Concrete

part 2

What ar the properties of fresh concrete (9)

1. Workability
2. Voids
3. Segregation
4. Bleeding
5. Shrinkage
6. Compaction and Honeycombing
7. Grout Loss
8. Curing
9. Surface Finishes

What is the workability of concrete

the ability of concrete to flow in a mold or
formwork, through congested
reinforcement, the ability to be compacted to a
minimum volume, to perform
satisfactorily in some transporting operation or
forming process.

Factors affecting workability

• Water content - higher water content leads to higher workability
• Fineness of cement - finer cement, faster loss of workability
• Aggregate shape -more angular (crushed) more water demand
• Grading - uniform grading leads to better workability
• Admixtures - accelerators / retarders / plasticisers
• Time - timing of cement hydration
• Temperature - higher temperatures lead to loss of workability

When are air voids important

sufficient air voids in concrete ensure sufficient freeze-thaw resistance - this improves durability in frost and marine climates.

How are air voids added and removed from concrete

Air-entraining admixtures - Generates evenly dispersed air bubbles in the mix.

Air voids in concrete may be
removed naturally by agitation
(vibrations etc.) during transport

What is segregation

Separation of constituents of the concrete mixture
caused by excessive handling or vibration, or improper
gradation of aggregates.

Coarse aggregates separate towards the
bottom and cement paste forms a thin layer on the top

How to reduce segregation

Increase air content
Reduce water-cement ratio
Increase rate of hydration

Causes of bleeding in concrete

Too much water (high water-cement ratio)
• Less fines in the mix (cement, sand)
• Poor grading of aggregates
• Overworking of concrete

What happens due to shrinkage

Cracking that occurs when the concrete is still
very young, caused my moisture loss from the surface, If bleed water replaces the water lost, cracking
is less likely

What is evaporation increased by

Low relative humidity
• High surface temperatures
• High wind speed

What is shrinkage minimised by

Moist aggregates
• Cover concrete
• Erection of windbreaks & sunshades
• Add plastic fibres

what is honeycombing

Honeycombing occurs when voids are
left in concrete due to failure of the
mortar to effectively fill the spaces
among the coarse aggregate particles

What causes honeycombing

Under/over compaction

What is curing

Curing is the maintenance of satisfactory
moisture content and temperature in
concrete for a period of time immediately
following placing and finishing so as to
develop the desired properties of concrete