CIV2235—STRUCTURAL MATERIALS
Week 3 Practice Class
Properties of Fresh Concrete
QUESTION 1: Select the appropriate option as answer and write down in the answer sheet (5 marks each)
1. Workability of concrete is inversely proportional to
a) time of transit
b) water-cement ratio
c) the air in the mix
d) size of aggregate
2. Which one of the following statements is true regarding the workability of fresh concrete?
a) A higher free water content in concrete mixes leads to increased workability.
b) A higher free water content in concrete mixes results in loss of workability.
c) Aggregate grading has no impact on concrete workability.
d) Workability loss only occurs when concretes are improperly mixed.
3. For compacting plain concrete road surface of thickness less than 20 cm, we use
a) form. vibrator
b) screed vibrator
c) internal vibrator
d) none of the above
4. A possible solution to reduce bleeding is
a) Use cement with high calcium chloride
b) Use coarser cements
c) Use cement with low tri-calcium aluminate content
d) All of the above
5. Which of the following statements is wrong about concrete bleeding?
a) It is a form. of segregation
b) Aggregates settle downwards under gravity
c) It should be always eliminated
d) It can be useful to finishing operations
e) None of the above
6. What is the mechanism for bleeding of fresh concrete?
a) Aggregates settle due to gravity, displacing free water and cement paste which accumulates at the highest surface of the concrete
b) Aggregates settle due to gravity, displacing free water which rises to the highest surface of the concrete
c) Aggregates settle due to gravity, displacing free water and cement paste which accumulates at the lowest surface of the concrete
d) Aggregates settle due to gravity, displacing free water which accumulates at the lowest surface of the concrete
7. After casting, an ordinary cement concrete on drying
a) expands
b) mixes
c) shrinks
d) none of the above
8. Which of the following statements is correct?
a) Keeping the cement moist when storing it will increase the strength of the hardened concrete.
b) Keeping the cement moist when storing it will increase the early strength of concrete.
c) Keeping the cement moist when storing it does not affect the strength of concrete.
d) None of the above
9. Concrete must be cured for at least
a) 1 day
b) 7 days
c) 28 days
d) Until it hardens
10. What is the maximum drop height of concrete on site
a) 20 m
b) 2 m
c) 30 m
d) 3 m
e) None of these
QUESTION 2: Write BRIEF answers for the following questions. (10 marks each)
1. Define workability of concrete and discuss the factors affecting the workability of concrete.
2. Why does bleeding occur within concrete? Mention three adjustments you would make to the concrete mix proportions to reduce it?
3. What is plastic shrinkage cracking? Is it detrimental to concrete strength? What are the solutions to plastic shrinkage cracking?
4. Why compaction is needed for concrete? List at least three types of compaction.
5. Air temperature is 23 °C, relative humidity is 50%, concrete temperature is 30 °C, and wind speed is 20 km/h:
(i) Draw in the following figure and determine rate of water evaporation
(ii) The maximum evaporation rate is 1.0 kg/m2/h. What is the maximum wind speed allowed if other parameters remain unchanged?
(ii) The maximum evaporation rate is 1.0 kg/m2/h. What is the concrete temperature allowed if other parameters remain unchanged?