WORKSHEET 4

CONCRETE

Q1

a) what are the two main properties that concrete must have?

(i) strength

b) how do we achieve them?

by using an acceptable water / cement ratio

(ii) workability

strength also depends on: mix proportions - more cement, stronger the concrete quality of materials - strength of cement, aggregate proper vibration proper curing

Q1

c) what is the main factor which affects them and how?

water / cement ratio

the higher the water / cement ratio - concrete more workable

the higher the water / cement ratio (more water) - weaker the concrete

Q2

a) what are the main ingredients of concrete?

cement

coarse aggregate (gravel)

water

(i) cement to aggregate?

b) what are the main proportions (by weight) of:

1:4 to 1:7

fine aggregate (sand)

additives

(ii) water to cement? 0.4:1 to 0.6:1

Q3

a) what happens when water and cement are mixed?

• hydration

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

b) why do we use aggregate?

• to provide bulk - cheaper than cement

• aggregate reduces shrinkage - only cement shrinks

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

Q3

c) why do we need to grade the aggregate?

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

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

d) what is the role of sand?

• to fill the gaps between the coarse aggregate

• sand is cheaper than cement

• aggregates are considered well-graded if the various size of the particles are uniformly distributed - but doesn’t have to be continuous

Q4

a) why do we want concrete workable?

b) how do we measure workability? Describe

• 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

Q4

c) how do we measure concrete strength? When?

d) how can one reduce the water/cement ratio yet keep the concrete workable?

• by compression testing of test cylinders

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

• after 28 days to test full strength

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

Q5

a) what slump indicates good workability?

• 50-100 mm for normal work

b) when would we use high slump concrete?

• when we need high workability

c) when would we use low slump concrete?

• 25-50 mm for mass and lightly reinforced concrete

• difficult formwork - thin walls, lots of detail

• lots of closely spaced reinforcement

• when we don’t need high workability

• mass concrete - light reinforcement

• dams, footings, paths

Q6

a) what are air pockets?

• voids in the concrete - honeycombing

b) what do they do?

• reduce the strength of concrete

c) how do we get rid of them?

• through proper compaction - by adequate vibration

Q7

a) what is meant by setting of concrete?

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

b) what is meant by hardening of the concrete?

• the process of gaining strength over time

c) how does age affect the strength of concrete?

• concrete gains strength with age

• gains almost all its strength after 1 year

Q7

c) what is meant by curing of concrete?

• 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

Q8

describe some methods for curing concrete?

• 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