1 What are the basic ingredients in concrete 2 What is the d

1. What are the basic ingredients in concrete? 2. What is the difference between density (solid density) with bulk density? 3. Define initial set and final set. Briefly discuss one method used to determine them. What is a false set and flash set of portland cement? 5. What is meant by air-entraining? 6. What type of cement would you use in each of the following cases a. Construction of a large pier b. Construction in cold weather c. Concrete structure without any specific exposure condition d. Building foundation in a soil with severe sulfate exposure 7. You are an engineer in charge of mixing concrete in an undeveloped area where no potable water is available for mixing concrete. A source of water is available that has some impurities. What tests would you run to evaluate the suitability of this water for concrete mixing? What criteria would you use? 8. Three standard mortar cubes were made using nonpotable water available at the job site, and three others were made using potable water. The cubes were tested for compressive strength after 7 days of curing and produced the failure loads in kips shown in Table P6.23. The Vicat test was conducted on the cement paste made with the questionable water and showed that the set time was 45 minutes more than the set time of paste made with potable water. Based on these results, would you accept that water for mixing concrete according to ASTM standards? Explain why TABLE P6.23 Nonpotable Water 15.8 164 16.6 Potable Water 16.0 18.7 17.4 9. How does the addition of air-entraining materials to Portland cement improve the concrete? 10. How does the fineness of the cement affect the concrete? 11. What natural aggregates are used in making concrete?

Solution

(1) Answer:- Basic ingredients in concrete:-

(i) Coarse aggregate

(ii) Fine aggregate

(iii) Cement

(iv) Water

(2) Answer:- Difference between solid density and bulk density:-

Solid density:- It is mass per unit volume.

Bulk density:- It is dry weight of soil per unit volume of soil.

(3) Answer:-

Initial setting time of cement:- Time at which cement starts hardens and completely losses its plasticity is called initial setting time of cement.

Initial setting time of cement is 30 minutes.

Final setting time of cement:- Time at which cement completely losses its plasticity and became hard is a final setting of cement .

It is 600 minutes for cement.

(4) Answer:-

Flash set:- Reaction of C₃A with water is violent .The hydration completes in a day .Hydration completes in a day. Hydration leads to immediate stiffening of the paste known as flash setting.

False setting :- It is the name given to the abnormal premature stiffening of cement within a few minutes of mixing with water.

(5) Answer:-

Air entraining is in creation of small air bubbles in concrete.

(6) Answer:-

(a) Answer:- Waterproof Portland cement

(b) Answer:- Portland cement type III

(c ) Answer:- Hydrophobic cement.

(d ) Answer:- Sulfate resisting cement

(9) Answer:- Air entraining material improve strength of concrete . because air entraining material reduces air voids of concrete.

(10) Answer:- Fineness of cement is very important for concrete. Fineness of cement improve workability of concrete.

(11) Answer:- National aggregates like sand , gravel , crushed stone are used in concrete.