properties of concrete.docx

7/26/2019 properties of concrete.docx

1/35

Properties of concreteFrom Wikipedia, the free encyclopedia

Concretehas relatively highcompressive strength,but significantly lower tensile strength, and assuch is usually reinforced with materials that are strong in tension (often steel). The elasticity of

concrete is relatively constant at low stress levels but starts decreasing at higher stress levels asmatri cracking develops. Concrete has a very low coefficient of thermal epansion, and as itmatures concrete shrinks. !ll concrete structures will crack to some etent, due to shrinkage andtension. Concrete which is sub"ected to long#duration forces is prone tocreep.

Tests can be made to ensure the properties of concretecorrespond to specifications for theapplication. The density of concrete varies, but is around $,%&& kilograms per cubic metre('& lbcu ft).*'+!s a result, without compensating, concrete would almost always fail from tensilestresses even whenloaded in compression. The practical implication of this is that concreteelements sub"ected to tensile stresses must be reinforced with materials that are strong in tension.

-einforced concreteis the most common form of concrete. The reinforcement is oftensteel, rebar(mesh, spiral, bars and other forms). tructural fibersof various materials are available.Concrete can also be prestressed(reducing tensile stress) using internal steel cables (tendons),

allowing for beamsor slabs with a longer spanthan is practical with reinforced concrete alone./nspection of eisting concrete structures can be non#destructive if carried out with e0uipment suchas a chmidt hammer,which is sometimes used to estimate relative concrete strengths in the field.

The ultimate strength of concrete is influenced by the water#cementitious ratio (w/cm), the designconstituents, and the miing, placement and curing methods employed. !ll things being e0ual,concrete with a lower water#cement (cementitious) ratio makes a stronger concrete than that with ahigher ratio. The total 0uantity of cementitious materials (portland cement,slag cement, po11olans)can affect strength, water demand, shrinkage, abrasion resistance and density. !ll concrete willcrack independent of whether or not it has sufficient compressive strength. /n fact, high 2ortlandcement content mitures can actually crack more readily due to increased hydration rate. !sconcrete transforms from its plastic state, hydrating to a solid, the material undergoes shrinkage.2lastic shrinkage cracks can occur soon after placement but if the evaporation rate is high they often

can actually occur during finishing operations, for eample in hot weather or a bree1y day. /n veryhigh#strength concrete mitures (greater than 3& 42a) the crushing strength of the aggregate can bea limiting factorto the ultimate compressive strength. /n lean concretes (with a high water#cementratio) the crushing strength of the aggregates is not so significant. The internal forces in commonshapes of structure, such as arches,vaults, columns and walls are predominantly compressiveforces, with floors and pavements sub"ected to tensile forces. Compressive strength is widely usedfor specification re0uirement and 0uality control of concrete. 5ngineers know their target tensile(fleural) re0uirements and will epress these in terms of compressive strength.

Wired.com reported on !pril '6, $&&3 that a team from the 7niversity of Tehran, competing in acontest sponsored by the!merican Concrete /nstitute, demonstrated several blocks of concreteswith abnormally high compressive strengths between 6%& and %'& 42a (%8,&&& and 8,&&& psi) at$9 days.*$+The blocks appeared to use an aggregate of steelfibres and 0uart1 a mineral with a

compressive strength of ''&& 42a, much higher than typical high#strength aggregates suchas granite('&&'%& 42a or ',&&&$&,&&& psi). -eactive 2owder Concrete, also known as 7ltra#:igh 2erformance Concrete, can be even stronger, with strengths of up to 9&& 42a ('';,&&& 2/).*6+These are made by eliminating large aggregate completely, carefully controlling the si1e of the fineaggregates to ensure the best possible packing, and incorporating steel fibers (sometimes producedby grinding steel wool) into the matri. -eactive 2owder Concretes may also make use of silica fumeas a fine aggregate. Commercial -eactive 2owder Concretes are available in the '3$' 42a($,&&6,&&& psi) strength range.
https://en.wikipedia.org/wiki/Concretehttps://en.wikipedia.org/wiki/Compressive_strengthhttps://en.wikipedia.org/wiki/Compressive_strengthhttps://en.wikipedia.org/wiki/Compressive_strengthhttps://en.wikipedia.org/wiki/Tensile_strengthhttps://en.wikipedia.org/wiki/Coefficient_of_thermal_expansionhttps://en.wikipedia.org/wiki/Creep_(deformation)https://en.wikipedia.org/wiki/Creep_(deformation)https://en.wikipedia.org/wiki/Properties_of_concrete#cite_note-1https://en.wikipedia.org/wiki/Stress_(mechanics)#Mohr.27s_circlehttps://en.wikipedia.org/wiki/Reinforced_concretehttps://en.wikipedia.org/wiki/Rebarhttps://en.wikipedia.org/wiki/Fiber_reinforced_concretehttps://en.wikipedia.org/wiki/Fiber_reinforced_concretehttps://en.wikipedia.org/wiki/Prestressed_concretehttps://en.wikipedia.org/wiki/Tensile_stresshttps://en.wikipedia.org/wiki/Beam_(structure)https://en.wikipedia.org/wiki/Span_(architecture)https://en.wikipedia.org/wiki/Schmidt_hammerhttps://en.wikipedia.org/wiki/Schmidt_hammerhttps://en.wikipedia.org/wiki/Portland_cementhttps://en.wikipedia.org/wiki/Slag_cementhttps://en.wikipedia.org/wiki/Slag_cementhttps://en.wikipedia.org/wiki/Pozzolanhttps://en.wikipedia.org/wiki/Limiting_factorhttps://en.wikipedia.org/wiki/Archhttps://en.wikipedia.org/wiki/Vault_(architecture)https://en.wikipedia.org/wiki/Vault_(architecture)https://en.wikipedia.org/wiki/University_of_Tehranhttps://en.wikipedia.org/wiki/American_Concrete_Institutehttps://en.wikipedia.org/wiki/Properties_of_concrete#cite_note-2https://en.wikipedia.org/wiki/Steelhttps://en.wikipedia.org/wiki/Steelhttps://en.wikipedia.org/wiki/Quartzhttps://en.wikipedia.org/wiki/Granitehttps://en.wikipedia.org/wiki/Properties_of_concrete#cite_note-3https://en.wikipedia.org/wiki/Compressive_strengthhttps://en.wikipedia.org/wiki/Tensile_strengthhttps://en.wikipedia.org/wiki/Coefficient_of_thermal_expansionhttps://en.wikipedia.org/wiki/Creep_(deformation)https://en.wikipedia.org/wiki/Properties_of_concrete#cite_note-1https://en.wikipedia.org/wiki/Stress_(mechanics)#Mohr.27s_circlehttps://en.wikipedia.org/wiki/Reinforced_concretehttps://en.wikipedia.org/wiki/Rebarhttps://en.wikipedia.org/wiki/Fiber_reinforced_concretehttps://en.wikipedia.org/wiki/Prestressed_concretehttps://en.wikipedia.org/wiki/Tensile_stresshttps://en.wikipedia.org/wiki/Beam_(structure)https://en.wikipedia.org/wiki/Span_(architecture)https://en.wikipedia.org/wiki/Schmidt_hammerhttps://en.wikipedia.org/wiki/Portland_cementhttps://en.wikipedia.org/wiki/Slag_cementhttps://en.wikipedia.org/wiki/Pozzolanhttps://en.wikipedia.org/wiki/Limiting_factorhttps://en.wikipedia.org/wiki/Archhttps://en.wikipedia.org/wiki/Vault_(architecture)https://en.wikipedia.org/wiki/University_of_Tehranhttps://en.wikipedia.org/wiki/American_Concrete_Institutehttps://en.wikipedia.org/wiki/Properties_of_concrete#cite_note-2https://en.wikipedia.org/wiki/Steelhttps://en.wikipedia.org/wiki/Quartzhttps://en.wikipedia.org/wiki/Granitehttps://en.wikipedia.org/wiki/Properties_of_concrete#cite_note-3https://en.wikipedia.org/wiki/Concrete


2/35

Contents

*hide+

'5lasticity

$5pansion and shrinkage

6Cracking

o 6.'hrinkage cracking

o 6.$Tension cracking

%Creep

Water retention

;Concrete testing

o ;.'Testing fraud in


3/35

where

correction factor for aggregate source (taken as '.& unless determined otherwise)

weight of concrete (kips per cubic foot), where and

specified compressive strength of concrete at $9 days (ksi)

For normal weight concrete (wcD&.'% kips per cubic feet) Ecmay betaken as?

(ksi)

Expansion and shrinkage*edit+

Concrete has a very low coefficient of thermal epansion. :owever,if no provision is made for epansion, very large forces can becreated, causing cracks in parts of the structure not capable ofwithstanding the force or the repeated cycles ofepansion andcontraction. The coefficient of thermal epansion of 2ortlandcement concrete is &.&&&&&9 to &.&&&&'$ (per degree Celsius) (9

to '$ microstrainsEC)(9#'$ '4).*+

!s concrete matures it continues to shrink, due to the ongoingreaction taking place in the material, although the rate of shrinkagefalls relatively 0uickly and keeps reducing over time (for all practicalpurposes concrete is usually considered to not shrink due tohydration any further after 6& years). The relative shrinkage andepansion of concrete and brickwork re0uire carefulaccommodation when the two forms of construction interface.

Because concrete is continuously shrinking for years after it isinitially placed, it is generally accepted that under thermalloading it will never expand to its originally placed volume.

Bue to its lowthermal conductivity,a layer of concrete is fre0uentlyused for fireproofing of steel structures.

Cracking*edit+

alginatobel Gridge, wit1erland.

!ll concrete structures will crack to some etent. @ne of the earlydesigners of reinforced concrete,-obert 4aillart, employedreinforced concrete in a number of arched bridges. :is first bridgewas simple, using a large volume of concrete. :e then reali1ed thatmuch of the concrete was very cracked, and could not be a part ofthe structure under compressive loads, yet the structure clearly
https://en.wikipedia.org/wiki/Pounds_per_square_inchhttps://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=2https://en.wikipedia.org/wiki/Coefficient_of_thermal_expansionhttps://en.wikipedia.org/wiki/Thermal_expansionhttps://en.wikipedia.org/wiki/Thermal_expansionhttps://en.wikipedia.org/wiki/Thermal_expansionhttps://en.wikipedia.org/wiki/Properties_of_concrete#cite_note-5https://en.wikipedia.org/wiki/Thermal_conductivityhttps://en.wikipedia.org/wiki/Thermal_conductivityhttps://en.wikipedia.org/wiki/Thermal_conductivityhttps://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=3https://en.wikipedia.org/wiki/Salginatobel_Bridgehttps://en.wikipedia.org/wiki/Switzerlandhttps://en.wikipedia.org/wiki/Switzerlandhttps://en.wikipedia.org/wiki/Robert_Maillarthttps://en.wikipedia.org/wiki/Robert_Maillarthttps://en.wikipedia.org/wiki/Pounds_per_square_inchhttps://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=2https://en.wikipedia.org/wiki/Coefficient_of_thermal_expansionhttps://en.wikipedia.org/wiki/Thermal_expansionhttps://en.wikipedia.org/wiki/Thermal_expansionhttps://en.wikipedia.org/wiki/Properties_of_concrete#cite_note-5https://en.wikipedia.org/wiki/Thermal_conductivityhttps://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=3https://en.wikipedia.org/wiki/Salginatobel_Bridgehttps://en.wikipedia.org/wiki/Switzerlandhttps://en.wikipedia.org/wiki/Robert_Maillart


4/35

worked. :is later designs simply removed the cracked areas,leaving slender, beautiful concrete arches. ThealginatobelGridgeis an eample of this.

Concrete cracks due to tensile stress induced by shrinkage orstresses occurring during setting or use. Harious means are used toovercome this.Fiber reinforced concreteuses fine fibers distributedthroughout the mi or largermetal or other reinforcementelementsto limit the si1e and etent of cracks. /n many large structures "ointsor concealed saw#cuts are placed in the concrete as it sets to makethe inevitable cracks occur where they can be managed and out ofsight. Water tanks and highways are eamples of structuresre0uiring crack control.

Shrinkage cracking*edit+

hrinkage cracks occur when concrete members undergorestrained volumetric changes (shrinkage) as a result of eitherdrying, autogenous shrinkage or thermal effects. -estraint isprovided either eternally (i.e. supports, walls, and other boundary

conditions) or internally (differential drying shrinkage,reinforcement). @nce the tensile strength of the concrete iseceeded, a crack will develop. The number and width of shrinkagecracks that develop are influenced by the amount of shrinkage thatoccurs, the amount of restraint present and the amount andspacing of reinforcement provided.These are minor indications andhave no real structural impact on the concrete member.

2lastic#shrinkage cracks are immediately apparent, visible within &to $ days of placement, while drying#shrinkage cracks develop overtime. !utogenous shrinkage also occurs when the concrete is 0uiteyoung and results from the volume reduction resulting from thechemical reaction of the 2ortland cement.

Tension cracking*edit+

Concrete members may be put into tension by applied loads. Thisis most common in concrete beamswhere a transversely appliedload will put one surface into compression and the opposite surfaceinto tension due to induced bending. The portion of the beam that isin tension may crack. The si1e and length of cracks is dependenton the magnitude of the bending moment and the design of thereinforcing in the beam at the point under consideration. -einforcedconcrete beams are designed to crack in tension rather than incompression. This is achieved by providing reinforcing steel whichyields before failure of the concrete in compression occurs andallowing remediation, repair, or if necessary, evacuation of an

unsafe area.

Creep*edit+

Creepis the permanent movement or deformation of a material inorder to relieve stresses within the material. Concrete that issub"ected to long#duration forces is prone to creep. hort#durationforces (such as wind or earth0uakes) do not cause creep. Creepcan sometimes reduce the amount of cracking that occurs in a
https://en.wikipedia.org/wiki/Salginatobel_Bridgehttps://en.wikipedia.org/wiki/Salginatobel_Bridgehttps://en.wikipedia.org/wiki/Salginatobel_Bridgehttps://en.wikipedia.org/wiki/Fiber_reinforced_concretehttps://en.wikipedia.org/wiki/Rebarhttps://en.wikipedia.org/wiki/Rebarhttps://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=4https://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=5https://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=5https://en.wikipedia.org/wiki/Beam_(structure)https://en.wikipedia.org/wiki/Bendinghttps://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=6https://en.wikipedia.org/wiki/Creep_(deformation)https://en.wikipedia.org/wiki/Creep_(deformation)https://en.wikipedia.org/wiki/Salginatobel_Bridgehttps://en.wikipedia.org/wiki/Salginatobel_Bridgehttps://en.wikipedia.org/wiki/Fiber_reinforced_concretehttps://en.wikipedia.org/wiki/Rebarhttps://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=4https://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=5https://en.wikipedia.org/wiki/Beam_(structure)https://en.wikipedia.org/wiki/Bendinghttps://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=6https://en.wikipedia.org/wiki/Creep_(deformation)


5/35

concrete structure or element, but it also must be controlled. Theamount of primary and secondary reinforcing in concrete structurescontributes to a reduction in the amount of shrinkage, creep andcracking.

Water retention*edit+

2ortland cement concrete holds water. :owever, some types ofconcrete (like 2ervious concreteallow water to pass, hereby beingperfect alternatives to 4acadamroads, as they do not need to befitted with storm drains.

Concrete testing*edit+

Compression testing of a concrete cylinder

ame cylinder after failure

5ngineers usually specify the re0uired compressive strength of

concrete, which is normally given as the $9 day compressivestrength in megapascals (42a) or pounds per s0uare inch (psi).Twenty eight days is a long wait to determine if desired strengthsare going to be obtained, so three#day and seven#day strengthscan be useful to predict the ultimate $9#day compressive strengthof the concrete. ! $I strength gain between 3 and $9 days isoften observed with '&&I @2C (ordinary 2ortland cement)mitures, and between $I and %&I strength gain can be reali1edwith the inclusion of po11olans and supplementary cementitious
https://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=7https://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=7https://en.wikipedia.org/wiki/Pervious_concretehttps://en.wikipedia.org/wiki/Macadamhttps://en.wikipedia.org/wiki/Storm_drainshttps://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=8https://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=7https://en.wikipedia.org/wiki/Pervious_concretehttps://en.wikipedia.org/wiki/Macadamhttps://en.wikipedia.org/wiki/Storm_drainshttps://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=8


6/35

materials (C4s) such as fly ash andor slag cement. trength gaindepends on the type of miture, its constituents, the use ofstandard curing, proper testing by certified technicians, and care ofcylinders in transport. For practical immediate considerations, it isincumbent to accurately test the fundamental properties of concretein its fresh, plastic state.

Concrete is typically sampled while being placed, with testingprotocols re0uiring that test samples be cured under laboratoryconditions (standard cured). !dditional samples may be field cured(non#standard) for the purpose of early JstrippingJ strengths, that is,form removal, evaluation of curing, etc. but the standard curedcylinders comprise acceptance criteria. Concrete tests canmeasure the plastic (unhydrated) properties of concrete prior to,and during placement. !s these properties affect the hardenedcompressive strength and durability of concrete (resistance tofree1e#thaw), the properties of workability (slumpflow),temperature, density and age are monitored to ensure theproduction and placement of J0ualityJ concrete. Bepending on

pro"ect location, tests are performed per!T4/nternational,5uropean Committee for tandardi1ationor Canadiantandards !ssociation. !s measurement of 0uality must representthe potential of concrete material delivered and placed, it isimperative that concrete technicians performing concrete tests arecertified to do so according to these standards. tructural design,concrete material design and properties are often specified inaccordance with nationalregional design codes such as!mericanConcrete /nstitute.

Compressive strengthtests are conducted by certified techniciansusing an instrumented, hydraulic ramwhich has been annuallycalibrated with instruments traceable to the Cement and Concrete

-eference Aaboratory(CC-A) of the


7/35

See also*edit+

egregation in concrete# particle segregation in concrete

applications

Creep and shrinkage of concrete

Properties of Concrete

2roperties of concrete are divide into two ma"or groups

roperties of !resh Concrete

roperties of "ardened Concrete

Fresh Concrete

Fresh concrete is that stage of concrete in which concrete can be moulded and it is in plastic state.

This is also called >reen Concrete. !nother term used to describe the state of fresh concrete

is consistence, which is the ease with which concrete will flow.

2roperties of Fresh Concrete

Following are the important properties of fresh concrete

#. Setting

$. %orka&ility

'. Bleeding and Segregation

a. Bleeding

&. Segregation

(. "ydration

). *ir +ntrainment

. etting of Concrete

The hardening of concrete before its hydration is known as setting of concrete. @-
https://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=10https://en.wikipedia.org/wiki/Segregation_in_concretehttps://en.wikipedia.org/wiki/Creep_and_shrinkage_of_concretehttp://www.aboutcivil.org/properties-of-hardened-concrete.htmlhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sehttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#wohttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sghttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#hyhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#aihttps://en.wikipedia.org/w/index.php?title=Properties_of_concrete&action=edit&section=10https://en.wikipedia.org/wiki/Segregation_in_concretehttps://en.wikipedia.org/wiki/Creep_and_shrinkage_of_concretehttp://www.aboutcivil.org/properties-of-hardened-concrete.htmlhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sehttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#wohttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sghttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#hyhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#ai


8/35

The hardening of concrete before it gains strength. @-

The transition process of changing of concrete from plastic state to hardened state. etting of

concrete is based or related to the setting of cement paste. Thus cement properties greatly affect the

setting time.

Factors affecting setting?

Following are the factors that affect the setting of concrete.

'. Water Cement ratio

$. uitable Temperature

6. Cement content

%. Type of Cement

. Fineness of Cement

;. -elative :umidity

3. !dmitures

9. Type and amount of !ggregate

Workability is often referred to as the ease with which a concrete can be transported, placed and

consolidated without ecessive bleeding or segregation.

@-

The internal work done re0uired to overcome the frictional forces between concrete ingredients for

full compaction. /t is obvious that no single test can evaluate all these factors. /n fact, most of these

cannot be easily assessed even though some standard tests have been established to evaluate

them under specific conditions.

/n the case of concrete, consistence is sometimes taken to mean the degree of wetnessL within

limits, wet concretes are more workable than dry concrete, but concrete of same consistence may

vary in workability.

Gecause the strength of concrete is adversely and significantly affected by the presence of voids in

the compacted mass, it is vital to achieve a maimum possible density. This re0uires sufficient

workability for virtually full compaction to be possible using a reasonable amount of work under the

given conditions. 2resence of voids in concrete reduces the density and greatly reduces the

strength? I of voids can lower the strength by as much as 6&I.

Slump Testcan be used to find out the workability of concrete. Hiew 2rocedure of lump Test

Factors affecting concrete workability?

i. Water#Cement ratio
http://www.aboutcivil.org/tests-on-concrete.htmlhttp://www.aboutcivil.org/tests-on-concrete.html


9/35

ii. !mount and type of !ggregate

iii. !mount and type of Cement

iv. Weather conditions

'. Temperature

$. Wind

v. Chemical !dmitures

vi. and to !ggregate ratio

i. Water content or Water Cement -atio

4ore the water cement ratio more will be workability of concrete. ince by simply adding water theinter particle lubrication is increased. :igh water content results in a higher fluidity and greater

workability but reduces the strength of concrete. Gecause with increasing wc ratio the strength

decreases as more water will result in higher concrete porosity. o, the lower the wc, the lower is

the void volumesolid volume, and the stronger the hardened cement paste. Also See:-ate of

trength >ain of Concrete

/ncreased water content also results in bleeding, hence, increased water content can also mean that

cement slurry will escape through the "oints of the formwork (huttering).


ince larger !ggregate si1es have relatively smaller surface areas (for the cement paste to coat) and

since less water means less cement, it is often said that one should use the largest practicable

!ggregate si1e and the stiffest practical mi. 4ost building elements are constructed with a

maimum !ggregate si1e of 6% to ', larger si1es beingprohibited by the closeness of the

reinforcing bars. Also See:5ffects of Bifferent !ggregates on 2roperties of Concrete

Gecause concrete is continuously shrinking for years after it is initially placed, it is generally

accepted that under thermal loading it will never epand to itJs originally#placed volume. 4ore the

amount of aggregate less will be workability.

7sing smooth and round aggregate increases the workability. Workability reduces if angular

and rough aggregate is used.

>reater si1e of !ggregate# less water is re0uired to lubricate it, the etra water is available

for workability
http://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/design-of-shuttering-formwork.htmlhttp://www.aboutcivil.org/effects-of-different-properties-of-aggregates-on-concrete.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/design-of-shuttering-formwork.htmlhttp://www.aboutcivil.org/effects-of-different-properties-of-aggregates-on-concrete.html


10/35

!ngular aggregates increases flakiness or elongation thus reduces workability. -ound

smooth aggregates re0uire less water and less lubricationand gretaer workability in a given

wc ratio

2orous aggregates re0uire more water compared to non absorbent aggregates for achieving

sam degree of workability.

iii. !ggregate Cement ratio

4ore ratio, less workability. ince less cement mean less water, so the paste is stiff.

iv. Weather Conditions

1. Temperature

/f temperature is high, evaporation increases, thus workability decreases.

2. Wind:/f wind is moving with greater velocity, the rate of evaporation also increase reduces the amount of

water and ultimately reducing workability.

v. !dmitures

Chemical admitures can be used to increase workability.

7se of air entraining agent produces air bubbles which acts as a sort of ball bearing between

particles and increases mobility, workability and decreases bleeding, segregation. The use of fine

po11olanic materials also have better lubricating effect and more workability.


/f the amount of sand is more the workability will reduce because sand has more surface area and

more contact area causing more resistance. The ingredients of concrete can be proportioned by

weight or volume. the goal is to provide the desired strength and workability at minimum epense. !

low water#cement ratio is used to achieve a stronger concrete. /t would seem therefore that by

keeping the cement content high one could use enough for god workability and still have a low wc

ratio. the problem is that cement is the most costly of the basic ingredients. the dilema is easily seen

in the graphs below.

6(a). Concrete Gleeding

Gleeding in concrete is sometimes referred as water gain. /t is a particular form of segregation, in

which some of the water from the concrete comes out to the surface of the concrete, being of the

lowest specific gravity among all the ingredients of concrete. Gleeding is predominantly observed in

a highly wet mi, badly proportioned and insufficiently mied concrete. /n thin members like roof slab

or road slabs and when concrete is placed in sunny weather show ecessive bleeding.


11/35

Bue to bleeding, water comes up and accumulates at the surface. ometimes, along with this water,

certain 0uantity of cement also comes to the surface. When the surface is worked up with the trowel,

the aggregate goes down and the cement and water come up to the top surface. This formation of

cement paste at the surface is known as MaitanceN. /n such a case, the top surface of slabs and

pavements will not have good wearing 0uality. This laitance formed on roads produces dust in

summer and mud in rainy season.

Water while traversing from bottom to top, makes continuous channels. /f the water cement ratio

used is more than &.3, the bleeding channels will remain continuous and un segmented. These

continuous bleeding channels are often responsible for causing permeability of the concrete

structures. While the miing water is in the process of coming up, it may be intercepted by

aggregates. The bleeding water is likely to accumulate below the aggregate. This accumulation of

water creates water voids and reduces the &ond &etween the aggregates and the paste.

The above aspect is more pronounced in the case of flaky aggregate. imilarly, the water that

accumulates below the reinforcing bars reduces the bond between the reinforcement and the

concrete. The poor bond between the aggregate and the paste or the reinforcement and the paste

due to &leeding can &e remedied &y re vi&ration of concrete. The formation of laitance and the

conse0uent bad effect can be reduced by delayed finishing operations.

Gleeding rate increases with time up to about one hour or so and thereafter the rate decreases but

continues more or less till the final setting time of cement.

2revention of Gleeding in concrete

Gleeding can be reduced by proper proportioning and uniform and complete miing.

7se of finely divided po11olanic materials reduces bleeding by creating a longer path for the

water to traverse.

!ir#entraining agent is very effective in reducing the bleeding.

Gleeding can be reduced by the use of finer cement or cement with low alkali content. -ich

mies are less susceptible to bleeding than lean mies.

The bleeding is not completely harmful if the rate of evaporation of water from the surface is e0ual to

the rate of bleeding. -emoval of water, after it had played its role in providing workability, from the

body of concrete by way of bleeding will do good to the concrete.

5arly bleeding when the concrete mass is fully plastic, may not cause much harm, because concrete

being in a fully plastic condition at that stage, will get subsided and compacted. /t is the delayed

bleeding, when the concrete has lost its plasticity, which causes undue harm to the concrete.

Controlled re vibration may be adopted to overcome the bad effect of bleeding.

6(b). egregation in concrete


12/35

Gack to top

egregation can be defined as the separation of the constituent materials of concrete. ! good

concrete is one in which all the ingredients are properly distributed to make a homogeneous miture.

There are considerable differences in the si1es and specific gravities of the constituent ingredients of

concrete. Therefore, it is natural that the materials show a tendency to fall apart.

egregation may be of three types

'. Coarse aggregateseparating out or settling down from the rest of the matri.

$. asteseparating away from coarse aggregate.

6. %aterseparating out from the rest of the material being a material of lowest specific gravity.

! well made concrete, taking into consideration various parameters such as grading, si1e, shape

and surface teture of aggregate with optimum 0uantity of waters makes a cohesive mi. uch

concrete will not ehibit any tendency for segregation. The cohesive and fatty characteristics of

matri do not allow the aggregate to fall apart, at the same timeL the matri itself is sufficiently

contained by the aggregate. imilarly, water also does not find it easy to move out freely from the

rest of the ingredients.

The conditions favorable for segregation are?

'. Gadly proportioned mi where sufficient matri is not there to bind and contain the

aggregates

$. /nsufficiently mied concrete with ecess water content

6. Bropping of concrete from heights as in the case of placing concrete in column concreting

%. When concrete is discharged from a badly designed mier, or from a mier with worn out

blades

. Conveyance of concrete by conveyor belts, wheel barrow, long distance haul by dumper,

long lift by skip and hoist are the other situations promoting segregation of concrete

Hibration of concrete is one of the important methods of compaction. /t should be remembered thatonly comparatively dry mi should be vibrated. /t too wet a mi is ecessively vibratedL it is likely that

the concrete gets segregated. /t should also be remembered that vibration is continued "ust for

re0uired time for optimum results. /f the vibration is continued for a long time, particularly, in too wet

a mi, it is likely to result in segregation of concrete due to settlement of coarse aggregate in matri.

%. :ydration in concrete
http://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#tophttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#top


13/35

Concrete derives its strength by the hydration of cement particles. The hydration of cement is not a

momentary action but a process continuing for long time. @f course, the rate of hydration is fast to

start with, but continues over a very long time at a decreasing rate /n the field and in actual work,

even a higher watercement ratio is used, since the concrete is open to atmosphere, the water used

in the concrete evaporates and the water available in the concrete will not be sufficient for effective

hydration to take place particularly in the top layer.

/f the hydration is to continue, etra water must be added to refill the loss of water on account of

absorption and evaporation. Therefore, the curing can be considered as creation of a favorable

environment during the early period for uninterrupted hydration. The desirable conditions are, a

suitable temperature and ample moisture.

Concrete, while hydrating, releases high heat of hydration. This heat is harmful from the point of

view of volume stability. Oeat of hydration of concrete may also shrinkage in concrete, thus producing

cracks. /f the heat generated is removed by some means, the adverse effect due to the generation of

heat can be reduced. This can be done by a thorough water curing.

. !ir 5ntrainment

!ir entrainment reduces the density of concrete and conse0uently reduces the strength. !ir

entrainment is used to produce a number of effects in both the plastic and the hardened

concrete.These include?

'. -esistance to free1ethaw action in the hardened concrete.

$. /ncreased cohesion, reducing the tendency to bleed and segregation in the plastic concrete.

6. Compaction of low workability mies including semi#dry concrete.

%. tability of etruded concrete.

. Cohesion and handling properties in bedding mortars.

2roperties of normal strength 2ortland cement concrete

Typical properties of normal strength 2ortland cement concrete?

Bensity? 2240 - 2400 kg/m3(140 - 150 l/ft3)

Compressive strength ? 20 - 40 !"a (3000 - #000 $si)

Fleural strength ? 3 - 5 !"a (400 - %00 $si)

Tensile strength ? 2 - 5 !"a (300 - %00 $si)
http://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.htmlhttp://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.html


14/35

4odulus of elasticity? 14000 - 41000 !"a (2 - # & 10#$si)

2ermeability ? 1 & 10-10cm/sec

Coefficient of thermal epansion ? 10-5'-1 (55 & 10-#'*-1)

Brying shrinkage ? 4 - + & 10-4

Brying shrinkage of reinforced concrete ? 2 - 3 & 10-4

2oissonJs ratio? 020 - 021

hear strength? # - 1% !"a

pecific heat capacity? 0%5 k,/kg (01+ .tu/lm'* (kcal/kg'))

trength?

The strength of concrete is basically referred to compressive strength and it depends upon three

factors.

'# 2aste trength

$# /nterfacial Gonding

6# !ggregate trength

'. 2aste strength?

/t is mainly due to the binding properties of cement that the ingredients are compacted together. /fthe paste has higher binding strength, higher will be strength of concrete.

$. /nterfacial bonding?

/nterfacial bonding is very necessary regarding the strength. Clay hampers the bonding between

paste and aggregate. The aggregate should be washed for a better bonding between paste and

aggregate.

6. !ggregate strength?

/t is mainly the aggregate that provide strength to concrete especially coarse aggregates which act"ust like bones in the body. -ough and angular aggregate provides better bonding and high strength.

Factors affecting Strength of concrete:

Following are the factors that affect the strength of concrete?

'. Water#Cement ratio

$. Type of cementing material
http://www.engineeringtoolbox.com/young-modulus-d_417.htmlhttp://www.engineeringtoolbox.com/poissons-ratio-d_1224.htmlhttp://www.engineeringtoolbox.com/poissons-ratio-d_1224.htmlhttp://www.engineeringtoolbox.com/stress-strain-d_950.htmlhttp://www.engineeringtoolbox.com/specific-heat-solids-d_154.htmlhttp://www.engineeringtoolbox.com/young-modulus-d_417.htmlhttp://www.engineeringtoolbox.com/poissons-ratio-d_1224.htmlhttp://www.engineeringtoolbox.com/stress-strain-d_950.htmlhttp://www.engineeringtoolbox.com/specific-heat-solids-d_154.html


15/35

6. !mount of cementing material

%. Type of aggregate

. !ir content

;. !dmitures

1. Water-Cement ratio:/t is water cement ratio that basically governs the property of strength. Aesser the water cement

ratio, greater will be strength.

2. Type of cement:

Type of cement affect the hydration process and therefore strength of concrete.

!mount of cementing material? it is the paste that holds or binds all the ingredients. Thus greater

amount of cementing material greater will be strength.

3. Type of Aggregate:

-ough and angular aggregates is preferable as they provide greater bonding.

4. Admixtures:

Chemical admitures like plastici1ers reduce the water cement ratio and increase the strength of

concrete at same water cement ratio. 4ineral admitures affect the strength at later stage and

increase the strength by increasing the amount of cementing material

Creep in concrete

Befinition?

Concrete creep is defined as? deformation of structure under sustained load. Gasically, long term

pressure or stress on concrete can make it change shape. This deformation usually occurs in the

direction the force is being applied. Aike a concrete column getting more compressed, or a beam

bending. Creep does not necessarily cause concrete to fail or break apart. Creep is factored in when

concrete structures are designed.

!actors *ffecting Creep

'. !ggregate

$. 4i 2roportions

6. !ge of concrete

#. -nfluence of *ggregate


16/35

!ggregate undergoes very little creep. /t is really the paste which is responsible for the creep.

:owever, the aggregate influences the creep of concrete through a restraining effect on the

magnitude of creep. The paste which is creeping under load is restrained by aggregate which do not

creep. The stronger the aggregate the more is the restraining effect and hence the less is the

magnitude of creep. The modulus of elasticity of aggregate is one of the important factors influencing

creep.

/t can be easily imagined that the higher the modulus of elasticity the less is the creep. Aight weight

aggregate shows substantially higher creep than normal weight aggregate.

$. /nfluence of 4i 2roportions?

The amount of paste content and its 0uality is one of the most important factors influencing creep. !

poorer paste structure undergoes higher creep. Therefore, it can be said that creep increases with

increase in watercement ratio. /n other words, it can also be said that creep is inversely proportional

to the strength of concrete. Groadly speaking, all other factors which are affecting the watercement

ratio are also affecting the creep.

6. /nfluence of !ge?

!ge at which a concrete member is loaded will have a predominant effect on the magnitude of

creep. This can be easily understood from the fact that the 0uality of gel improves with time. uch

gel creeps less, whereas a young gel under load being not so stronger creeps more. What is said

above is not a very accurate statement because of the fact that the moisture content of the concrete

being different at different age also influences the magnitude of creep.

5ffects of Creep on Concrete and -einforced Concrete

/n reinforced concrete beams, creep increases the deflection with time and may be a

critical consideration in design.

/n eccentrically loaded columns, creep increases the deflection and can load to buckling.

/n case of statically indeterminate structures and column and beam "unctions creep may

relieve the stress concentration induced by shrinkage, temperatures changes or movement

of support. Creep property of concrete will be useful in all concrete structures to reduce the

internal stresses due to non#uniform load or restrained shrinkage.

/n mass concrete structures such as dams, on account of differential temperature conditions

at the interior and surface, creep is harmful and by itself may be a cause of cracking in the

interior of dams. Therefore, all precautions and steps must be taken to see that increase in

temperature does not take place in the interior of mass concrete structure.

Aoss of prestress due to creep of concrete in prestressed concrete structure.


17/35

Durai!ity of Concrete

Befinition

The ability of concrete to withstand the conditions for which it is designed without deterioration for a

long period of years is known as durability.

Burability of concrete may be defined as the ability of concrete to resist weathering action, chemical

attack, and abrasion while maintaining its desired engineering properties.

Burability is defined as the capability of concrete to resist weathering action, chemical attack and

abrasion while maintaining its desired engineering properties. /t normally refers to the duration or life

span of trouble#free performance. Bifferent concretes re0uire different degrees of durability

depending on the eposure environment and properties desired. For eample, concrete eposed to

tidal seawater will have different re0uirements than indoor concrete.

Concrete will remain durable if?

The cement paste structure is dense and of low permeability

7nder etreme condition, it has entrained air to resist free1e#thaw cycle.

/t is made with graded aggregate that are strong and inert

The ingredients in the mi contain minimum impurities such as alkalis, Chlorides, sulphatesand silt

!actors *ffecting /ura&ility of Concrete

Burability of Concrete depends upon the following factors?

Cement content

4i must be designed to ensure cohesion and prevent segregation and bleeding. /f cement is

reduced, then at fied wc ratio the workability will be reduced leading to inade0uate compaction.

:owever, if water is added to improve workability, water cement ratio increases and resulting inhighly permeable material.

Compaction

The concrete as a whole contain voids can be caused by inade0uate compaction. 7sually it is being

governed by the compaction e0uipments used, type of formworks, and density of the steelwork


18/35

Curing

/t is very important to permit proper strength development aid moisture retention and to ensure

hydration process occur completely

Cover

Thickness of concrete cover must follow the limits set in codes

ermea&ility

/t is considered the most important factor for durability. /t can be noticed that higher permeability is

usually caused by higher porosity .Therefore, a proper curing, sufficient cement, proper compaction

and suitable concrete cover could provide a low permeability concrete

Types of /ura&ility of Concrete

There are many types but the ma"or Concrete Burability types are?

'. hysical dura&ility

$. Chemical dura&ility

hysical /ura&ility

2hysical durability is against the following actions

'. Free1ing and thawing action

$. 2ercolation 2ermeability of water

6. Temperature stresses i.e. high heat of hydration

Chemical /ura&ility

Chemical durability is against the following actions

'. !lkali !ggregate -eaction

$. ulphate !ttack

6. Chloride /ngress

%. Belay 5ttringite Formation


19/35

. Corrosion of reinforcement

Causes for the ack of /ura&ility in Concrete#. +xternal Causes0

a. 5treme Weathering Conditions

b. 5treme Temperature

c. 5treme :umidity

d. !brasion

e. 5lectrolytic !ction

f. !ttack by a natural or industrial li0uids or gases

$. -nternal Causesa1 hysical

Holume change due to difference in thermal properties of aggregates and cement paste

Frost !ction

&1 Chemical

!lkali !ggregate -eactions

i. !lkali ilica -eaction

ii. !lkali ilicate -eaction

iii. !lkali Carbonate -eaction

Corrosion of teel

Shrinkage in Concrete

Concrete is sub"ected to changes in volume either autogenous or induced. Holume change is one of

the most detrimental properties of concrete, which affects the long#term strength and durability. To

the practical engineer, the aspect of volume change in concrete is important from the point of view

that it causes unsightly cracks in concrete.


20/35

We have discussed elsewhere the effect of volume change due to thermal properties of aggregate

and concrete, due to alkaliaggregate reaction, due to sulphate action etc. 2resently we shall discuss

the volume change on account of inherenet properties of concrete MshrinkageN.

@ne of the most ob"ectionable defects in concrete is the presence of cracks, particularly in floors and

pavements. @ne of the important factors that contribute to the cracks in floors and pavements is thatdue to shrinkage. /t is difficult to make concrete which does not shrink and crack. /t is only a 0uestion

of magnitude.


21/35

strength. From the above it can be inferred that high watercement ratio, badly proportioned

concrete, rapid drying, greater bleeding, unintended vibration etc., are some of the reasons for

plastic shrinkage. /t can also be further added that richer concrete undergoes greater plastic

shrinkage.

2lastic shrinkage can be reduced mainly by preventing the rapid loss of water from surface. This canbe done by covering the surface with polyethylene sheeting immediately on finishing operationL by

fog spray that keeps the surface moistL or by working at night. 7se of small 0uantity of aluminium

powder is also suggested to offset the effect of plastic shrinkage.

imilarly, epansive cement or shrinkage compensating cement also can be used for controlling the

shrinkage during the setting of concrete.

b. Brying hrinkage

Oust as the hydration of cement is an ever lasting process, the drying shrinkage is also an ever

lasting process when concrete is sub"ected to drying conditions. The drying shrinkage of concrete is

analogous to the mechanism of drying of timber specimen. The loss of free water contained in

hardened concrete, does not result in any appreciable dimension change. /t is the loss of water held

in gel pores that causes the change in the volume. 7nder drying conditions, the gel water is lost

progressively over a long time, as long as the concrete is kept in drying conditions. Cement paste

shrinks more than mortar and mortar shrinks more than concrete. Concrete made with smaller si1e

aggregate shrinks more than concrete made with bigger si1e aggregate. The magnitude of drying

shrinkage is also a function of the fineness of gel.The finer the gel the more is the shrinkage.

c. !utogeneous hrinkage

/n a conservative system i.e. where no moisture movement to or from the paste is permitted, when

temperature is constant some shrinkage may occur. The shrinkage of such a conservative system is

known as autogeneous shrinkage.!utogeneous shrinkage is of minor importance and is not

applicable in practice to many situations ecept that of mass of concrete in the interior of a concrete

dam.

d. Carbonation hrinkage

Carbon dioide present in the atmosphere reacts in the presence of water with hydrated cement.

Calcium hydroide *Ca(@:)$+ gets converted to calcium carbonate and also some other cementcompounds are decomposed. uch a complete decomposition of calcium compound in hydrated

cement is chemically possible even at the low pressure of carbon dioide in normal atmosphere.

Carbonation penetrates beyond the eposed surface of concrete very slowly.

The rate of penetration of carbon dioide depends also on the moisture content of the concrete and

the relative humidity of the ambient medium. Carbonation is accompanied by an increase in

eight of the concrete and by shrinkage.


22/35

Carbonation shrinkage is probably caused by the dissolution of crystals of calcium hydroide and

deposition of calcium carbonate in its place. !s the new product is less in volume than the product

replaced, shrinkage takes place.

Carbonation of concrete also results in increased strength and reduced permeability, possibly

because water released by carbonation promotes the process of hydration and also calciumcarbonate reduces the voids within the cement paste. !s the magnitude of carbonation shrinkage is

very small when compared to long term drying shrinkage, this aspect is not of much significance

Factors !ffecting hrinkage

@ne of the most important factors that affects shrinkage is the drying condition or in other words, the

relative humidity of the atmosphere at which the concrete specimen is kept. /f the concrete is placed

in '&& per cent relative humidity for any length of time, there will not be any shrinkageL instead there

will be a slight swelling. The typical relationship between shrinkage and time for which concrete is

stored at different relative humidities is shown in Figure. The graph shows that the magnitude of

shrinkage increases with time and also with the reduction of relative humidity.

The rate of shrinkage decreases rapidly ith time. /t is observed that '% to 6% per cent of the $&

year shrinkage occurs in $ weeks, %& to 9& per cent of the $& year shrinkage occurs in 6 months and

;; to 9 per cent of the $& year shrinkage occurs in one year. !nother important factor which

influences the magnitude of shrinkage is watercement ratio of the concrete. The richness of the

concrete also has a significant influence on shrinkage. !ggregate plays an important role in the

shrinkage properties of concrete. The 0uantum of an aggregate, its si1e, and its modulus of elasticity

influence the magnitude of drying shrinkage.

:arder aggregate with higher modulus of elasticity like 0uart1 shrinks much less than softeraggregates such as sandstone.

4oisture 4ovement Concrete shrinks when allowed to dry in air at a lower relative humidity and it

swells when kept at '&& per cent relative humidity or when placed in water.

Oust as drying shrinkage is an ever continuing process, swelling, when continuously placed in water

is also an ever continuing process. /f a concrete sample sub"ected to drying condition, at some

stage, is sub"ected to wetting condition, it starts swelling. /t is interesting to note that all the initial

drying shrinkage is not recovered even after prolonged storage in water which shows that the

phenomenon of drying shrinkage is not a fully reversible one.

Oust as the drying shrinkageis due to loss of adsorbed water around gel particles, swelling is dueto the adsorption of water by the cement gel. The water molecules act against the cohesive force

and tend to force the gel particles further apart as a result of which swelling takes place. /n addition,

the ingress of water decreases the surface tension of the gel.

The property of swelling when placed in wet condition, and shrinking when placed in drying condition

is referred as moisture movement in concrete


23/35

Acid Attac" on Concrete

*cid *ttack0

@rdinary 2ortland cement is alkaline in nature. When the cement paste comes into contact with the

acids its components break down, this phenomenon is known as acid attack.

When acid attack concrete it dissolves both hydrated and unhydrated cement compounds as well as

calcareous aggregates. /n many of the cases the chemical reaction results in water soluble calcium

compounds which are leached away. Concrete vulnerability to acid attack increases as the p: of the

acid in contact decreases from ;.. Begree of aggression is Slightfor p:? 2.)to ).), Severefor

p:? ).)to(.)and 3ery Severefor p: less than (.).

Note that the aggression of the acid attack is not only because of its p: value but also due to the

presence of C@$in relation with the hardness of water.

#ate of $trengt% &ain of Concrete

Strength can be defined as ability to resist change. @ne of the most valuable properties of the

concrete is its strength. trength is most important parameter that gives the picture of overall 0uality

of concrete. trength of concrete usually directly related to cement paste.

4any factors influence the rate at which the strength of concrete increases after miing.

Gefore coming toward the factors that influence the strength gain of concrete, it is important to have

concept of these terminologies?

"ardening is the process of growth of strength. This is often confused with JsettingJ but setting and

hardening are not the same.

Settingis the stiffening of the concrete after it has been placed. :ardening may continue for weeks

or months after the concrete has been mied and placed.

Factors affecting strength gain P rate of strength gain of concrete

Concrete porosity

Hoids in concrete can be filled with air or with water. Groadly speaking, the more porous the

concrete, the weaker it will be. 2robably the most important source of porosity in concrete is the ratio

of water to cement in the mi, known as the Jwater to cementJ ratio.Watercement ratio


24/35

This is defined as the mass of water divided by the mass of cement

in a mi. The watercement ratio may be abbreviated to Jwc ratioJ or "ust JwcJ. /n mies where the

wc is greater than approimately &.%, all the cement can, react with water to form cement hydration

products. !t higher wc ratios it follows that the space occupied by the additional water above wc D

&.% will remain as pore space filled with water, or with air if the concrete dries out.

Conse0uently, as the wc ratio increases, the porosity of the cement paste in the concrete also

increases. !s the porosity increases, the compressive strength of the concrete will decrease.

oundness of aggregate

/f the aggregate in concrete is weak, the concrete will also be weak. -ocks with low strength, such

as chalk, are clearly unsuitable for use as aggregate.

!ggregate paste bond

The compactness of the bond between the paste and the aggregate is critical. /f there is no bond,

the aggregate effectively represents a void P voids are a source of weakness in concrete.

Cement#related parameters

4any parameters relating to the composition of the cement constituents and their proportions in the

cement can affect the rate of strength gain and the final strength achieved. These include?


25/35

'. !lite content (Tri#Calcium silicates) P Gelite contents (Bi#calcium silicates)

$. !lite P belite reactivity

6. ulfate contents

!lite is the most reactive cement mineral that contributes significantly to concrete strength. 4ore

!lite should give better early strengths (JearlyJ means up to about 3 days).

ulfate in cement, both the clinker sulfate and added gypsum, retards the hydration phase. /f there is

insufficient sulfate, a flash set (rapid hardening of freshly mied cement paste with noticeable heat

evolution) may occur. on the other hand too much sulfate contents can cause false#setting(rapid

hardening of freshly mied cement paste with minimum heat evolution)

ome physical parameters of cement also play role in strength gain of concrete like Cement

surface area and particle si4e distri&ution.

Fineness is often epressed in terms of total particle surface area. 4ore fine is cementL greater will

be its hydration rate. 2article si1e distribution is also very important prospect in strength gain of

concrete. Cement with very finely#ground gypsum and clinker particles results in slower hydration.

Tests To Betermine The trength >ain P -ate of trength >ain @f Concrete

/n concrete practice the strength of concrete is characteri1ed by the $9 day value and some other

properties are also related to the $9 day strength. !fter $9 days, different tests are usually performed

to determine the strength gain of the concrete. These are as under?

For trength >ain?

Compressi!e Strength Test

'. Cylinder test

$. Cube test

Tensile Strength Test

Split cylinder test

Fle"ural Strength Test

'. Two point loading test

$. Three point loading test

-ate of strength gain of concrete?


26/35

To determine the rate of gain of strength of concrete, there is a need to select period shorter than $9

day, as $9 day is considered to be the reference time. /n concrete practice, it is accepted that after

$9 days concrete usually gains most of its strength. trength determined at an early stage say after

3th day of placing of concrete can be compared to strength determined after $9 days, which is

considered to be the reference time. /n this way, rate of gain of strength of concrete can be

determined.

2roperties of concrete are divide into two ma"or groups

roperties of !resh Concrete

roperties of "ardened Concrete

Fresh Concrete

Fresh concrete is that stage of concrete in which concrete can be moulded and it is in plastic state.

This is also called >reen Concrete. !nother term used to describe the state of fresh concreteis consistence, which is the ease with which concrete will flow.

2roperties of Fresh Concrete

Following are the important properties of fresh concrete

#. Setting

$. %orka&ility

'. Bleeding and Segregation

a. Bleeding

&. Segregation

(. "ydration

). *ir +ntrainment

'. etting of Concrete

The hardening of concrete before its hydration is known as setting of concrete. @-

The hardening of concrete before it gains strength. @-
http://www.aboutcivil.org/properties-of-hardened-concrete.htmlhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sehttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#wohttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sghttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#hyhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#aihttp://www.aboutcivil.org/properties-of-hardened-concrete.htmlhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sehttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#wohttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sghttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#hyhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#ai


27/35

The transition process of changing of concrete from plastic state to hardened state. etting of

concrete is based or related to the setting of cement paste. Thus cement properties greatly affect the

setting time.

Factors affecting setting?

Following are the factors that affect the setting of concrete.

'. Water Cement ratio

$. uitable Temperature

6. Cement content

%. Type of Cement

. Fineness of Cement

;. -elative :umidity

3. !dmitures

9. Type and amount of !ggregate

$. Workability of Concrete

Workability is often referred to as the ease with which a concrete can be transported, placed and

consolidated without ecessive bleeding or segregation.

@-

The internal work done re0uired to overcome the frictional forces between concrete ingredients for

full compaction. /t is obvious that no single test can evaluate all these factors. /n fact, most of these

cannot be easily assessed even though some standard tests have been established to evaluatethem under specific conditions.

/n the case of concrete, consistence is sometimes taken to mean the degree of wetnessL within

limits, wet concretes are more workable than dry concrete, but concrete of same consistence may

vary in workability.

Gecause the strength of concrete is adversely and significantly affected by the presence of voids in

the compacted mass, it is vital to achieve a maimum possible density. This re0uires sufficient

workability for virtually full compaction to be possible using a reasonable amount of work under the

given conditions. 2resence of voids in concrete reduces the density and greatly reduces the

strength? I of voids can lower the strength by as much as 6&I.

Slump Testcan be used to find out the workability of concrete. Hiew 2rocedure of lump Test

Factors affecting concrete workability?

i. Water#Cement ratio
http://www.aboutcivil.org/tests-on-concrete.htmlhttp://www.aboutcivil.org/tests-on-concrete.html


28/35


iii. !mount and type of Cement

iv. Weather conditions

'. Temperature

$. Wind

v. Chemical !dmitures


i. Water content or Water Cement -atio

4ore the water cement ratio more will be workability of concrete. ince by simply adding water theinter particle lubrication is increased. :igh water content results in a higher fluidity and greater

workability but reduces the strength of concrete. Gecause with increasing wc ratio the strength

decreases as more water will result in higher concrete porosity. o, the lower the wc, the lower is

the void volumesolid volume, and the stronger the hardened cement paste. Also See:-ate of

trength >ain of Concrete

/ncreased water content also results in bleeding, hence, increased water content can also mean that

cement slurry will escape through the "oints of the formwork (huttering).


ince larger !ggregate si1es have relatively smaller surface areas (for the cement paste to coat) and

since less water means less cement, it is often said that one should use the largest practicable

!ggregate si1e and the stiffest practical mi. 4ost building elements are constructed with a

maimum !ggregate si1e of 6% to ', larger si1es beingprohibited by the closeness of the

reinforcing bars. Also See:5ffects of Bifferent !ggregates on 2roperties of Concrete

Gecause concrete is continuously shrinking for years after it is initially placed, it is generally

accepted that under thermal loading it will never epand to itJs originally#placed volume. 4ore the

amount of aggregate less will be workability.

7sing smooth and round aggregate increases the workability. Workability reduces if angular

and rough aggregate is used.

>reater si1e of !ggregate# less water is re0uired to lubricate it, the etra water is available

for workability
http://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/design-of-shuttering-formwork.htmlhttp://www.aboutcivil.org/effects-of-different-properties-of-aggregates-on-concrete.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/design-of-shuttering-formwork.htmlhttp://www.aboutcivil.org/effects-of-different-properties-of-aggregates-on-concrete.html


29/35

!ngular aggregates increases flakiness or elongation thus reduces workability. -ound

smooth aggregates re0uire less water and less lubricationand gretaer workability in a given

wc ratio

2orous aggregates re0uire more water compared to non absorbent aggregates for achieving

sam degree of workability.

iii. !ggregate Cement ratio

4ore ratio, less workability. ince less cement mean less water, so the paste is stiff.

iv. Weather Conditions

1. Temperature

/f temperature is high, evaporation increases, thus workability decreases.

2. Wind:/f wind is moving with greater velocity, the rate of evaporation also increase reduces the amount of

water and ultimately reducing workability.

v. *dmixtures

Chemical admitures can be used to increase workability.

7se of air entraining agent produces air bubbles which acts as a sort of ball bearing between

particles and increases mobility, workability and decreases bleeding, segregation. The use of fine

po11olanic materials also have better lubricating effect and more workability.

vi. Sand to *ggregate ratio

/f the amount of sand is more the workability will reduce because sand has more surface area and

more contact area causing more resistance. The ingredients of concrete can be proportioned by

weight or volume. the goal is to provide the desired strength and workability at minimum epense. !

low water#cement ratio is used to achieve a stronger concrete. /t would seem therefore that by

keeping the cement content high one could use enough for god workability and still have a low wc

ratio. the problem is that cement is the most costly of the basic ingredients. the dilema is easily seen

in the graphs below.

'5a1. Concrete Bleeding

Gleeding in concrete is sometimes referred as water gain. /t is a particular form of segregation, in

which some of the water from the concrete comes out to the surface of the concrete, being of the

lowest specific gravity among all the ingredients of concrete. Gleeding is predominantly observed in

a highly wet mi, badly proportioned and insufficiently mied concrete. /n thin members like roof slab

or road slabs and when concrete is placed in sunny weather show ecessive bleeding.


30/35

Bue to bleeding, water comes up and accumulates at the surface. ometimes, along with this water,

certain 0uantity of cement also comes to the surface. When the surface is worked up with the trowel,

the aggregate goes down and the cement and water come up to the top surface. This formation of

cement paste at the surface is known as MaitanceN. /n such a case, the top surface of slabs and

pavements will not have good wearing 0uality. This laitance formed on roads produces dust in

summer and mud in rainy season.

Water while traversing from bottom to top, makes continuous channels. /f the water cement ratio

used is more than &.3, the bleeding channels will remain continuous and un segmented. These

continuous bleeding channels are often responsible for causing permeability of the concrete

structures. While the miing water is in the process of coming up, it may be intercepted by

aggregates. The bleeding water is likely to accumulate below the aggregate. This accumulation of

water creates water voids and reduces the &ond &etween the aggregates and the paste.

The above aspect is more pronounced in the case of flaky aggregate. imilarly, the water that

accumulates below the reinforcing bars reduces the bond between the reinforcement and the

concrete. The poor bond between the aggregate and the paste or the reinforcement and the paste

due to &leeding can &e remedied &y re vi&ration of concrete. The formation of laitance and the

conse0uent bad effect can be reduced by delayed finishing operations.

Gleeding rate increases with time up to about one hour or so and thereafter the rate decreases but

continues more or less till the final setting time of cement.

revention of Bleeding in concrete

Gleeding can be reduced by proper proportioning and uniform and complete miing.

7se of finely divided po11olanic materials reduces bleeding by creating a longer path for the

water to traverse.

!ir#entraining agent is very effective in reducing the bleeding.

Gleeding can be reduced by the use of finer cement or cement with low alkali content. -ich

mies are less susceptible to bleeding than lean mies.

The bleeding is not completely harmful if the rate of evaporation of water from the surface is e0ual to

the rate of bleeding. -emoval of water, after it had played its role in providing workability, from the

body of concrete by way of bleeding will do good to the concrete.

5arly bleeding when the concrete mass is fully plastic, may not cause much harm, because concrete

being in a fully plastic condition at that stage, will get subsided and compacted. /t is the delayed

bleeding, when the concrete has lost its plasticity, which causes undue harm to the concrete.

Controlled re vibration may be adopted to overcome the bad effect of bleeding.

'5&1. Segregation in concrete


31/35

Gack to top

egregation can be defined as the separation of the constituent materials of concrete. ! good

concrete is one in which all the ingredients are properly distributed to make a homogeneous miture.

There are considerable differences in the si1es and specific gravities of the constituent ingredients of

concrete. Therefore, it is natural that the materials show a tendency to fall apart.

Segregation may &e of three types

'. Coarse aggregateseparating out or settling down from the rest of the matri.

$. asteseparating away from coarse aggregate.

6. %aterseparating out from the rest of the material being a material of lowest specific gravity.

! well made concrete, taking into consideration various parameters such as grading, si1e, shape

and surface teture of aggregate with optimum 0uantity of waters makes a cohesive mi. uch

concrete will not ehibit any tendency for segregation. The cohesive and fatty characteristics of

matri do not allow the aggregate to fall apart, at the same timeL the matri itself is sufficiently

contained by the aggregate. imilarly, water also does not find it easy to move out freely from the

rest of the ingredients.

The conditions favora&le for segregation are0

'. Gadly proportioned mi where sufficient matri is not there to bind and conta

'. contain the aggregates

$. /nsufficiently mied concrete with ecess water content

6. Bropping of concrete from heights as in the case of placing concrete in column concreting

%. When concrete is discharged from a badly designed mier, or from a mier with worn out

blades

. Conveyance of concrete by conveyor belts, wheel barrow, long distance haul by dumper,

long lift by skip and hoist are the other situations promoting segregation of concrete

Hibration of concrete is one of the important methods of compaction. /t should be remembered that

only comparatively dry mi should be vibrated. /t too wet a mi is ecessively vibratedL it is likely that

the concrete gets segregated. /t should also be remembered that vibration is continued "ust for

re0uired time for optimum results. /f the vibration is continued for a long time, particularly, in too wet

a mi, it is likely to result in segregation of concrete due to settlement of coarse aggregate in matri.

(. "ydration in concrete
http://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#tophttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#top


32/35

Concrete derives its strength by the hydration of cement particles. The hydration of cement is not a

momentary action but a process continuing for long time. @f course, the rate of hydration is fast to

start with, but continues over a very long time at a decreasing rate /n the field and in actual work,

even a higher watercement ratio is used, since the concrete is open to atmosphere, the water used

in the concrete evaporates and the water available in the concrete will not be sufficient for effective

hydration to take place particularly in the top layer.

/f the hydration is to continue, etra water must be added to refill the loss of water on account of

absorption and evaporation. Therefore, the curing can be considered as creation of a favorable

environment during the early period for uninterrupted hydration. The desirable conditions are, a

suitable temperature and ample moisture.

Concrete, while hydrating, releases high heat of hydration. This heat is harmful from the point of

view of volume stability. Oeat of hydration of concrete may also shrinkage in concrete, thus producing

cracks. /f the heat generated is removed by some means, the adverse effect due to the generation of

heat can be reduced. This can be done by a thorough water curing.

). *ir +ntrainment

!ir entrainment reduces the density of concrete and conse0uently reduces the strength. !ir

entrainment is used to produce a number of effects in both the plastic and the hardened

concrete.These include?

'. -esistance to free1ethaw action in the hardened concrete.

$. /ncreased cohesion, reducing the tendency to bleed and segregation in the plastic concrete.

6. Compaction of low workability mies including semi#dry concrete.

%. tability of etruded concrete.

. Cohesion and handling properties in bedding mortars.

De 'cing of concrete

Batching, 6ixing, lacing 7 Compaction of Concrete

Befinition of Be /cing?

/t is similar to free1ing and

thawing action

/n winters to remove the ice

over large structures e.g. deck

of bridge etc the chemical

sodium chloride


33/35

roperties of Concrete

!resh Concrete roperties

'. etting

$. Workability

6. Gleeding and

egregation

a. Gleeding

b. egregation

%. :ydration

. !ir 5ntrainment

"ardened

Concrete

roperties

'. trength

$. Creep

6. hrinkage

%. 4odulus

@f

5lasticity

. Water

tightness

(imperme

ability)

;. -ate of

trength

gain of

Concrete

Concrete is a construction material that

consists, cement, aggregate i.e. gravel and

sand and... [Read More]

'. Gatching of Concrete

$. 4iing of Concrete ingredients

6. 2lacing of Concrete# Concreting

%. Compaction of Concrete

. Transportation of Concrete

a1 Concrete Slump Test

This test is performed to check the consistency of freshly made concrete. The slump test is done to

make sure a concrete mi is workable. The measured slump must be within a set range, or

tolerance, from the target slump.

Workabilityof concrete is mainly affected by consistency i.e. wetter mies will be more workable

than drier mies, but concrete of the same consistency may vary in workability. /t can also be

defined as the relative plasticity of freshly mied concrete as indicative of its workability.

elated ages Tools and apparatus used for slump test 5e8uipment10

'. tandard slump cone ('&& mm top diameter $&& mm

bottom diameter 6&& mm high)

$. Small scoop
http://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.htmlhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.htmlhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sehttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#wohttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sghttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#hyhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#aihttp://www.aboutcivil.org/properties-of-hardened-concrete.htmlhttp://www.aboutcivil.org/properties-of-hardened-concrete.htmlhttp://www.aboutcivil.org/properties-of-hardened-concrete.htmlhttp://www.aboutcivil.org/properties-of-hardened-concrete.html#sthttp://www.aboutcivil.org/creep-in-concrete.htmlhttp://www.aboutcivil.org/shrinkage-in-concrete.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.htmlhttp://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.html#bahttp://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.html#mihttp://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.html#plhttp://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.html#cohttp://www.aboutcivil.org/concrete-transportation.htmlhttp://www.aboutcivil.org/concrete-workability-factors.htmlhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.htmlhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.htmlhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sehttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#wohttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#blhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#sghttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#hyhttp://www.aboutcivil.org/Properties-of-concrete-factors-affecting-them.html#aihttp://www.aboutcivil.org/properties-of-hardened-concrete.htmlhttp://www.aboutcivil.org/properties-of-hardened-concrete.htmlhttp://www.aboutcivil.org/properties-of-hardened-concrete.htmlhttp://www.aboutcivil.org/properties-of-hardened-concrete.html#sthttp://www.aboutcivil.org/creep-in-concrete.htmlhttp://www.aboutcivil.org/shrinkage-in-concrete.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/concrete-strength-gain-rate.htmlhttp://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.htmlhttp://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.html#bahttp://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.html#mihttp://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.html#plhttp://www.aboutcivil.org/batching-mixing-placing-compaction-of-concrete.html#cohttp://www.aboutcivil.org/concrete-transportation.htmlhttp://www.aboutcivil.org/concrete-workability-factors.html


34/35

'. Gullet#nosed rod (;&& mm long '; mm diameter)

6. ule

2rocedure of slump test for concrete?

'. Clean the cone. Bampen with water and place on the slump plate. The slump plate should

be clean, firm, level and non#absorbent. Collect a sample of concrete to perform the slum

test.

$. tand firmly on the footpieces and fill '6 the volume of the cone with the sample. Compact

the concrete by JroddingJ $ times. -odding means to push a steel rod in and out of the

concrete to compact it into the cylinder, or slump cone. !lways rod in a definite pattern,

working from outside into the middle.

6.


35/35

specified as a characteristic strength of concrete measured at $9 days after miing. The

compressive strength is a measure of the concreteRs ability to resist loads which tend to crush it.

*pparatus for compression test

Cylinders ('&& mm diameter $&& mm high or '& mm diameter 6&& mm high) (The smallcylinders are normally used for most testing due to their lighter weight)

'. mall scoop

$. Gullet#nosed rod (;&& mm '; mm)

6. teel float

%. teel plate

How to do a compression test?

rocedure for compression test of concrete

'. Clean the cylinder mould and coat the inside lightly with form oil, then place on a clean, level

and firm surface, ie the steel plate. Collect a sample.

$. Fill '$ the volume of the mould with concrete then compact by rodding $ times. Cylinders

may also be compacted by vibrating using a vibrating table.

6. Fill the cone to overflowing and rod $ times into the top of the first layer, then top up the

mould till overflowing.

%. Aevel off the top with the steel float and clean any concrete from around the mould.

. Cap, clearly tag the cylinder and put it in a cool dry place to set for at least $% hours.

;. !fter the mould is removed the cylinder is sent to the laboratory where it is cured and

crushed to test compressive strength

Documents

properties of concrete.docx