You are responsible for the construction of a concrete slab for a residential ga

Question

You are responsible for the construction of a concrete slab for a residential garage located in Coffs Harbour. It is summer and there is a high chance of rain on the days scheduled for the pour. Question 1 (15 Marks) a) Describe what type of cement and aggregate; the required concrete strength, slump and cover that should be used for this project? (4 marks) b) Describe the required mixing, batching, handling, transporting, placing, compacting, curing and finishing operations of concrete for the slab. (9 marks) c) Do you recommend any type of admixture (specify) for this purpose? (2 marks) Note: In your answers, you can refer to American, British or Australian Standards

Explanation / Answer

1. Type of Cement and Aggregates used in High Rainfall Areas:-

(i) Hydrophobic cement-it is used for heavy rain fall areas.

(ii) Granite small size used (2-5mm) as a aggregates.

-its 28 days strength is equal to 28 days portland cement strength.

2.(A)Mixing of concrete:

(a)Hand mixing: Mixing done on a steel plate or on a hard surface manually. Used when the quantity of concrete needed is in a small quantity. First the sand and cement taken in correct proportion and mixed in dry state. Then the coarse aggregate is added and mixed well using shovels. He predetermined amount of water is added then and mixed till the colour is homogeneous and the mix is workable.

(b)Machine mixing: Concrete mixed by means of mechanical mixer. The ingredients are mixed well in drums. The drums made of steel with the blades in it at an inclined manner. The mixers are usually electrically operated. The coarse aggregate is fed first then the fine aggregate and last the cement. The water is added after the aggregate and the cement is thoroughly mixed. After adding the water the ingredients mixed till a uniform colour is achieved [not less than 2 minutes].

(B)Transporting concrete : The concrete should be handled from place of mixing to the final deposit as fast as possible without segregation. If segregation happened then concrete should be mixed at the site once again. During hot or cold climate the concrete should be transported in deep containers to avoid loss of water or heat.
(c)Placing concrete: The concrete should be placed before the setting starts. Before placing the forms should be rigid and well oiled. Concrete should be poured into the forms at one point and uniformly spread before the compaction.

(D)Compacting concrete:Compacting remove the entrapped air and the dense arrangement of the constituents. The density, strength and the durability of concrete depend on the compaction. There are three types of vibrators that can be used for compaction namely: internal vibrator, external vibrator and surface vibrator. Mechanical compacting is used when the mix is stiff. Mechanical compacting are three types : rodding, tamping and hammering. Over vibration is harmful.

(e)Curing concrete:Curing is to keep the concrete damp so as to gain the strength. He period of curing depend on the temperature, humidity and wind velocity. The normal period is 7 to 10 days. The methods adopted are wetting the surface, impounding water in earthen to sandy bunds, curing using steam or hot water etc.

3. Admixture-Hydrophobic cement contains admixtures which decreases the wetting ability of cement grains. The usual hydrophobic admixtures are acidol, naphthenesoap, oxidized petrolatum.
These substances form a thin film around cement grains. Hydrophobic cement is obtained by grinding portland cement clinker with a film-forming substance such as oleic acid in order to reduce the rate of deterioration when the cement is stored under unfavourable conditions.

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