Concrete containing cement, water, aggregate and discontinuous or uniformly dispersed or discrete fibers is term as Fiber Reinforced Concrete. It is a composite obtain by adding a single type or a blend of fibers to the conventional concrete mix.

Fibers can be in form of steel fibers, glass fibers, natural fibers, synthetic fibers etc.

Read More – 20 types of Concrete Used In Building Construction

Why Fiber Reinforced Concrete are use ?

• Main role of fibers is to bridge the cracks that develop in concrete and increase the ductility of concrete elements.
• There is considerable improvement in the post-cracking behavior of concrete containing fibers due to both plastic shrinkage and drying shrinkage.
• They also reduce the permeability of concrete and thus reduce bleeding of water.
• Some types of fibers produce greater abrasion and shatter resistance to impart load.

Types of Fiber Reinforced Concrete

1. Steel Fibers
2. Glass Fibers
3. Carbon Fibers
4. Cellulose Fibers
5. Synthetic Fibers
   1. Polypropylene Fibers
   2. Nylon Fibers
6. Natural Fibers
   1. Coir
   2. Hay

Steel Fibers ( Fiber Reinforced Concrete )

• Aspect ratios of 30 to 250.
• Diameters vary from 0.25 mm to 0.75 mm.
• High structural strength.
• Reduce crack widths and control the crack width tightly. Thus improving durability.
• Improve impact and abrasion resistance.
• Use in precast and structural applications, highway and airport pavements, refractory and canal linings, industrial flooring, bridge decks etc.

Glass Fibers ( Fiber Reinforced Concrete )

• High tensile strength 1020 to 4080 N/mm².
• Generally, fibers of length 25mm are use.
• Improvement in impact strength.
• Increase flexural strength, ductile and resistance to thermal shock.
• Use in formwork, swimming pools, ducts and roots, sewer lining etc.

Synthetic Fibers ( Fiber Reinforced Concrete )

• Man made fibers from petrochemicals and textile industries.
• Cheap, abundantly available.
• High chemical resistance.
• High melting point.
• Low modulus elasticity.
• It’s types are acrylic, aramid, carbon, nylon, polyester, polyethylene, polypropylene etc.
• Applications in cladding panels and shotcrete.

Natural Fibers ( Fiber Reinforced Concrete )

• Obtain at low cost and low level of energy using local manpower and technology.
• Jute, coir and bamboo are examples.
• They may undergo organic decay.
• Low modulus of elasticity, high impact strength.

Advantages of Fiber Reinforced Concrete

• High modulus of elasticity the effective long-term reinforcement even in the harden concrete.
• Does not rust nor corrode and requires no minimum cover.
• Ideal aspect ratio which makes them excellent for early-age performance.
• Easily place, Cast, Spray and less labour intensive than placing rebar.
• Greater retain toughness in conventional concrete mixes.
• Higher flexural strength, depending on addition rate.
• Can be make into thin sheets or irregular shapes.
• FRC passes enough plasticity to go under large deformation once the peak load has been reach.

Disadvantage of Fiber Reinforced Concrete

• Greater reduction of workability.
• High cost of materials.
• Generally fibers do not increase the flexural strength of concrete and so cannot replace moment resisting or structural steel reinforcement.

Applications of Fiber Reinforced Concrete

Runway, Aircraft Parking and Pavements.

For the same wheel load FRC slabs could be about one half the thickness of plain concrete slab. FRC pavements offers good resistance even in severe and mild environments.

It can be use in runways, taxiways, aprons, seawalls, dock areas, parking and loading ramps.

Tunnel Lining and slope stabilization.

Steel fiber reinforce concrete are being use to line underground openings and rock slope stabilization. It eliminates the need for mesh reinforcement and scaffolding.

Dams and Hydraulic Structure.

FRC is being use for the construction and repair of dams and other hydraulic structure to provide resistance to cavitation severe erosion cause by the impact of large debris.

Thin Shell, Walls, Pipes, and Manholes.

Fibrous concrete permits the use of thinner flat and curve structural elements. Steel fibrous shotcrete is use in the construction of hemispherical domes.

Agriculture

It is use in animal storage structures, walls, silos, paving, etc.

Precast Concrete and Products.

It is use in architectural panels, tilt-up construction, walls, fencing, septic tanks, grease trap structures, vaults and sculptures.

Commercial

It is use for exterior and interior floors, slabs and parking areas, roadways, etc.

Warehouse / Industrial

It is use in light to heavy duty loading floors.

Residential

It includes application in driveways, sidewalks, pool construction, basements, colored concrete, foundation, drainage, etc.

Fiber Reinforced Concrete	Normal Reinforced concrete
High Durability.	Lower Durability.
Protect steel from Corrosion.	Steel potential to corrosion.
Lighter materials.	Heavier materials.
More expensive.	Economical.
With the same volume, the strength is greater.	With the same volume, the strength is less.
Less workability.	High workability as compare to FRC.

Is Bamboo stronger than Steel? This question has many answers depending upon the quality, usage and availability of Bamboo. Let’s discuss according to various aspects.

Aspect : 1

No.

Bamboo is not stronger than steel, however it is sustainable; i.e steel is limited in quantity on earth plus has big carbon footprint and environmental hazards of mining. While it grows and absorbs carbon dioxide, it also rots and degrades.

But same when you have to decrease the weight of whole structure mainly in roof that time we can go for bamboo. When you have to decrease the dead load of the structure that time you should use bamboo. In other case when it is in use of for support of form-work that time it is more durable. And quality of it is good than it can beat steel reinforcement.

There is one another wood, which is also more durable than bamboo. That is Sag wood. Sag wood is more durable and has anti-corrosive property.

Aspect : 2

Yes.

Bamboo is stronger than steel only in tension. Steel has a tensile strength of 23,000 pounds per square inch. But it surpasses steel with a noticeable lead at 28,000 pounds per square inch. In compression, it is weaker than steel. But it withstands better in compression than concrete. Therefore, it is mostly used as reinforcement. It has impermeable layer outside it which protects it from moisture.

Let’s discuss some of merits and demerits while using bamboo as reinforcement.

Merits of Bamboo in reinforcement

1. It is extremely strong natural fiber.
2. Has high tensile strength.
3. It is highly flexible due to its hollow interior.
4. Light in weight as comparatively to steel. So it is used to construct light weight structures. Also easy to transport and carry.
5. It is available at very low price and easily available.
6. It is environmental friendly also.
7. And has great shock absorbing capacity.

Demerits of Bamboo in reinforcement

1. It is less durable as it rots and degrades. So cannot be used in permanent structures.
2. More prone to insects attack.
3. Thus it has less life span as compared to steel.
4. It has shrinking problem.
5. Also can’t be used in cold climatic region.
6. It has low adherence with concrete mix, thus difficult to maintain bond between concrete and Bamboo.
7. Due to its low modulus of elasticity, it gets cracks and deflects more than steel reinforcement.

Read more about – Is top to down construction possible ?

Admixture is defined as a material, other than cement, water, sand and aggregates, that is used as an ingredient of concrete. It is added to the batch immediately before or during mixing. Whereas additive is a material which is added at the time of grinding cement clinker at the cement factory.

To improve the quality performance or durability of concrete, admixtures are added. These modify the properties of ordinary concrete so as to make it more suitable for any situation.

Get more knowledge about 20 types of cement used across the world.

Classification of Admixture

1. Plasticizers
2. Superplasticizers
3. Retarders
4. Regarding Plasticizers
5. Accelerators
6. Accelerating Plasticizers
7. Air entraining Admixture
8. Pozzolanic or Mineral Admixture
9. Damp proofing and Water proofing Admixture
10. Gas forming Admixture
11. Air detraining Admixture
12. Alkali aggregate expansion inhibiting Admixture
13. Workability Admixture
14. Grouting Admixture
15. Corrosion inhibiting Admixture
16. Bonding Admixture
17. Fungicidal, Germicidal, Insecticidal Admixture
18. Colouring Admixture
19. Miscellaneous Admixture

1. Plasticizers

Plasticizers can help difficult conditions for obtaining higher workability without using excess of water. One should remember that addition of excess water, will only improve the fluidity or the consistency. But it will not improve the workability of concrete. It reduces the tendency of segregation and bleeding of concrete.

Action of Plasticizers

The action of plasticizers is mainly to fluidify the mix and improve the workability of concrete, mortar or grout. The mechanisms that are involved could be explained in the following way:

Dispersion

Portland cement, being in fine state of division, will have a tendency to flocculate in wet concrete. These flocculation entraps certain amount of water used in the mix and thereby all the water is not freely available to fluidify the mix.

When plasticizers are_used, they gets_adsorb on the cement particles. The adsorption of charged polymer on the particles of cement creates particle-to-particle repulsive forces which overcome the attractive forces. This repulsive force is termed Zeta Potential, which depends on the base, solid content, quantity of plasticizer used. The overall result is_that the cement particles are_deflocculated and dispersed. When cement particles are_deflocculated, the water trapped inside the flocs gets release and now available to fluidify the mix.

When cement particles get flocculate there will_be interparticles friction between particle to particle and floc to floc. But in the dispersed condition there is water in between the cement particle and hence the interparticle friction is_reduced.

Retarding Effect

It is_mentioned earlier that plasticizer gets adsorb on the surface of cement particles and form a thin sheath. This thin sheath inhibits the surface hydration reaction between water and cement as long as sufficient plasticizer molecules are available at the particle/solution interface. The quantity of available plasticizers will progressively decrease as the polymers become entrapped in hydration products.

Many research workers explained that one or more of the following mechanisms may take place simultaneously:

• Reduction in the surface tension of water.
• Induced electrostatic repulsion between particles of cement.
• Lubricating film between cement particles.
• Dispersion of cement grains, releasing water trapped within cement flocs.
• Inhibition of the surface hydration reaction of the cement particles, leaving more water to fluidify the mix.
• Change in the morphology of the hydration products.
• Induced steric hindrance preventing particle-to-particle contact.
• It may be noted that all plasticizer are to some extent set retarders, depending upon the
• base of plasticizers, concentration and dosage used.

2. Superplasticizers (High Range Water Reducers)

Superplasticizers constitute a relatively new category and improved version of plasticizer, the use of which was_developed in Japan and Germany during 1960 and 1970 respectively. They are chemically different from normal plasticiszers. Use of superplasticizers permit the reduction of water to the extent upto 30 per cent without reducing workability in contrast to the possible reduction up to 15 per cent in case of plasticizers.

The use of superplasticizer is_practiced for production of flowing, self levelling, self compacting and for the production of high strength and high performance concrete. The mechanism of action of superplasticizers are more or less same as explained earlier in case of ordinary plasticizer. Only thing is that the superplasticizers are more powerful as dispersing agents and they are high range water reducers. They are also termed High Range Water Reducers in American literature, It is the use of superplasticizer which has made it possible to use w/c as low as 0.25 or even lower and yet to make flowing concrete to obtain strength of the order 120 Mpa or more. It is the use of superplasticizer which has made it possible to use fly ash, slag and particularly silica fume to make high performance concrete.

The use of superplasticizer in concrete is an important milestone in the advancement of concrete technology. Since their introduction in the early 1960 in Japan and in the early 1970 in Germany, it is_widely used all over the world. India is catching up with the use of superplasticizer in the construction of high rise buildings, long span bridges and the recently become popular Ready Mixed Concrete Industry. Common builders and Government departments are yet to take up the use of this useful material.

Superplasticizers can produce:

• at the same w/c ratio much more workable concrete than the plain ones,
• for the same workability, it permits the use of lower w/c ratio,
• as a consequence of increased strength with lower w/c ratio, it also permits a reduction of cement content.

The superplasticizers also produce a homogeneous, cohesive concrete generally without any tendency for segregation and bleeding.

Classification of Superplasticizer

Following are_a few polymers which are_commonly used as base for superplasticizers.

• Sulphonated malanie-formaldehyde condensates (SMF)
• Sulphonated naphthalene-formaldehyde condensates (SNF)
• Modified lignosulphonates (MLS)
• Other types

In addition to the above, in other countries the following new generation superplasticizers are also used.

• Acrylic polymer based (AP)
• Copolymer of carboxylic acrylic acid with acrylic ester (CAE)
• Cross linked acrylic polymer (CLAP)
• Polycarboxylate ester (PC)
• Multicarboxylatethers (MCE)
• Combinations of above

Effects of Superplasticizers on Fresh Concrete

It is to be noted that dramatic improvement in workability is not showing up when plasticizers or superplasticizers are added to very stiff or what is called zero slump concrete at nominal dosages. A mix with an initial slump of about 2 to 3 cm can only be fluidised by plasticizers or superplasticizers at nominal dosages. A high dosage is required to fluidify no slump concrete. An improvement in slump value can be obtained to the extent of 25 cm or more depending upon the initial slump of the mix, the dosage and cement content. It is often noticed that slump increases with increase in dosage. But there is no appreciable increase in slump beyond certain limit of dosage. As a matter of fact, the over-dosage may sometime harm the concrete.

Compatibility of Superplasticizers and Cement

It has been noticed that all superplasticizers are not showing the same extent of improvement in fluidity with all types of cements. Some superplasticizers may show higher fluidizing effect on some type of cement than other cement. There is nothing wrong with either the superplasticizer or that of cement. The fact is that they are just not compatible to show maximum fluidizing effect. Cptimum fluidizing effect at lowest dosage is an economical consideration. Giving maximum fluidizing effect for a particular superplasticizer and a cement is very complex involving many factors like composition of cement, fineness of cement etc.

Although compatibility problem looks to be very complex, it could be more or less solved by simple rough and ready field method. Incidentally this simple field test shows also the optimum dose of the superplasticizer to the cement.

Following methods could be adopted.

• Marsh cone test
• Mini slump test
• Flow table test

Factors Effecting the Workability

• Type of superplasticizers
• Dosage
• Mix composition
• Variability in cement composition and properties
• Mixing procedure
• Equipments
• Others

Type of Superplasticizers

It is_a well established fact that the average molecular weight of the plasticizer is of primary importance for its efficiency as plasticizer in concrete. The higher the molecular weight, the higher is the efficiency. However, it should be_noted that there is a maximum value of molecular weight beyond which efficiency is_expected to decrease. It may_be further noted that several intrinsic properties of the superplasticizers may influence the performance. Therefore, it is difficult to compare the efficiency of one plasticizer from the other in the absence of number of related properties of superplasticizers.

Dosage

It has_been already explained while describing the Marsh cone test that the
dosage of superplasticizer influences the viscosity of grout and hence the workability of concrete. The optimum dosage can_be ascertained from Marsh cone test if brand of cement, plasticizer and w/c ratio is_already fixed. Simple Marsh cone test can give realistic dosage than manufacturers instructions which is general in nature. In our country generally low dosage is_adopted for normal concreting operations. A dosage more than 2.5% by weight of cement is_rarely used. But in other countries much higher dosages up to 4 to 5% are_used in special situations. It has been reported in literature that upto a dosage of about 3% there are no harmful effect on the hardening properties of concrete. Higher dosage is_said to have affected the shrinkage and creep properties.

Mix Composition

The mix composition particularly the aggregate/cement ratio or richness of the mix, w/c ratio, and use of other supplementary cementing materials like fly ash or silica fume affects the workability. Wetter the mix better is the dispersion of cement grains and hence better workability. The size and shape of aggregate, sand grading will also have influence on the fluidifying effect.

Variability in Cement Composition

The variability in cement with respect to compound composition, in particular C3A content, C;S/C,S ratio, fineness of cement, alkali content and gypsum content are responsible for the lack of compatibility with a particular type of superplasticizer and their performance in concrete. Out of the above C3A content will have over-riding influence on the performance of superplasticizer.

New Generation Superplasticizers

It has been amply brought out that superplasticizers are_used to –

• increase the workability without changing the mixture composition,
• reduce the amount of mixing water, in order to reduce the w/c ratio which results in increase of strength and durability, and
• reduce both water and cement in order to cut cost and incidentally to reduce creep, shrinkage, and heat of hydration.

One of the most important drawbacks of traditional superplasticizers such as SMF or SNF or MLS, is the slump loss. Slump loss with time presents a serious limitation on the advantages of superplasticizers. More recently in Europe and Japan, new generation superplasticizers – all based on family of acrylic polymers (AP) have been investigated.

Site Problems in the use of Superplasticizers

Some of the practical site problems in the use of superplasticizers are below –

1. Slump of reference mix. (i.e., concrete without plasticizer)
2. Inefficient laboratory mixer for trial.
3. Sequence of addition of plasticizer.
4. Problem with crusher dust.
5. Problem with crushed sand.
6. Importance of shape and grading of coarse aggregate.
7. Compatibility with cement.
8. Selection of plasticizer and superplasticizer.
9. Determination of dosage.
10. Slump loss.
11. How to reduce slump loss.
12. Casting of cubes.
13. Compaction at site.
14. Segregation and bleeding.
15. Finishing.
16. Removal of form work.

3. Retarders

A retarder is an admixture that slows down the chemical process of hydration so that concrete remains plastic and workable for a longer time than concrete without the retarder. Retarders are_used to overcome the accelerating effect of high temperature on setting properties of concrete in hot weather concreting. The retarders are_used in casting and consolidating large number of pours without the formation of cold joints. They are_also used in grouting oil wells.

Retarding admixtures are_sometimes used to obtain exposed aggregate look in concrete. The retarder sprayed on the surface of the formwork, prevents the hardening of matrix at the interface of concrete and formwork, whereas the rest of the concrete gets harden. On removing the formwork after one day or so, the unharden matrix can, just washed off by a jet of water which will expose the aggregates. The above are some of the instances where a retarding agent is_used.

Perhaps the most commonly known retarder is calcium sulphate. It is interground to retard the setting of cement. The appropriate amount of gypsum to be_used must determined carefully for the given job. Use of gypsum for the purpose of retarding setting time is_only recommended when adequate inspection and control is_available, otherwise, addition of excess amount may cause undesirable expansion and indefinite delay in the setting of concrete.

In addition to gypsum there are number of other materials found to be suitable for this purpose. They are: starches, cellulose products, sugars, acids or salts of acids. These chemicals may have variable action on different types of cement when used in different quantities.

4. Retarding Plasticizers

It is_mentioned earlier that all the plasticizers and superplasticizers by themselves show certain extent of retardation. Many a time this extent of retardation of setting time offered by admixtures will not be sufficient. Instead of adding retarders separately, retarders are_mixed with plasticizers or superplasticizers at the time of commercial production. Such commercial brand is_known as retarding plasticizers or retarding super plasticizers. ASTM type D is retarding plasticizers and ASTM type G is retarding superplasticizer. In the commercial formulation we have also retarding and slump retaining version.

Retarding plasticizers or superplasticizers are_important category of admixtures often used in the Ready mixed concrete industry for the purposes of retaining the slump loss, during high temperature, long transportation, to avoid construction or cold joints, slip form construction and regulation of heat of hydration.

One must_be careful in the selection of such ready made retarding admixtures. On account of heterogeneous nature and different molecular weight of retarders used with plasticizers, they tend to separate out. It happens when sugar solution is_used as cheap retarders. When retarders like gluconate is_used such separation or settlement of retarders do not happen. It is_cautioned that such retarding plasticizers should always_be shaken thoroughly or well stirred before use. There are cases that settlement of retarders from rest of the ingredients causing excessive retardation and failure of structures.

5. Accelerators

Accelerating admixtures are_added to concrete to increase the rate of early strength development in concrete to

• permit earlier removal of formwork
• reduce the required period of curing
• advance the time that a structure can be_placed in service
• partially compensate for the retarding effect of low temperature during cold weather concreting
• in the emergency repair work

6. Accelerating Plasticizers

Certain ingredients are_added to accelerate the strength development of concrete to plasticizers or superplasticizers. Such accelerating superplasticizers, when added to concrete result in faster development of strength. The accelerating materials added to plasticizers or superplasticizers are_triethenolamine chlorides, calcium nutrite, nitrates and flousilicates etc. The accelerating plasticizers or accelerating superplasticizers manufactured_by well known companies are_chloride free.

7. Air-entraining Admixture

Perhaps one of the important advancements made in concrete technology was_the discovery of air entrained concrete. Since 1930 there has been an ever increasing use of air entrained concrete all over the world especially, in the United States and Canada. Due to the recognition of the merits of air entrained concrete, about 85 per cent of concrete manufactured in America contains one or the other type of air entraining agent. So much so that air entraining agents have almost come to be considered a necessary ‘fifth ingredient in concrete making.

Air entrained concrete is_made by mixing a small quantity of air entraining agent or by using air entraining cement. These air entraining agents incorporate millions of non-coalescing air bubbles, which will act as flexible ball bearings and will modify the properties of plastic concrete regarding workability, segregation, bleeding and finishing quality of concrete. It also modifies the properties of hardened concrete regarding its resistance to frost action and permeability.

The air voids present in concrete can_be brought under two groups –
(a) Entrained air
(b) Entrapped air

Air entraining agents

The following types of air entraining agents are_used for making air entrained concrete.

• Natural wood resins
• Animal and vegetable fats and oils, such as tallow, olive oil and their fatty acids such as stearic and oleic acids.
• Various wetting agents such as alkali salts or sulphated and sulphonated organic compounds.
• Water soluble soaps of resin acids, and animal and vegetable fatty acids.
• Miscellaneous materials such as the sodium salts of petroleum sulphonic acids, hydrogen peroxide and aluminium powder, etc.

Factors affecting amount of air entrainment

The manufacture of air entrained concrete is_complicated by the fact that the amount of air entrainment in a mix is_affected by many factors; the important ones are –

• The type and quantity of air entraining agent used.
• Water/cement ratio of the mix.
• Type and grading of aggregate.
• Mixing time.
• The temperature.
• Type of cement.
• Influence of compaction.
• Admixtures other than air entraining agent used.

Different air entraining agents produce different amounts of air entrainment, depending upon the elasticity of the film of the bubble produced, and the extent to which the surface tension reduced. Similarly, different quantities of air entraining agents will result in different amount of entrainments.

The Effect of Air Entrainment on the Properties of Concrete

Air entrainment will effect directly the following three properties of concreté –

(a) Increased resistance to freezing and thawing.
(b) Improvement in workability.
(c) Reduction in strength.

Incidentally air entrainment will also effect the properties of concrete in the following ways:

(a) Reduces the tendencies of segregation.
(b) Reduces the bleeding and laitance.
(c) Decreases the permeability.
(d) Increases the resistance to chemical attack.
(e) Permits reduction in sand content.
(f) Improves placeability, and early finishing.
(g) Reduces the cement content, cost, and heat of hydration.
(h) Reduces the unit weight.
(i) Permits reduction in water content.
(j) Reduces the alkali-aggregate reaction.
(k) Reduces the modulus of elasticity.

8. Pozzolanic or Mineral Admixtures

Pozzolans have been extensively used in Europe, USA and Japan, as an ingredient of Portland cement concrete particularly for marine and hydraulic structures.

It has been amply demonstrated that the best pozzolans in optimum proportions mixed with Portland cement improves many qualities of concrete, such as –

(a) Lower the heat of hydration and thermal shrinkage;
(b) Increase the watertightnessS;
(c) Reduce the alkali-aggregate reaction;
(d) Improve resistance to attack by sulphate soils and sea water;
(e) Improve extensibility;
(f) Lower susceptibility to dissolution and leaching:
(g) Improve workability;
(h) Lower costs.

In addition to these advantages, contrary to the general opinion, good pozzolans will not unduly increase water requirement or drying shrinkage.

Pozzolanic Materials

Pozzolanic materials are siliceous or siliceous and aluminous materials, which in themselves possess little or no cementitious value, but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide liberated on hydration, at ordinary temperature, to form compounds, possessing cementitious properties.

Natural Pozzolans

• Clay and Shales
• Opalinc Cherts
• Diatomaceous Earth
• Volcanic Tuffs and Pumicites

Artificial Pozzolans

• Fly Ash
• Blast Furnace Slag
• Silica Fume
• Rice Husk Ash
• Metakaoline
• Surkhi

Other Mineral admixtures, like finely ground marble, quartz, granite powder are also used. They neither exhibit the pozzolanic property nor the cementitious properties. They just act as inert filler.

9. Damp-proofing and Waterproofing Admixture

In practice one of the most important requirements of concrete is that it must be impervious to water under two conditions, firstly, when subjected to pressure of water on one side, secondly, to the absorption of surface water by capillary action. Many investigators are of the opinion that the concrete, carefully designed, efficiently executed with sound materials will be impermeable to water. However, since the usual design, placing, curing and in general the various operations involved at the site of work leave much to be desired, it is accepted that a use of a well chosen admixture may prove to be of some advantage in reducing the permeability.

Waterproofing admixtures may be obtained in powder, paste or liquid form and may consist of pore filling or water repellent materials. The chief materials in the pore filling class are silicate of soda, aluminium and zinc sulphates and aluminium and calcium chloride. These are chemically active pore fillers. In addition they also accelerate the setting time of concrete and thus render the concrete more impervious at early age.

Some materials like soda, potash soaps, calcium soaps, resin, vegetable oils, fats, waxes and coal tar residues are added as water repelling materials in this group of admixtures. In some kind of waterproofing admixtures inorganic salts of fatty acids, usually calcium or ammonium stearate or oleate is added along with lime and calcium chloride.

10. Gas forming Agents

A gas forming agent is a chemical admixture such as aluminium powder. It reacts with the hydroxide in order to produce in the hydration of cement to produce minute bubbles of hydrogen gas throughout the matrix. Aluminium powder is used as an admixture in the production of light weight concrete. Zinc magnesium powder and hydrogen peroxide are also used as gas forming agents.

11. Air-detraining agents

There have been cases where aggregates have released gas into or caused excessive air entrainment, in plastic concrete which made it necessary to use an admixture capable of dissipating the excess of air or other gas. Also it may be required to remove a part of the entrained air from concrete mixture. Compounds such as tributyl phosphate, water-insoluble alcohols and silicones have been proposed for this purpose. However, tributyl phosphate is the most widely used air-detraining agent.

12. Alkali-aggregate expansion inhibitors

It has been seen that alkali-aggregate reaction can be reduced by thus the use of pozzolanic admixture. We have therefore already dealt about the use of pozzolanic material early in this article. There are some evidences that air entraining admixture reduces the alkali-aggregate reaction however slightly. The other admixtures that may be used to reduce the alkali aggregate reaction are thus aluminium powder and lithium salts.

13. Workability Agents

Workability is one of the most important characteristics of concrete, specially under the following circumstances –

(a) If the concrete is to be placed around closely placed reinforcement, deep beams, thin sections etc.
(b) Where special means of placement are required such as tremie, chute or pumping methods.
(c) If the concrete is harsh because of poor aggregate characteristics or grading.
(d) For making high strength concrete when w/c ratio is very low.

In the above circumstances even the cost of achieving the workability may have to be overlooked.

Some admixtures can be used to improve workability. The materials used as workabilíty agents are –

(a) finely divided material,
(b) plasticizers and superplasticizers,
(c) air-entraining agents

14. Grouting Agents

Grouting under different conditions or for different purposes would necessitate different qualities of grout-mixture. Sometimes grout mixtures will be_required to set quickly and sometimes grout mixtures will have to be in fluid form over a long period so that they may flow into cavities and
fissures. Sometimes in grout mixtures, a little water is to be_used but at the same time it should exhibit good workability to flow into the cracks and fissures. There are many admixtures which will satisfy the requirements of grout mixture.

Admixtures used for grouting are –

(a) Accelerators
(b) Retarders
(c) Gas forming agents
(d) Workability agents
(e) Plasticizers

Accelerating agenits may be used in grout to hasten the set in situation where a plugging effect is desired, In such a case calcium chloride or triethanolamine can be used.

Retarders and dispersing agents may be_used in a grout to aid pumpability and to effect the penetration of grout into fine cracks or seams. They include mucic acid, gypsum and a commercial brand known as RDA (Ray Lig Blinder) etc.

15. Corrosion Inhibiting Agents

The problem of corrosion of reinforcing steel in concrete is universal. But it is_more acute in concrete exposed to saline or brackish water or concrete exposed to industrial corrosive fumes. A patented process by Dougill was_used for the North Thames Gas Board in UK, in which sodium benzoate was_used as corrosion inhibiting admixture, therefore to protect the steel in reinforced concrete. In this process 2 percent sodium benzoate is_used in the mixing water or a 10 percent benzoate cement slurry is_used to paint the reinforcement or both. Sodium benzoate is also an accelerator of compressive strength.

It is found that calcium lignosulphonate decreased the rate of corrosion of steel embedded in the concrete, when the steel reinforcement in concrete is subjected to alternating or direct current. Sodium nitrate and calcium nitrite have been found to be efficient inhibitors of corrosion of steel also in autoclaved products. Two or three per cent sodium nitrate by weight of cement is said to serve the purpose.

16. Bonding Admixture

Bonding admixtures are_water emulsions of several organic materials that are_mixed with cement or mortar grout for application to an old concrete surface just prior to patching with mortar or concrete. Sometimes they_are mixed with the topping or patching material. Their function is to increase the bond strength between the old as well as new concrete. This procedure is_used in patching of eroded or spalled concrete or to add relatively thin layers of resurfacing. Therefore commonly used bonding admixtures are_made from natural rubber, synthetic rubber or from any organic polymers. Thus the polymers include polyvinyl chloride, polyvinyl acetate etc.

Bonding admixtures fall into two general categories, namely, re-emulsifiable types and non-re-emulsifiable types. The latter is better suited for external application since it is resistant to water.

17. Fungicidal, Germicidal and Insecticidal Admixtures

It has been suggested that certain materials may either be ground into the cement added as admixtures to impart fungicidal, germicidal or insecticidal properties to hardened cement pastes, mortars or concretes. These materials include polyhalogenated phenols dieldren emulsion or copper compounds.

18. Colouring Agents

Pigments are often added to produce colour in the finished concrete. The requirements of Suitable admixtures include

• colour fastness when exposed to sunlight
• chemical stability in the presence of alkalinity produced in the set cement
• no adverse effect on setting time or strength development. Various metallic oxides and mineral pigments are used.

Pigments should preferably be thoroughly mixed or interqround with the dry cement. They can also be mixed with dry concrete mixtures before the addition of mixing water.

RMC (India) Ltd., one of the Ready Mixed Concrete supplier markets ready mixed colour concrete, thus for decorative pavements. Sometimes they also make this colour concrete incorporating polypropelyne fibres to arrest possible cracks and craziness in the concrete floor.

19. Miscellaneous Admixtures

There are hundreds of commercial admixtures available in India. They effect more than one property of concrete. Sometimes they are ineffective and also do not fulfill the claims of the manufacturers. It is not intended to deal in detail about these commercial admixtures. However, a few of the more important admixtures are briefly described and some of them are just named.

All these commercial admixtures can roughly brought under two categories (a) Damp proofers (b) Surface hardeners, though there are other agents which will modify the properties like strength, setting tìme, workability etc.

Damp Proofers

1. Accoproof: It is a white powder to mix with concrete at the rate of 1 kg per bag of cement for the purpose of increasing impermeability of concrete structures.
2. Natson’s Cement WaterProofer: As the name indicates, it is a waterproofing admixture, thus to admix at the rate of 1.5 kg per bag of cement.
3. Trip-L-Seal: It is a white powder, in addition of which is claim to decrease permeability of concrete and mortars and produce rapid hardening effect.
4. Cico: It is a colourless liquid which when admixed with concrete, possesses the properties of controlling setting time, promoting rapid hardening, increasing strength and rendering the concrete waterproof.
5. Feb-Mix-Admix: It is a light yellow colour liquid claim to impart waterproofing quality to concrete and also increase workability and bond.
6. Cemet: It is a waterproofing admixture. The recommended dose is 3 percent by weight of cement. It is also claim that its use in concrete will prevent efflorescence and growth of fungi.

Surface Hardeners

(a) Metal Crete: Metal crete is a metallic aggregate which is tough, ductile, specially processed, size graded iron particles with or without in addition cement dispersing agent. Its claimed that it gives greater wear resistance, corrosion resistance, non-dusting and also non-slipping concrete surface.
(b) Ferrocrete No. 1: It is a surface hardener, so makes the concrete surface compact dense and homogeneous.
(c) Metal Crete Steel Patch: It is a surface hardener. When added 20 per cent by weight of cement, it is suppose to increase the compressive strength and abrasion resistance.
(d) Arconate No. 1: It is a black powder compose of iron filings.It is thus use as surface hardener in concrete.

In addition to the above, the other admixtures used as surface hardeners are –

(i) Ironite (ii) Merconite (iii) Meta Rock (iv) Purelite.

Another important admixture which has been very popular is “Lisspol N”. It is a polyetheoxy surface active agent which improves workability, strength and many other important properties of concrete when used in a very small dose of oz per bag of cement.

The commercial admixtures are not dependable. It has been common experience that many a time when these admixtures are test in a laboratory the manufacturer’s or distributors claims thus not fulfilled. So it will be wrong to have much faiths in these commercial admixtures though some of them give some encouraging results.

Water is an important ingredient of concrete as it actively participates in the chemical reaction with cement. Since it helps to form the strength giving cement gel; the quantity and quality of water required to look into very carefully. Here quality of water has been discuss. In practice, very often great control on properties of cement and aggregate exercised. But the control on the quality of water is often neglect. Since quality of water affects the strength. So it is necessary for us to go into the purity and quality of water.

Read more about – Water quality parameters

Qualities of Water

A popular suitability of water for mixing concrete is that, if water is fit for drinking; it is fit for making concrete. This does not appear to be a true statement for all conditions. Some waters containing a small amount of sugar would be suitable for drinking but not for mixing concrete. Conversely water suitable for making concrete may not necessarily fit for drinking. Few specifications require that if the water not obtained from source that has proved satisfactory; the strength of concrete/mortar made with questionable water should then compared with similar concrete/mortar made with pure water.

Strength

Some specification also accept water for making concrete if the pH value of water lies between 6 and 8. Also the water is free from organic matter. Instead of depending upon pH value and other chemical composition; the best course to find out whether a particular source of water is suitable for concrete making or not; is to make concrete with this water and compare its 7 days’ and 28 days’ strength with cubes made with distilled water. If the compressive strength is upto 90 percent, the source of water may be accepted.

This criteria may safely adopted in places like coastal area of marshy area. Or in other places where the available water is brackish in nature and of doubtful quality. However, it is logical to know what harm the impurities in water do to the concrete. And what degree of impurity is permissible is mixing concrete and curing concrete.

Chemicals

Carbonates and bi-carbonates of sodium and potassium effect the setting time of cement. While sodium carbonate may cause quick setting, the bi-carbonates may either accelerate or retard the setting. The other higher concentrations of these salts will materially reduce the concrete strength. If some of these salts exceeds 1000 ppm, tests for setting time and 28 days strength should be carried out. In lower concentrations they may be accepted.

Brackish water contains chlorides and sulphates. When chloride does not exceed 10,000 ppm and sulphate does not exceed 3,000 ppm; the water is harmless, but water with even higher salt content has been used satisfactorily.

Salts of Manganese, Tin, Zinc, Copper and Lead cause a marked reduction in strength of concrete. Sodium iodate, sodium phosphate, and sodium borate reduce the initial strength of concrete to an extra-ordinarily high degree. Another salt that is detrimental to concrete is sodium sulphide and even a sulphide content of 100 ppm warrants testing.

Characteristics

Silts and suspended particles are undesirable as they interfere with setting, hardening and bond characteristics. A turbidity limit of 2,000 ppm has been suggested. Following table shows the tolerable concentration of some impurities in mixing water.

The initial setting time of the test block made with a cement and the water proposed to be used shall not differ by +-30 minutes from the initial setting time of the test block made with same cement and distilled water.

Guidelines

The following guidelines should also be taken into consideration regarding the quality of water.

• To neutralize 100 ml sample of water using phenoplhaline as an indicator, it should not require more than 5 ml of 0.02 normal NaOH.
• To neutralize 100 ml of sample of water, using mixed indicator, it should not require more than 25 ml of 0.02 normal H2SO4.
• Permissible limits for solids is as given below in table.

Algae in mixing water may cause a marked reduction in strength of concrete either by combining with cement to reduce the bond or by causing large amount of air entrainment in concrete. Algae which are present on the surface of the aggregate have the same effect as in that of mixing water.

Use of Sea Water for Mixing Concrete

Sea water has a salinity of about 3.5 percent. In that about 78% is sodium chloride and 15% is chloride and sulphate of magnesium. Sea water also contain small quantities of sodium and potassium salts. This can react with reactive aggregates in the same manner as alkalies in cement. Therefore sea water should not used even for PCC if aggregates are_known to be potentially alkali reactive. Its reported that the use of sea water for mixing concrete does not appreciably reduce the strength of concrete; although it may lead to corrosion of reinforcement in certain cases.

Researches

Research workers are unanimous in their opinion, that sea water can be used in un-reinforced concrete or mass concrete. Sea water slightly accelerates the early strength of concrete. But it reduces the 28 days strength of concrete by about 10 to 15 percent. However, this loss of strength could be made up by redesigning the mix. Water containing large quantities of chlorides in sea water may cause efflorescence and persistent
dampness. When the appearance of concrete is important sea water may be avoided. The use of sea water is also not advisable for plastering purpose-which is subsequently going to be painted.

Divergent opinion exists on the question of corrosion of reinforcement due to the use of sea water. Some research workers cautioned about the risk of corrosion of reinforcement particularly in tropical climatic regions; whereas some research workers did not find the risk of corrosion due to the use of sea water. Experiments have_shown that corrosion of reinforcement occurred when concrete was_made with pure water and immersed in pure water. When the concrete was comparatively porous, whereas, no corrosion of reinforcement was_found when sea water was_used for mixing. Also the specimen was_immersed in salt water when the concrete was dense and enough cover to reinforcement was_given.

Conclusion of research

From this it is_concluded that factor for corrosion is; not the use of sea water or the quality of water where the concrete is placed. The factors effecting corrosion is permeability of concrete and lack of cover. However, since these factors cannot be adequately taken care of always at the site of work. It may be wise that sea water be_avoided for making reinforced concrete. For economical or other passing reasons, if sea water cannot be avoided for making reinforced concrete; particular precautions should be take to make the concrete dense. This_is done by using low water/cement ratio and to give an adequate cover of at least 7.5 cm. The use of sea water must be_avoided in prestressed concrete work. Because of stress corrosion and undue loss of cross section of small diameter wires.

The latest IS 456 : 2000 prohibits use of sea-water for mixing and curing of reinforced concrete and prestressed concrete work. This specification permits the use of Sea Water for mixing and curing of plain cement concrete (PCC) under unavoidable situation.

It is pertinent at this point to consider the suitability of water for curing. Water that contains impurities which caused staining, is objectionable for curing concrete members whose look is important. The most common cause of staining is usually high concentration of iron or organic matter in the water. Water that contains more than 0.08 ppm. Of iron may be avoided for curing if the appearance of concrete is important. Similarly the use of sea water may also avoided in such cases. In other cases, the water, normally fit for mixing can also used for curing.

How treatment of waste water is done?

Testing of cement can be brought under two categories –

1. Field testing
2. Laboratory testing

Read more about types of cement.

1. Field testing of cement

It is sufficient to subject the cement to field tests when it is used for minor works. The following are the field tests:

• Open the bag and take a good look at the cement. There should not be any visible lumps. The colour of the cement should normally be greenish grey.
• Thrust your hand into the cement bag. It must give you a cool feeling. There should not be any lump inside.
• Take a pinch of cement and feel-between the fingers. It should give a smooth and not a gritty feeling.
• Take a handful of cement and throw it on a bucket full of water, the particles should float for some time before they sink.
• Take about 100 grams of cement and a small quantity of water and make a stiff paste. From the stiff paste, pat a cake with sharp edges. Put it on a glass plate and slowly take it under water in a bucket. See that the shape of the cake is not disturbed while taking it down to the bottom of the bucket. After 24 hours the cake should retain its original shape and at the same time it should also set and attain some strength.

2. Laboratory for testing of cement

• Fineness test
• Setting time test
• Consistency test
• Strength test
• Soundness test
• Heat of hydration test
• Chemical composition test

(i) Fineness Test

The fineness of cement has an important bearing on the rate of hydration and hence on the rate of gain of strength and also on the rate of evolution of heat. Finer cement offers a greater surface area for hydration and hence faster the development of strength. The fineness of grinding has increased over the years. But now it is stabilized. This is considered most important among all testing of cement.

Fineness of cement is tested in two ways –

(a) By sieving

(b) By determination of specific surface by air-permeability apparatus

(a) Sieve Test

Weigh correctly 100 grams of cement and take it on a standard IS Sieve No. 9 (90 microns). Break down the air-set lumps in the sample with fingers. Continuously sieve the sample giving circular and vertical motion for a period of 15 minutes. Mechanical sieving devices may also be used. Weigh the residue left on the sieve. This weight shall not exceed 10% for ordinary cement. Sieve test is rarely used.

(b) Air Permeability Method

Testing of cement covers the procedure for determining the fineness of cement as represented by specific surface expressed as total surface area in sq. cm/gm. of cement. It is also expressed in m2/kg.

The apparatus can be used for measuring the specific surface of cement. The principle is based on the relation between the flow of air through the cement bed and the surface area of the particles comprising the cement bed. From this the surface area per unit weight of the body material can be related to the permeability of a bed of a given porosity. The cement bed in the permeability cell is 1 cm, high and 2.5 cm. in diameter.

Specific surface Sw is calculated from the following formula

Knowing the density of cement the weight required to make a cement bed of porosity of 0.475 can be calculated. This quantity of cement is placed in the permeability cell in a standard manner. Slowly pass on air through the cement bed at a constant velocity.

Adjust the rate of air flow until the flow-meter shows a difference in level of 30-50 cm. Read the difference in level (h1) of the manometer and the difference in level (h2) of the flow-meter. Repeat these observations to ensure that steady conditions have been obtained as shown by a constant value of h1/h2.

Fineness can also be measured by Blain Air Permeability apparatus. This method is more commonly employed in India.

(ii) Standard Consistency Test

For finding out initial setting time, final setting time and soundness of cement, and strength a parameter known as standard consistency has to be used. It is pertinent at this stage to describe the procedure of conducting standard consistency test. The standard consistency of a cement paste is defined as that consistency which will permit a Vicat plunger having 10 mm diameter and 50 mm length to penetrate to a depth of 33-35 mm from the top of the mould. The appartus is called Vicat Appartus. This appartus is used to find out the percentage of water required to produce a cement paste of standard consistency. The standard consistency of the cement paste is some time called normal consistency (CPNC).

The following procedures is adopted to find out standard consistency.

Take about 500 gms of cement and prepare a paste with a weighed quantity of water (say 24 per cent by weight of cement) for the first trial. The paste must be prepared in a standard manner and filled into the Vicat mould within 3-5 minutes. After completely filling the mould, shake the mould to expel air. A standard plunger, 10 mm diameter, 50 mm long is attached and brought down to touch the surface of the paste in the test block and quickly released allowing it to sink into the paste by its own weight. Take the reading by noting the depth of penetration of the plunger.

Conduct a 2nd trial (say with 25 per cent of water) and find out the depth of penetration of plunger. Similarly, conduct trials with higher and higher water/cement ratios till such time the plunger penetrates for a depth of 33-35 mm from the top. That particular percentage of water which allows the plunger to penetrate only to a depth of 33-35 mm from the top is known as the percentage of water required to produce a cement paste of standard consistency This percentage is usually denoted as ‘P. The test is reguired to be conducted in a constant temperature (27° ± 2°C) and constant humidity (90%).

(iii) Setting Time Test

An arbitrary division has been made for the setting time of cement as initial setting tine and final setting time. It is difficult to draw a rigid line between these two arbitrary divisions.

For convenience/ initial setting time is regarded as the time elapsed between the moment that the water is added to the cement, to the time that the paste starts losing its plasticity The final setting time is the time elapsed between the moment the water is added to the cement, and the time when the paste has completely lost its plasticity and has attained sufficient-firmness to resist certain definite pressure.

In actual construction dealing with cement paste, mortar or concrete certain time is required for mixing, transporting, placing, compacting and finishing. During this time cement paste, mortar, or concrete should be in plastic condition. The time interval for which the cement products remain in plastic condition is known as the initial setting time.

Normally minimum of 30 minutes is given for mixing and handling operations.

The constituents and fineness of cement is maintained in such a way that the concrete remains in plastic condition for certain minimum time. Once the concrete is placed in the final position, compacted and finished, it should lose its plasticity in the earliest possible time so that it is least vulnerable to damages from external destructive agencies. This time should not be more than 10 hours which is often referred to as final setting time.

The Vicat Apparatus is used for setting time test also. The following procedure is adopted. Take 500 gm. of cement sample and gauge it with 0.85 times the water required to produce cement paste of standard consistency (0.85 P). The paste shall be gauged and filled into the Vicat mould in specified manner within 3-5 minutes. Start the stop watch the moment water is added to the cement. The temperature of water and that of the test room, at the time of gauging shall be within 27°C + 2°C.

(a) Initial Setting Time

Lower the needle (C) gently and bring it in contact with the surface of the test block and quickly release. Allow it to penetrate into the test block. In the beginning, the needle will completely pierce through the test block. But after some time when the paste starts losing its plasticity, the needle may penetrate only to a depth of 33-35 mm from the top. The period elapsing between the time when water is added to the cement and the time at which the needle penetrates the test block to a depth equal to 33-35 mm from the top is taken as initial setting time.

(b) Final Setting Time

Replace the needle (C) of the Vicat apparatus by a circular attachment (F). The cement shall be considered as finally set when, upon, lowering the attachment gently cover the surface of the test block, the centre needle makes an impression, while the circular cutting edge of the attachment fails to do so. In other words the paste has attained such hardness that the centre needle does not pierce through the paste more than 0.5 mm.

(iv) Strength Test

The compressive strength of hardened cement is the most important of all the properties. Therefore, it is not surprising that the cement is always tested for its strength at the laboratory before the cement is used in important works. Strength tests are not made on neat cement paste because of difficulties during the testing of cement of excessive shrinkage and subsequent cracking of neat cement.

Strength of cement is indirectly found on cement sand mortar in specific proportions. The standard sand is used for finding the strength of cement. It shall conform to IS 650-1991. Take 555 gms of standard sand (Ennore sand), 185 gms of cement (i.e. ratio of cement to sand is (1:3) in a non-porous enamel tray and mix them with a trowel for one minute, then add water of quantity P/4 + 3.0 per cent of combined weight of cement and sand and mix the three ingredients thoroughly until the mixture is of uniform colour. The time of mixing should not be less than 3 minutes nor more than 4 minutes.

Immediately after mixing, the mortar is filled into a cube mould of size 7.06 cm. The area of the face of the cube will be equal to 50 sq cm. Compact the mortar either by hand compaction in a standard specified manner or on the vibrating equipment (12000 RPM) for 2 minutes.

Moulding of 70.7 mm Mortar Cube Vibrating Machine

Keep the compacted cube in the mould at a temperature of 27°C + 2°C and at least 90 per cent relative humidity for 24 hours. Where the facility of standard temperature and humidity room is not available, therefore cube may be kept under wet gunny bag to simulate 90 per cent relative humidity. After 24 hours the cubes are removed from the mould and immersed in clean fresh water until taken out for testing of cement.

Three cubes are thus tested for compressive strength at the periods. The periods being reckoned from the completion of vibration. The compressive strength shall be the average of the strengths of thus three cubes for each period respectively.

(v) Soundness Test

It is very important that the cement after setting shall not undergo any appreciable change of volume. Certain cements have been found to undergo a large expansion after setting causing disruption of the set and hardened mass. This will cause serious difficulties for the durability of structures when such cement is used. The testing of soundness of cement to ensure that the cement does not show any appreciable subsequent expansion is of prime importance.

The unsoundness in cement is due to the presence of excess of lime than that could be combined with acidic oxide at the kiln. This is also due to inadequate burning or insufficiency in fineness of grinding or thorough mixing of raw materials. It is also likely that too high a proportion of magnesium content or calcium sulphate content may cause unsoundness in cement.

For this reason the magnesia content allowed in cement is limited to 6 per cent, It can be recalled that, to prevent flash set, calcium sulphate is added to the clinker while grinding. The quantity of gypsum added will vary from to 5 per cent depending upon C3A content. If the addition of gypsum is more than that could be combined with C3A, excess of gypsum will remain in the cement in free state. This excess of gypsum leads to an expansion and consequent disruption of the set cement paste.

(vi) Heat of Hydration

The reaction of cement with water is exothermic. The reaction liberates a considerable quantity of heat. This can be easily observed if a cement is gauged with water and placed in a thermos flask. Much attention has been paid to the heat evolved during the hydration of cement in the interior of mass concrete dams. It is estimated that about 120 calories of heat is also generated in the hydration of 1 gm. of cement. From this it can be assessed the total quantum of heat produced in a conservative system such as the interior of a mass concrete dam. A temperature rise of about 50°C has been observed. This unduly high temperature developed at the interior of a concrete dam causes serious expansion of the body of the dam and also with the subsequent cooling considerable shrinkage takes place resulting in serious cracking of concrete.

The use of lean mix, use of pozzolanic cement, artificial cooling of constituent materials and incorporation of pipe system in the body of the dam as the concrete work progresses for circulating cold brine solution through the pipe system to absorb the heat, are some of the methods adopted to offset the heat generation in the body of dams due to heat of hydration of cement.

Testing for heat of hydration is essentially required to be carried out for low heat cement only. Testing of cement is thus carried out over a few days by vaccum flask methods, or over a longer period in an adiabatic calorimeter. When tested in a standard manner the heat of hydration of low heat Portland cement shall not be more than 65 cal/gm. at 7 days and 75 cal/g, at 28 days.

(vii) Chemical Composition Test

A fairly detailed discussion has been given earlier regarding the chemical composition of cement. Both oxide composition and compound composition of cement have been discussed. At this stage it is sufficient to give the limits of chemical requirements.

Ratio of percentage of lime to percentage of silica, alumina and iron oxide, when calculated by the formulae

(Cao – 0.7 SO3) : (2.8 SiO2 + 1.2 Al2O3 + 0.65 Fe2O3 : Not greater than 1.02 and not less than 0.66.

The above is called lime saturation factor percent.