Mortar Concrete Module 1

7/29/2019 Mortar Concrete Module 1

1/24

Mortar:Definition,

Cement mortarwhen used for protective plaster provides a water proof layer andprotectsthe elements covered from weathering effects i.e., provides resistance to weathering agencies

such as rain, temperature variation, frost action

Preparation of Cement Mortar1. Cement mortar is prepared in the following way:2. First, clean dry sand is spread in a uniform layer on a platform.3. On it the requisite quantity of cement is uniformly spread.4. Then the whole mass is mixed dryby working with spades till the whole mass

becomes uniform in color.5. Then make small depression in the middle and add water and mix it thoroughly and

make paste.6. When mortar is required in large quantity, it isprepared by mixing in mechanical

mixers.7. Normally, a pan mixer is used for mixing the mortar. The mixer consists of a

cylindrical container in which a rotor with blades rotate for turning the material formixing. This rotor is rotated mechanically by an electric motor or a power engine.

8. Sand, cement and water are added to the mixer and mixed for some time till themixture has a uniform consistency.9. The mixed mortar is then poured out for use.10.Concrete mixers of tilting and non-tilting type can also be used for mixing of cement

mortar. The details of these mixers can be studied from any book of ConcreteTechnology.

11.Cement mortars should be tested for crushing strength, adhesiveness, setting time andtensile strength.

Properties of Mortar1. For the mortar to be workable and strong the ratio of cement to sand should normally be

1:3 to 1:6 by weight.2. The use of much weaker cement mortar(i.e. mortars of lower cement content) is not

satisfactory since any notable reduction in cement content leads to reduction inworkability (i.e. it leads to harshness) and less cohesion and will produceporous jointswith a tendency for low frost resistance.

3. The setting and hardening of cement mortar depends upon the setting and hardening ofthe cement-water paste which binds the particles of sand.

4. On hardening of mortar in thejoints, the members get united and provide a fairly strongstructural element.

The mortars are classified on the basis of the following:

1. Bulk density2. Kind of binding material3. Nature of application

Special mortars1. Bulk density: According to the bulk density of mortar in dry state, there are two types of

mortars:

i. Heavy mortarsii. Lightweight mortars


2/24

i. Heavy mortars: The mortars having bulk density of 15 kN/m3 or more are known as theheavy mortars and they are prepared from heavy quartzs or other sands.

ii. Lightweight mortars: The mortars having bulk density less than 15 kN/m3 are known asthe lightweight mortars and they are prepared from light porous sands from pumice and

other fine aggregates.

2. Kind of binding material: The kind of binding material for a mortar is selected bykeeping in mind several factors such as expected working conditions, hardening

temperature, moisture conditions, etc. According to the kind of binding material, the

mortars are classified into the followingfive categories:

i. Lime mortarii. Surkhi mortar

iii. Cement mortariv. Gauged mortarv. Gypsum mortar.

i. Lime mortar: In this type of mortar, the lime is used as binding material. The limemay be fat lime or hydraulic lime. The fat lime shrinks to a great extent and hence itrequires about 2 to 3 times its volume of sand. The lime should be slaked before use .This mortar is unsuitable for water-logged areas or in damp situations. It possessesgood cohesiveness with other surfaces and shrinks very little. It is sufficientlydurable,but it hardens slowly. It is generally used for lightly loaded above-groundparts of buildings

ii. Surkhi mortar: This type of mortar is prepared by using fully surkhi instead of sandor by replacing half of sand in case of fat lime mortar. The powder of surkhi shouldbe fine enough to pass BIS No. 9 sieve and the residue should not be more than 10%by weight. The surkhi mortar is used for ordinary masonry work of all kinds in

foundation and superstructure. But it cannot be used for plastering or pointing sincesurkhi is likely to disintegrate after some time"

iii. Cement mortar: In this type of mortar, the cement is used as binding material.Depending upon the strength required and importance of work, the proportion of

cement to sand by volume varies from 1:2 to 1:6 or more. It should be noted that

surkhi and cinder are not chemically inert substances and hence they cannot be used

as adulterants with matrix as cement. Thus the sand only can be used to form cement

mortar. The proportion of cement with respect to sand should be determined with due

regard to the specified durability and working conditions. The cement mortar is used

where a mortar of high strength and water-resisting properties is required such as

underground constructions, water saturated soils, etc.

iv. Gauged mortar: To improve the quality of lime mortar and to achieve early strength,the cement is sometimes added to it. This process is known as the gauging. It makes

lime mortar economical, strong and dense. The usual proportion of cement to lime by

volume is about 1:6 to 1 :8. It is also known as the composite mortar or lime-cement

mortar and it can also be formed by the combination of cement and clay. This mortar

may be used for bedding and for thick brick walls.

v. Gypsum mortar: These mortars are prepared from gypsum binding materials such as


3/24

building gypsum and anhydrite binding materials.

3. Nature of application: According to the nature of application, the mortars are classifiedinto two categories:

i. Bricklaying mortarsii. Finishing mortars.i. Bricklaying mortars: The mortars for bricklaying are intended to be used for

brickwork and walls. Depending upon the working conditions and type ofconstruction, the composition of masonry mortars with respect to the kind of bindingmaterial is decided.

ii. Finishing mortars: These mortars include common plastering work and mortars fordeveloping architectural or ornamental effects. The cement or lime is generally usedas binding material for ordinary plastering mortar. For decorative finishing, themortars are composed of suitable materials with due consideration of mobility, waterretention, resistance to atmospheric actions, etc.

Special mortars: Following are tile various types of special mortars which are used for certainconditions:

1. Fire-resistant mortar2. Lightweight mortar3. Packing mortar4. Sound-absorbing mortar5. X-ray shielding mortar1. Fire-resistant mortar: This mortar is prepared by adding aluminous cement to the finely

crushed powder of fire-bricks. The usual proportion is 1 part of aluminous cement to 2

parts of powder of fire-bricks.2. This mortar is fire-resistant and it is therefore used with fire-bricks for lining furnaces,fire places, ovens, etc.

3. Lightweight mortar: This mortar is prepared by adding materials such as saw dust, woodpowder, etc. to the lime mortar or cement mortar. Other materials which may be addedare asbestos fibres,jute fibres, coir, etc. This mortar is used in the sound-proof and heat-proof constructions.

4. Packing mortar: To pack oil wells, special mortars possessing the properties of highhomogeneity, waterresistance,predetermined setting time, ability to form solid water-

proof plugs in cracks and voids ofrocks, resistance to subsoil water pressure, etc. have tobe formed

5. Sound-absorbing mortar: To reduce the noise level, the sound-absorbing plaster isformed with the help of sound-absorbing mortar. Thebulk density ofsuch a mortar varies

from 6 to 12 kN/m3

and thebinding materials employed in its composition may bePortland cement, lime, gypsum, slag, etc. The aggregates are selected from lightweight

porous materials such as pumice, cinders, etc

6. X-ray shielding mortar: This type ofmortar is used for providing the plastering coat towalls and ceiling ofX-ray cabinets. It is a heavy type of mortar with bulk density over22

kN/m3. The aggregates are obtained from heavy rock and suitable admixtures are added

to enhance the protective property of such a mortar.


4/24

COMPOSITION AND USES OF MORTAR.


5/24

Concrete:Concrete is a construction material obtained by mixing as cement, lime, mud

etc.), aggregate (sand and gravel crushed aggregate) and water in certainproportions.the various ingredients are mixed, these form a plasticbe molded into the desired shapeand size. The molded mass allowed to cure in suitable environment, hardens capable ofmaintaining its shape and size

Hardening in concrete is a result of the binding material, water and air.Hardened concrete so obtained serves different purposes the ingredients used inconcrete component..

Properties of cement:

Following are the important properties of a good cement

1. It gives strength to the masonry,2. It is an excellent binding material,3. It is easily workable,4. It offers good resistance to the moisture,5. It possesses a good plasticity,6. It stiffens or hardens early.

Functions Of Ingredients

The ingredients of ordinary cement, as mentioned above, perform the following

functions:

Lime (CaO): This is the important ingredient of cement and I s proportion is to be

carefully maintained. The lime in excess makes the cement unsound and causes the

cement to expand and disintegrate. On the other hand, if lime is in deficiency, thestrength of cement is decre ed and it causes cement to set quickly.

Silica (Si02): This is also an important ingredient' of cement and it gives or imparts

strength to the cement due to the formation of dicalcium and tricalcium silicates. If

silica is present in excess quantity, the strength of cement increases but at the same

time, its setting time is p. longed.

Alumina (AI203): This ingredient imparts quick setting property to the cement. It

acts as a flux and it lowers the clinkering temperature. However the high temperature

is essential for the formation of a suitable type of cement and hence the alumina

should not be present in excess amount as it weakens the cement.

Calcium sulphate (CaS04): This ingredient is in the form of gypsum and its

function is to increase the initial setting time of cement.Iron oxide (Fe203): This ingredient imparts color, hardness and strength to the

cement.

Magnesia (MgO): This ingredient, if present in small amount, imparts hardness and

color to the cement. A high content of magnesia makes the cement unsound.

Sulphur (S): A very small amount of sulphur is useful in making sound cement. If it

is in excess, it causes cement to become unsound.


6/24

Alkalis: The most of the alkalis present in raw materials are carried away by the flue

gases during heating and the cement contains

Quality of mixing water,Strength development of concrete is the result of the reaction of waterwith cement

particles. The reaction always starts between water and the cement particle at its surface.

Thus, larger the surface area available forreaction, greater is the rate of hydration. It mustbe noted that the fineness of grinding does not alter the total quantity of heat liberatedbut it

changes the rate of heat development due to change in surface areas.Following are the main properties of cement which are important to civil engineers,

because it is by these properties that the engineer judges the suitability of cements:1. Fineness2. Setting3. Soundness4. Crushing strength5. Heat of hydration.1. Fineness of cement

Fineness of cement is measured eitherin terms of(i)percentage of weight retainedafter sieving cement through 90 micron sieve or(ii) surface area ofcement in cm2

perg.IS:269-1967 specifies that the maximum residue after sieving through a 90-micron

IS sieve should be limited to 10%by weight for ordinary Portland and 5%by

weight for rapid hardening Portland cement.

2. Setting TimeWhen water is mixed with cement to form a paste, reaction starts. In its pure form,the finely ground cement is extremely sensitive to water.Out of the three main compounds, viz. C3A, C3S and C2S, reacts quickly withwater to produce a jelly-like compound which starts solidifying. The action ofchanging from a fluid state to a solid state is called 'setting' and should not be

confused with 'hardening'.During the next stage of hydration, cement paste starts hardening owing to thereaction of C3S and C2S with water and the paste gains strength. II). the first fewminutes, the setting action is more predominant and later on the hardening action

becomes dominant.In practice, such solidifying action or loss of plasticity is required to be delayed,

because some time is needed for mixing, transporting and placing of concrete intofinal position before the mix loses its plasticity due to the setting action.It is usually specified that the plastic concrete should be placed and consolidated

before initial set has occurred and it should not then be disturbed until concrete hashardened. This initial setting time should not be too small and therefore, thestandard specifies minimum initial setting time.Once initial stiffening of concrete has taken place, it is desirable that it should harden or

gain strength as rapidly as possible, so that there is a minimum of delay before shutteringcan be removed and the risk of frost damage in cold climate is minimized. The standard,

therefore, specifies the maximum value of final setting time.

It is not however, possible in practice to exactly locate the initial setting time andfinal setting time. The Indian Standards have selected two arbitrary points whichrelate setting of cement to the time measured from the moment the water is added.'Initial setting time' is defined as the period elapsing between the time when wateris added to the cement and the time at which the needle of 1 mm square section


7/24

(Fig. 5.6) fails to pierce the test block to a depth of about 5 mm from the bottom ofthe mould. A period of 30 minutes is the minimum initial setting time, specified byISI for ordinary and rapid hardening Portland cements and 60 minutes for low heatcementThe 'final setting time' is defined as the period elapsing between the time when

water is added to cement and the time at which the needle of 1 mm square section

with 5 mm diameter attachment makes an impression on the test block. 600 minutesis the maximum time specified for the final set for all the above mentioned Portland

cementsIS: 269-1976 specifies the following strengths in compression on the standardmortar-cube:Compressive strength test has two functions to fulfill. Firstly, it is a final check onthe quality of cement. Secondly, in case of doubt, it also helps us to classify thecement as ordinary Portland cement, rapid hardening Portland cement or low heatPortland cement, according to the strength it gives after 3 days and 7 days curing.It is important to note the difference between setting and hardening of cement at thisstage. As has been explained earlier C3A reacts first with water forming hydratedcalcium alumino silicates. These compounds contribute very little to the mechanical

strength of concrete, but cement starts losing its plasticity because of loss of waterdue to reaction and formation of gel. This loss of plasticity without development ofstrength is called setting action.Cement is said to harden when the cement paste further reacts with water bringingC2S and C3S into action. These compounds contribute to the mechanical strength.Hardening, therefore, is associated with the development of strength.

Water content or Water Cement Ratio

More the water cement ratio more will be workability of concrete. Since by simply

adding water the inter particle lubrication is increased.

High water content results in a higher fluidity and greater workability. Increased watercontent also results in bleeding. another effect of increased water content can also be that cementslurry will escape through joints of formwork.

Workability:

The term workability is used to describe the ease or difficulty with which the concrete ishandled, transported and placed between the forms with minimum loss of homogeneity.

However this gives a very loose description of this vital property of concrete which also depends

on the means of compaction available. For instance, the workability suitable for mass concrete is

not necessarily sufficient for thin, inaccessible or heavily reinforced sections.

The compaction is achieved either by ramming or by vibrating.The workability, as a physical property of concrete alone irrespective of a particular type

of construction, can be defined as the amount of useful internal work necessary to produce full

compaction.

Factors affecting workability

If the concrete mixture is too wet, the coarse aggregates settle at the bottom of concrete


8/24

mass and the resulting concrete becomes of non-uniform composition. On the other hand, if theconcrete mixture is too dry, it will be difficult to handle and place it in position. Both theseconflicting conditions should be correlated by proportioning carefully various components ofconcrete mixture. The important facts in connection with workability are as follows:

1. If more water is added to attain the required degree of workmanship, it results intoconcrete of low strength and poor durability.

2. If the strength of concrete is not to be affected, the degree of workability can be obtained:i. by slightly changing the proportions of fine and coarse aggregates, in case the

concrete mixture is too wet; andii. by adding a small quantity of water cement paste in the proportion of original

mix, in case the concrete mixture is too dry.3. A concrete mixture for one work may prove to be too stiff or too wet for another work.

For instance, the stiff concrete mixture will be required in case of vibrated concrete workwhile wet concrete mixture will be required for thin sections containing reinforcing bars.

4. The workability of concrete is affected mainly by water content, water-cement ratio andaggregate-cement ratio.

5. The workability of concrete is also affected by the grading, shape, texture and maximumsize of the coarse aggregates to be used in the mixture.

Measurement of workability,

In order to measure the workability of concrete mixture, the various tests are developed.

The tests such as flow test and compacting test are used in great extent in laboratory. The slump

test, which is commonly used in the field, is briefly described belowIt should however be remembered that numerous attempts have been made to correlate

workability with some easily determinable physical measurement. But none of these tests is fully

satisfactory although they may provide useful information within a range of variation in

workability. At the same time, the slump test does not measure the workability of concrete. It issimply useful in detecting variations in the uniformity of a mix of given nominal proportions

slump test: The standard slump cone, is placed on the round. The operator holds the cone firmly

by standing on the foot pieces. The cone is filled with about one-fourth portion and then rammed

with a rod which is provided with bullet nose at the lower end. The . diameter of the rod is 16

mm and its length is 60 mm. The strokes to be given for ramming vary from 20 to 30. The

remaining portion, of the cone is filled in with similar layers and then the top of concrete surface

is struck off so that the cone is completely full of concrete. The cone is then gradually raised

vertically and removed. The concrete is allowed to subside and then the height of concrete is

measured. The slump of concrete is obtained by deducting height of concrete after subsidence

from 30 cm.


9/24

FIGURE SHOWING SLUMP CONE

Segregation in concrete

Segregation can be defined as the separation of the constituent materials of concrete. A

good concrete is one in which all the ingredients are properly distributed to make a homogeneousmixture. There are considerable differences in the sizes and specific gravities of the constituent

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

Segregation may be of three types

1. Coarse aggregate separating out or settling down from the rest of the matrix.2. Paste separating away from coarse aggregate.3. Water separating out from the rest of the material being a material of lowest specific

gravity.


10/24

A well made concrete, taking into consideration various parameters such as grading, size,

shape and surface texture of aggregate with optimum quantity of waters makes a cohesive mix.

Such concrete will not exhibit any tendency for segregation. The cohesive and fattycharacteristics of matrix do not allow the aggregate to fall apart, at the same time; the matrix

itself is sufficiently contained by the aggregate. Similarly, water also does not find it easy to

move out freely from the rest of the ingredients.

The conditions favorable for segregation are:

1. Badly proportioned mix where sufficient matrix is not there to bind and contain theaggregates

2. Insufficiently mixed concrete with excess water content3. Dropping of concrete from heights as in the case of placing concrete in column

concreting

4. When concrete is discharged from a badly designed mixer, or from a mixer with worn outblades

5.

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

Vibration of concrete is one of the important methods of compaction. It should be

remembered that only comparatively dry mix should be vibrated. It too wet a mix is excessively

vibrated; it is likely that the concrete gets segregated. It should also be remembered that vibrationis continued just for required time for optimum results. If the vibration is continued for a long

time, particularly, in too wet a mix, it is likely to result in segregation of concrete due to

settlement of coarse aggregate in matrix.

Concrete Bleeding

Bleeding in concrete is sometimes referred as water gain. It 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. Bleeding ispredominantly observed in a highly wet mix, badly proportioned and insufficiently mixed

concrete. In thin members like roof slab or road slabs and when concrete is placed in sunny

weather show excessive bleeding.

Due to bleeding, water comes up and accumulates on the surface. Sometimes, along with

this water, certain quantity 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 Laitance. In such a case, the top

surface of slabs and pavements will not have good wearing quality. This laitance formed on

roads produces dust in summer and mud in rainy season.

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

used is more than 0.7, the bleeding channels will remain continuous and unsegment. Thesecontinuous bleeding channels are often responsible for causing permeability of the concrete

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


11/24

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

of water creates water voids and reduces the bond between the aggregates and the paste.

The above aspect is more pronounced in the case offlaky aggregate. Similarly, 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 pastedue to bleeding can be remedied by revibration of concrete. The formation of laitance and the

consequent bad effect can be reduced by delayed finishing operations.

Bleeding 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.

Prevention of Bleeding in concrete

1. Bleeding can be reduced by proper proportioning and uniform and complete mixing.2. Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for

the water to traverse.3. Air-entraining agent is very effective in reducing the bleeding.4. Bleeding can be reduced by the use of finer cement or cement with low alkali content.

Rich mixes are less susceptible to bleeding than lean mixes.

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

equal to the rate of bleeding. Removal 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.

Early 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. It is the

delayed bleeding, when the concrete has lost its plasticity, which causes undue harm to theconcrete. Controlled re vibration may be adopted to overcome the bad effect of bleeding.

Uniformity of mixing

A concrete mixer (also commonly called a cement mixer) is a device thathomogeneously combines cement, aggregate such as sand or gravel, and water to form concrete.

A typical concrete mixer uses a revolving drum to mix the components. For smaller volume

works portable concrete mixers are often used so that the concrete can be made at the

construction site, giving the workers ample time to use the concrete before it hardens. Analternative to a machine is mixing concrete or cement by hand. This is usually done in a

wheelbarrow; however, several companies have recently begun to sell modified tarps for this

purpose.


12/24

FIGURE SHOWING CONCRETE MIXING BY HAND

FIGURE SHOWING CONCRETE MIXING BY MIXER

ADVANCED CONCRETE MIXING SYSTEMS

Dry mixing cycle of precisely batched raw materials


13/24

1. With reference to the scheme in the previous page the sand is fed to a hopper installed onload cells for precise weighing and equipped with vibrator and sliding valve for fast

discharge into the homogenizer.2. Beside this there are a second smaller hopper for the cement batching and a third gate,

that can be used to feed other materials like oxide pigments and/or fibers.At this stage, all

the components are dry.3. Once the loading of the hoppers are completed, everything is discharged in a pre-mixerequipped with continuously rotating paddles. Due to high velocity rotation, the dry

materials are perfectly mixed and homogenized.

4. At the end of the sand cement premixing cycle the batch of dry mix is discharged in ahopper and conveyed via a belt conveyor to the continuous screw mixer.

Continuous mixing cycle of wet materials: In this final phase water is added by some nozzleinstalled on the open top mixer. The quantity of water can be checked at any time by the

operator. The final mix is then fed to the extruder for tile production. Automatic moisture control

can be supplied under request. This new concrete batching and mixing system with capacity up

to 25m3/hour has the following advantages compared to the standard ones

FIGURE SHOWING WET MIXING OF CONCRETE

1. Thanks to the concrete high velocity dry premixing cycle, the quality of the mix at theoutlet of the continuous mixer is on the average much better than the one made with

traditional pan mixer. This means that tiles with lower weight and lower quantity of

cement can be produced with consequent big savings.


14/24

2. It combines the advantages of the continuous mixing system with the ones of the systemsworking for batches, so raw materials are perfectly proportioned.

3. Majority of standard systems works for batches. This means that they have to bedimensioned considering the mixing time and not the instantaneous needed quantity of

material. Vortex Hydra s.r.l. concrete mixing system (EasyMix) is continuous and has

been designed to supply the instantaneous request of mixed materials.4. No dead times are foreseen because of the continuous mixing cycle. The reduction ofcycle time doesnt bring to any reduction of efficiency as happens for the standard

mixers.5. Less power is required as the quantity of materials to be moved and worked is lower.6. In case of unexpected stoppages, just a small part of concrete has to be rejected as the

water is added to the cement only in the final part of the cycle. This means that also the

maintenance of the system is much easier than usual.

Mixing time

This is directed at evaluating the effect of mixing time on the physical characteristics ofthe finished Portland cement concrete pavement. It considered three mix designs, two difference

concrete mixers and four mixing times. The results of the research, compared to the researchobjectives, indicate the following conclusions:

1. Potential segregation in the hauling units: Dump truck type hauling units do notsignificantly change or decrease the quality of the material being delivered to the paver

and should continue to be allowed in addition to agitator type hauling vehicles fortransport of Portland cement concrete paving materials.

2. Concrete consolidation and workability quality at the paving site: The results of theANOVA indicate that mixing times of 60 seconds or greater do have a positive influence

on the physical characteristics of the concrete product. It is recommended that the 60second minimum mixing time be retained for all mixer types at this time.

3. Hardened air content and distribution: The data from this set of tests indicates that forIowa DOT designed mixes the mixing time did not effect the physical attributes of the

concrete significantly. The results did show a conflicting ideas for the contractordesigned mix. We suggest that this is the result of both a different matrix of coarse and

fine aggregate in the contractor mix as related to previous Iowa DOT mixes. It does open

a new set of parameters for mix approval when coupled with the impact of mixadmixtures being used or considered. It is recommended that contractor mix designs be

thoroughly laboratory tested prior to construction to determine the impact of admixtures

and the differences in aggregate/cement matrix on the desired physical performance

factors desired by the agency.4. Concrete mixer type and mixing times: Visual and physical test data indicate that reduced

mixing times for alternative type mixers should only be allowed when steps have been

taken to change the mixing process to eliminate any particles of aggregate that are notcoated upon discharge into the hauling unit.


15/24

VIBRATION OF CONCRETE

1. When you are working with concrete, you need to make sure it is free of air bubbles andexcess water after it has been poured.

2. That's why we at Speedcrete have the Multivibe Hummer Lightweight Concrete Vibratoron sale - a user friendly and reliable tool that is perfect for the job.

3. It's great for releasing air and water and ensures your concrete pour is even, strong andends up with a smooth finish.

4. The Multivibe Hummer Lightweight Concrete Vibrator works with flexible shafts andheads that can be purchased separately.

5. Various head sizes are available, while you can also pick from a number of flex lengths,depending on your needs and the size of your project.

6. And if you are carrying out column work, you might wish to purchase the "Fishing Pole"version of this tool.

7. You'll find the concrete vibrator easy to use on all sorts of concreting tasks, such asworking on foundations, slabs and stem walls.

Lightweight Concrete Vibrators

See our range of concrete vibrators from lightweight concrete vibrator to traditional

engine framed pendulum pokers. We have a concrete vibrator for every job site application. The

Hummer is our latest lightweight portable concrete poker. The Hummer is lighter and muchmore manageable than traditional pokers and gives greater performance and reliability. It is

especially suitable as a concrete slab poker. The hip mounted concrete vibrator has proved a

great success as it is alot lighter than older backpack poker models.

High Frequency Concrete Vibrators

We have a complete range of Technoflex high frequency electric pokers. Our high

frequency concrete vibrators come with a variety of flex lengths and head sizes to suit many

typical applications including a concrete poker for floor slabs and wall and concrete columnvibrators. The excel is our top of the range electric high frequency concrete vibrator featuring

7meters of flexible hose and a choice of poker head sizes to 60mm [2"]. We also have a unique

mechanical shaft high frequency concrete poker vibrator called the 'Rabbit' (Who named thatone!). With the mechanical connection this allows the ability to alter the flex and head sizes

providing you with a more versatile poker. The electric drive unit keeps this concrete vibrators

noise to a minimum.

Concrete Pokers

We have a variety of concrete pokers and concrete vibrators available. Including highfrequency electric models available in 110 volt and 230 volt for export. Concrete pokers are used

to consolidate the concrete ad remove air pockets and voids created as part of the concrete

placement process..
https://www.speedcrete.co.uk/concrete-tents-c51.htmlhttps://www.speedcrete.co.uk/technoflex-excel-high-frequency-i134.htmlhttps://www.speedcrete.co.uk/concrete-tents-c51.htmlhttps://www.speedcrete.co.uk/concrete-tents-c51.htmlhttps://www.speedcrete.co.uk/technoflex-excel-high-frequency-i134.htmlhttps://www.speedcrete.co.uk/concrete-tents-c51.html


16/24

Pneumatic Air Concrete Vibrators

Pneumatic concrete vibrators or Air pokers as their commonly known are becoming lesspopular with recent developments in the high frequency electric poker models. However if you

still demand the most powerful concrete vibrator then air is the best option unless you have

access to hydraulics. Our pneumatic air pokers run up to 80mm vibrator heads to give youextreme amplitude to tackle even the most aggressive concrete mix designs.

FIGURE SHOWING NEEDLE VIBRATORS

Concrete Mix Design

Introduction

The process of selecting suitable ingredients of concrete and determining their relative

amounts with the objective of producing a concrete of the required, strength, durability, and

workability as economically as possible, is termed the concrete mix design. The proportioning ofingredient of concrete is governed by the required performance of concrete in 2 states, namely

the plastic and the hardened states. If the plastic concrete is not workable, it cannot be properly

placed and compacted. The property of workability, therefore, becomes of vital importance.

The compressive strength of hardened concrete which is generally considered to be an

index of its other properties, depends upon many factors, e.g. quality and quantity of cement,

water and aggregates; batching and mixing; placing, compaction and curing. The cost of concreteis made up of the cost of materials, plant and labor. The variations in the cost of materials arise

from the fact that the cement is several times costly than the aggregate, thus the aim is to produce

as lean a mix as possible. From technical point of view the rich mixes may lead to high shrinkageand cracking in the structural concrete, and to evolution of high heat of hydration in mass

concrete which may cause cracking.
https://www.speedcrete.co.uk/concrete-tents-c51.htmlhttps://www.speedcrete.co.uk/concrete-tents-c51.html


17/24

The actual cost of concrete is related to the cost of materials required for producing a

minimum mean strength called characteristic strength that is specified by the designer of the

structure. This depends on the quality control measures, but there is no doubt that the qualitycontrol adds to the cost of concrete. The extent of quality control is often an economic

compromise, and depends on the size and type of job. The cost of labor depends on the

workability of mix, e.g., a concrete mix of inadequate workability may result in a high cost oflabor to obtain a degree of compaction with available equipment.

Requirements of concrete mix design

The requirements which form the basis of selection and proportioning of mix ingredients

are :1. The minimum compressive strength required from structural consideration2. The adequate workability necessary for full compaction with the compacting equipment

available.3. Maximum water-cement ratio and/or maximum cement content to give adequate

durability for the particular site conditions4. Maximum cement content to avoid shrinkage cracking due to temperature cycle in massconcrete.

Types of Mixes

1.Nominal Mixes

In the past the specifications for concrete prescribed the proportions of cement, fine and

coarse aggregates. These mixes of fixed cement-aggregate ratio which ensures adequate strength

are termed nominal mixes. These offer simplicity and under normal circumstances, have a

margin of strength above that specified. However, due to the variability of mix ingredients thenominal concrete for a given workability varies widely in strength.

2.Standard mixes

The nominal mixes of fixed cement-aggregate ratio (by volume) vary widely in strengthand may result in under- or over-rich mixes. For this reason, the minimum compressive strength

has been included in many specifications. These mixes are termed standard mixes.

IS 456-2000 has designated the concrete mixes into a number of grades as M10, M15,

M20, M25, M30, M35 and M40. In this designation the letter M refers to the mix and the number

to the specified 28 day cube strength of mix in N/mm

2

. The mixes of grades M10, M15, M20 andM25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2)respectively.

3.Designed Mixes

In these mixes the performance of the concrete is specified by the designer but the mixproportions are determined by the producer of concrete, except that the minimum cement content


18/24

can be laid down. This is most rational approach to the selection of mix proportions with specific

materials in mind possessing more or less unique characteristics. The approach results in the

production of concrete with the appropriate properties most economically. However, thedesigned mix does not serve as a guide since this does not guarantee the correct mix proportions

for the prescribed performance.

Factors affecting the choice of mix proportions

The various factors affecting the mix design are:

1.Compressive strength

It is one of the most important properties of concrete and influences many other

describable properties of the hardened concrete. The mean compressive strength required at a

specific age, usually 28 days, determines the nominal water-cement ratio of the mix. The otherfactor affecting the strength of concrete at a given age and cured at a prescribed temperature is

the degree of compaction. According to Abrahams law the strength of fully compacted concreteis inversely proportional to the water-cement ratio.

2.Workability

The degree of workability required depends on three factors. These are the size of the

section to be concreted, the amount of reinforcement, and the method of compaction to be used.For the narrow and complicated section with numerous corners or inaccessible parts, the concrete

must have a high workability so that full compaction can be achieved with a reasonable amount

of effort. This also applies to the embedded steel sections. The desired workability depends on

the compacting equipment available at the site.

3.Durability

The durability of concrete is its resistance to the aggressive environmental conditions.

High strength concrete is generally more durable than low strength concrete. In the situations

when the high strength is not necessary but the conditions of exposure are such that highdurability is vital, the durability requirement will determine the water-cement ratio to be used.

4.Maximum nominal size of aggregate

In general, larger the maximum size of aggregate, smaller is the cement requirement for a

particular water-cement ratio, because the workability of concrete increases with increase inmaximum size of the aggregate. However, the compressive strength tends to increase with the

decrease in size of aggregate.

IS 456:2000 and IS 1343:1980 recommend that the nominal size of the aggregate should

be as large as possible.


19/24

5.Grading and type of aggregate

The grading of aggregate influences the mix proportions for a specified workability andwater-cement ratio. Coarser the grading leaner will be mix which can be used. Very lean mix is

not desirable since it does not contain enough finer material to make the concrete cohesive.

The type of aggregate influences strongly the aggregate-cement ratio for the desired

workability and stipulated water cement ratio. An important feature of a satisfactory aggregate is

the uniformity of the grading which can be achieved by mixing different size fractions.

6.Quality Control

The degree of control can be estimated statistically by the variations in test results. The

variation in strength results from the variations in the properties of the mix ingredients and lack

of control of accuracy in batching, mixing, placing, curing and testing. The lower the differencebetween the mean and minimum strengths of the mix lower will be the cement-content required.

The factor controlling this difference is termed as quality control.

Mix Proportion designations

The common method of expressing the proportions of ingredients of a concrete mix is in

the terms of parts or ratios of cement, fine and coarse aggregates. For e.g., a concrete mix of

proportions 1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or the mixcontains one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The

proportions are either by volume or by mass. The water-cement ratio is usually expressed in

mass

Factors to be considered for mix design

The grade designation giving the characteristic strength requirement of concrete. The type of cement influences the rate of development of compressive strength of

concrete.

Maximum nominal size of aggregates to be used in concrete may be as large as possiblewithin the limits prescribed by IS 456:2000.

The cement content is to be limited from shrinkage, cracking and creep. The workability of concrete for satisfactory placing and compaction is related to the size

and shape of section, quantity and spacing of reinforcement and technique used for

transportation, placing and compaction.

Procedure

1. Determine the mean target strength ft from the specified characteristic compressivestrength at 28-day fckand the level of quality control.

ft = fck+ 1.65 S

where S is the standard deviation obtained from the Table of approximatecontents given after the design mix.


20/24

2. Obtain the water cement ratio for the desired mean target using the emperical relationshipbetween compressive strength and water cement ratio so chosen is checked against the

limiting water cement ratio. The water cement ratio so chosen is checked against thelimiting water cement ratio for the requirements of durability given in table and adopts

the lower of the two values.

3.

Estimate the amount of entrapped air for maximum nominal size of the aggregate fromthe table.4. Select the water content, for the required workability and maximum size of aggregates

(for aggregates in saturated surface dry condition) from table.

5. Determine the percentage of fine aggregate in total aggregate by absolute volume fromtable for the concrete using crushed coarse aggregate.

6. Adjust the values of water content and percentage of sand as provided in the table for anydifference in workability, water cement ratio, grading of fine aggregate and for rounded

aggregate the values are given in table.7. Calculate the cement content form the water-cement ratio and the final water content as

arrived after adjustment. Check the cement against the minimum cement content from the

requirements of the durability, and greater of the two values is adopted.8. From the quantities of water and cement per unit volume of concrete and the percentageof sand already determined in steps 6 and 7 above, calculate the content of coarse and

fine aggregates per unit volume of concrete from the following relations:

where V = absolute volume of concrete

= gross volume (1m

3

) minus the volume of entrapped airSc = specific gravity of cement

W = Mass of water per cubic metre of concrete, kg

C = mass of cement per cubic metre of concrete, kgp = ratio of fine aggregate to total aggregate by absolute volume

fa, Ca = total masses of fine and coarse aggregates, per cubic metre of concrete,

respectively, kg, and

Sfa, Sca = specific gravities of saturated surface dry fine and coarse aggregates,

respectively

9.

Determine the concrete mix proportions for the first trial mix.10.Prepare the concrete using the calculated proportions and cast three cubes of 150 mm sizeand test them wet after 28-days moist curing and check for the strength.

11.Prepare trial mixes with suitable adjustments till the final mix proportions are arrived at.


21/24

Admixtures,

Admixtures are materials other than cement, aggregate and water that are added to

concrete either before or during its mixing to alter its properties, such as workability, curing

temperature range, set time or color. Some admixtures have been in use for a very long time,

such as calcium chloride to provide a cold-weather setting concrete. Others are more recent andrepresent an area of expanding possibilities for increased performance. Not all admixtures are

economical to employ on a particular project. Also, some characteristics of concrete, such as low

absorption, can be achieved simply by consistently adhering to high quality concreting practices.

The chemistry of concrete admixtures is a complex topic requiring in-depth knowledge andexperience. A general understanding of the options available for concrete admixtures is

necessary for acquiring the right product for the job, based on climatic conditions and job

requirements. Based on their functions, admixtures can be classified into the following five

major categories:

1. Retarding admixtures2. Accelerating admixtures3. Super plasticizers4. Water reducing admixtures5. Air-entraining admixturesAmong other important admixtures that do not fit into these categories are admixtures whose

functions include bonding, shrinkage reduction, damp proofing and coloring. The followingparagraphs provide details on the above-mentioned categories of concrete admixtures.

Admixtures

IS:1343 - 1980 allows to use admixtures that conform to IS:9103 - 1999, Concrete

Admixtures Specification. The admixtures can be broadly divided into two types: chemical

admixtures and mineral admixtures. The common chemical admixtures are as follows.1. Air-entraining admixtures2. Water reducing admixtures3. Set retarding admixtures4. Set accelerating admixtures5. Water reducing and set retarding admixtures6. Water reducing and set accelerating admixtures.

The common mineral admixtures are as follows.1. Fly ash2. Ground granulated blast-furnace slag3. Silica fumes4. Rice husk ash5. Metakoline

These are cementitious and pozzolanic materials.


22/24

Retarding admixtures

Retarding admixtures slow down the hydration of cement, lengthening set time. Retardersare beneficially used in hot weather conditions in order to overcome accelerating effects of

higher temperatures and large masses of concrete on concrete setting time. Because most

retarders also act as water reducers, they are frequently called water-reducing retarders. As perchemical admixture classification by ASTM-ASTM C 494, type B is simply a retarding

admixture, while type D is both retarding and water reducing, resulting in concrete with greater

compressive strength because of the lower water-cement ratio.

Retarding admixtures consists of both organic and inorganic agents. Organic retardants

include unrefined calcium, sodium, NH4, salts of lignosulfonic acids, hydrocarboxylic acids, andcarbohydrates. Inorganic retardants include oxides of lead and zinc, phosphates, magnesium

salts, fluorates and borates. As an example of a retardant's effects on concrete properties,

lignosulfate acids and hydroxylated carboxylic acids slow the initial setting time by at least an

hour and no more than three hours when used at 65 to 100 degrees Fahrenheit. The concrete

contractor, however, need not memorize these chemical-specific results. Given the specific jobrequirements and goals, the concrete supplier should offer appropriate admixtures and concrete

mixes from which to choose.

Accelerating admixtures

Accelerators shorten the set time of concrete, allowing a cold-weather pour, early

removal of forms, early surface finishing, and in some cases, early load application. Proper care

must be taken while choosing the type and proportion of accelerators, as under most conditions,commonly used accelerators cause an increase in the drying shrinkage of concrete.

Calcium chloride is a common accelerator, used to accelerate the time of set and the rateof strength gain. It should meet the requirements of ASTM D 98. Excessive amounts of calcium

chloride in concrete mix may result in rapid stiffening, increase in drying shrinkage and

corrosion of reinforcement. In colder climates, calcium chloride should not be used as an anti-freeze. Large amount of calcium chloride is required to lower the freezing point of the concrete,

which may ruin the concrete.

Super plasticizers

Super plasticizers, also known as plasticizers, include water-reducing admixtures.Compared to what is commonly referred to as a "water reducer" or "mid-range water reducer",

super plasticizers are "high-range water reducers". High range water reducers are admixtures that

allow large water reduction or greater flowability (as defined by the manufacturers, concrete

suppliers and industry standards) without substantially slowing set time or increasing airentrainment.

Each type of super plasticizer has defined ranges for the required quantities of concrete

mix ingredients, along with the corresponding effects. They can maintain a specific consistency

and workability at a greatly reduced amount of water. Dosages needed vary by the particular


23/24

concrete mix and type of super plasticizer used. They can also produce a high strength concrete.

As with most types of admixtures, super plasticizers can affect other concrete properties as well.

The specific effects, however, should be found from the manufacturer or concrete supplier.

Water reducing admixtures

Water reducing admixtures require less water to make a concrete of equal slump, or

increase the slump of concrete at the same water content. They can have the side effect of

changing initial set time. Water reducers are mostly used for hot weather concrete placing and toaid pumping. A water-reducer plasticizer, however, is a hygroscopic powder, which can entrain

air into the concrete mix via its effect on water's surface tension, thereby also, obtaining some of

the benefits of air-entrainment (see below).

Air-entraining admixtures

Air-entraining agents entrain small air bubbles in the concrete. The major benefit of this

is enhanced durability in freeze-thaw cycles, especially relevant in cold climates. While somestrength loss typically accompanies increased air in concrete, it generally can be overcome byreducing the water-cement ratio via improved workability (due to the air-entraining agent itself)

or through the use of other appropriate admixtures. As always, admixtures should only be

combined in a concrete mix by a competent professional because some of them can interact inundesirable ways.

Bonding admixtures, including addition of compounds and materials such as polyvinylchlorides and acetates, acrylics and butadiene-styrene co-polymers, can be used to assist in

bonding new / fresh concrete with old / set concrete.

Coloring agents have become more commonly used, especially for patios and walkways.Most are surface applied and often have the additional effect of surface hardening. Such surface

applied coloring admixtures generally should not be used on air-entrained concrete. Integrallycolored concrete is also available.

Waterproofing and damp proofing admixtures, including soaps, butyl stearate, mineral oiland asphalt emulsions, are used to decrease the amount of water penetration into the larger pores

of concrete. "Antifreeze" admixtures typically are accelerators used in very high doses, with a

corresponding high price, to achieve a very fast set-time, though they do not have properties to

protect against freezing on their own. However, in general, these are not used for residentialwork.

Cement substitutes also change concrete properties, but typically are not classified asadmixtures. See the Technology Inventory article, "Cement Substitutes."

Most organic chemical-type admixtures are affected by cement type and brand, water-

cement ratio, aggregate grading, and temperature. Damp proofing and waterproofing admixtures

still have uncertain value and hazards. These are just two cases that point to the learning curve

required of anyone working with admixtures. In some cases, if exacting directions are not


24/24

followed, including addition of supplemental materials to balance the negative or undesirable

side effects of an admixture, the resulting concrete mix may be compromised. For example,

retarding admixtures generally have a possibility of rapid concrete stiffening, resulting indifficulty in concrete placement and finishing. Therefore, an in-depth knowledge of the

potentially complex interrelated effects, besides specifications, is required to successfully use a

number of admixtures. This is even more critical when a number of parties are involved in themanufacture of the concrete, for example the producer, the placing contractor and the builder,where the finished concrete is a combined result of a number of individual decisions. Choosing

an appropriate admixture for a specific job should be the responsibility of an experienced expert.

Alternatives to the use of admixtures should always be considered.

The environmental impact of certain admixtures is questionable. Some super plasticizers

may impact the environment through pollution of ground and surface waters. More researchremains to be carried out in this area.

Finally, admixtures cannot compensate for bad practice and low quality materials.

Grade and strength of Concrete.

S.NO. GRADE STRENGTHAFTER 28DAYS

CURING N/mm2

remarks

1 M15 15

2 M20 20

3 M25 25

4 M30 30

5 M35 35

6 M40 407 M45 45

8 M50 50

9 M55 55

Date post:	04-Apr-2018
Category:	Documents
Upload:	andrea-deleon
View:	221 times
Download:	0 times

Mortar Concrete Module 1

Documents