MSE 220 HOMEWORK #4 Solutions Ch. 3: CEMENT
1. What are the three main compounds in the raw materials for the manufacture of cement?
1. Lime (CaO)
2. Silica (SiO2)
3. Alumina (Al2O3)
2. What are the four major compounds of Portland cement?
1. Dicalcium Silicate C2S
2. Tricalcium Silicate C3S
3. Tricalcium Aluminate C3A
4. Tetracalcium Aluminoferrite C3AF
3. Which is the most reactive compound in Portland cement?
Tricalcium Aluminate
4. Which is the major chemical compound in cement?
Calcium Silicate
5. What would you add to cement to make it cure faster in cold weather?
Tricalcium Aluminate
6. How would you adjust the proportions of C3S and C2S if you want to have:
(a) High early strength
Increase the amount of tricalcium Silicate
(b) Minimum heat of hydration
Increase the amount of dicalcium Silicate
7. When would you want to use a cement that has a low heat of hydration?
If you are pouring large, thick sections of concrete. This will minimize the strain gradients that result from uneven temperature distribution in the concrete, reducing the chance that it will crack.
8. What is slump and how is it measured?
Slump is a measure of the moisture content and aggregate gradation in concrete. Concrete mix is put into a cone-shaped mold. The mold is removed, and the amount that the concrete “sags” or “slumps” is measured. Proper slump is typically 2-4 inches, depending upon the application.
12. What are the three considerations for mix design?
1. Workability (consistency)
2. Strength and durability
3. Economy
15. If the size of aggregate is increased from ½ to 1-inch, how does that affect the compressive strength?
The compressive strength will improve with larger aggregate, but only if the amount of water used is decreased. Using the same amount of water will actually decrease the strength of the concrete when increasing the aggregate size.
16. Draw a typical relationship between compressive strength and water-to-cement ratio.
19. What is freeze-thaw resistance, and how can it be improved?
In the freeze-thaw process, water that is stored in voids in the concrete expands from freezing temperatures, causing cracks and deterioration in the concrete. If the top surface of concrete had water applied to it during floating or troweling, had bleed water worked into the surface during finishing, was floated too early or over-troweled, or exposed to deicers, the top thin mortar layer may flake off from freeze-thaw effects in a process called scaling. Porous aggregates may form popouts or cracking of the surfaces of the concrete.
Freeze-thaw damage may be averted by minimizing the water-to-cement ratios, and by using admixtures that increase the entrained air, allowing room for expansion upon freezing.
20. Calculate the weight of cement required to make 2 yd3 of concrete of mix proportions: 1C:2.5FA:3.5CA:0.5W, by weight with 2% air. (The specific gravity values of cement, FA, and Ca are 3.15, 2.65, 2.75).
Volume of concrete = 2 yd3 =
Solving for Wc: Wc = 39.9 lbs
22. 14.3 lb of cement are used to make three 6 x 12-in. cylinders. How many sacks are required per cubic yards of concrete?
One cubic yard is: 1 yd3 x (36 in.)3/yd3 = 46,656, in3.
The volume of the cylinders is: 3 x 12 x p/4 x 62 = 1,017.36 in2
Therefore, 14.3 lb/1,017.36 in3 = X/46,656 in3
Solving for X gives: X = 656 lb cement
A standard, US bag of cement weighs 94 lbs, thus the number of bags is: 656 lbs/94 lbs/bag = 7 bags per yd3.
23. Mix design calculations show a dry mix of 1C:2FA:3.5CA:0.55W by weight. The moisture content and absorption of coarse aggregate are 3% and 1%, respectively, and of fine aggregate, 8% and 3%, respectively. Find the field mix proportions.
The moisture content of coarse aggregate is 3%, but it can only absorb 1%, so there is a net 2% extra water weight on the coarse aggregate.
Similarly, the fine aggregate has (8-3) or 5% extra weight due to water.
The total water must be reduced by 7% to account for that added by the aggregate.
The given mix proportions are as follows:
1 lb. cement
2 lb. fine aggregate
3.5 lb. coarse aggregate
0.55 lb. water
The net reduction in water is: coarse aggregate: 0.02 x 3.5 lb = 0.07 lb.
fine aggregate: 0.05 * 2 = 0.1 lb.
The net mixing water required is: 0.55 lb. – 0.17 lb = 0.38 lb.
The required weight of the coarse aggregate is: 1.03 x 3.5 = 3.605 lb.
The required weight of the fine aggregate is: 1.08 x 2 = 2.16 lb.
Thus, the field mix quantities are:
1 lb. cement
2.16 lb. FA
3.605 lb. CA
0.38 lb. water
The proportions are 1C:2.16FA:3.605CA:00.38W
29. What is bleeding, and how is it controlled?
Bleeding happens when a concrete mix cannot hold the mix water, which separates and rises to the top of the form. Excess bleeding causes aggregate and cement segregation in the concrete, resulting in a nonhomogeneous mix and a weaker concrete. It may be exacerbated by overvibration, overtroweling, and lean mixtures. Air entrainment is an effective method of controlling bleeding, as is increasing the fineness of the cement, and decreasing the water-to-cement ratio. Cement rich mixes bleed less; mixes that lack fine sand bleed more.
30. What is segregation, and how is it prevented?
Segregation is the tendency for separation between large and fine particles of fresh concrete, resulting in rock pockets (or honeycombing), sand streaks, crazing and surface scaling. This is usually caused by very wet mixtures, or mixtures deficient in finer particles. It may also be caused by moving concrete over long distances before being placed in the forms. Fresh concrete must be dropped vertically and not at an angle. Segregation can be minimized by reducing the water-to-cement ratio, adding air entraining admixtures, or finely divided materials such as fly ash, powdered limestone and silica-fume).