School of Vocational Engineering, Science and Health MIET 7785 and MIET 7793 Reinforced Concrete Design and Drawing Tutorial Week 1 Solution 1. Define water/cement ratio and provide an example. Ans: wt. of water/ wt. of cement in concrete. For example, 0.5 water/cement ratio means 10 kg water to 20 kg cement. 2. Which concrete is likely to have greater strength, the one with water cement ratio of 0.5 or 0.6? Ans: 0.5 because less water. For example, 5 kg water instead of 6 kg water to 10 kg cement. 3. Why would you add more water to concrete on site? Ans: to increase workability. 4. How does the addition on site of water to concrete affect your guarantee of strength from the concrete supplier? Ans: Adding water make it more workable, but reduce strength and cause bleeding. 5. What does a slump test indicate? Provide an example. Ans: workability of concrete and water content. 6. When concrete is delivered to site, how much air does it contain? Does this adversely affect the concrete and if so, by how much? Ans: 4-5% and it reduces 20% strength of concrete. 7. Why is it necessary to vibrate the concrete and which are the effects of vibration of the concrete? Ans: to compact and reduce air in the concrete mix. Vibrator increases the lubrication around it and causes the concrete to settle down by friction between aggregates breaks down. Air bubbles travel to the surface through concrete around the vibrator. 8. List methods used to vibrate the concrete? Ans: Poker vibrator, beam vibrator, vibrating screed. In case of > 200 mm or more thickness of concrete, use poker then beam vibrator. Use poker vertically and do not touch the bottom formworks, remove it slowly from concrete. For flat surfaces beam vibrator and vibrating screed suitable. 9. When vibrating concrete slabs, at approximately what spacing should you insert the poker? Ans: 500 mm 10. When vibrating layers, approximately how far should you insert the poker into the lower layer? Ans: 100 mm 11. Which are the effects of poor vibration of concrete? Ans: air remains in the concrete; make low cohesive and honeycomb structure, segregation of concrete. 12. What does the term “cover” mean? Ans: thickness of concrete from inner steel reinforcement, ie distance from finished surface to the nearest inner steel reinforcement. 13. Explain ways to ensure that steel reinforcement is placed accurately? Ans: use chair/spaces, prefabricated blocks, keep gaps between form and reinforcement, tie up all reinforcement. 14. In which situations will more cover than usual be required? Ans: near to industrial areas or coastal areas. 15. Explain the concept “concrete bleeding”? Ans: water comes up/ flows to the surface of the concrete through tiny pores and channel inside the concrete. 16. Explain the effect of trowelling concrete while there is still water on the surface? Ans: surface concrete will be weak, less resistant to wear and will produce lots of dust on the surface. 17. Explain the difficulties encountered when placing concrete during hot weather conditions, and ways to alleviate these? Ans: evaporation and chemical reaction get faster, quick stiffening and reduce time for placing and finishing of concrete, reduce strength and make cracks. To alleviate these problems, all formworks and reinforcing must be completed, easy access to the site, enough manpower, standby other workers (eg carpenter to set/adjust formworks). Fill up checklist including pre-wetting. Avoid any delays to place concrete. 18. List the worst weather conditions for pouring concrete? Ans: >30° C temperature, windy, low relative humidi ty.
