What is the problem of over vibration of fresh concrete?

For proper compaction of concrete by immersion vibrators, the vibrating part of the vibrators should be completely inserted into the concrete. The action of compaction is enhanced by providing a sufficient head of concrete above the vibrating part of the vibrators. This serves to push down and subject the fresh concrete to confinement within the zone of vibrating action.

Over vibration should normally be avoided during the compaction of concrete. If the concrete mix is designed with low workability, over vibration simply consumes extra power of the vibration, resulting in the wastage of energy. For most of concrete mixes, over-vibration creates the problem of segregation in which the denser aggregates settle to the bottom while the lighter cement paste tends to move upwards. If the concrete structure is cast by successive lifts of concrete pour, the upper weaker layer (or laitance) caused by segregation forms the potential plane of weakness leading to possible failure of the concrete structure during operation. If concrete is placed in a single lift for road works, the resistance to abrasion is poor for the laitance surface of the carriageway. This becomes a critical problem to concrete carriageway where its surface is constantly subject to tearing and traction forces exerted by vehicular traffic.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Should large diameter or small-diameter vibrators be used in compacting concrete?

There is a general rule regarding the size of vibrators in compacting concrete. The diameter of the vibrator should be a quarter of the wall thickness of the concrete being cast. In general, large-diameter internal vibrators have higher amplitude with lower frequency while small-diameter internal vibrators have lower amplitude with higher frequency. In particular, small-diameter internal vibrators with high frequency are normally used in compacting high slump concrete.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is the mechanism of formation of pedestrian level winds around buildings?

When a building blocks the wind blowing across it, part of the wind will escape over the top of the building. Some will pass around the edges of the building while a majority of the wind will get down to the ground. The channeling effect of wind for an escaping path, together with the high wind speeds associated with higher elevations, generates high wind speeds in the region at the base of the building. At the base level of the building, there are three locations of strong pedestrian level winds:

(i) Arcade passages – wind flow is generated by the pressure difference between the front and the back of the building.

(ii) At the front of the building – high wind is produced by standing vortex.

(iii) At the corners of the building – high wind is induced by corner flow.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is the function of cladding in concrete buildings?

Cladding refers to the external layer of the building which provides the aesthetic effect. Apart from the external appearance of the building, the main use of cladding is to protect the building structure from weather in one of the following ways:

(i) Cladding is made of impermeable materials so that rain water could only access the building through joints. However, properly designed joints with sealant could completely keep out the rain infiltration.

(ii) Cladding is made up of porous material (bricks) which absorbs rain during rainfall and subsequently dries out. Water could hardly penetrate into the building given that the cladding is of sufficient thickness with low permeability.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is the purpose of installation of resilient bearings in buildings?

When railway tunnels are built close to buildings, ground-borne vibration is transmitted to the building by means of compression and shear waves.

When structural members (e.g. wall) of a building have natural frequencies similar to the frequency at the source, the response of the structures would be magnified. This effect is even more significant when the building is designed with small number of movement joints. Consequently, the vibration can be felt inside the building and noise associated with such vibration is produced. To avoid this, vibration isolation can be implemented, sometimes by providing resilient bearings at column heads in buildings.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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