WORKSHEET 4 CONCRETE

1
WORKSHEET 4CONCRETE
2
Q1
(i) strength
(ii) workability
by using an acceptable water / cement ratio
strength also depends on mix proportions -
more cement, stronger the concrete quality of
materials - strength of cement, aggregate
proper vibration proper curing
3
Q1
water / cement ratio
the higher the water / cement ratio (more water)
- weaker the concrete
the higher the water / cement ratio - concrete
more workable
4
Q2
cement
coarse aggregate (gravel)
fine aggregate (sand)
water
additives
(i) cement to aggregate?
14 to 17
(ii) water to cement?
0.41 to 0.61
5
Q3

• hydration

• a series of chemical reactions occur after the
  cement is mixed
• with water forming new compounds

• a gel or paste is formed which progressively
  hardens and
• the concrete gains strength

• to provide bulk - cheaper than cement

• aggregate reduces shrinkage - only cement
  shrinks

6
Q3

• well- graded aggregate ensures that no
  bridging occurs
• and voids are filled

• coarse aggregate ranges from 5 - 20mm (approx.)

• aggregates are considered well-graded if the
  various size of the
• particles are uniformly distributed - but
  doesnt have to be continuous

• to fill the gaps between the coarse aggregate

• sand is cheaper than cement

7
Q4

• so that it can be easily moved into all parts of
  the formwork

• we measure workability using the slump test

• a standard slump cone should be filled with a
  sample of concrete within
• 20 mins of the concrete arriving on site.
• fill 1/3 of the cone and compact the concrete
  by rodding with a steel rod
• 25 times (pushing the rod in and out of the
  concrete).
• fill to 2/3 and rod again 25 times.
• fill and rod again.
• lift the cone straight up.
• turn the cone upside down and place the rod
  across the top of the cone.
• measure the average distance to the top of the
  sample.
• if the sample is outside the tolerance (too
  high or too low) another
• must be taken. If this also fails, the batch
  should be rejected

8
Q4

• by compression testing of test cylinders

• after 28 days to test full strength

• can test after 7 days or 24 hrs to get a
  statistical estimate

• by the addition of various admixtures,
  including superplaticizers,
• which increase the fluidity of the concrete

9
Q5

• 50-100 mm for normal work

• 25-50 mm for mass and lightly reinforced concrete

• when we need high workability

• difficult formwork - thin walls, lots of detail

• lots of closely spaced reinforcement

• when we dont need high workability

• mass concrete - light reinforcement

• dams, footings, paths

10
Q6

• voids in the concrete - honeycombing

• reduce the strength of concrete

• through proper compaction - by adequate vibration

11
Q7

• the initial hardening of the concrete - up to
  2hrs after placing

• the process of gaining strength over time

• concrete gains strength with age

• gains almost all its strength after 1 year

12
Q7

• the procedure for retaining moisture in concrete
  for several days

• it prolongs the process of hydration

• it improves the compressive strength and reduces
  the
• incidence of drying shrinkage cracking

• curing should be maintained for a minimum period
  of 3 days

• it should begin no more than 3 hours after
  finishing

13
Q8

• polythene sheeting secured over the concrete to
  prevent evaporation

• curing compounds applied by spray or roller

• ponding water on the surface where practicable

• steam curing