P.Hema Durga Rao

By

• Need of curing

• What are the different types of methods

• Prefabrication

• What is curing ?

Curing can be defined as a procedure for insuring the hydration

of the Portland cement in newly-placed concrete. It generally

implies control of moisture loss and sometimes of

temperature.

Saturated

Saturated

Evaporation from water surface

Concrete

Water supplied from external source

• Hydration reaction of cement with water

• Loss of water by evaporation can be prevented

• Self-desiccation (desiccation = dry ness due to loss of water)

cause internal water loss

• Maintain conductive Temperature

• For completing of Hydration reaction

• For capillary segmentation

• It is most important in producing a strong, durable, and

watertight or impermeable concrete.

• Capillary porosity

capillary pores larger in size compared to that of gel pores, will allow water and hence aggressive chemicals into the concrete, which adversely affects the durability of concrete in long term

• C-S-H gel porosity

Less impact on concrete strength and durability

Fig: a)Capillary pores interconnected b) Partially connected capillary pores

• Inadequate strength

• Drying shrinkage and cracks at high temperatures

• Poorly segmented capillaries

• Immersion

• Ponding

• Spraying

• Covering with wet sand

• Wetted Hessian (gunny bags)

• Membrane curing

• Water proof plastic sheeting

• Steam curing

• Immersion

The precast concrete items are normally immersed in curing tanks

for certain duration.

• Ponding

Pavement slabs, roof slab etc. are covered under water by

making small ponds.

Fig: Ponding of slab

• Spraying

Vertical retaining wall or plastered surfaces or concrete

columns etc. are cured by spraying water.

• Wet covering

such as wet gunny bags, hessian cloth, jute matting, straw etc., are

wrapped to vertical surface for keeping the concrete wet.

Covering with Gunny bags

Curing with water proof paper or sheet

Hessians for concrete curing

• Sometimes concrete works are carried out in places where there is

acute shortage of water.

• Curing does not mean only application of water; it means also

creation of conditions for promotion of uninterrupted and

progressive hydration.

• The quantity of water, normally mixed for making concrete is more

than sufficient to hydrate the cement, provided this water is not

allowed to go out from the body of concrete. For this reason, concrete

could be covered with membrane which will effectively seal off

the evaporation of water from concrete.

• Sometimes, such films have been used at the interface of the ground

and concrete to prevent the absorption of water by the ground from

the concrete.(bituminous compounds, polyethylene or polyester film,

waterproof paper, rubber compounds etc.)

• Membrane curing is a good method of maintaining a

satisfactory state of wetness in the body of concrete to

promote continuous hydration when original water/cement

ratio used is not less than 0.5.

• Curing compound is an obvious choice for curing canal

lining, sloping roofs and textured surface of concrete

pavements.

• No doubt that curing compounds are not as efficient and as

ideal as water curing. The efficiency of curing compounds can

be at best be 80% of water curing.

Membrane curing by spraying

• The development of strength of concrete is a function of not only time but also that of temperature. When concrete is subjected to higher temperature it accelerates the hydration process resulting in faster development of strength.

• Subjecting the concrete to higher temperature and maintaining the required wetness can be achieved by subjecting the concrete to steam curing.

• Advantages of faster attainment of strength read it in rdg. Material.

• subjecting the concrete to higher temperature is done in the following manner:

a) Steam curing at ordinary pressure.

b) Steam curing at high pressure.

c) Curing by Infra-red radiation.

d) Electrical curing.

• Steam curing at ordinary pressure:

This method of curing is often adopted for prefabricated

concrete elements.

An accelerated hydration takes place at this higher

temperature and the concrete products attain the 28 days

strength of normal concrete in about 3 days.

Effect of curing temperature

on strength gain relative to

28-day strength of concrete

at 23°C (73°F)

Relation between strength, curing age at different temperatures

Relation between strength and curing temperatures at 1 & 28 days

Relation between strength and curing age at different temperatures

• The phenomenon of Retrogression of strength explains that

faster hydration will result in the formation of poor quality gels

with porous open structure.

• Concrete subjected to higher temperature at the early period

of hydration is found to lose some of the strength gained

at a later age. Such concrete is said to undergo

“Retrogression of Strength”.

Compressive strength Vs. Age

• Steam curing often adopted for precast elements, specially

prestressed concrete sleepers. Concrete sleepers are being

introduced on the entire Indian Railway.

• For rapid development of strength, they use special type

of cement namely IRST 40 and also subject the sleepers to

the steam curing.

• A steam-curving cycle consists of:

• an initial delay prior to steaming,

• a period for increasing the temperature,

• a period for retaining the temperature,

• a period for decreasing the temperature.

1 Initial delay prior to steaming 3 to 5 hours 2 Temperature increase period 2½ hours 3 Constant temperature period 6 to 12 hours* 4 Temperature decrease period 2 hours

Fig: A typical steam-curing cycle

Typical strength of steam cured concrete

• Steam curing High pressure:

The superheated steam at high pressure and high

temperature is applied on the concrete. This process is also called “Autoclaving”. This process is used for lightweight concrete products.

Advantages:

i) The strength developed does not show retrogression.

[ Why because? At high temperature the amorphous calcium

silicates are probably converted to crystalline forms. Probably due to high pressure the frame work of the gel will become more compact and dense. ]

ii) High pressure steam cured concrete exhibits lower drying shrinkage, and moisture movement.

iii) higher resistance to sulphate attack, freezing and

thawing action and chemical action. It also shows less efflorescence.

• Minimum days of curing required for segmentation of capillary

depends on W/C fro W/C of 0.4 3 days

W/C of .7 1 year

• For Dry and Hot conditions 10 and 14 days.

• As per IS 456: At least 7 days for OPC (ordinary Portland

cement)

• For Blended/concrete with mineral Admixture require 10 days.

• Concrete must be cured till it attains about 70% of

specified strength.

• Mass concrete, heavy footings, large piers, abutments, should

be cured for at least 2 weeks.

• Pozzolanic cement or concrete admixed with Pozzolanic

material is required to be cured for longer duration.

• Strength as a function of hydration time and temperature.

• The concept of “Maturity”, defined as function of product of curing

time „t‟ and curing temperature „T‟ [i.e. Maturity = f ( t, T)]

(or)

• As an equivalent age for concrete cured at some specified

temperature.

Fig:

Influence of curing temperature on the

strength of concrete cured at 10 °C (50 OF)

for the first 24 hours before storing at the

temperature

Fig: Compressive strength as a function of maturity

• Structural components are standardized and produced in

plants in a location away from the building, and then

transported to the site for assembly.

• main features of this construction process are:

The division and specialization of the human workforce.

The use of tools, machinery, and other equipment, usually

automated, in the production of standard, interchangeable parts

and products .

Advantages:

• Parallel activities and reduced time.

• Standardization is possible in this case. The advantage of

modular co- ordination can be realized in the long run.

• Modular Coordination:

Modular coordination is a set of dimensional rules.

Reduces variability of dimension of building components.

Uses a basic measurement(modular size) and restricted component

sizes.

Disadvantages of pre-cast concrete:

i) The initial cost of setting up such precast concrete yard is

very high.

ii) As it introduces monotony (i.e. all the structures in a particular

location look alike) may not be pleasant be look at.

Prefabrication Plants & Activities:

i) Permanent plants ii) Field plants iii) Fabrication on building site

• Three main groups of activities in prefabricated concrete

production:

i) Main production activity: activities such as preparation of

concrete mix, casting of mixed concrete, curing, inspection and hauling

of the hardened elements etc.

ii) Auxiliary production activities: This group of activities include,

raw materials procurement, storage and preparation, reinforcement

preparation, moulds preparation and

maintenance of equipment etc.

iii) Managerial, Technical and General Support: This group involves

engineering, production planning etc.

• Stationary moulds : activities such as reinforcement

preparation, casting, curing etc. will be carried out without

disturbing the position of the moulds with the aid of equipment.

• Movable moulds : at one place, the moulds are prepared,

at another place on the assembly line reinforcement is

installed, then the moulds moves to the next position on

the assembly line where mixed concrete is poured into the

moulds.

• Industrialized building construction.

• Bridges ( Segmental construction, Long or short line casting)

• Railway sleepers

• Industry and sport complexes

1. Cracks are formed in concrete why ?

Ans: i) Due to content in sand which absorbs water the concrete mixes

faces water stress and causes crakes. If curing is not done properly it can

happen .

Ii) So curing should be done as soon as concrete dries.

iii) Property burnt bricks should be use and they must be sued after 24

hours soaking.

2. What is compaction? Why it is done? What is the best way to do it?

Ans: when you mix various material together, there will be air entrapped

between them. It will effect the strength substantially.

The air has to be removed to attain maximum strength.

Steel rods or paddling sticks can be used but Mechanical

vibrators are ideal.

Any compacting device has to reach the bottom of form and

should be small enough to pass between reinforcement rods.