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WORKSHEET 4 CONCRETE

WORKSHEET 4 CONCRETE. a). b). what are the two main properties that concrete must have?. how do we achieve them?. Q1. (i) strength. (ii) workability . by using an acceptable water / cement ratio . strength also depends on: mix proportions - more cement, stronger the concrete

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WORKSHEET 4 CONCRETE

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  1. WORKSHEET 4CONCRETE

  2. a) b) what are the two main properties that concrete must have? how do we achieve them? 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. c) what is the main factor which affects them and how? 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. a) b) what are the main ingredients of concrete? what are the main proportions (by weight) of: Q2 cement coarse aggregate (gravel) fine aggregate (sand) water additives (i) cement to aggregate? 1:4 to 1:7 (ii) water to cement? 0.4:1 to 0.6:1

  5. a) b) what happens when water and cement are mixed? why do we use aggregate? 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. c) d) why do we need to grade the aggregate? what is the role of sand? 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 doesn’t have to be continuous • to fill the gaps between the coarse aggregate • sand is cheaper than cement

  7. a) b) why do we want concrete workable? how do we measure workability? Describe 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. c) d) how do we measure concrete strength? When? how can one reduce the water/cement ratio yet keep the concrete workable? 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. a) b) c) what slump indicates good workability? when would we use high slump concrete? when would we use low slump concrete? 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 don’t need high workability • mass concrete - light reinforcement • dams, footings, paths

  10. a) b) c) what are air pockets? what do they do? how do we get rid of them? Q6 • voids in the concrete - honeycombing • reduce the strength of concrete • through proper compaction - by adequate vibration

  11. a) b) c) what is meant by setting of concrete? what is meant by hardening of the concrete? how does age affect the strength of concrete? 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. c) what is meant by curing of concrete? 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. describe some methods for curing concrete? 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

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