Define Concrete and discuss the content, properties and usage of concrete in construction

1. Define “Concrete” and discuss the content, properties and usage of concrete in construction.

Answer:

Concrete = A mixture of cement, water and aggregates. The cement and water combined to form a paste and when hardened, binds the aggregates particles together to form a monolithic whole. The cement and water hardened by a chemical reaction, called hydration.

Basic desired properties of concrete =

(a) good workability when the concrete is fresh (newly mixed); i.e, the concrete can be transported from the mixer, handled, placed in the moulds (or formwork) and compacted satisfactorily;

(b) high strength and hardness;

(c) adequate durability.

Usage = beam, column, wall, foundation, slab and etc.

2. Some other materials (besides concrete) can also be made using cement as part of their constituents. Please provide the example and its application.

Answer:

i) Grout = cement + water

ii) Mortar = cement + fine aggregate + water

Give examples of usage for Grout and Mortar:

Grout ==> patching concrete, re-surface concrete structural members, to fill space and or cavities and provide continuity between structural elements.

Mortar ==> brick-laying, bedding of small concrete component such as ridge tiles, and for the sealing of certain types of paving, screed is a mortar applied to a floor, render is a mortar applied to a wall (ratio 4 Sand:1 Cement)

(Source: http://www.axp.mdx.ac.uk/~john49/cemfaq.htm, http://www.pavingexpert.com/mortars.htm,

http://www.nrmca.org/aboutconcrete/cips/22p.pdf,

http://www.maconline.org/tech/materials/grout/whatisgrout/whatisgrout.html )

3. Explain Hydration in concrete. (you might use a graph to help your explanation)

Answer:

– the chemical combination of cement and water

Heat is given off during the setting and early hardening of cement paste (concrete).

Fig 13.2 shows a typical graph on the heat given off during hydration of cement. Immediately after mixing, there is a high but short peak (A) of heat released, lasting only a few minutes. This quickly declines to a low constant for the dormant period when the cement is relatively inactive; this may last for up to 2 or 3 hours. The rate of heat released start to increase rapidly at a time corresponding to the initial set. This reaches a peak (B) sometime after the final set. The reactions then gradually slow down, with sometimes a short spurt of heat release after 1 or 2 days giving a narrow peak (C).

4. Discuss other types of Portland Cement apart from OPC and give examples of usage for each of them.

Answer:

Rapid-hardening Portland Cement

In cold weather, the high rate of heat output helps to prevent damage by frosts. However, the higher rate of heat output in the early stages of hydration will increase the risk of thermal cracking in large pours.

Ultra-high early strength Portland cement

In this cement, the cement clinkers are ground to extremely fine particles. Concrete made with this cement achieves the 3-day strength of rapid-hardening cement in 16 hours and its 7-day strength in 24 hours. Even higher early strength can be achieved by steam curing.

Low-heat Portland cement

This cement is required for thick concrete work, where the heat generated by ordinary cements will be excessive and lead to serious cracking.

Sulphate resisting Portland cement

This cement is used where there is presence of sulphates (sulfates) from external sources, such as in industrial wastes, sulphate bearing soils and in sewage wastes.

White Portland cement

Use when required for appearance.

Portland blast-furnace cement

It has good resistance to dilute acids and sulphates and can be used for construction in sea water.

Pozzolanic cement

Low-heat cement – initially slower in hardening but attains strength equal to that of opc after 3 months. Good resistance to sea water and sulphates (sulfates).

5. List out and briefly explain the general classification of aggregates.

Answer:

Heavy Aggregate – Effectively and economically used for radiation shielding and for weight coating of submerged pipelines.

Normal Aggregate – Suitable for most purposes. Generally consists of various combinations of naturally occurring sands, gravels and stones and of different sizes of crushed rocks.

Lightweight Aggregate – For the production of lightweight concrete. Bulk density ranges from 350 to 850 kg/m3 for coarse aggregates and from 750 to 1100 kg/m3 for fine aggregates. Lightweight aggregates can be processed from natural materials (for example expanded clay or expanded shale), processed by-products (for example foamed slag or sintered pulverised fuel ash)

6. Briefly explain the procedure for compaction test. Provide sketches.

Answer:

7. Explain admixture and their usage.

Answer:

Admixtures are defined as materials or substances other than aggregates, cement and water which are added to the concrete batch immediately before or during mixing.

It is used to modify one or more of its properties in the plastic or hardened state.

8. Explain these terms of aggregates:

Answer:

i) Porosity

Aggregates contain pores which can absorb and hold water

ii) Toughness

Defined as resistance to failure by impact.

iii) Hardness

Defined as resistance of an aggregate to wear.

iv) Soundness

The ability of aggregate to resist excessive changes in volume

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