Concrete Exam 2
Explain which ASTM type cement you would use for: (a) Cold-weather construction (b) Construction of a dam (c) Making reinforced concrete sewer pipes
(a) type III cement would be useful for cold-weather due to additional heat of hydration. • (b) Type II or Type IV would be useful for low heat of hydration to avoid thermal shrinkage issues. • (c) Type V sulfate resistant cement would be useful for sulfate resistance in sewers (if it were still widely available). Blended Type IS cement could serve as an effective replacement.
What is a truly elastic material? Is concrete truly elastic? If not, why? Describe the various stages of microcracking when a concrete specimen is loaded to failure.
. A truly elastic material is one that deforms in a readily predictable way. It will always act the same way until failure. It has a constant stiffness. Concrete is not a truly elastic material because it does not act the same way each time and it does not have the same stiffness between mixtures. The four steps of cracking until failure are one microcrack exist in the ITZ and up to 30% of failure stress does not propagate any. Step two between 30 and 50% of failure stress the micro cracks begin to grow in the ITZ, and the strain/stress curve increases and deviate from a straight line. The third step between 50 and 75% the ITZ starts to become unstable the curve bends greatly to the horizontal and cracks begin in the matrix as well meeting up with the ITZ cracks. At 75% there is spontaneous crack growth under sustained loads. Above 75% the material starts failing rapidly until complete failure
Explain the mechanism by which mineral admixtures can improve the pumpability and finishability of concrete mixtures. In the amounts normally used, some mineral admixtures are waterreducing whereas others are not. Discuss the subject with the help of examples.
. Finely divided, glassy particles in mineral admixtures improve workability by reducing the size and volume of voids. Also, the mineral admixtures will reduce the tendency of mixtures to bleed or segregate, which will improve pumpability and workability. Some mineral admixtures, such as silica fume, have a very high surface area which will increase the water requirements
In their composition and mechanism of action, how do the superplasticizers differ from the normal water-reducing admixtures? The addition of 1 to 2 percent of a normal water-reducing agent to a concrete mixture may cause segregation and severe retardation. These effects do not take place in the super plasticized concrete. Explain why.
. Long-chain high molecular weight anionic surfactants. When absorbed on cement particles, the surfactant imparts a strong negative charge which lowers the surface tension of the surrounding water considerably and enhances the fluidity of the system. The colloidal size of the long-chain particles obstructs bleeding and prevent segregation.
Approximately, what is the combined percentage of calcium silicates in portland cement? What are the typical amounts of C3Aand C4AF in ordinary (ASTM Type I) portland cement?
55% C3S and 20% C2S. 8% C3A and 8% C4AF.
The aggregate in concrete is looked down upon as an "inert filler." Explain why this viewpoint is erroneous.
Aggregate can influence strength, dimensional stability, and durability of concrete, in addition to the cost and workability. In addition to composing 60 to 80 percent of the concrete volume.
What are the essential differences in composition and mode of action between the surfactants used for air entrainment and those used for water reduction?
Air entraining surfactants are composed of nonpolar hydrocarbon chains with an anionic polar group. The polar groups are oriented toward the water phase lowering the surface tension and promoting the formation of air bubbles. Also, the directive forces present on the surface of the cement interact with the nonpolar groups making the cement surface hydrophobic and allowing air to displace the water and remain attached to the soli particles as bubbles. Water reducing surfactants - the anionic polar group is joined to a hydrocarbon chain that is polar or hydrophilic, this chain is adsorbed alongside the cement particle and lowers the surface tension of water making the cement particle hydrophilic. A layer of water dipoles surrounds the hydrophilic cement particles preventing flocculation and creating a well-dispersed system.
What do you understand by the following terms: alite, belite, periclase, langbeinite, plaster of paris, tobermorite gel?
Alite - Tricalcium silicate (C3S) of triclinic, monoclinic, and trigonal structure of high reactivity. • Belite - irregular, but the interstitial holes are much smaller, and this makes belite far less reactive than alite. • Periclase - crystalline MgO. • Langbeinite - Depending on the amount of sulfate available, soluble double-sulfates of alkalies such as langbeinite (2CS⋅NS) and aphthitalite (3NS-ΚS) are known to be present in portland cement clinker. • Plaster of Paris - soluble form of Calcium sulfate • Tobermorite gel - naturally occurring mineral of seemingly similar structure to that of CSH gel.
What is the cause of the bulking phenomenon and what role does it play in concrete manufacturing practice?
An increase in the bulk volume which occurs in the damp condition, due to the surface tension keeping the aggregate particles apart.
In concrete technology, what distinction is made between the terms of specific gravity and bulk density? With the help of suitable sketches, explain the following terms and discuss their significance: absorption capacity, saturated-surface dry condition, damp condition.
Apparent specific gravity - the density of the material including the internal pores. • Bulk Density - the mass of the aggregate fragments that would fill a unit volume. The volume is occupied by both aggregates and voids. • Absorption capacity - the total amount of moisture required to bring an aggregate from the oven-dry to SSD condition. • Saturated Surface Dry - saturated, no water film on the surface. • Damp - saturated with free moisture on the surface.
Write a short report on the influence of the rock-forming process on characteristics of aggregates derived from the rock. Explain why:
Basalts, which are generally fine-grained or glassy are not alkali reactive. The glassy minerals present in basalt are basic and do not react well with alkaline pore solution. • Limestone tends to form flat aggregate particles. Limestone is stratified and will produce flat or elongated fragments. • Pumice is useful to produce lightweight aggregate. Pumice is a volcanic glassy mineral that is cellular and lightweight.
Assuming the chemical composition of the calcium silicate hydrate formed on hydration of C3S or C2S corresponds to C3S2H3, make calculations to show the proportion of calcium hydroxide in the final products and the amount of water needed for full hydration.
Based on the stoichiometric equations given, 61% of C3S and 82% of C2S would react to form C2S2H3 and the remainder CH. • 2C3S + 6H → C3S2H3 + 3CH • 2C2S + 4H → C3S2H3 + CH • 55% C3S, 20% C2S, 8% C3A, 8% C4AF • 100 units of cement will contain 55u C3S and 20u C2S (205u of C and 75u of S) each unit of S will form 0.5u of C3S2H3. o The hydration products will contain 37.5u of C2S2H3 and 92.5u of CH o This will require 205u of H o The actual required mass or volume would be a function of molecular weight
What do you understand by the terms basic creep, specific creep, drying creep, and creep coefficient?
Basic creep is the application of constant stress on a concrete specimen under conditions of 100% relative humidity which leads to an increase of strain over time. Specific creep is the creep strain per unit of applied stress. Drying creep is the additional creep occurring when the specimen under load is drying. Creep coefficient is the ratio of creep strain to elastic strain.
what are the typical ranges of drying shrinkage strain and creep strain in concrete: what is their significance? How are the two phenomena like each other?
Both drying shrinkage strain and creep strain are on the order of magnitude of 400 to 1000 times 10 to the negative 6. Both drying shrinkage and creep originate from the same source which is the hydrated cement paste. The strain/time curves are very similar, and both are affected by the same factors. The strain they produce is large and must be considered for the effect it will cause.
Why is C3S more reactive, and gC2S nonreactive with water at normal temperatures? MgO and CaO have similar crystal structures, but their reactivities are very different from each other. Explain why.
C3S - The coordination of oxygen ions around the calcium is irregular so that the oxygen ions are concentrated on one side of each of the calcium ion. This arrangement leaves large structural holes, which account for the high lattice energy and reactivity. • gC2S - has a regularly coordinated structure thus rendering this compound nonreactive. • MgO vs CaO - Both MgO and CaO form cubic structures, with each magnesium or calcium ion surrounded by six oxygens in a regular octahedron. The size of the Mg2+ ion is such that, in the MgO structure, the oxygen ions are in close contact and the Mg2+ ions are well packed in the interstices. However, in the case of the CaO structure, due to the much larger size of the Ca2+ ion, the oxygen ions are forced apart so that the Ca2+ ions are not well packed. Consequently, the crystalline MgO formed from a high-temperature (>1400°C) melt in a portland cement kiln is much less reactive with water than the crystalline CaO formed under the same temperature conditions.
What maximum strength levels have been attained in recently developed high strength concrete mixtures? Explain the role played by admixtures in the development of these concretes
Concrete has been developed with compressive strengths of over 200 MPa! Without the use of silica fume and superplasticizers, strength and workability would not be possible
How does the moisture state of a concrete test specimen affect the elastic modulus and strength values? Explain why both properties are not affected in the same manner.
Concrete specimens that are tested in a wet condition show a 15% higher elastic modulus than corresponding dry specimens. On the other hand, dry specimens test 15% higher than wet specimens in compressive strength. By drying the concrete, the cement matrix is helped because the Van der Waals force of attraction can move water toward anhydrous cement to whereas in a saturated condition those forces are negated. On the other hand, the microcracking around the ITZ increases as the specimen dries out which lowers the elastic modulus of the concrete in a dry condition.
Which factors affect creep only, and why?
Curing history, the temperature of exposure and magnitude of applied stresses are known to affect creep more than drying shrinkage. It is thought that the reason creep is affected more than drying shrinkage is since the properties affect the ITZ and its characteristics.
What do you know about the following rocks and minerals: dolomite, greywacke, flint, opal, plagioclase, smectites, and marcasite?
Dolomite - Usually, they contain both the calcium and magnesium carbonate minerals (CaCO3⋅MgCO3) in various proportions, and significant amounts of noncarbonate impurities, such as clay and sand. • Greywacke - gray sandstones containing angular and sand-sized rock fragments in an abundant matrix of clay, shale, or slate. • Flint - poorly crystalline quartz, chalcedony, or opal. Denser than Chert. Good aggregates, reactive with alkalis. • Opal - hydrous silica mineral that is non-crystalline. • Plagioclase - also known as soda-lime feldspars includes sodium aluminum silicates, calcium aluminum silicates or both. • Smectite - Montmorillonites clays that undergo large expansions on wetting. • Marcasite - sulfide of iron, found in sedimentary rocks, oxides to form sulfuric acid and iron hydroxides. May lead to the corrosion of reinforcing steel
You are the civil engineer in charge of rehabilitating some old concrete pavements in your area. In a brief note to your superiors, discuss the equipment needed, deleterious constituents to be avoided, and the cost economy of using the crushed concrete from old pavements as a source of aggregate for the construction of new pavements.
Equipment - crushing, grading, dust control. • Deleterious constituents - separation of undesirable constituents. • Cost economy - becomes economic when good quality natural aggregates are scarce.
When producing a certain type of portland cement it is important that the oxide composition remains uniform. Why?
Even small differences in the oxide analyses of two cements can result in large differences in their compound composition.
How are aggregates made from expandable clays, fly ash, and blast-furnace slag? What are some of the interesting characteristics of the products?
Expanded Clays - thermal treatment of clays to produce an expanded structure that is used as a lightweight aggregate. • Fly ash - produced in the combustion of coal, spherical particles of aluminosilicate glass, fly ash is pelletized sintered in a kiln. • Blast-furnace slag - basic slags tend to produce a vesicular structure with a lower apparent specific gravity, density between the normal and lightweight aggregate. Acidic slags produce denser aggregates.
List the most important factors that affect drying shrinkage and creep and discuss when the effects are similar or opposite
Factors that affect drying shrinkage and creep are materials and mix proportions, time and humidity, the geometry of the concrete element, and curing history. The aggregate can affect the shrinkage and creep of concrete because the aggregate is directly related to the elastic modulus. The shrinkage will be similar for set w/c values with equal volumes of coarse aggregate, but the creep may be different as different aggregates have different ITZ's which affect their creep. In general, the influence of cement content and w/c ratio of concrete on the drying shrinkage and creep is not direct, because an increase in the cement paste volume means a decrease in the aggregate fraction and an increase in the moisture dependent deformations in the concrete. Time tends to effect creep and shrinkage similarly. Humidity effects both in a similar manner. Temperature can have different effects on creep and shrinkage. If temps are higher than normal during curing before loading creep will decrease but if the temp is elevated and load added during curing creep will increase.
Compare and contrast industrial fly ashes and ground iron blast-furnace slag concerning mineralogical composition and particle characteristics.
Fly ash is comprised of spherical particles of glass. There are two classes of fly ash, Class C ash has a higher CaO content and is slightly pozzolanic, while class f ash has less CaO and is not pozzolanic. Quickly cooled, or quenched, slag produces a glassy non-crystalline structure which is cementitious. Both fly ash and blast furnace slag are non-crystalline and produce similar reactions in the presence of calcium. Class F, low calcium, fly ash will take two weeks or more to produce any significantly improved strength in concrete. Class C fly ash and blast furnace slag will produce improved strength in as few as 7 days.
Name some of the commonly available industrial by-products that show pozzolanic or cementitious properties when used in combination with Portland cement.
Fly ash, Blast-furnace slag, silica fume, rice husk ash.
Formic acid is an accelerator, while gluconic acid is a retarder. Explain why
Formic acid accelerates hydration through the increased solubility of calcium, however, gluconic acid promotes the formation of insoluble calcium salts which prevent cement hydration
Define the terms grading and maximum aggregate size, as used in concrete technology. What considerations control the choice of the maximum aggregate size of aggregate in concrete? Discuss the reason why grading limits are specified
Grading - the distribution of particles of a granular material among various size ranges, expressed in terms of cumulative percentage larger or smaller than each of a series of sizes. • Maximum aggregate size - designated by the sieve size on which 15 percent or more particles are retained. • Aggregate gradation has considerable effects on the cement paste requirements of a concrete mixture.
What is high-performance concrete?
High-performance concrete provides the following characteristics: constructability, productivity and durability, ultimate and early strength, environmental consideration, and economic effectiveness.
State several important reasons why it is desirable to use pozzolanic admixtures in concrete.
Improved resistance to thermal cracking due to low heat of hydration, enhancement of ultimate strength and impermeability due to pore refinement, strong ITZ and high durability to sulfate attack and AAR.
When added to Portland cement paste in very small amounts, calcium chloride acts as a retarder, but in large amounts, it behaves as an accelerator. Can you explain the phenomenon?
In small concentrations, the salts of weak bases and strong acids or strong bases and weak acids retard the set of concrete, while in high concentrations the solubility of the silicate and aluminate ions of cement becomes dominant and the set is accelerated.
Define the terms initial set and final set. For a normal portland cement draw a typical heat evolution curve for the setting and early hardening period, label the ascending and descending portions of the curve with the underlying chemical processes at work, and show the points where the initial set and final set are likely to take place.
Initial Set - beginning of solidification. the point in time when the paste has become unworkable • Final Set - complete solidification and beginning of hardening. portland cement paste has little or no strength because it represents only the beginning of the hydration of C3S, the principal compound present • A - represents the heat of solution of aluminates and sulfates. This initial heat evolution ceases quickly (descending portion of peak A) when the solubility of aluminates is depressed in the presence of sulfate in the solution. • B - represents the heat of formation of ettringite (ascending portion of peak B). Researchers believe that the heat evolution period includes some heat of solution due to C3S and heat of formation of C-S-H. The paste of a properly retarded cement will retain much of its plasticity before the commencement of this heat cycle and will stiffen and show the initial set (beginning of solidification) before reaching the apex at B, which corresponds to the final set (complete solidification and beginning of hardening).
Ideally, from the standpoint of crack resistance, some concrete should have low shrinkage and high extensibility. Give examples to show why this may not be possible to achieve in practice.
It may not be possible because if the aggregate is changed to reduce shrinkage then it may cause restraint internally which leads to more cracking to relieve the stress. In the same sense, the shrinkage and stress relaxation may not be the same as in the given graphs from the design code. There is a bit of interpretation needed for concrete placement.
Give typical ranges of aggregate unit weight for making structural lightweight, normal-weight, or heavyweight concretes. What types of natural and synthetic aggregates are used for making lightweight masonry blocks and insulating concrete; what types of natural minerals are useful for producing heavyweight aggregates?
Lightweight - < 120 lb./ft3 • Normal weight - 150 lb./ft3 • Heavyweight - 180 to 380 lb./ft3 • Natural - Pumice, scoria, or tuff. • Synthetic - clays, shale, slate, diatomite, pearlite, vermiculite, blast-furnace slag, and fly ash. Heavyweight - Witherite, Barite, Magnetite, Hematite, and Ilmenite
Commercial lignin-based admixtures, when used as water-reducing agents, may exhibit certain side effects. Discuss the possible side effects and explain how they are corrected
Lignin based water reducers entrain considerable air and usually contain air detraining agents.
Discuss the physical-chemical factors involved in explaining the development of strength in products containing the following cementitious materials, and explain why portland cement has come to stay as the most used cements for structural purposes:
Lime - Hydrated lime, Ca(OH)2 is also not stable in water. However, it can slowly carbonate in air to form a stable product (CaCO3). When a pozzolan (reactive silica) is present in the system, the calcium silicate hydrates formed as a result of the reaction between lime and pozzolan are stable in water. • Plaster of Paris - Setting times as low as 10 minutes can be achieved by using mixtures of either portland cement and plaster of paris (CaSO4 ⋅ 1/2H2O) or portland cement and calcium aluminate cement. The durability and ultimate strength of the hardened product are generally low. Calcium aluminate cement - However, several structural failures due to gradual strength loss with concrete containing CAC have been instrumental in limiting the use of this cement for structural applications. In most countries, now CAC is used mainly for making castable refractory lining for high-temperature furnaces.
When used as an accelerator, what effect would calcium chloride have on the mechanical properties, dimensional stability, and durability of concrete?
Mechanical - Short term compressive strength is improved; however long term compressive and tensile strength will be reduced. • Dimensional Stability - increased shrinkage and creep, in addition to increased volume change Durability - reduced sulfate resistance, in addition to increased risk of ASR. Improved early age resistance to freeze-thaw and permeability related problems
Discuss the mechanisms by which mineral admixtures improve the durability of concrete to acidic waters. Why is that all fly ash-portland cement or slag-portland cement combinations may not turn out to be sulfate-resisting?
Mineral admixtures will reduce the porosity and therefore permeability of concrete which will improve the resistance to intrusion of acidic water. At low replacement rates, high calcium fly ash or blast furnace slag may increase the probability of internal sulfate attack through an increased concentration of aluminate. High calcium fly ash and high alumina slag both have a higher concentration of glassy or non-crystalline alumina which adversely effects the sulfate resistance of portland cement.
Besides the magnitude of shrinkage strain, which other factors determine the risk of cracking in a concrete element?
Other factors affecting cracking are modulus of elasticity, creep, and tensile strength.
Why are plasticizing admixtures called water-reducing? What is the distinction between normal and high-range water-reducing admixtures according to the ASTM Standard Specification?
Plasticizing admixtures increase the consistency without increasing water content, therefore they are referred to as water reducers. High range water reducers reduce three to four times the mixing water of a normal range water reducer. Normal range water reducers must reduce at least 5 percent of mixing water while the high range must reduce 12 percent.
List any three characteristics of concrete aggregates and discuss their influence on both the properties of fresh concrete and hardened concrete.
Porosity - density, moisture absorption, strength, hardness, elastic modulus, and soundness. • Exposure and processing factors - particle size, shape, and texture. • Chemical and mineralogical composition - strength, hardness, elastic modulus, and deleterious substances.
What do you know about the origin and characteristics of the following mineral admixtures: pumice, zeolitic tuff, rice husk ash, and silica fume?
Pumice - Natural volcanic material that requires treatment before reacting with lime to form CSH (cementitious). • Zeolitic tuff - the product of hydrothermal alteration of volcanic glass, minerals include phillipsite and herschelite which show considerable reactivity with lime. • Rice husk ash -produced through controlled combustion of rice hulls, which produces pozzolanic non-crystalline silica. • Silica fume - a byproduct of induction arc furnaces in the production of silicon metals. SiO vapors oxidize and condense to form tiny spherical particles (0.1 um) consisting of non-crystalline silica. Highly pozzolanic, although hard to handle and unlike fly ash, it increases the required water content of the concrete mixture.
The aluminate-sulfate balance in solution is at the heart of several abnormal setting problems in concrete technology. Justify this statement by discussing how the phenomena of quick-set, flash set, and false set occur in freshly hydrated portland cements.
Quick-set - When the amount of reactive C3Ais high but the soluble sulfate present is less than required for normal retardation, hexagonal-plate crystals of monosulfate and calcium aluminate hydrates form quickly and in large amounts causing the cement paste to set in less than 45 min after the addition of water. • Flash set - When little or no gypsum has been added to a ground portland cement clinker, the hydration of C3Ais rapid and the hexagonal-plate calcium aluminate hydrates start forming in large amounts soon after the addition of water, causing almost an instantaneous set. • False set - When the C3Ain cement is of low reactivity, as is the case in partially hydrated or carbonated cements which have been improperly stored, and at the same time a large amount of calcium sulfate hemihydrate is present in the cement, the solution phase will contain a low concentration of aluminate ions but will quickly become supersaturated with respect to calcium and sulfate ions. This situation will lead to the rapid formation of large crystals of gypsum with a corresponding loss of consistency.
Write short notes on the compositions and special characteristics of the following cements: regulated-set cement, very high early strength cement, API Class J cement, white cement, and calcium aluminate cement.
Regulated-set cement - Using a modified portland cement clinker containing mainly alite and a calcium fluoroaluminate (11CaO ⋅ 7Al2O3 ⋅ CaF2), a proportion of the clinker is blended with normal portland cement clinker and calcium sulfate so that the final cement contains 20 to 25 percent of the fluoroaluminate compound and about 10 to 15 percent calcium sulfate. The cement is generally very fast setting (2 to 5 min setting time). • Very high early strength cement - With the very high-early-strength (VHE) cement, C4A3S is the main source of aluminate for the ettringite formation • API Class J cement - Since organic retarders are unstable at high temperatures, API Class J cement represents a relatively recent development in the field of modified portland cements that can be used for case-cementing at temperatures above 150°C without the addition of a retarder. The cement, composed mainly of a bC2S clinker, is ground to about 200 m2/kg Blaine, with 40 mass percent silica flour. • White cement - pulverized a white portland-cement clinker. The gray color of ordinary portlandcement clinker is generally due to the presence of iron. By lowering the iron content of clinker, light-colored cements can be produced. Consequently, white cements are approximately three times as expensive as conventional portland cement. • Calcium aluminate cement - pulverized calcium aluminate cement clinker; the clinker is a partially fused or a completely fused product consisting of hydraulic calcium aluminates. Thus, unlike portland and modified portland cements, in which C3S and C2S are the principal cementing compounds, CAC contains monocalcium aluminate (CA) as the principal cementing compound with C12A7, CA, C2AS, bC2S, and Fss as minor compounds. Typically, the chemical analysis of ordinary CAC corresponds to approximately 40 percent Al2O3 and some cements contain even higher alumina content (50 to 80 percent); therefore, this cement is also called high-alumina cement (HAC).
What type of admixtures would you recommend for concreting in (i) hot weather, (ii) cold weather?
Retarding admixtures overcome the accelerating effects of high temperatures. Accelerating admixtures allow placement of concrete in cold weather and decrease the time before insulation can be applied
What is the distinction between the terms rock and minerals?
Rocks are composed of minerals. The composition, grain size, and crystal structure are determined by the process by which the rock is formed.
As an accelerator why isn't sodium chloride as effective as calcium chloride?
Sodium reduces the solubility of calcium and improves the solubility of silicate and aluminate ions. As the concentration increases, the solubility of silicate and aluminate ions outweighs the reduced solubility of calcium. Sodium has a single charge while calcium has a double charge
With the help of appropriate sketches, explain the terms sphericity, flat particles, and angular particles. Explain how the surface texture of the fine aggregate may influence the properties of concrete.
Sphericity -the ratio of surface area to volume as compared to a sphere • Flat particles - thickness is small relative to two other dimensions • Angular particles - well-defined edges and corners • The surface texture will increase the required cement paste to produce workable concrete as the surface area increases.
What is the significance of tensile strain capacity? How can you determine it?
Tensile strain capacity is defined as the failure strain under tension. For risk analysis against thermal cracking, it is suggested that the determination of the tensile strain capacity is a better criterion than the converting of the thermal strain to induced elastic stress. It can be determined by taking the ratio of the modulus of rupture to the elastic modulus in compression.
After reviewing the ASTM C618 and C989 Standard Specifications and other published literature, write a critical note comparing the two standards
The ASTM C618 includes arbitrary requirements that have no proven relationship with the performance of the mineral admixture in concrete, the ASTM C989 simply requires that a concrete mixture containing slag cement meets the strength requirements.
Which one of the four major compounds of portland cement contributes most to the strength development during the first few weeks of hydration? Which compound or compounds are responsible for rapid stiffening and early setting problems of the cement paste?
The C3S is primarily responsible for strength gain at early age. The formation of ettringite due to the hydration of C3A is responsible for stiffening and early setting problems of cement paste.
4.6 Can we control the CTE of concrete? If so, how?
The coefficient of thermal expansion is the change in unit length per degree of temperature change. The coefficient of thermal expansion can be changed by selecting an aggregate with a low coefficient of thermal expansion. Since the aggregate is the primary constituent of concrete, by changing the aggregate the coefficient of linear thermal expansion may be changed.
Why are clays and shales heat-treated to make them suitable for use as a pozzolan?
The crystal structure of the clay must be melted and rearranged by heat treatment to produce pozzolanic activity
What is the significance of fineness in cement? How is it determined? Can you give some idea of the fineness range in industrial portland cements?
The fineness of cement also affects its reactivity with water. Generally, the finer the cement, the more rapidly it will react. the fineness is easily determined as the residue on standard sieves such as No. 200 mesh (75 μm) and No. 325 mesh (45 μm). It is a common practice in the industry to obtain a relative measure of the particle size distribution from surface area analysis of the cement by the Blaine Air Permeability Method (ASTM C 204). Typical values range from 300 - 500 m2 /kg.
Mineral acids are accelerators for Portland cement, but organic acids do not show consistent behavior. Explain why.
The organic acids will lead to the development of highly insoluble calcium salts which form a layer around the cement particle and prevent hydration.
With the help of the "pozzolanic reaction," explain why under given conditions, compared to portland cement, portland pozzolan, and portland blast-furnace slag cements are likely to produce concrete with higher ultimate strengths and superior durability to sulfate attack.
The reaction between a pozzolan and calcium hydroxide is called the pozzolanic reaction. The technical advantage of using pozzolan cements and slag cements is derived mainly from three features of the pozzolanic reaction. (1) the reaction is slow; therefore, the rates of heat liberation and strength development will be accordingly slow. (2) the reaction is lime-consuming instead of lime producing, which has an important bearing on the durability of the hydrated paste in acidic environments. (3) pore size distribution studies of hydrated IP and IS cements have shown that the reaction products are very efficient in filling up capillary spaces, thus improving the strength and impermeability of the system
What is the significance of adiabatic temperature in concrete? How much adiabatic temperature rise can occur in a typical low-strength concrete containing ASTM Type II cement? How can this be reduced?
The significance of adiabatic temperature is that it affects the amount of thermal shrinkage in the concrete. The adiabatic temperature is a function of the amount, composition, and fineness of the cement and temperature during hydration. Type II cement will generally produce around 32 degrees C of adiabatic heat. This can be reduced by adding a pozzolan such as silica fume or slag. These materials take slightly longer to set up but do not produce as much heat as other cement types.
Why doesn't calcium sulfate behave like an accelerator for Portland cement as calcium chloride?
The solubility of gypsum is very low, and the sulfate ions will not accelerate hydration until they go into solution. When the sulfate is dissolved ettringite crystallization will occur
5 Some manufacturers claim that the application of water-reducing admixtures can lower cement content and increase the consistency and strength of a reference concrete mixture. Explain why all three benefits may not be available at the same time
The surfactants have a relatively short period of effectiveness and begin to breakdown as cement hydration products develop.
4.2 Draw a typical stress-strain curve for concrete. From this, how would you determine the dynamic modulus of elasticity and the different types of static moduli? Typically, what are their magnitudes for a medium-strength concrete?
The three types of static modulus of elasticity are tangent modulus, secant modulus, and chord modulus. The tangent modulus (18.8 GPa) can be determined by the slope of the line drawn tangent to the stress-strain curve at any point on the curve. The secant modulus (24.9 GPa) is given by the slope of a line drawn from the origin to a point on the curve corresponding to a 40% stress of the failure load. The chord modulus (24.0 GPa) is given by the slope of a line drawn between two points on the stress- strain curve. The dynamic modulus (34.9 GPa) is approximately given by the tangent modulus which is the tangent modulus for a line drawn at the origin. It is generally 20, 30, and 40% higher than high, medium, and low strength concrete.
What are the assumptions underlying the formulas used by the ACI Building Code and FIP Model Code for predicting the static elastic modulus of concrete? Can you point out any limitations of these formulas?
The underlying assumptions of these equations are that the elastic modulus is directly dependent on the strength and density of the concrete. In many instances, this is a safe assumption but there are instances where a lower density concrete could have good matrix bonding with a higher than the calculated elastic modulus. The formulas generally underestimate the elastic modulus of normal weight concrete which makes them conservative.
What is the usefulness of the extensibility concept? Why would high-strength concrete be more prone to cracking than low-strength concrete?
The usefulness of extensibility is in understanding what needs to be reduced to produce concrete that will not crack under the loading. It is more prone to cracking because of higher thermal shrinkage and lower stress relaxation.
What is the significance of the term theoretical thickness?
Theoretical thickness is equal to the area of the section divided by the semi perimeter in contact with the atmosphere. This helps to relate creep and drying shrinkage on graphs.
In regard to sulfate resistance and rate of strength development, evaluate the properties of the portland cement which has the following chemical analysis: SiO2 = 20.9 percent; Al2O3 = 5.4 percent; Fe2O3 = 3.6 percent; CaO = 65.1 percent; MgO = 1.8 percent; and SO2 = 2.1 percent
This cement is like Cement no. 2 in Table 6-2 and will have a lower C3A content resulting in lower heat of hydration and moderate sulfate resistance, a Type II cement. (see equations 6-1 - 6-4)
Can you list and define the seven types of chemical admixtures, four classes of mineral admixtures, and three grades of iron blast-furnace slag that are used as admixtures for concrete?
Type A - water reducing • Type B - retarding • Type C - accelerating • Type D - water-reducing and retarding • Type E - water reducing and accelerating • Type F - high range water reducing • Type G - high range water reducing and retarding • Class N - raw or calcined pozzolans (diatomaceous earth, opaline cherts and shales, tuffs, volcanic ash, or pumicite and calcined materials such as clays and shales). • Class F - low calcium fly ash from burning anthracite or bituminous coal • Class C - high calcium fly ash from lignite or subbituminous coal • Grade 80 - finely ground granulated blast furnace slag, with a strength index greater than 75 • Grade 100 - strength index greater than 95 • Grade 120 - strength index greater than 115
Discuss the major differences in the physical and chemical composition between an ordinary (ASTM Type I) and a high early strength (ASTM Type III) portland cement.
Type III cement is finer (500 m2/kg) than Type I cement (300 m2/kg) and contains a greater percentage of C3S (50-60% as compared to 40-50%). Type III is high-early strength not fast-setting!
Discuss the two methods that the cement industry employs to produce cements having different rates of strength development or heat of hydration. Explain the principle behind the maximum limit on the C3A content in the ASTM C 150 Standard Specification for Type V portland cements.
Type IV - For use when a low heat of hydration is desired. As C3S and C3A produce high heats of hydration, and C2S produces much less heat, the specification calls for maximum limits of 35 and 7 percent on C3S and C3A, respectively, and requires a minimum of 40 percent C2S. • Type V - For use when high sulfate resistance is desired. The specification calls for a maximum limit of 5 percent on C3A to be applied when the sulfate expansion test is not required. It should be noted that the hydration product of cements with more than 5 percent potential C3A, as calculated by Bogue equations, contains monosulfate hydrate which is unstable when exposed to a sulfate solution. Ettringite is the stable product in a sulfate environment, and conversion of the monosulfate to ettringite is generally associated with expansion and cracking.
Why do the ASTM Specifications for Type IV cement limit the minimum C2S content to 40 percent and the maximum C3A content to 7 percent?
Type IV cement is used for low heat of hydration and the C2S and C3A content are primarily responsible for adiabatic temperature rise.
Why is gypsum added to the cement clinker? Typically, how much is the amount of added gypsum?
Unless the rapid hydration of C3A is slowed down by some method, portland cement cannot be used for most construction applications. This task is generally accomplished by the addition of gypsum. Therefore, for practical purposes, it is not the hydration reactions of C3A alone but the hydration reactions of C3A in the presence of gypsum which are important. 5 to 6 percent gypsum
Assuming that the workability of concrete is of no consequence, would sand be necessary for concrete mixtures? What is the significance of fineness modulus? Calculate the fineness modulus of sand with the following sieve analysis:
Weight retained on No. 8 sieve, g = 30, %R = 6, Cum. %R = 6 • Weight retained on No. 16 sieve, g = 70, %R = 14, Cum. %R = 20 • Weight retained on No. 30 sieve, g = 125, %R = 25, Cum. %R = 45 • Weight retained on No. 50 sieve, g = 135, %R = 27, Cum. %R = 72 • Weight retained on No. 100 sieve, g = 120, %R = 24, Cum. %R = 96 • Weight retained on No. 200 sieve, g = 20, %R = 4, Cum. %R = 100 • Is this sand suitable for making concrete? Yes, the sand is required to produce a well-graded aggregate with minimum voids. The fineness modulus is a measure of the coarseness of the aggregate gradation, with a smaller fineness modulus representing a finer aggregate which will require less cement paste. F.M. = [6+20+45+72+96]/100 = 2.39
The presence of high free lime in portland cement can lead to unsoundness. What is meant by the term, "unsoundness"? Which other compound can cause unsoundness in portland cement products?
cracking and pop-outs in cement-based products. Hydration of periclase to magnesium hydroxide is a slow and expansive reaction that, under certain conditions, can cause unsoundness.
When present as deleterious substances in concrete aggregates, how can the following materials affect propertied of concrete: clay lumps, silt, zinc sulfide, gypsum, and humus?
• Clay lumps -affect workability and abrasion resistance. • Silt - increase water requirement • Zinc sulfide - in the presence of zinc salts the concrete will be retarded and may fail to set. • Gypsum - May alter (delay) set time of the cement or lead to sulfate durability issues. • Humus - may cause concrete setting and hardening problems.
What is the distinction between shrinkage-compensating and self-stressing cements? What are Types K, M, S, and O expansive cements? Explain how the expansive cements function to make concrete crack-free.
• Shrinkage compensating - When the magnitude of expansion is small such that the prestress developed in concrete is on the order of 15 to 100 psi (0.1 to 0.7 MPa), which is usually adequate to offset the tensile stress from restrained drying shrinkage, the cement is known as shrinkage compensating. • Self-Stressing - When the magnitude of expansion is large enough to produce prestress levels on the order of 1000 psi (6.9 MPa), the cement is called self-stressing and can be used to produce chemically prestressed concrete elements. • Type-K - The cement produced by grinding a sulfoaluminate-type clinker is called Type K expansive cement. • Type M - Type M expansive cement is a mixture of portland cement, calcium aluminate cement (with CA is the principal compound), and calcium sulfate. • Type S - Type S expansive cement is composed of a very high C3A portland cement (approximately 20 percent C3A) and large amounts of calcium sulfate. • Type O - the expansive portland cement deriving its expansion from hard-burnt CaO is called Type O expansive cement. • Expansive cements are hydraulic cements which, unlike portland cement, expand during the early hydration period after setting. Large expansion occurring in an unrestrained cement paste can cause cracking; however, if the expansion is properly restrained, its magnitude will be reduced, and a prestress will develop.
Briefly discuss the following propositions:
• Specific gravity and absorption capacity are good indicators of aggregate quality. Specific gravity and absorption capacity influence the density and strength • The crushing strength of aggregate can have a large influence on the compressive strength of concrete. In lightweight aggregates where the crushing strength is relatively low, the compressive strength of the concrete may be limited by the aggregate. • Using mineral aggregates, the unit weight of structural quality concrete can be varied between 2700 and 5400 lb./yd3. Lightweight aggregates can be used to produce lightweight concrete, also heavy-weight aggregates can be used to produce heavyweight concrete. • Pores in aggregate that are smaller than 4 µm can become critically saturated. water cannot move out of these pores rapidly during freezing and will cause stress on the aggregate matrix. For small pores, the Kelvin Equation (Robert von Helmholtz 1886) indicates ab increased pore pressure (vapor pressure)-due to curvature of the boundary between the vapor and liquid phase-may lead to transport of water to and from the pore space leading to a critical net tensile pressure in the capillaries due to transport of liquid water phases.
If the dry-rodded unit weight of an aggregate is 105 lb./ft3 and its specific gravity is 2.65, determine the void content. (b) A sample of sand weighs 500 g and 480 g in "as-received" and ovendried conditions, respectively. If the absorption capacity of the sand is 2 percent, calculate the percentage of free moisture.
• e = Vv/Vs, Vs = [DRUW/(Gs*γw)], Vv = 1 - Vs • w% = (500-480)/480*100% - 2% = 2.16%