Question

b) Describe how the water reducer and accelerator admixture affect the quality of fresh and hardened concrete. (CLO2; PLO1; C3) (6 marks) c) It is important to control the workability of concrete. Please elaborate this statement in relation to concreting work on site and state two (2) factors that can affect concrete workability (CLO2; PLO1; C2) (7 marks) d) In placing concrete, briefly explain the precautions that must be taken to ensure a lasting surface free from defects. (CLO2; PLO2, C2) (8 marks) c) Why is it necessary to do compacting on fresh concrete? What would happen if concrete is not compacted? Briefly explain how to compact concrete. (CLO4; PLO6; C2) (7 marks)

Answer 1

and acceleration on Reducer 6) Affect of water i) fresh concrete - increases when tempera 1. workability of len and it is fresh concrete vice versa, 2. seggregation and bleeding om by addition du super plasticgeva Bleeding increases considerably but it did not have any adverse effect on the finishing of concrete. 3. Initial & final setting time ay Initial setting time in delayed due to the addition of super - plasticazers. f) Hardened concrete => 1.wmpressive shength + super plasticizere increase the early strength as well as strength after 28 days. 2. Freeze - thaw Resistanu will be deterioated by the addition to super plasticizera. 3. chloride Resistance was not effected. Normal water reducing admixtures have no significant affart on the hydration of rument if added to concrete at normal doen However, the improved dispersion of cement grain & caure of stiffening time (initial setting time). e ment grain cause sharlening

c) workability The concrete mix proportions chosen should be such that the concrete in of adequate worlability for the placing conditions of the concrete and can be properly comparted with the mear available → In the very low worleability category, shick controle in neccessory → The property is work ability determines the amount of useful internal work, necessary to produu full compaction ie, the workability in the amount ob energy to overcome frictim that while comparting while meine SE - It is the relative case with which conorete can be mixed transported, moulded and compacted. 1. water content only increase of water content has no effect on workability, be cause shength decreases. 2. Aggregate sige - size increases then word ability increases. 3. surface texture ob Aggregata => smooth → High workability 4. shape of Aggregate - Round=> more workable.

0 T 8° 0m 1 1 tu au avu- ..a The precautions to be taken during the placing of concrete are as follows: (i) The formwork or the surface which is to receive the fresh concrete should be properly cleaned prepared and well-watered.

e compaction of concrete → The main aim ao comparting of concrete is to eliminate air bubbles and thus to give maximum density to the concrete. conc ty to the An initimate contact between concrete and Reinforcement in ensured by propper compartion. → The importance de consolidation of concrete can be seen from fact that a presence of 54. ob voide reduux 30). strength of concrete → The process of compaction du concrete can be carried out either with hand or with the help of vibratorn. Methoda do compacting concrete => 1. Tamping 2. wibrating 3. vi bro pressing 4. Rolling with Rollers 5. centrifugation for piles books 6. High pressure & shock → over comparting may lead to bleeding.