. Tetra calcium aluminate ferrite: Poor cementing value and less active.

Types of cement:

• Ordinary Portland cement
• Portland pozzolana cement
• Rapid hardening cement
• White cement
• Colored cement
• Acid resistant cement
• Low heat cement
• Quick setting cement

Test of cement:

• Fineness
• Setting time
• Soundness
• Specific gravity
• Tensile strength
• Compressive strength

3. Admixture:

• A material other than basic ingredient like cement, aggregate and water which are used to alter the properties of concrete for specific purpose such as workability, w/c, setting time and strength can be changed.
• Added in powder or liquid form.

Purpose:

1. To modify fresh properties:

• Increasing the workability without changing water cement ratio.

(This process is also known as plasticizer.)

• Retard or accerate the intial setting time.
•  Modify rate of bleeding.

2. To modify harden properties:

• Reduce the heat of evolution.
• Accelerate the rate of strength developed at early stage.
• Increase durability.

Types of admixture:

1. Chemical admixture:

• Material is form of powder or fluid that are added to concrete to give certain characteristics.
• Less than 5% added at the time of mixing.
• Accelerator: Increase rate of hydration.
• Retarder: Slow down rate of hydration.
• Entrapped air:

               Air bubble are formed inside concrete unintentionally due to less compaction.

• Entrained air:

Air bubble is added intentionally to concrete to change the properties of concrete.

2. Mineral admixture:

• Very fine grain inorganic material which are added to concrete to improve properties of concrete.
• Needs large amount of volume compare to chemical admixture.

Use:

• Increase water tightness.
• Increase early strength.
• Increase workability.
• Decrease heat evolution.

References:

• Dayaratnam, P. Brick and reinforced brick structure.
• Neville, A.M. Properties of Concrete. England: Pearson Education Limited.
• Hendry, A.W., Sinha, B.P. & Davies, S.R. Design of Masonry Structure. London: E & FN Spon.

The post Introduction to Concrete Technology and Constituents of Concrete appeared first on OnlineEngineeringNotes.