Q: What is cement?A: Cement is a fine, soft, powdery substance, made from a mixture of elements found in naturalmaterials such as limestone, clay, sand and/or shale. When cement is mixed with water, it canbind sand and gravel into a hard, solid mass called concrete. Cement is usually grey. Whitecement is also available, but is usually more expensive.1. Cement mixed with water, sand and gravel, forms concrete.2. Cement mixed with water and sand, forms cement plaster.3. Cement mixed with water, lime and sand, forms mortar.Cement powder is extremely fine; one kilo (2.2lbs) contains over 300 billion grains. The powder isso fine it will pass through a sieve capable of holding water.In India, Ordinary Portland Cement (OPC) is manufactured in three grades, viz. 33 grade, 43 gradeand 53 grade. The numbers indicate the compressive strength obtained after 28 days, when testedas per the stipulated procedure.Apart from OPC, there are several other types of cement, mostly meant for special purposes, e.g.sulphate resistant cement, coloured cement, oil well cement etc. However, there are somegeneral-purpose cements, the commonest one being Portland Pozzolana Cement (PPC).

Q: What is natural cement?A: Natural cements are hydraulic cements, produced by mining natural deposits of limestone andclay with a specific chemical composition within a narrow range. When heated in a kiln and groundto a fine powder, a type of cement is produced, which through chemical reactions sets and hardenswhen mixed with water. The strength and uniformity of natural cements are lower than those ofPortland cements; but these are more historically accurate materials for restoration projects,which is their primary application. Natural cements were extensively used in 19th and early 20thcentury construction in several historic structures. However, with improved technology forproducing Portland cements, sales of natural cements began to decline in the late 1800s, stoppingentirely by the mid 1970s.

Q: How is cement made?A: 1) Limestone, the major ingredient needed for making cement is quarried. Small quantities ofsand and clay are required as well. Limestone, sand and clay contain the four essential elementsrequired to make cement: calcium, silicon, aluminium and iron.2) Boulder-size limestone rocks are transported from the quarry to the cement plant and fed into acrusher, which crushes the boulders into marble-size pieces.3) The limestone pieces then go through a blender where they are mixed with the other rawmaterials in the right proportion.4) Raw materials are then ground to a powder. This is sometimes done with rollers that crush thematerials against a rotating platform.5) This mixture then goes into a huge, extremely hot, rotating furnace to undergo a process called‘sintering’. Sintering means: to cause to become a coherent mass by heating without melting. Inother words, the raw materials become partially molten. The raw materials reach about 2700° F(1480°C) inside the furnace. This causes chemical and physical changes to the raw materials andthey come out of the furnace as large, glassy, red-hot cinders called ‘clinker’.6) This clinker is cooled and ground into a fine grey powder. A small amount of gypsum is addedduring the final grinding. The finished product is Portland cement.The cement is then stored in silos (large holding tanks) where it awaits distribution.The cement is usually shipped in bulk in purpose-made trucks, by rail or even by barges and ships.Some is bagged for those who want small quantities.

Q: What are the different types of Cements?A: Portland cement: Portland cement is made by heating limestone with small quantities of othermaterials (such as clay) to 1450°C in a kiln, in a process known as calcination. The resulting hardsubstance, called ‘clinker,’ which is then ground with a small amount of gypsum into a powder tomake ‘Ordinary Portland Cement,’ the most commonly used type of cement (often referred to asOPC).Portland cement is the basic ingredient of concrete, mortar and most non-speciality grout. Itsmost common use is in the production of concrete. Concrete is a composite material consisting ofaggregate (gravel and sand), cement, and water. As a construction material, concrete can be castin almost any shape desired, and once hardened can become a structural (load bearing) element.Portland cement may be grey or white.Portland cement blends: These are often available as inter-ground mixtures from cementmanufacturers, but similar formulations are often also mixed from ground components at theconcrete mixing plant.

Portland Blastfurnace Cement contains up to 70% ground granulated blast furnace slag,Portland clinker and a little gypsum. All compositions produce high ultimate strength, but as theslag content is increased, the early strength is reduced, while the sulphate resistance increasesand heat evolution diminishes. Portland Blastfurnace Cement is used as an economic alternative toPortland sulphate-resisting and low-heat cements.Portland Flyash Cement contains up to 30% fly ash. The fly ash is pozzolanic, so that ultimatestrength is maintained. Because fly ash addition allows for lower concrete water content, earlystrength can be maintained. This can be an economic alternative to ordinary Portland cementwhere good quality, cheap fly ash is available.Portland Pozzolan Cement includes fly ash cement, since fly ash is a pozzolan, in addition tocements made from other natural or artificial pozzolans. In countries where volcanic ashes areavailable (e.g. Italy, Chile, Mexico, the Philippines) these cements are often the most commonform in use.Portland Silica Fume Cement is produced by the addition of silica fume to cement, andexceptionally high strength substance. Cements containing 5–20% silica fume are occasionallyproduced. However, silica fume is more usually added to Portland cement at the concrete mixer.Masonry Cements are used for preparing bricklaying mortars and stuccos, and must not be usedin concrete. They are usually complex proprietary formulations containing Portland clinker and anumber of other ingredients that may include limestone, hydrated lime, air-entrainers, retarders,waterproofers and colouring agents. They are formulated to yield workable mortars that allowrapid and consistent masonry work. Subtle variations of Masonry cement in the US are PlasticCements and Stucco Cements. These are designed to produce controlled bonds with masonryblocks.Expansive Cements contain, in addition to Portland clinker, expansive clinkers (usuallysulfoaluminate clinkers) and are designed to offset the effects of drying shrinkage that is normallyencountered with hydraulic cements. This allows large floor slabs (up to 60m2) to be preparedwithout contraction joints.White blended cements may be made using white clinker and white supplementary materials suchas high-purity metakaolin.Coloured cements are used for decorative purposes. Some standards allow the addition ofpigments to produce ‘coloured Portland cement’. In other standards (e.g. ASTM), pigments are notallowed constituents of Portland cement, and coloured cements are sold as ‘blended hydrauliccements’.Very finely ground cements are made from mixtures of cement with sand or slag or other pozzolantype minerals, which are finely ground together. Such cements can have the same physicalcharacteristics as normal cement but with 50% less cement, particularly due to their increasedsurface area for the chemical reaction. Even with intensive grinding they can use up to 50% lessenergy for fabrication than ordinary Portland cements.Non-Portland hydraulic cementsPozzolan-lime cements: Mixtures of ground pozzolan and lime were the cements used by theRomans, and are found in Roman structures still standing (e.g. the Pantheon in Rome). Theydevelop strength slowly, but their ultimate strength can be very high. The hydration products thatproduce strength are essentially the same as those of Portland cement.Slag-lime cements: Ground granulated blast furnace slag is not hydraulic on its own, but is‘activated’ by the addition of alkalis, most economically using lime. They are similar to pozzolanlime cements in their properties. Only granulated slag (i.e. water-quenched, glassy slag) iseffective as a cement component.Supersulphated cements: These contain about 80% ground granulated blast furnace slag, 15%gypsum or anhydrite and small quantities of Portland clinker or lime as an activator. They producestrength by formation of ettringite, with strength growth similar to a slow Portland cement. Theyexhibit good resistance to aggressive agents, including sulphates.Calcium aluminate cements are hydraulic cements made primarily from limestone and bauxite.The active ingredients are monocalcium aluminate CaAl2O4 (CA in Cement chemist notation) andMayenite Ca12Al14O33 (C12A7 in CCN). Strength forms by hydrating calcium aluminate hydrates.They are well adapted for use in refractory (high-temperature resistant) concretes, e.g. furnacelinings.Calcium sulfoaluminate cements are made from clinkers that include ye'elimite(Ca4(AlO2)6SO4 or C4A3 in CCN) as a primary phase. They are used in expansive cements, inultra-high early strength cements, and in ‘low-energy’ cements. Hydration produces ettringite, andspecialised physical properties (such as expansion or rapid reaction) are obtained by adjustment ofthe availability of calcium and sulphate ions. Their use as a low-energy alternative to Portlandcement has been pioneered in China, where several million tonnes per year are produced. Energyrequirements are lower because of the lower kiln temperatures required for reaction and the lower

amount of limestone (that has to be endothermically decarbonised) in the mix. In addition, thelower limestone content and lower fuel consumption leads to a CO2 emission around half thatassociated with Portland clinker. However, SO2 emissions are significantly higher.‘Natural’ Cements correspond to certain cements of the pre-Portland era, produced by burningargillaceous limestone at moderate temperatures. The level of clay components in the limestone(around 30–35%) is so that large amounts of belite (the low-early strength, high-late strengthmineral in Portland cement) are formed without the formation of excessive amounts of free lime.As with any natural material, such cements have highly variable properties.Geopolymer cements are made from mixtures of water-soluble alkali metal silicates andaluminosilicate mineral powders such as fly ash and metakaolin.

Q: How is Portland cement made?A: Materials that contain appropriate amounts of calcium compounds like silica, alumina and ironoxide are crushed, screened and placed in a rotating cement kiln. Ingredients used in this processare typically materials such as limestone, marl, shale, iron ore, clay and fly ash.The kiln resembles a large horizontal pipe with a diameter of 10–15ft (3–4.1m) and a length of300ft (90m) or more. One end is raised slightly and the raw mix is placed in the high end; as thekiln rotates, the materials move slowly toward the lower end. Flame jets are at the lower end andall the materials in the kiln are heated to high temperatures that range between 2700 and 3000°F(1480 and 1650°C). This high heat drives off, or calcines, the chemically combined water andcarbon dioxide from the raw materials and forms new compounds (tricalcium silicate, dicalciumsilicate, tricalcium aluminate and tetracalcium aluminoferrite). For each ton of material that goesinto the feed end of the kiln, two thirds of a ton of clinker comes out the discharge end. Thisclinker is in the form of marble sized pellets. The clinker is very finely ground to produce Portlandcement. A small amount of gypsum is added during the grinding process to control the cement’sset or rate of hardening.

Q: What is Fibre Reinforced Concrete?A: Low Fibre volume composite concrete contains less than 1% fibre. It is used for fieldapplications involving large volumes of concrete. The fibres do not significantly increase thestrength of the concrete. Low fibre volume concrete is used for paving roads.High Fibre Volume Concrete: Typically used for thin sheets with cement mortar mix. The fibrevolume in this mix ranges from 5% to 15%.High Fibre Volume Composite: The fibre volume in this mix can be as high as 40%. Thissignificantly increases the strength and toughness of the mix. The reinforcement in High FibreVolume Composite concrete is usually in sheet form. This reinforced concrete type is used in roofand wall panels.

Q: What is the difference between cement and concrete?A: Concrete should not be confused with cement because the term cement refers only to the drypowder substance used to bind the aggregate materials of concrete. Upon the addition of waterand/or additives the cement mixture is referred to as concrete, especially if aggregates have beenadded.

Q: What is concrete?A: Concrete is a mixture of cement, water, sand and gravel (stones, crushed rock). The mixtureeventually hardens into a stone-like material. Cement and water are the two ingredients thatchemically react; the gravel and sand give strength.

Q. How was concrete made in the earlier times?A: During the Roman Empire, Roman concrete (or Opus caementicium) was made from quicklime,pozzolanic ash/pozzolana and an aggregate of pumice. Its widespread use in many Romanstructures, a key event in the history of architecture termed the Concrete Revolution, freed Romanconstruction from the restrictions of stone and brick material and allowed for revolutionary newdesigns both, in terms of structural complexity and dimension. Concrete, as the Romans knew it,was in effect a new and revolutionary material. Laid in the shape of arches, vaults and domes, itquickly hardened into a rigid mass, free from many of the internal thrusts and strains, whichtroubled the builders of similar structures in stone or brick.

Q: How is modern structural concrete different from the earlier form of concrete?A: Modern structural concrete differs from Roman concrete in two important details. First, its mixconsistency is fluid and homogeneous, allowing it to be poured into forms rather than requiringhand layering together with the placement of aggregate, which in Roman practice often consisted

of rubble. Second, integral reinforcing steel gives modern concrete assemblies great tensilestrength, whereas Roman concrete could depend only upon the strength of the concrete bondingto resist tension.

Q: What does ‘curing’ concrete mean?A: Curing is one of the most important steps in concrete construction, because proper curinggreatly increases concrete strength and durability. Concrete hardens as a result of hydration: thechemical reaction between cement and water. However, hydration occurs only in the presence ofwater and if the concrete’s temperature stays within a suitable range. During the curing period-from, five to seven days after placement for conventional concrete, the concrete surface needs tobe kept moist to permit the hydration process. New concrete can be wet with soaking hoses,sprinklers or covered with wet burlap, or can be coated with commercially available curingcompounds, which seal in moisture.

Q: What is Reinforced concrete?A: Reinforced concrete contains steel reinforcing that is designed and placed in structural membersat specific positions to cater for the stress conditions that the member is required toaccommodate.

Q. What is Prestressed concrete?A: The principle behind Prestressed concrete is that compressive stresses induced by high-strengthsteel tendons in a concrete member before loads are applied will balance the tensile stressesimposed in the member during service.For example a horizontal beam will tend to sag down. However, if the reinforcement along thebottom of the beam is prestressed, it can counteract this.In pre-tensioned concrete, prestressing is achieved by using steel or polymer tendons or bars thatare subjected to a tensile force prior to casting; and for post-tensioned concrete, after casting.

Q. What are the sought after properties of concrete?A. 1. The concrete mix is extremely workable. It can be placed and consolidated properly.2. Desired qualities of the hardened concrete are met. For example, resistance to freezingand thawing and deicing chemicals, watertightness (low permeability), wear resistance andstrength.3. Economy. Since the quality depends mainly on the water to cement ratio, the waterrequirement should be minimised to reduce the cement requirement (and thus reduce the cost).The following steps reduce water and cement requirements:Use the stiffest mix possibleUse the largest size aggregate practical for the jobUse the optimum ratio of fine to coarse aggregate

Q: What is the composition of ConcreteA: 11% Cement (usually Portland)16% Water6% Air26% Sand41% Gravel or crushed stoneQ: Descriptive composition of Concrete.A: There are many types of concrete available, created by varying the proportions of its mainingredients.The mix design depends on the type of structure being built, how the concrete will be mixed,delivered and how it will be placed to form the structure.

CementPortland cement is the most widely used cement. It is the basic ingredient in concrete, mortar andplaster. English engineer, Joseph Aspdin patented Portland cement in 1824; it was named becauseof its similar colour to Portland limestone, quarried from the Isle of Portland and used extensivelyin London architecture. It consists of a mixture of oxides of calcium, silicon and aluminium and ismanufactured by heating limestone (source of calcium) and clay, then grinding this product(clinker) with a source of sulphate (most commonly gypsum). The manufacturing of Portlandcement creates about 5% of human CO2 emissions.

WaterCombining water with a cementitious material forms a cement paste by the process of hydration.

The cement paste glues the aggregate together, fills voids within it and allows it to flow moreeasily.Lower amounts of water in the cement paste will yield a stronger, more durable concrete; morewater will give an easier-flowing concrete with a higher slump.Impure water used to make concrete can cause problems when setting or premature failure of thestructure.Hydration involves many different reactions, often occurring at the same time. As the reactionsproceed, the products of the cement hydration process gradually bind the individual sand andgravel particles with other components of the concrete to form a solid mass.

ReactionCement chemist notation: C3S + H2O → CSH(gel) + CaOHStandard notation: Ca3SiO5 + H2O → (CaO)•(SiO2)•(H2O)(gel) + Ca(OH)2Balanced: 2Ca3SiO5 + 7H2O → 3(CaO)•2(SiO2)•4(H2O)(gel) + 3Ca(OH)2

AggregatesFine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel andcrushed stone are mainly used for this purpose. Recycled aggregates (from construction,demolition and excavation waste) are increasingly used as partial replacements of naturalaggregates, while a number of manufactured aggregates, including air-cooled blast furnace slagand bottom ash are also permitted.Decorative stones such as quartzite, small river stones or crushed glass are sometimes added tothe surface of concrete for a decorative ‘exposed aggregate’ finish, popular among landscapedesigners.

ReinforcementConcrete is strong in compression, as the aggregate efficiently carries the compression load.However, it is weak in tension as the cement holding the aggregate in place can crack, allowingthe structure to fail. Reinforced concrete solves these problems by adding either metal reinforcingbars, glass fibre or plastic fibre to carry tensile loads.

Chemical admixturesChemical admixtures are materials in the form of powder or fluids that are added to the concreteto give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixturedosages are less than 5% by mass of cement, added to the concrete at the time ofbatching/mixing.

Mineral admixtures and blended cementsThere are inorganic materials that also have pozzolanic or latent hydraulic properties. These veryfine-grained materials are added to the concrete mix to improve the properties of concrete(mineral admixtures) or as a replacement to Portland cement (blended cements).A by-product of coal fired electric generating plants, Fly ash is used to partially replace Portlandcement (up to 60% by mass). The properties of fly ash depend on the type of coal burnt. Ingeneral, silicious fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties.

Ground granulated blast furnace slag (GGBFS or GGBS), a by-product of steel production, isused to partially replace Portland cement (up to 80% by mass). It has latent hydraulic properties.

Silica fume is one of the by-products of the production of silicon and ferrosilicon alloys. Silicafume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surfaceto volume ratio and a much faster pozzolanic reaction. Silica fume is used to increase strength anddurability of concrete, but generally requires the use of superplasticisers for workability.

High Reactivity Metakaolin (HRM): Metakaolin produces concrete with strength and durabilitysimilar to concrete made with silica fume. While silica fume is usually dark grey or black in colour,high reactivity metakaolin is usually bright white, making it the preferred choice for architecturalconcrete where appearance is important.

Q: What is the moisture content of concrete?A: The moisture content of concrete is viewed from the context of total water content of the freshconcrete mixture and the available moisture content of the hardened concrete. The total watercontent of a fresh concrete mixture is a function of the total cementitious materials and watercement ratio (w/cm). Typical fresh concrete mixtures vary in cementitious material content in a

range of 279 kg/m3 to 415 kg/m3 (470 lb/yd3 to 700 lb/yd3). Water cement ratios typically varyfrom 0.4 to 0.55. To estimate the available moisture content of hardened concrete one must beginwith the total water content of the fresh mixture and define the service condition of the hardenedconcrete with regard to relative humidity (%). In addition, the water that is chemically bound withthe cement in the hydration process must be accounted for. The water bound with the cement is inthe range of 0.22 to 0.24 of the cement content.

As an example, the moisture content of a concrete mixture with 334 kg/m3 (564 lb/yd3) ofcement and a w/c of 0.45 and in a service environment with a 50% relative humidity could beestimated as follows:

Total water content:334kg cement/m3 times 0.45 w/c ~ 150kg water/m3(564lb cement/yd3 times 0.45 w/c ~ 254lb water/yd3)Chemically bound water at 0.24 w/c:334kg cement/m3 times 0.24 w/c ~ 80kg water/m3(564lb cement/yd3 times 0.24 ~ 135lb water/yd3)Moisture content:150kg water/m3 – 80kg water/m3times .50 relative humidity ~ 35kg water/m3(254lb water/yd3 – 135lb water/yd3 times .50 relative humidity ~ 60lb water/yd3)In reality, the relative humidity of the concrete will only reach 50% at the near surface of theconcrete and the moisture gradient with depth will increase toward 100% relative humidity; hence,this method of estimation would typically overstate the quantity of moisture available to leave theconcrete due to the initial mixing of water.This is only an estimate of the moisture available to leave the concrete, but it may help in gaininga perspective to the limited amount of water that the concrete can contribute when considering thedrying time of hardened concrete.

Relative Humidity Profile

Q: When was concrete first made?A: 500BC

Q: What is the purpose of cement in concrete?A: It acts as a primary binder that joins the aggregate into a solid mass.

Q: Why does concrete harden?A: The chemical process called cement hydration produces crystals that interlock and bindtogether.

Q: How strong can concrete or cement be (in pounds per square inch (psi))?A: 50,000

Q: How long can concrete last (in years)?A: 50,000

Q: What are Type I/II or Type II/V cements?A: Type I/II and Type II/V cements simply means that the cement complies with the requirementsof ASTM C 150, Standard Specification for Portland Cement. It is quite common to find cementsthat comply with multiple cement designations such as Type I/II and Type II/V.

Q: How is white cement different and why is it used in decorative concrete?A: There are only slight chemical and physical differences between grey Portland cement andwhite Portland cement. This is due to raw material differences and sometimes, though not always,slight differences in manufacturing. White cement has small amounts of the oxides (particularlyiron and manganese) that impart the greyish colour normally associated with Portland cement.

Q. What are the decorative finishes that can be applied to concrete surfaces?A: Adding pigment before or after the concrete is placed and using white cement rather thanconventional grey cement, using chemical stains or exposing colourful aggregates at the surfacemay add colour to concrete. Textured finishes can vary from a smooth polish to the roughness of

gravel.Geometric patterns can be scored, stamped, rolled, or inlaid into the concrete to resemble stone,brick or tile paving. Other interesting patterns are obtained by using divider strips (commonlyredwood) to form panels of various sizes and shapes rectangular, square, circular or diamond.Special techniques are available to make concrete slip-resistant and sparkling.

Q: What are the different forms of sulphate in Portland cement and how can we analysecement for SO3?A: Sulphates in Portland cement can be broadly categorised as:1. Added sulphates – gypsum, hemihydrates, anhydrite, several synthetic forms of sulphates(typically by-products like flue gas desulphurisation materials). Clinker sulphates include arcanite,aphthitalite, calcium langbeinite and thenardite. Although normally reported as SO3 (% by mass)for consistency, sulphur can be found in any combination of forms. Elemental sulphur is almostnever found in Portland cement, except in trace amounts.Added sulphates are blended with clinker during the final grinding of the cement, in amountsneeded to control early setting properties as well as shrinkage and strength development. Theamount needed varies depending on the chemistry and fineness of the cement, but is typically onthe order of 5% by mass. The most common form of sulphate added to Portland cement isgypsum, some of which is intentionally dehydrated by the heat of grinding to form hemihydrates,which are more soluble and therefore available to control early hydration reactions.Clinker sulphates form naturally during clinker production. These sulphates tend to volatilise at thetemperatures of cement kilns (up to about 1450ºC) and condense on the outer surface of clinkernodules as alkali sulphates, during the last stage of clinker production (rapid cooling). Again, theamount depends on the chemistry of the raw materials and kiln operating conditions, making thecement somewhat unique. These alkali sulphates also are soluble enough to help control earlyhydration reactions. Some clinker sulphate is also incorporated into other cement phases.

Since cement is unique, chemical analyses are the best method of determining the SO3 content ofcements. Typically the total SO3 content is measured (or elemental S measured and converted toSO3) through methods in ASTM C 114 (or AASHTO T 105). XRF analysis is probably the mostcommon technique.

Q: What is air-entrained concrete?A: Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pocketsrelieve the internal pressure on the concrete by providing tiny chambers for water to expand intowhen it freezes. Air-entrained concrete is produced through the use of air-entraining Portlandcement, or by the introduction of air-entraining agents, under careful engineering supervision. Theamount of entrained air is usually between 4% and 7% of the volume of the concrete, but may bevaried as required by special conditions.

Q: What are recommended mix proportions for good concrete?A: Good concrete can be obtained by using a wide variety of mix proportions if proper mix designprocedures are used. The general custom is the rule of 6’s:A minimum cement content of 6 bags per cubic yard of concreteA maximum water content of 6 gallons per bag of cementA curing period (keeping concrete moist) a minimum of 6 daysAn air content of 6% (if concrete will be subject to freezing and thawing)

Q: Will concrete harden under water?A: Portland cement is a hydraulic cement, which means that it sets and hardens due to a chemicalreaction with water. Consequently, it will harden under water.

Q: What does 28 -day strength mean?A: Concrete hardens and gains strength as it hydrates. The hydration process continues over along period of time; beginning rapidly and progressively slowing down. To measure the ultimatestrength of concrete would require a wait of several years. This would be impractical, so a timeperiod of 28 days was selected, by specification writing authorities, as the age that all concreteshould be tested. At this age, a substantial percentage of the hydration has taken place.

Q: What is 3,000 pound concrete?A: Concrete that is strong enough to carry a compressive stress of 3,000psi (20.7MPa) at 28 daysis 3,000 pound concrete. Concrete may be specified at other strengths as well. Conventional

concrete has strengths of 7,000psi or less; concrete with strengths between 7,000 and 14,500psiis considered high-strength concrete.

Q: How do you control the strength of concrete?A: The easiest way to add strength is to add cement. The factor that most predominantlyinfluences concrete strength is the water to cement ratio in the cement paste that binds theaggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Everydesirable physical property will be adversely affected by adding more water.

Q: What is alkali-silica reactivity (ASR)?A: Alkali-silica reactivity is an expansive reaction between reactive forms of silica in aggregatesand potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans,admixtures and mixing water. External sources of alkali from soil, deicers and industrial processescan also contribute to ASR. The reaction forms an alkali-silica gel that swells as it draws waterfrom the surrounding cement paste, thereby inducing pressure, expansion and cracking of theaggregate and surrounding paste. This often results in map-pattern cracks, sometimes referred toas alligator pattern cracking. ASR can be avoided through

Proper aggregate selection Use of blended cements Use of proper pozzolanic materials Contaminant-free mixing water

Q. What are Supplementary Cementations Materials (SCM)?A: Supplementary Cementations Materials (SCM) like silica fumes, meta-kaolin, fly ash, slag arethe substances which improve the properties of concrete and enhance its durability, by reducingpore size in concrete through better particle distribution and through increased packing density ofthe concrete.

Q: Are there different types of Portland cement?A: Though all Portland cement is basically the same, eight types of cement are manufactured tomeet different physical and chemical requirements for specific applications:Type I is a general purpose Portland cement suitable for most uses.Type II is used for structures in water or soil containing moderate amounts of sulphate, or whenheat build-up is a concern.Type III cement provides high strength at an early state, usually in a week or less.Type IV moderates heat generated by hydration that is used for massive concrete structures suchas dams.Type V cement resists chemical attacks by soil and water high in sulphates.Types IA, IIA and IIIA are cements used to make air-entrained concrete. They have the sameproperties as types I, II and III, except that they have small quantities of air-entrained materialscombined with them.White Portland cement is made from raw materials containing little or no iron or manganese.

Q. Is there any shelf life of cement?A: Cement is a hygroscopic material, meaning that in presence of moisture it undergoes chemicalreaction termed as hydration. Therefore cement remains in good condition as long as it does notcome in contact with moisture. If cement is more than three months old then it should be testedfor its strength before being employed.

Q. How fineness of cement affects strength gain?A: Finer cement particles imply more particles in unit weight. This enhances the reaction rate,which in turn will result in faster gain of strength at earlier stages.

Q: Why do concrete surfaces flake and spall?A: Concrete surfaces can flake or spall for one or more of the following reasons:In areas subjected to freezing and thawing, the concrete should be air-entrained to resist flakingand scaling of the surface. If air-entrained concrete is not used, there will be subsequent damageto the surface.The water/cement ratio should be as low as possible to improve durability of the surface. Toomuch water in the mix will produce a weaker, less durable concrete, in turn leading to early flaking

and spalling of the surface.The finishing operations should not begin until the water sheen on the surface is gone and excessbleed water on the surface has had a chance to evaporate. If this excess water is worked into theconcrete because the finishing operations are begun too soon, the concrete on the surface willhave too high a water content and will be weaker and less durable

Q: How do you remove stains from concrete?A: Stains can be removed from concrete with dry or mechanical methods, or by wet methodsusing chemicals or water.Common dry methods include sandblasting, flame cleaning, shotblasting, grinding, scabbing,planning and scouring. Steel-wire brushes should be used with care because they can leave metalparticles on the surface that later rust and stain the concrete.Wet methods involve the application of water or specific chemicals according to the nature of thestain. The chemical treatment either dissolves the staining substance so it can be blotted up fromthe surface of the concrete or bleaches the staining substance so it will not show.To remove bloodstains, for example, wet the stains with water and cover them with a layer ofsodium peroxide powder. Let stand for a few minutes, rinse with water and scrub vigorously.Follow with an application of a 5% solution of vinegar to neutralise any remaining sodiumperoxide.

Q: What is Self-Consolidating concrete (SCC)?A: SCC is a high-performance concrete that can flow easily into tight and constricted spaceswithout segregating and without requiring vibration. The key to creating SCC, also referred to asself-compacting, self-levelling, or self-placing concrete, is a mixture that is fluid, but also stable toprevent segregation.To achieve the desired flowability a new generation of superplasticisers based on polycarboxylateethers works best. Developed in the 1990s, they produce better water reduction and slower slumploss than traditional superplasticisers. The required level of fluidity is greatly influenced by theparticular application under consideration. Obviously the most congested structural membersdemand the highest fluidity. However, element shape, desired surface finish, and travel distancecan also determine the required fluidity.Generally, the higher the required flowability of the SCC mix, the higher the amount of finematerial needed to produce a stable mixture. However, in some cases, a viscosity-modifyingadmixture (VMA) can be used instead of, or in combination with, an increased fine content tostabilize the concrete mixture.

Q: The size of concrete cube is 150mm x 150mm x 150mm as per Indian Standards.Why?A: Because the shape effect is the least for the 15cm cube and we get a fairly accurate idea of thestrength of the concrete as such.

Q: How do you protect a concrete surface from aggressive materials like acids?A: Many materials have no effect on concrete. However, there are some aggressive materials,such as most acids, that can have a deteriorating effect on concrete. The first line of defenceagainst chemical attack is to use quality concrete with maximum chemical resistance, followed bythe application of protective treatments to keep corrosive substances from contacting theconcrete. Principles and practices that improve the chemical resistance of concrete include using alow water-cement ratio, selecting a suitable cement type (such as sulphate-resistant cement toprevent sulphate attacks), using suitable aggregates, water- and air-entrainment. A large numberof chemical formulations are available as sealers and coatings to protect concrete from a variety ofenvironments; detailed recommendations should be requested from manufacturers, formulators ormaterial suppliers.

Q: Why does concrete crack?A;Concrete, by nature, shrinks as it hardens. When concrete is placed on supporting soil or aroundsteel reinforcement, the concrete mass is prevented from shrinking. This restraint creates internalforces exceeding the strength of concrete; cracks form to relieve these forces.

Q: Does the presence of cracks indicate a structural problem?A: In most instances, the answer is no. Very narrow ‘hairline’ cracks are aesthetic in nature and donot indicate any structural problem. Cracks that have movement, i.e. where one side of the crackmoves relative to the opposite side, should be investigated by a professional engineer.

Q: Why does concrete harden?A: Concrete solidifies and hardens after mixing with water and placement due to a chemicalprocess known as hydration. The water reacts with the cement, which bonds the other componentstogether, eventually creating a stone-like material.

Q: What is concrete used for?A: Concrete is used to make pavements, pipe, architectural structures, foundations,motorways/roads, bridges/overpasses, parking structures, brick/block walls and footings for gates,fences and poles.Concrete is used more than any other man-made material in the world. As of 2006, about 7.5km3of concrete is made each year—more than 1m3 for every person on earth.

Q: What are the more popular types of concrete in use?A: Reinforced concrete and prestressed concrete are the most widely used modern kinds offunctional concrete extensions.

Q: What evidence is there for the long life of concrete?A: The widespread use of concrete in many Roman structures has ensured that many of themhave survived. The Baths of Caracalla is just one example of the longevity of concrete, whichallowed the Romans to build this and similar structures across their Empire. Many Romanaqueducts and Roman bridges have masonry cladding to a concrete core, a technique they used instructures such as the Pantheon, the dome of which is concrete.

Q: Who discovered concrete?A: The Romans used concrete in their structures but the secret had been lost for 13 centuries until1756, when the British engineer John Smeaton pioneered the use of hydraulic lime in concrete,using pebbles and powdered brick as aggregate. Portland cement was first used in concrete in theearly 1840s. This version of history has been challenged however, as the Canal du Midi wasconstructed using concrete in 1670.

Q: What is the role of water in concrete mix?A: Combining water with cementitious material forms a cement paste by the process of hydration.The cement paste glues the aggregate together, fills voids within it and allows it to flow moreeasily.Less water in the cement paste will yield a stronger, more durable concrete; more water will givean easier-flowing concrete with a higher slump.Impure water used to make concrete can cause problems when setting or in causing prematurefailure of the structure.Hydration involves many different reactions, often occurring at the same time. As the reactionsproceed, the products of the cement hydration process gradually bind the individual sand andgravel particles with other components of the concrete, to form a solid mass.

Q: How do aggregates affect the strength of concrete?A: Concrete has a high compressive strength, as the aggregate efficiently carries the compressionload. However, it is weak in tension as the cement holding the aggregate in place can crack,allowing the structure to fail. Reinforced concrete solves these problems by adding either metalreinforcing bars, steel fibres, glass fibre or plastic fibre to carry tensile loads.

Q. What are the reasons for slow or fast setting of concrete or mortar?A: the rate of setting normally depends on the nature of the cement. It could also be due toextraneous factors not related to the cement. Ambient conditions also play an important role. Inhot weather, concrete sets faster, whereas in cold weather, setting is delayed. Some salts,chemicals, clay etc., if inadvertently mixed with the sand, aggregate and water could accelerate ordelay the setting of concrete.

Q: What do grade numbers indicate?A: The grade number indicates the minimum compressive strength of cement sand mortar inN/mm2 at 28 days.

Q. What is slag?A: Slag is a non-metallic product, essentially consisting glass containing silicates, alumino-silicates

of lime and other bases, and is obtained as a by-product in the manufacture of pig iron in blast orelectric furnaces. Granulated slag is used in the manufacture of Portland Slag Cement (PSC).

Q. How is PSC made?A: PSC is made by intergrading clinker, granulated blast furnace slag and gypsum or by blendingground slag with Portland cement.

Q. Where can PSC be used?A: Slag cement can be used for all plain and reinforced concrete constructions and massconcreting structures such as dams, reservoirs, swimming pools, river embankments, bridge piersetc. It is used with advantage where low heat of hydration and resistance to alkali-silica reactionsare desired; for structures in aggressive environments where chemical and mildly acidic waters areencountered (where the use of OPC is not recommended) and for marine constructions, dykes,wharves, etc where sulphuric water is encountered. In short, PSC can be used wherever OPC isused.