CE 60 Part 2

After hydration, a cement paste has 10% porosity and the amount of hydration products is twice the amount of unhydrated cement. Calculate the original w/c ratio.

1. (Vw_2)/(2Vc_2 + Vc_2 + Vw_2) = 0.1, Vw_2 = 1/3*Vc_2. Initial state: 4/3*Vc_2 of water and 2Vc_2 of cement, w/c ratio = 0.212.

Give 4 ways to reduce the temperature of fresh concrete.

1. Cast at night. 2. Lower temperature of water (use ice chips). 3. Use liquid nitrogen to cool. 4. Pre-cool the aggregate.

Give examples where creep is beneficial and undesirable.

1. Creep can relieve stress caused by shrinkage. 2.

Give 3 ways to reduce the temperature rise.

1. Decrease the amount of cement. 2. Use cement with lower C3S and C3A. 3. Use fly ash.

After 14 days, the hydration degree is 40% and the cement contains 50% of hydration products. Compute porosity and original w/c ratio.

1. Find Vw_2 in terms of Vc_1. (0.8Vc_1)/(Vw_2 + 0.8Vc_1 + 0.6Vc_1) = 0.5, Vw_2 = 0.2Vc_1, porosity = 12.5%. 2. Vw_1 = Vw_2 + 0.4Vc_1, w/c ratio = 0.191.

An aggregate blend of 40 wt% sand and 60 wt% gravel has a dry-rodded unit weight of 1920 kg/m3. If the BSG of the sand is 2.5 and the gravel is 2.65, calculate the volume percent of void space.

1. Find mass or sand and gravel by multiplying proportions by 1920. 2. Find corresonding volume by dividing by density. Subtract total from 1.

What is the effect of the CA being covered by very fine materials on a) fresh concerete and b) hardened concrete? What would be your recommendation to avoid this problem?

1. High surface area of fine powder increases the water demand, so workability will decrease if same amount of water is used. 2. if same amount of water is used and workability decreased, there may be more voids and defects leading to lower strength.

Compute the w/c ratio with final porosity 25% and 65% hydration.

1. Intial volume of cement is 100 and final volume of water is 55. 2. Initial volume of water is 120 giving a 0.381 w/c ratio.

Describe the two sources of CO2 during the firing of the clinker in the kiln.

1. Limestone (CaCO3) --> CaO + CO2. 2. The kiln is heated to 1450 to 1550C.

Describe the tangent, chord, and secant modulus of elasticity methods.

1. Tangent: slope of tangent line drawn at point. 2. Secant: slope of a line drawn from the origin to the point (so simply stress/strain). 3. Slope of a line drawn between a point corresponding to 50 micrometers to another point corresponding to 40% of ultimate load.

A portland cement has 40% C3S, 45% C2S, 4% C3A, 11% C4AF. Will the cement develop high early strength? High heat of hydration? Applications where you would not recommend this cement?

1. Will not develop high early strength because low C3S and C3A. 2. Low heat of hydration because low C3S and C3A. 3. Not recommended for emergency construction that requires high early strength.

What is the influence of type and amount of aggregate on the modulus of elasticity of concrete?

1. aggregate with high E leads to concrete with high E. 2. more aggregate leads to higher E in concrete.

Compute the maximum percentage of cement that will ever hydrate in a 0.25 w/c ratio cement paste.

100cm3 of water, 127.39cm of cement for 0.25 w/c ratio. Maximum degree of hydration is 78.5%.

C2S

2(CaO)SiO2

At what relative humidity do you expect a maximum in the corrosion rate?

80-83% relative humidity. You want high humidity and exposure to oxygen but really high humidity fills pores completely with water and does not allow O2 to propogate.

A steel is heated until 85% austenite, with a carbon content of 1% forms. Estimate the overall carbon content.

85% * 1% + 15% * 6.67% = 1.85% C

A 4500-psi concrete mix is used in location A with aggregate of E = 40Gpa and in location B with aggregate of E = 29Gpa. Which will have: higher compressive strength, higher creep, higher shrinkage, higher thermal stress?

A will generally not have higher strength, less creep, less shrinkage, but higher thermal stress.

How does the strength of wood vary above and below FSP?

Above FSP: does not change, since no shrinkage occurs. Below FSP: strength increases below FSP because wood shrinks and is closer together.

Large amounts of aggregate increase the elastic modulus of concrete and thermal stresses are proportional to the elastic modulus of concrete. Why do large concrete dams use a large amount of aggregate in the mix proportion.

Aggregate is still a better option than cement, because cement generates much more heat.

Non-air entrained concrete has a higher w/c ratio than air-entrained concrete but they still have the same compressive strength. Why?

Air entrainment causes a decrease in compressive strength because it introduces air bubbles (increases porosity), which is compensated for in the ACI method by a reduction in w/c ratio.

What is the alkali-aggregate reaction? List rock types vulnerable to this.

Alkali in cement paste reacts with silica in aggregate leading to a loss of strength. Opal, obsidian, fulgurite.

How does aggregate size affect permeability of concrete? List other factors that determine permeability.

An increase in aggregate size leads to an increase in permeability, because there will be more microcracks in the ITZ, allowing more capillary cavities. However, the permeability of cement paste is typically higher than that of aggregates, so w/c ratio and curing time will influence permeability more.

Explain basic creep, specific creep, drying creep, and creep coefficient.

Basic creep: sustained stress. Drying creep: additional creep when the stressed specimen is also drying. Specific creep: creep per unit of applied stress. Creep coefficient: ratio of creep strain to elastic strain.

What is bleed water and how can you minimize it?

Bleeding is the process where free water in the mix is pusehd upward to the surface due to the settlement of heavier solid particles. To reduce bleed water, add more fine aggregate and use less coarse aggregate. Reduce w/c ratio.

What is the difference between portland clinker and portland cement?

By grinding portland clinker and adding gpsum, the portland clinker is turned into portland cement.

Which compound in cement generates the greatest heat of hydration?

C3A

Describe the chemical reaction responsible for the formation of ettringite.

C3A + Gypsum (CS_barH2) --> Ettringite + 300 cal/g

Which clinker compound is most abundant in Type III cement?

C3S is the most abundant material in all except Type IV cement where C2S is.

Which hydration product of cement is responsible for the long term strength of concrete?

CSH has a high surface area accompanied by high Van der Waals forces

Which of the hydration products is responsible for the shrinkage of concrete? Describe the hydration reaction that generates this product.

CSH is responsible for the shrinkage of concrete because it has water in its interlayer spaces. C3S and C2S hydrate to from CSH.

Just after casting, a cement paste had a porosity of 60%. After 3 moonths, the porosity was reduced to 48%. Compute degree of hydration and original w/c ratio.

Can treat percentage as volume. Initial: 60% water, 40% cement. Final 48% water, 24% hydrates, 28% cement.

Describe the anodic and cathodic reactions occurring in the corrsion of concrete.

Cathode: O2 + 2H2O + 4e --> 4OH-. Anode: Fe --> Fe(2+) + 2e.

Cement A Composition: 50 C3S, 35 C2S, 5 C3A, 10 C4AF. B: 65 C3S, 15 C2S, 10 C3A, 10 C4AF. Which is better for a hot summer?

Cement A is far better as it has less C3S and C3A, which will lead to a lower heat of hydration for cement A than cement B.

Explain the difference between hydraulic and non-hydraulic cements.

Cements that not only harden by reacting with water but also form a water resistant product are called hydraulic cements (e.g. Portland cement). Cements dervied from the calcination of gypsum or calcium carbonates are nonhydraulic because their products of hydration are not resistant to water.

What raw material of Portland cement provides aluminmum for the formation of C3A?

Clay

How does aggregate affect the drying shrinkage of concrete?

Concrete with higher amounts of aggregate will shrink less. Shrinkage occurs in the cement paste, so having less cement will cause less shrinking.

Why is concrete weaker in tension than in compression?

Cracks proprogate in tension but in compression the cracks close on each other. This is a property displayed in most brittle materials.

Explain the difference between creep and relaxation of concrete.

Creep: fixed load, the absolute value of strain keeps increasing. Relaxation: fixed strain, internal stress keeps decreasing.

What is dry rodded unit weight and how can you measure it?

Dry rodded unit weight is perfomed on aggregate and is a measure of density. It is determined by compacting dry aggregate into a container of known specific volume. Place the aggregate in 3 layers, compacting each with a rod.

Question 3 II)

Dry-rodded unit weight is found by dividing mass by volume found from volume percentage. Bulk density is found from dividing mass by volume found from container subtraction.

Explain how entrained air voids reduce frost damage.

Entrained air voids provide pre-allocated space for the water to expand as it freezes into ice. Instead of expanding the cracks and causing damage, the ice squeezes into the air voids.

Explain how tensile stresses develop in a splitting test.

Explain how tensile stresses develop in a splitting test.

Discuss the factors that influence the compressive strength of concrete exposed to fire. How is elastic modulus affected compared to compressive strength?

Fire takes away from the water content, killing the CSH layers.

Why does the failure of concrete cylinders cause the formation of cones at the extremities?

Friction between the bearing faces of the testing machine and the test specimen restrains the specimen laterally, thereby inducing lateral compression in the specimen ends. This effect wears off the further you get from the ends of the specimen, so cross-sections at mid-height should be free of the effect of the end restraint. In the middle there is Poisson expansion which pushes the concrete outward.

How do you develop high early-strength cement wihtout changing the composition of the cement?

Grind finer.

What would happen to the concrete you're batching if the cement factory made a mistake and used too little gypsum?

Gypsum is added to the clinker with the portland cement materials to retard the rapid hydration of C3A. Without adequate gypsum, the concrete will harden too fast and not be usable.

Explain why drying shrinkage of concrete can lead to cracking.

If the concrete is constrained, the volume/length decrement from drying shrinkage will lead to tensile stress in the concrete. These tensile stresses can cause cracking.

60cm3 of unhydrous cement, 30cm3 of hydration products and 10% pores filled with water. Compute the hydration degree and the original w/c ratio.

In order to get 10% porosity, final volume of water is 10cm3. Cement and water react in 1:1 ratio to form product, so the intial volume of water is 25cm3 and the intial of cement is 75cm3. Hydration degree is thus 25% and w/c ratio is (25*1)/(75*3.14)

Explain the formation of ettringite during a sulfate attack.

In the presence of sulfates, monosulfate hydrates convert to ettringite, causing volume expansion and cracking.

After a few days of curing, a cement with initial 0.637 w/c ratio has a porosity of 63%. Compute the degree of hydration.

Initial: 200cm3 water, 100cm3 cement. Porosity = (200-x)/300.

Why is Type IV cement used for mass concrete?

It has a low heat of hydration because of its low C3S and C3A levels.

How does MSA influence strength of concrete?

Large aggregates normally form weaker ITZ which contains more microcracks. Consequently, the concrete with larger aggregates has lower compressive strengths than those with smaller aggregate.

How do earlywood cells compare to latewood cells in their contribution to drying shrinkage?

Latewood dominates shrinkage since it contains more adsorbed water.

Does the removal of free water cause shrinkage in the wood?

No, because free water is in air voids between wood.

Is gypsum blended with limestone and clays and then transported to a kiln with a temperature of 1450C to make Portland cement?

No, gypsum is blended with clinker and then grinded.

Describe a test to measure the segregation of fresh concrete.

Place the fresh concrete in a special container and divide it horizontally into three equal section. Measure the mass of each section--the difference in mass gives an indication of the degree of segregation.

Why is the pH of fresh concrete very high?

Reactions involving C2S and C3S form CH, which is very basic.

What is the difference between setting and hardening?

Setting is the stiffening of cement paste from fluid to rigid. Hardening refers to the gaining of strength.

What happens if silica sand is used instead of clay in the manufacture of portland cement?

Silica sand is harder to grind than clays and the lack of Al and Fe will require a higher reaction temperature. The final product will not contain C3A or C4AF.

Will testing 4x4x4 concrete cubes result in a lower or higher compressive strength than testing a 4x8 cylinder?

Smaller volume leads to less flaws, so higher strength for smaller test.

How can you measure the creep of concrete?

Take a concrete cylinder and apply constant compressive stress to it. Measure the compressive strain and plot the strain against time to evaluate creep behavior.

Identify the direction of maximum drying shrinkage.

Tangential because there is an unbroken alignment of latewood cells, which dominate shirnkage since they adsorb more water.

Why is the chord modulus recommended instead of the secant modulus for elastic modulus?

The chord modulus shifts the base line by 50 microstrain to correct for the slight concavity often observed at the beginning of the stress-strain curve.

A: cast and cured at 13C, B: cast and cured at 38C

The early strength of B will be higher. Note that an increase in early strength is generally compensated before by a decrease in ultimate strength.

Explain why the elastic modulus of aggregate influences the elastic modulus of concrete but the strength of the aggregates does not affect the strength of the concrete.

The elastic modulus of the concrete is the volumetric average of the modulus of each component. Aggregate consumes a large percentage of volume, so it has a direct impact. Strength of concrete is largely determined by the weakest link: ITZ, which is mostly affected by the amount and size of aggregates, not the aggregate strength.

A: cast at 21C and cured at 10C, B: cast at 21C and cured at 21C.

The strength of B will be higher due to more energy to accelerate hydration reactions.

Graphically show how the fineness of the cement incluences the rate of heat development and strength gain.

The surface area for the reaction will increase, thereby increasing heat of development and early strength gain.

A: cast at 21 C and cured at 21C, B: cast at 10C and cured at 21C.

The ultimate strength of B will be higher due to the more uniform microstructure of the hydrated cement paste for the lower cast temperature.

Describe the potential problems of using very finely ground Portland cement.

The very fine particle size leads to high surface area of cement, increasing the rate of hydration reaction. This leads to a high heat of hydration, which may lead to thermal cracking in the concrete. The concrete will also set fast, but it will have a high early strength.

How does the microstructure of the ITZ differ from that of the bulk cement paste?

The volume and size of voids in the transition zone are larger than in the bulk cement paste due to internal bleeding. The size and concentration of crystalline compounds such as CH and ettringite are larger in ITZ and cracks are more easily formed.

What changes in fresh concrete property will occur if FM is decreased from 3.1 to 2.7 of FA?

The water and cement demand will increase. With an increased w/c ratio, the compressive strength will decrease.

Client requests a laboratory test for measuring the workability of concrete mixes. What do you suggest?

There is no test for workability. Workability is broken up into two parts: consistency and cohesiveness. Use slump test to test for consistency and segregation and bleeding test for cohesiveness.

A contractor wants to eliminate bleeding by dusting cement on top of the slab, is this a good practice?

This is not a good practice, because there will be a layer with really high w/c ratio at the top.

A sewer line is damaged due to an earthquake, should you use Type III cement to expedite the repair?

Type III has high levels of C3A, which is vulnerable to sulfate attack, so use a sulfate resistant cement with lower than 5% C3A.

Question 3 II)

Use algebra involving w/c ratio and volume summing to 27.

Why are very fine particles a problem in concrete?

Very fine particles decrease slump/workability, increase water/cement requirement, and drive up material costs

Can you batch by volume instead of batching by weight if you can measure specific densities very accurately?

Volume batching is only suitable for very small jobs. There is too much variability in the amount contained in a specific volume (for example a different mass of aggregate will take up a container each time).

What are the benefits of well-graded aggregate?

Well-graded means there is a high variety of grain sizes, which implies that compaction is better. Less water and cement is needed making for a more economical batch.

Describe what SSD conditions are.

When all the permeable pores are full and there is no water film on the surface, the aggregate is said to be in the saturated-surface dry condition.

What is bulking of sand and how does it affect the batching of concrete?

When sand is moist, water films form on surface of sand particles, resulting in an increase in volume. When using bulked sand for batching, its real volume (of actual sand) is smaller than the measured volume, so less sand is used.

Explain the creep mechanism in cement pastes.

When there is sustained stress, the CSH layers will lose absorbed water and the paste will show a creep strain.

What happens when you load concrete very close to its compressive strength?

When you sustain loads close to compressive strength, the concrete will reach failure over time.

Explain why using aggregates with larger MSA lead to more economical concrete mixtures.

With the same cement content and consistency, concrete mixtures containing higher MSA require less mixing water. This makes it more economical. BUT larger aggregates tend to form weaker ITZ containing more microcracks, so usually a lower MSA leads to higher compressive stength.