6-27. Of the following descriptions, which most accurately describes pretensioning of concrete members? 1. After the concrete has been placed and has reached a specified  strength, reinforcement strands are pulled through formed channels, and a predetermined amount of stress is  applied 2. Reinforcement strands are pulled through inflated tubes and are stressed  before  placement  of  the concrete 3. Reinforcement strands are stressed to a predetermined point before placement of the concrete and are released just before the concrete has set 4. Reinforcement strands are placed in the forms and are stressed to a predetermined point before the concrete is placed; the strands are then released after the concrete has reached a specified strength 6-28. In what part of a prestressed beam does the tensioned steel produce high  compression? 1. Upper 2. Lower 3. Exact center 4. Approximate center 6-29. What condition occurs when a load (force) is placed on a prestressed beam? 1. The camber is forced out, leaving  a  beam  with  positive deflection 2. The upward bow is increased 3. The camber is forced out, leaving a level beam with no deflection 4. The upward bow is forced out, creating deflection in the beam 6-30. What is the approximate weight of conventional   concrete? 1. 175 lb/cu ft 2. 150 lb/cu ft 3. 130 lb/cu ft 4. 115 lb/cu ft IN  ANSWERING  QUESTIONS  6-31  THROUGH  6-35, SELECT THE TYPE OF CONCRETE FROM THE FOLLOWING LIST THAT BEST MATCHES THE CHARACTERISTIC  GIVEN. A. HEAVYWEIGHT CONCRETE B. SEMI-LIGHTWEIGHT CONCRETE C. INSULATING LIGHTWEIGHT CONCRETE D. STRUCTUTWL LIGHTWEIGHT CONCRETE 6-31. Weighs 115 to 140 lb/cu ft and has a compressive strength comparable to normal  concrete. 1.   A 2.   B 3.   C 4.   D 6-32. Weighs 20 to 70 lb/cu ft and is used for  fireproofing. 1.   A 2.   B 3.   C 4.   D 6-33. Weighs up to 400 lb/cu ft. 1.   A 2.   B 3.   C 4.   D 6-34. Weighs up to 115 lb/cu ft and is used to decrease the dead-load weight  of  structural  members. 1.   A 2.   B 3.   C 4.   D 6-35. Normally has a compressive strength of 1,000 psi or less. 1.   A 2.   B 3.   C 4.   D 6-36. In what type of construction are concrete  walls  poured  horizontally, lifted upright, and then secured in place? 1. Tilt-up 2. Lift-up 3. Cast-in-place 4. Prefab 6-37. In  tilt-up  panel  construction,  where is  additional  reinforcement generally  needed? 1. At the top 2. At the bottom 3. Around the edges 4. Around any openings 39