Nature Of Concrete

Question 1

What are the three ingredients that make up cement

Answer 1

This is formed by baking calcium (limestone – calcium carbonate) and silicon (clay) until it forms nodules called clinker. This clinker is then ground with gypsum to a fine powder to form the highly alkaline cement.

Question 2

Why is aggregate important in the making of concrete

Answer 2

Aggregate is a hard material that is responsible for the compression strength & stiffness of concrete.
Aggregates may be:
 Quarried from land sites.
 Manufactured by crushing larger rocks.  Harvested from beaches.
If from a beach, it may contain significant levels of salt, & must be washed thoroughly in fresh water

Question 3

Why is salt contamination bad

Answer 3

If salt-contaminated aggregate is used:
 May accelerate the corrosion of built-in
steelwork.
 Salt crystals on the surface of the concrete, is
termed Efflorescence.
 Salt crystals will not form underwater so, their presence in the splash-zone is not definitive.

Question 4

Explain different types of aggregates and what the make

Answer 4

Aggregates’ size & shape are important factors that determine the mechanical properties of the concrete.
 Sand - fine aggregate, less than 5mm dia.
 Mortar - If only sand is used it will be more
flexible, but will have less strength than
concrete made with sand & gravel.
 Gravel - coarse aggregate more than 5mm dia.
 Concrete - made with sand & gravel is stronger
than mortar but, stiffer & less flexible.

Question 5

What is hydraulic cement

Answer 5

Hydraulic Cement is the most common type of cement.
Formed by baking limestone & clay into clinker. Then ground with gypsum to a fine powder.
This reacts with water in a process called Hydration. This forms a binder that holds the aggregate together to form a mass of material – the mixture is said to Set.

Question 6

What is the correct ratio of water to cement

Answer 6

Cement to hydrate properly, 25% water is required.
 Often, 40% or more is added to improve
workability.
 Better workability means concrete flows more easily through pipes & into moulds.
Pouring of concrete at this stage is termed Placement.

Question 7

Why is too much water bad

Answer 7

Excess water forms capillary voids within the concrete that must be lost from the bulk of the material.
This happens when:
 The water migrates to the surface - Bleed Water.
 By evaporation from the surface.
 The less excess water in the mix; the stronger it
is.
 Beneficial in terms of strength, to limit water to
a minimum, while still maintaining good
workability.

Question 8

Why is too much water bad 2

Answer 8

 Too high a water content, results in a
porous/weak material.
 As excess water migrates to the surface,
volume shrinkage occurs in the outer layers.
 This effectively ’tightens’ the surface, forming
a skin of tensile stress, which leads to Plastic
Shrinkage Cracking.
 Excessive water may also lead to settlement of
aggregate around the steel reinforcement,
which may result in Plastic Settlement
Cracking.

Question 9

Why is too much heat bad

Answer 9

 In a large pour, the heat produced causes the
sub-surface material to expand.
 Cooling at the surface causes contraction.  A skin of tensile stress forms, which may
cause Thermal Cracking.
 As concrete hydrates, it moves from a liquid
state, becoming increasingly viscous, until it
sets as a solid.
 The alkalinity stabilises at a pH 12.5.

Question 10

Explain curing

Answer 10

After concrete has set, a period of Curing continues, during which, silicon polymers form.
 The initial cure may last from days to weeks.
 Temperature & evaporation must be
controlled.
 70% of its final strength after the initial cure.
 90% of its final strength after one month.
 Full maturity after around 27 years.

Question 11

How do you slow rate of evaporation

Answer 11

 Sometimes a spray system is used to slow the
rate of evaporation to reduce the likelihood of
cracking.
 Concrete slabs may be flooded with water
during this time for the same reason
(Ponding).
 Walls may be painted with curing compounds
that form an evaporation-retarding
membrane.

Question 12

What are admixtures

Answer 12

Admixtures - chemicals added to the mix to enhance its properties, change its workability or hydration rate.
 Plasticizers - added to reduce the water
content.
 Accelerators/Retarders - used to change the
rate of hydration to suit the construction
method.
 Water-retardant - added to enhance resistance
to penetrating water.