Concrete is: a proportioned mixture of cement, aggregate and water. a plastic mass which can be cast, molded or formed into predetermined size or shape.

Concrete is:

• a proportioned mixture of cement, aggregate and water.

• a plastic mass which can be cast, molded or formed into predetermined size or shape

• upon hydration, becomes stone-like in strength, hardness and durability. The hardening of concrete is called setting.

• when mixed with water and a fine aggregate of less than 6mm (¼“) is known as mortar, stucco or cement plaster.

• when mixed with water, fine aggregate and a large aggregate of more than 6mm (¼”) in size produces concrete.

• when strengthened by embedded steel, is called reinforced concrete.

• when without reinforcement, is called plain or mass concrete.

CONCRETE

Concrete should be:

• Strong• Durable• of uniform quality, and • thoroughly sound.

These are obtained through:

• careful selection of materials • correct proportioning • thorough mixing• careful transporting and placing • proper curing or protection of the concrete after it is

placed

QUALITIES OF GOOD CONCRETE

a. Cement

soundness, or constancy of volume time of setting fineness tensile strength

Each bag of cement is equivalent to approximately 1 cu. ft. and weighs 94 lbs.

• in reinforced-concrete construction should be high-grade Type 1 Portland cement type C-150 conforming to the “Standard Specifications and Test for Portland Cement” of the American Society for Testing Materials (ASTM).

• The kind of tests usually made are:

MATERIALS OF CONCRETE

b. Aggregates are:

• Fine aggregates (aggregates smaller than 6mm (¼”) in size) consist of sand, stone screenings or other inert materials of similar characteristics.

Specs: 80 to 95% shall pass a No. 4 wire cloth sieve and not more than 30% nor less than 10% shall pass a No. 50 sieve.

inert mineral fillers used with cement and water in making concrete, should be particles that are durable strong, clean, hard and uncoated, and which are free from injurious amount of dusts, lumps, soft and flaky particles, shale, alkali, organic matter loam or other deleterious substances.


b. Aggregates


• Coarse aggregate (aggregate larger than ¼” in size) consists of crushed stones, gravel or other inert materials of similar characteristics.

Coarse aggregates should be well graded in size to a size which will readily pass between all reinforcing bars and between reinforcement and forms but not exceed 25mm (1”) in size for reinforced beams, floor slabs, & thin walls.

They may range up to 50mm (2”) for less highly reinforced parts of the structures such as footings, thick walls, and massive work.

b. Aggregates


• Special aggregates, such as cinders, blast furnace slag, expanded shale or clay, perlite, vermiculite, and sawdust, may produce:

- lightweight, nailable concrete- thermal insulating concrete.

b. Aggregates


c. Water

- should be free from oil, acid, alkali, vegetable matter, or other deleterious substances

- should be reasonably clear and clean.

- The use of sea or brackish water is not allowed.

- Water combines with the cement to form a paste which coats and surrounds the inert particles of aggregates.

- Upon hardening, it binds the entire mass together.

- The strength of the mixture therefore depends directly upon the strength of the paste. If there be an excess of water the paste becomes thin and weak and its holding power is reduced.


- The water-cement ratio is the amount of water used per bag of cement.

- This usually varies from 5 to 7 gallons, with 6.5 gallons as average for ordinary job conditions. The less water used in mixing, the better the quality of concrete.

- The ideal mix is one that is plastic and workable. It should not be too dry that it becomes too difficult to place in the forms, nor too wet that separation of the ingredients result.

WATER – CEMENT RATIO

Assumed 28-dayCompressive strength

(lbs. per sq. inch)

Maximum water-cement ratioU.S. gallons of water per sack

Cement of 94 lbs.

Pounds of water per 100 lbs. of cement

2,0002,5003,0003,750

7.006.505.755.00

62.057.551.044.5

c. Water


- used for measuring the consistency of a concrete mix. - Consistency may be defined as the “state of fluidity of

the mix”, and it includes the entire range of fluidity from the wettest to the driest possible mixtures.

In this test the tendency of a mix to “slump”, or reduce its height due to gravity action, is measured. The apparatus consist of metal cone, the bottom opening being 200mm (8”) in diameter, the top opening being 100mm (4”), and the height exactly 300mm (12”).

SLUMP TEST

In making the test, the slump tester is placed on a flat, smooth surface and is filled with newly mixed concrete from mixer. In filling the mold with concrete, the latter is tamped in with a 12mm (½”) rod pointed at one end and the top of the concrete is smoothed off exactly level. The mold is then slowly raised vertically and the height deducted from the original height of 300mm (12”) represents the slump.

SLUMPNo slumpCollapsed

slump

TOO WET SUITABLE TOO DRYBucket

SLUMP TEST

A harsh mix is efficient for slabs, pavements, or mass concrete where the lowest possible water-cement ratio is desirable.

The following table gives the permissible slump for various types of concrete in relation to their uses:

CONSISTENCY (SLUMP)Maximum Minimum

Reinforced foundation walls and footings

125mm (5”) 50mm (2”)

Plain footings, caissons, and substructure walls

100mm (4”) 25mm (1”)

Slabs, beams, thin reinforced walls & building columns

150mm (6”) 75mm (3”)

Pavements and floor laid on ground 75mm (3”) 25mm (1”)

Heavy mass construction 75mm (3”) 25mm (1”)

SLUMP TEST

Briefly stated, the principles of proper proportioning are as follows:

PROPORTIONING OF CONCRETE

a. Use good quality materials: Portland cement, water, and aggregate.

b. Determine the strength of the concrete using the water-cement ratio. (The strength increases as the water-cement ratio decreases).

c. Determine the consistency of the mix using the slump test using as dry a mix as practicable.

d. Add correct proportions of aggregates to the cement and water as will give a mix of the desired consistency.

e. Make a mix that’s workable, not harsh.

The strength of a workable concrete mix depends upon the water-cement ratio.

The economy of the mix depends upon the proper proportioning of the fine and coarse aggregates.

There are several methods of proportioning concrete:

a. Proportioning by arbitrary proportions b. Proportioning by the water-ratio and slump test c. Proportioning by water-ratio, slump and fineness

modulus


a. Proportioning by arbitrary proportions

Proportioning concrete by the arbitrary selection of the proportions is the oldest, the most commonly used, the most convenient and the least scientific method.

In this method, the aggregates are measured by loose volume, that is, its volume as it is thrown into a measuring box.

One sack of cement is taken as 1 cu. ft.

Enough water is used to give the desired consistency.

1 f

oo

t

1 foot

1 foot


Common mixes expressed in proportions by volumes of cement to fine aggregate to coarse aggregate are as follows:

CONCRETE PROPORTIONS

Class “AA” 1 : 1.5 : 3 For concrete under water, retaining walls

Class “A” 1 : 2 : 4 For suspended slabs, beams, columns, arches, stairs, walls of 100mm (4”) thickness

Class “B” 1 : 2.5 : 5 For walls thicker than 100mm (4”), footings, steps, reinforced concrete slabs on fill.

Class “C” 1 : 3 : 6 For concrete plant boxes, and any non-critical concrete structures.

Class “D” 1 : 3.5 : 7 For mass concrete works.

The proportion is to be read: Class A : 1 part cement is to 2 parts sand is to 4 parts gravel.

Each ‘part’ is equivalent to one cubic foot which is the measure of the box constructed to be 1 foot (12 inches) on each of the three sides. Each bag of cement is equivalent to approximately one cubic foot.


b. Proportioning by the water-ratio and slump test

There are two steps to be observed:

- Select the amount of water to be added to the cement to give the desired strength (see Table)

- Add just enough mixed aggregate to the water and cement to give a concrete mix the desired consistency.

It is customary to specify - the cement in sacks- the water in gallons per sack of cement and - the mixed aggregate in cu. ft. per sack of cement.

Proportions of cement to fine aggregate to coarse aggregate may be given if desired.


c. Proportioning by water-ratio, slump and fineness modulus

This method is the same as the second except that the proportions of the fine and coarse aggregate are determined by the fineness modulus method.

For economy, proportion the fine coarse aggregates so that the largest quantity of mixed aggregate may be used with a given amount of cement and water to produce a mix of the desired consistency of slump.

Comparatively, the coarse aggregate has a lesser total surface to be covered with cement paste and, therefore, is more economical.

However, there must be enough fine aggregate present to fill the voids in the coarse aggregate, or extra cement paste will be needed for this purpose. A well-graded aggregate contains all sizes of fine and coarse particles in such proportions that the voids in the combined aggregate will be a minimum.


• Reinforced-concrete work should be mixed by machine

• Machine-mixed concrete is usually of more uniform quality than that mixed by hand and is generally less expensive when in large volume.

• The strength of concrete is very largely dependent upon the thoroughness of mixing.

MIXING OF CONCRETE

a. MACHINE MIXING

In machine-mixing, the mixing of each batch should continue not less than one minute after all the materials are in the mixer and whenever practicable, the length of the mixing time should be increased to 1.5 or 2 minutes. The entire contents of the drum should be discharged before recharging the mixer. The mixer should be cleaned at frequent intervals while in use.

Concrete mixers may be divided into two general classes:

Batch mixers - into which sufficient materials are placed at one time to make a convenient size batch of concrete, the whole amount being discharged in one mass after it is mixed.

MIXING OF CONCRETE

Continuous mixers -into which the materials are fed constantly and from which the concrete is discharged in a steady stream.

Concrete mixers may also be classified as:

- drum mixers- trough mixers- gravity mixers, and - pneumatic mixers.

The drum mixers are the most common type.

a. MACHINE MIXING

MIXING OF CONCRETE

b. HAND MIXING

- hand-mixing must be done on a water-tight platform.

- cement and fine aggregate shall first be mixed dry until the whole is a uniform color.

- water and coarse aggregate shall then be added and the entire mass turned at least three times, or until a homogeneous mixture of the required consistency is obtained.

MIXING OF CONCRETE

CONCRETE

- since initial set of concrete takes place 1 to 3 hours after mixing, a batch may be used anytime before initial set takes place, provided that the mix is plastic.

- Regaging or retempering of concrete that has been allowed to stand more than ½ hour is not to be permitted.

b. HAND MIXING

MIXING OF CONCRETE

• The delivery of the concrete from the mixer to the forms should be fairly continuous and uninterrupted.

• The time of transportation should not exceed 30 minutes.

TRANSPORTING AND PLACING OF CONCRETE

• Fresh concrete should be transported from the mixer as rapidly as practicable by methods that will permit the placing of the concrete in the forms before initial set occurs and without loss or separation of materials.

• The concrete may be transported by means of barrows, buggies, buckets, cableways, hoists, chutes, belts and pipes.

• When chutes are used, the slope should not be more than 1 vertical to 2 horizontal or less than 1 vertical to 3 horizontal. The delivery end of the chutes shall be as close as possible to the point of deposit.


• Before placing concrete, the forms shall be cleaned and inspected, surfaces wetted or oiled, and reinforcement properly secured.

• Concrete should be deposited in approximately horizontal layers in wall, column and footing forms. They should not be piled up in the forms which may result in the separation of the cement mortar from the coarse aggregate.

• Concrete should never be allowed to drop freely over 5 ft. for unexposed work and over 3 ft. for exposed work.


• Shrinkage of concrete due to hardening and contraction from temperature changes, causes cracks the size of which depends on the extent of the mass. They cannot be counteracted successfully but they can be minimized by placing reinforcement so that large cracks can be broken up to some extent to smaller ones.

• In long continuous length of concrete, it is better to place shrinkage or contraction joints. Shrinkage cracks are likely to occur at joints where fresh concrete is joined to concrete which has already set, and hence in placing the concrete, construction joints should be made on horizontal and vertical lines.

SHRINKAGE OF CONCRETE & TEMPERATURE CHANGES

• Concrete must be allowed to “cure” or harden after it is placed.

• Hardening is a rather slow process in which the cement and water unite to form compounds that give strength and durability to the concrete. It continues as long as the temperatures are favorable and moisture is present.

• Three main factors that affect hardening are:

CURING OF CONCRETE

- age or time- temperature, and - moisture.

• In order that the hardening may proceed favorably, the fresh concrete, for about 7 days after placing, should be protected from excessive vibration, loads, extreme heat or cold, too rapid drying, and contact with impurities which may interfere with the chemical action.

• The strength of the concrete increases with age when the curing conditions remain favorable.

CURING OF CONCRETE

• The increase in strength is rapid during the early ages and continues more slowly as time goes on. The compressive strength reaches about 60% of its own maximum value at an age of 28 days and about 80% at an age of 3 months.

CURING OF CONCRETE

Curing consists primarily in keeping the concrete from drying out too rapidly. This may be done by:

a. Covering the concrete. Floors shall be covered with paper sacking wetted down at the edges or with burlap, sand or earth that is kept moist, after the concrete is hard enough to walk on.

b. Removal of forms at prescribed time. Forms shall not be removed until after the time specified.

c. Sprinkling with water. Beams, columns and walls are sprinkled or sprayed with water as soon as the forms are removed.

d. Using curing compounds (see ADMIXTURES).

CURING OF CONCRETE

Parts of Structure CURING PERIOD or TIME REQUIRED FOR THE REMOVAL OF FORMS

FOOTINGS a. Massive footings b. Cantilever footingsc. Slab footings

a. 1 day (24 hours)b. 5 days (120 hours)c. 5 days (120 hours)

WALLS AND PLASTERS

a. Massive walls, 30 cms. thick or more

b. Thin walls less than 30 cms. Thick

c. Cantilever walls, buttresses, counter forts, diaphragms.

a. Up to 2 M. high: 1 day (24 hours). Add 1 day (24 hours) for every additional meter or fraction thereof.

b. Up to 2 M. high: 2 days (48 hours. Add 1-1/2 days (36 hours) for every additional meter or fraction thereof

c. Without loads, same as (b).

COLUMNS a. Ratio of height to least diameter up to 4

b. Ratio of height to least diameter from 4 to 15.

a. 2 days (48 hours)

b. Add to the above number 1 day (24 hours) for every additional meter or height or fraction there of but not more than 28 days (672 hours).

CURING OF CONCRETE

SLABS a. 3 to 7 ft. spans

b. Over 7 ft. span

a. 3 ft. span, 5 days (120 hours). Add ½ day (12 hours) for every additional 1 ft. span or fraction thereof.

b. 7 ft. span, 7 days (168 hours). Add 1 day (24 hours) for every additional 1 ft. span or fraction thereof but not more than 28 days (672 hours).

BEAMS AND GIRDERS

a. Sides

b. Bottoms

a. 3 days

b. Up to 14 ft., 14 days (336 hours). Add 1 day for every 1 ft. additional span or fraction thereof but not more than 28 days (672 hours).

ARCHES a. Spandrel wallsb. Spandrel archesc. Main arches

a. 7 days (168 hours).b. 14 days (336 hours)c. 21 days (504 hours)

BALUSTRADES, COPINGS,ETC.

a. Steel & side forms a. 1 day (24 hours)

R.C. PILES and R.C. POSTS

a. Sides.b. Bottom

a. 3 days (72 hours)b. 14 days (336 hours)

Parts of Structure CURING PERIOD or TIME REQUIRED FOR THE REMOVAL OF FORMS

CURING OF CONCRETE

CURING OF CONCRETE

CURING OF CONCRETE

Requirements of Concrete in the Fresh State

• Ease of mixing and transportation

• Uniformity within a batch and between batches

• Ability to fill the form completely

• Ability to be placed and compacted fully without a high energy requirement

• Absence of segregation during placing and consolidation

• Capable of being finished properly

Factors that Affect Workability

• Water content• Increase in water content increases workability• Excess water causes segregation

• Cement content• Increase in cement content increases workability• Excess cement can lead to over cohesive and sticky mix

• Use of finer or more angular-shaped aggregates will lead to a higher water content for the same workability

• Mineral admixtures tend to increase the cohesiveness of the concrete

• Chemical admixtures affect workability positively or negatively, depending on the type and dosage

Bleeding of Placed Concrete

• The settling of the aggregates during consolidation of the concrete normally forces excess water and some cement to rise.

• This leads to weaker zones at the top surface (laitence), and below reinforcing bars and coarse aggregates due to the accumulation of water.

Composite Response of Concrete

Compressive FailureMode of crackingMode of cracking

Compressive FailureStress-strain responseStress-strain response

As the strength (or maximum stress) increases, the stress-strain curve under uniaxial compression usually shows a slightly higher elastic modulus, a longer linear response and a sharper post-peak descent.

Compressive FailureStress-strain responseStress-strain response

Beyond a certain axial Beyond a certain axial stress (stress (σσAA), the ), the

transversal strain transversal strain increases considerably increases considerably causing an apparent causing an apparent increase in the Poisson’s increase in the Poisson’s ratio. ratio.

Compressive FailureStress-strain response under confinementStress-strain response under confinement

As lateral confinement increases the effective strength and ductility of the concrete increase.

The effectiveness of the confinement is lower for higher strength concretes.

Multiaxial Stresses

Even though the Mohr-Coulomb theory is not directly applicable to Even though the Mohr-Coulomb theory is not directly applicable to concrete, it is a convenient way of representing failure under multiaxial concrete, it is a convenient way of representing failure under multiaxial stresses.stresses.

Mohr rupture diagram for concreteMohr rupture diagram for concrete

Fatigue StrengthFatigue load spectrum for concrete structuresFatigue load spectrum for concrete structures

Fatigue Strength

Fatigue life is often represented through an S-N diagram Fatigue life is often represented through an S-N diagram or Wöhler curveor Wöhler curve

Steel-Concrete Bond

Bond failure of steel reinforcing bars occurs due to Bond failure of steel reinforcing bars occurs due to crushing and cracking of the surrounding concrete crushing and cracking of the surrounding concrete

confining action of concrete

splitting cracks in the cover

wedging forces from the ribs of the bar

inclined cracks develop from the ribs

Modelling of Concrete Failure for Structural Analysis

Modelling of Concrete Failure in Materials Science

Nature of Shrinkage and Creep• Creep and shrinkage are time-dependent strains that involve

the movement of water• Shrinkage strains occur when water is lost• Creep strains occur when water is forced to move by stress• These strains are not completely recoverable when the load

is removed or the concrete is re-saturated

Effects of Shrinkage and Creep

• Axial strains increase with time; e.g., in columns under compressive loads and bridge piers.

• Deflections increase; e.g., beams and girders in flexure.

• Stress relaxation; e.g., the prestressing force decreases with time as the concrete shrinks and creeps.

• Cracks can occur in elements that are restrained and develop tensile stresses; e.g., in pavements and slabs-on-grade.

Types of Shrinkage• Plastic shrinkage: Due to the loss of water in the plastic state

due to evaporation.

• Autogenous shrinkage: Chemical shrinkage (lower volume of hydrates than cement and water) + Self-desiccation (reduction in the pore water due to hydration).

• Thermal contraction (or thermal shrinkage): Due to the decrease in temperature after setting.

• Drying shrinkage: Due to the loss of water to the environment in the hardened state.

• Carbonation shrinkage: Volume reduction due to the reaction of hydrated cement paste with CO2 in the presence of

moisture.

Static Fatigue or Creep

Effect of constant compressive loads (Rüsch, 1960)

Young et al.Young et al.

Creep

NevilleNeville

• Creep is the gradual increase in strain due to the applied load.

• Creep is defined in terms of the time-dependent strain due to a sustained load

• When concrete is restrained, creep is manifested as relaxation (or a gradual lowering of the stress).

Relaxation under constant strain of 360×10-6

Types of Creep

• Basic Creep: Creep strain that occurs without any loss of moisture to the environment.

• Drying creep: Creep strain that can be attributed to the effect of drying (= total creep - basic creep)

• Creep recovery: When sustained load is removed, there is an instantaneous decrease in strain, followed by a gradual decrease, called creep recovery

NevilleNeville

Creep recovery of mortar maintained in fog room under 15 MPa stress and then unloaded

Combined Effect of Shrinkage and Creep• Creep strains are always opposed to the applied

stress; i.e., creep will cause strains in the direction of the stress. They are always additive with the elastic strains.

• The Poisson’s ratio is about the same in creep as in the elastic regime. So, lateral expansion will increase due to creep.

• Shrinkage strains are volumetric; i.e., shrinkage strain is same in all directions.

• Under uniaxial loading, the elastic, creep and shrinkage strains will lead to axial contraction, whereas shrinkage and creep may compensate each other in the lateral direction.

Combined Effect of Shrinkage and Creep• Under conditions of restrained shrinkage, creep will

lead to a decrease in the stress (i.e., relaxation) resulting in a reduction of cracking.

NevilleNeville

Study of Shrinkage and Creep in Hardened Concrete

Age

Str

ain

Curing

Ins

tant

aneo

us s

trai

n =

i

i + Basic creep strain

i + Basic + Drying creep strains

Drying shrinkage strain

Autogenous shrinkage strain

sealed

sides exposed

Deterioration due to Chemical Reactions

• Leaching due to exposure to soft (acidic) water• Calcium hydroxide is dissolved and reacts with

carbon dioxide to be deposited as (white) calcium carbonate within the concrete and on the surface.

• Sulphate attack• Sulphates react with the calcium hydroxide to form

gypsum. The gypsum reacts with the hydrated compounds to form ettringite. This results in expansion and cracking of the concrete.

• In addition, attack by magnesium sulphate is more damaging since the magnesium hydroxide that is formed in the reaction with the C-S-H replaces the Ca2+ ions with Mg2+, which destroys the cementing effect.


• Alkali-silica reaction• Hydroxides of sodium and potassium present in the

cement can react with fine-grained porous silica aggregates. The product is a silicate gel that absorbs water and expands. When all the pores are filled, further expansion causes cracking. Dehydration of the gel leaves open cracks.

• When the silica has high surface area (as in silica fume) or the concentration of alkalis is low, non-swelling gels are formed and there is no damage.

• Alkali-carbonate reaction• Dolomitic limestone (CaCO3.MgCO3) aggregates can

react with alkalis resulting in the loss of bond strength and microcracking.


The entry of aggressive chemicals into concrete depends on:

• Permeability (ease with which water can flow into and through concrete)

• Governed by the volume and size of capillary pores• Low w/c and extended curing lowers permeability• Addition of a mineral admixture also decreases the

permeability due to more C-S-H formation (and a discontinuous pore structure)

• Diffusion of ions and gases through the empty pores and the pore solution in saturated pores

• w/c and curing are again of primary importance• Cracking

• Facilitates the entry of water and other aggressive substances

Deterioration due to Physical Effects

• Frost (Freeze-Thaw) Attack

• Fire Damage

• Thermal Cycles

• Shrinkage Stresses

Corrosion of Reinforcement

• Corrosion involves the formation of a cathode and an anode, with electric current flowing in a loop between the two.

• In the anode, iron atoms are oxidized to Fe2+ ions, which dissolve in the surrounding solution. At a distance, cathodic reactions occur with the consumption of electrons and the formation of OH- ions.

• For the cathodic reaction to occur moisture must be present and there should be supply of oxygen.

• The ions formed at the cathode and anode migrate through the aqueous solution present in the pores of the surrounding concrete.

Corrosion

Young et al.Young et al.

Chemical reactions Chemical reactions and charge and charge movementsmovements

Parameters Parameters controlling the controlling the corrosion rate corrosion rate - resistivity and - resistivity and diffusivity of Odiffusivity of O22

Effect of Corrosion

• The Fe2+ and OH- ions in the pore solution interact near the anode to produce iron oxide (rust).

• The corrosion of the steel in the concrete results in:

• Expansion created by the rust, which can lead to cracking and spalling of the concrete. (Rust has a volume that is two to six times that of the steel.)

• Reduction of the cross-section of the steel bar.

Corrosion Protection• In general, the high pH of concrete is sufficient to maintain

the steel in a passivated state. This leads to the spontaneous formation of a stable protective iron oxide film around the steel. Corrosion occurs only when these conditions are changed and pH drops.

• Depassivation of the concrete can occur when:• The calcium hydroxide has been carbonated by the

penetration of CO2 into the concrete (and the pH becomes lower than 11).

• Chloride ions are present in the concrete (more than 0.2-0.4%), even though the pH is high.

• Moisture and oxygen are necessary for corrosion to be sustained. Porous concrete and cracks permit the ingress of water and oxygen, and promote corrosion.

Increasing the Durability of Concrete

• Proper mix design

• Reduction of cracking

• Optimum cover thickness

• Adequate compaction and curing

• Quality of construction

• Correct maintenance

Special Concretes

• High Strength or High Performance Concrete

• Fibre Reinforced Concrete

• Lightweight Concrete

• Shotcrete

• Self Compacting Concrete

Concrete Testing

Introduction• Testing is the basic method to verify that concrete

complies with the specifications.• Strength of concrete is verified by testing samples

(cubes, cylinders, or prisms) made of fresh concrete.• Disadvantage: Suspected concrete may have been

placed and hardened when testing take place.• Accelerated strength tests are some times used to

offset this disadvantage.• When samples fail in testing further investigation of

concrete may be performed using non-destructive testing.

Precision of Testing

• Concrete properties vary.• Precision: general term used for the closeness of

agreement between replicate test results.– Repeatability: the value below which the absolute difference

between two single test results obtained with the same method on identical test material under the same conditions (i.e. same: operator, apparatus, Lab, and short interval of time) may be expected to lie within a specified probability (usually 95%).

– Reproducibility: the value below which the absolute difference between two single tests results, obtained with the same method on identical test material under different conditions (i.e. different: operators, apparatus, Labs, time) may be expected to lie within a specified probability (usually 95%).

Precision Cont.

• Values of repeatability & reproducibility are applied in a variety of ways:– To verify the experimental technique of a lab is up

to requirements.– To compare results of tests performed on a

sample from a batch of material with specification.

– To compare test results obtained by a supplier and by consumer on the same batch of material.

Analysis of Fresh Concrete• Determination of composition of concrete at an early age can

be of benefit since knowing that actual proportioning correspond to those specified will conclude that there is little need for testing the strength of hardened concrete.

• Properties of interest: (W/c) ratio & cement content (mainly responsible for ensuring that concrete is strong & durable).

• BS suggest five methods to assess cement content:– Buoyancy method– Chemical method– Constant volume method– Physical separation method– Pressure filter method

• Water content of fresh concrete can be found as in chemical method, or by rapid drying method (different in mass before and after heating).

Strength Tests

• Compressive strength• Tensile strength:

– Uniaxial tension (direct tension: very difficult)– Flexure test (Indirect)– Splitting test (Indirect)– Indirect methods yields higher strength values

than the true tensile strength under uniaxial loading for reason already stated.

Method of Testing Concrete In Tension

• Different test methods yield numerically different results, ordered as follows:Direct tension < Splitting < flexural tension

• Reasons for that:1. With the usual size of Lab. Specimen, the volume of concrete

subjected to tensile stress decrease in the order listed above. Statistically there is a greater chance of a weak element and therefore of failure for larger volume than in a small volume.

2. Both splitting and flexural test involve non-uniform stress distribution which impede the propagation of cracks, and therefore delay the ultimate failure. Whereas, in the direct test, the stress distribution is uniform, so that once a crack has formed, it can propagate quickly through the section of the specimen.

Compressive Strength• Determined using 150 x 300 mm (6 x 12 in) cylinders in US and

150 mm (6 in) cubes in UK.• Smaller specimen sizes can be used based on Agg. Max. size.• Molds (re-usable or non re-usable) must be oiled from inside to

prevent bond with concrete.• ACCORDING TO ASTM (Cylindrical molds are used).• For high-slump concrete: Concrete placed in cylindrical molds in

three layers, and each layer is compacted 25 times with a rod (3/8 in D).

• For low-slump concrete: Concrete placed in two layers and compacted using internal or external vibration.

• Top surface of cylinders must be plane, smooth, and normal to its axis.

• Plane surface can be obtained using two methods”– Grinding– Capping (expensive)

Compressive Strength/ Capping

• Materials used:– Stiff Portland cement paste on freshly-cast concrete.– Mixture of Sulphur and granular material (e.g. milled fire

clay)…… Best capping material– Mixture of sulphur and high strength Gypsum plaster.

• Cap should be thin (1.5 to 3 mm) and has strength similar to that of concrete.

• Caution: Toxic fumes are produced when capping with Sulphur mixtures.

Compressive Strength/ Curing• ASTM C 192-90a curing conditions for standard test cylinders.• Molded specimens are stored for not less than 20 and not

more than 48 hrs at temp. of 23 +- 1.7 oC so that moisture loss is prevented.

• After removing from molds, specimens are stored at the same temp. and under moist conditions or in a saturated lime water until the prescribed age of testing.

• Cured cylinders give potential strength.• Service cylinders may be used to determine the actual quality

of concrete in the structure by being subjected to the same conditions as the structure.

Compressive Strength Test

• ASTM C39-86 compressive strength of a cylinder.

• Loading rate for hydraulically operated machines is 0.15 – 0.34 MPa/ s (20 -50 psi/s)

• Deformation rate for mechanically operated machines is 1.3 mm/min (0.05 in/min).

• Compressive strength = (max. load / cross section area of cylinder) reported to the nearest 0.05 MPa (10 psi).

Compressive Strength Cont.• ACCORDING TO BS (Cube molds are used).• Molds filled in 3 layers.• 35 strokes/ layer for 150 mm cubes, or 25 strokes for 100 mm

cubes, using a 25 mm (1 in) square steel punner.• Alternatively vibration can be used.• Top surface finished by a trowel.• Cubes stored at temp. of 20 +- 5 oC and with 90% moist

condition.• De-molding after 16 – 28 hrs and specimens stored in a curing

tank at 20 +-2 oC until test age.• Testing ages: common 3, 7, 28 days, uncommon 1, 2, 14 days,

13, 26 weeks, and 1 year.

Platen Restraint• Failure of concrete under pure uniaxial compression is the ideal

mode of testing.• But compression test imposes more complex stress system

because of lateral forces developed between the end surface of the concrete specimen and the adjacent steel platen of the testing machine.

• These forces are induced by the restraint of the concrete, which attempts to expand laterally, by the several time stiffer steel, which has a much smaller lateral expansion.

• The degree pf platen restraint on the concrete section depends on the friction developed at the concrete platen interface, and on the distance of the end surfaces of the concrete.

• Consequently, in addition to the imposed uniaxial compresion, there is a lateral shearing stress, the effect of which is to increase the apparent compressive strength of concrete.

Typical Failure Modes/ Cubes• Influence of platen restraint can be seen from typical failure

modes.• Effect of shear is always present and decrease towards the

center of the cube, or disintegrate so as to leave undamaged central core (Non-explosive). Testing machine is rigid.

• Less rigid machine can store more energy so that the explosive failure is possible.

• Explosive: One face touching the platen disintegrate so as to leave a pyramid or a cone.

• Other modes failure are regarded as unsatisfactory and indicate a probable fault in the testing machine.

Typical Failure Modes of test Cubes

a) Non-Explosive b) Explosive

Typical Failure Modes Cont.

• When the ratio of height to width of the specimen increase, the influence of shear becomes smaller.

• The central part of the specimen may fail by lateral splitting.

• Situation in cylinders (H/W = 2).• Possible modes of failure in Cylinders: Splitting,

Shear, and splitting & shear.

Typical Failure Modes of Test Cylinders

a) Splitting b) Shear (Cone)

c) Splitting & Shear

Influence of Height/ Diameter Ratio on the Apparent Strength

• As (H/D) ratio increase the apparent strength will decrease.

• Due to larger effect of platen restraint on cubes mode of failure Cube strength = 1.25 Cylinder strength

• This relation depends also on strength level and moisture condition of concrete when tested.

• Cylinder strength is probably closer to the true uniaxial compressive strength of concrete than the cube strength because:– Less end restraint.– More uniform distribution of stress over the cross section.

Influence of Height/ Diameter Ratio on The Apparent Strength of A cylinder

Tensile Strength

• Tensile strength:– Uniaxial tension (direct tension: very difficult

because the ends have to be gripped and bending must be avoided)

– Flexure test (Indirect)– Splitting test (Indirect)

• Indirect methods yields higher strength values than the true tensile strength under uniaxial loading for reason already stated.

Flexural Test• The theoritical max. tensile stress reached in the bottom

fiber of a test beam is known as the “Modulus of Rupture”.

• Relevant to the design of highways and airfields.• The value of the modulus of rupture depends on the

dimensions of the beam and on the arrangement of loading.

• Symmetrical two-point loading (at third point of the span) is used in UK ans US.

• This produces a constant bending moment between the load points so that one third of the span is subjected to the max. stress.

• Therefore it is there where cracking is likely to take place.

Flexural Test Cont.

• BS– Beam 150 x 150 x 750 mm (6 x 6 x 30 in) – Or beam 100 x 100 x 500 mm for max. agg < 25 mm.– Curing as specified in BS.– Beams tested on their side (as-cast position), in a

moist condition.– Rate of increase stress in the bottom fiber (0.02 – 0.1

MPa/s (2.9 – 14.5 psi/s), lower rate for low strength concrete and higher rate for high strength concrete.

Arrangement for The Modulus of Rupture Test

Flexural Test Cont.• ASTM C78- 84

– Similar flexural test as in BS except:– Beam 152 x 152 x 508 mm (6 x 6 x 20 in).– Rate of loading 0.0143 – 0.02 MPa/s (2.1 – 2.9 psi/s).

• If fracture occurs within the middle one-third of the beam, the modulus of rupture (fbt) is given by:Fbt = (P L) / (b d2)P: Max total loadL: Span length between supportsd: depth of the beamb: width of the beam

Flexural Test Cont.• If fracture takes place outside the middle one-third of the beam,

then:– According to BS the test result should be discarded.– ASTM C78-84 allows for failure outside the load points, say, at at an

average distance (a) from the nearest support.– modulus of rupture (fbt) is given by:

Fbt = (3 P a) / (b d2)P: Max total loadL: Span length between supportsd: depth of the beamb: width of the beam

• If failure occurs at a section such that ((L/3) – a) > 0.05 L, then the results should be discarded.

Flexural Strength Using One-Point Loading

– Modulus of rupture (fbt) is given by:

Fbt = (3 P L) / (2 b d2)

P: Max total loadL: Span length between supportsd: depth of the beamb: width of the beam

Splitting Test• Concrete cylinder (or less commonly cube) of the type used in

compressive strength testing.• Placed, with its axis horizontal, between platens of a testing

machine.• Load is increased until failure takes place by splitting in the

plane containing the vertical diameter of the specimen• ASTM C496 -90• Plywood are placed between specimen and platen tp

preventd local compressive stresses at the load line.

Splitting Test Cont.

• Rate of loading:– BS: 0.02 – 0.4 MPa/s (2.9 – 5.8 psi/s).– ASTM: 0.011 – 0.023 MPa/s (1.7 – 3.3 psi/s).

• According to ASTM C 496 -90 the tensile splitting strength (fst) is given by:

fst = 2P/ πLdP: Max. loadL: length of specimend: diameter or width of specimen

Jigs for Supporting Test Specimen in Splitting Test

a) Cube or Prism

a) Cylinder

Test Cores

• If strength of the standard compression test specimen is below the specified value then either:– The concrete in the actual structure is

unsatisfactory.– Or specimens are not truly representative of the

concrete in the structure (test specimen not correctly prepared, handled or cured, or testing machine could be at fault).

– Argument is often resolved by testing cores of hardened concrete taken from the suspect part of the structure in order to its potential strength.

Test Cores Cont.

• Potential Strength: the strength equivalent to the 28 days strength of the standard test specimen.

• When translating core strength into potential strength, take into account differences in:– Type of specimen and curing conditions.– Age– Degree of compaction

• Note that core taking damages the structure, so test cores should be taken only when other, non-destructive, methods are inadequate.

Test Cores Cont.• ASTM C42-90 prescribe method of determining the

compressive strength of cores.• It is desirable to obtain cores free from reinforcement.• Normal cores refer to the cores being representative of the

concrete.• ACI 318-89 considers the concrete in the structure is

adequate if the average strength of three cores is equal to at least 85% of the specified strength, and if no single core has a strength lower than 75% of the specified value.

• ACI require testing in a dry state which leads to a higher strength than when tested in a moist condition as (ASTM &BS specify).

Concrete is: a proportioned mixture of cement, aggregate and water. a plastic mass which can be cast, molded or formed into predetermined size or shape.

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