SELF-CURING  CONCRETE

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.

 

TRADITIONAL METHODS OF CURING :

-Water curing

-Membrane Curing

PROPER CURING ENSURES :

-Increased Concrete Strength

-Increased Concrete Abrasion Resistance

-Less Chance of Concrete Scaling

-Less Chance of Concrete Cracking

WATER CURING

MEMBRANE CURING

SELF-CURING

DEFINITION:

 Self-­curing concrete is designed to hold water and reduce water evaporation while increasing water retention capacity. Hydrophilic water-­soluble polymers can be used as self-­curing agents in concrete as they can absorb water and keep the surrounding medium moist so that water can be used later by cement.

 

NEED OF SELF-CURING:

When the mineral admixtures react completely in a blended cement system, their demand for curing water (external or internal) can be much greater than that in a conventional ordinary Portland cement concrete. When this water is not readily available, due to depercolation of the capillary porosity, for example, significant autogenous deformation and (early-age) cracking may result. 

Due to the chemical shrinkage occurring during cement hydration, empty pores are created within the cement paste, leading to a reduction in its internal relative humidity and also to shrinkage which may cause early-age cracking. This situation is intensified in HPC (compared to conventional concrete) due to its generally higher cement content, reduced water/cement (w/ c) ratio and the pozzolanic mineral admixtures (fly ash, silica fume). The empty pores created during self-desiccation induce shrinkage stresses and also influence the kinetics of cement hydration process, limiting the final degree of hydration. The strength achieved by IC could be more than that possible under saturated curing conditions. 

Often specially in HPC, it is not easily possible to provide curing water from the top surface at the rate required to satisfy the ongoing chemical shrinkage, due to the extremely low permeabilities often achieved.

 

INTERNAL CURING :

Internal curing refers to the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing Water.

 

MATERIALS USED FOR INTERNAL CURING :

Lightweight Aggregate (natural and synthetic, expanded shale),

LWS Sand (Water absorption =17 %)

LWA 19mm Coarse (Water absorption = 20%)

Super-absorbent Polymers (SAP) (60-300 mm size)

SRA (Shrinkage Reducing Admixture) (propylene glycol type i.e. polyethylene-glycol)

Wood powder

 

CHEMICALS TO ACHIEVE SELF-CURING :

Some specific water-soluble chemicals added during the mixing can reduce water evaporation from and within the set concrete, making it ‘self-curing.’ The chemicals should have abilities to reduce evaporation from solution and to improve water retention in ordinary Portland cement matrix.

 

SELF-CURING AGENTS:

 High-performance self-curing agent  is added to concrete during mixing. The self-curing agent can absorb moisture from atmosphere and then release it to concrete. The self-curing concrete means that no curing is required for concrete, or no external supplied water is required after placing. The properties of this self-cured concrete  are at least comparable to and even better than those of concrete with traditional curing.

 

CHEMICALS USED FOR SELF-CURING:

 

Polyethylene glycol (PEG)

Polyacrylamide (PAM)

FX-338 Acrylic Latex Bonding Agent

Sodium gluconate

Super-absorbent Polymer (SAP)

BENEFITS OF SELF CURING:

 

·    Reduces autogenous cracking,

·    largely eliminates autogenous shrinkage,

·    Reduces permeability,

·    Protects reinforcing steel,

·    Increases mortar strength,

·    Increases early age strength sufficient to withstand strain,

·    Provides greater durability,

·    Higher early age (say 3 day) flexural strength

·    Higher early age (say 3 day) compressive strength,

·    Lower turnaround time,

·    Improved rheology

·    Greater utilization of cement,

·    Lower maintenance,

·    use of higher levels of fly ash,

·    higher modulus of elasticity, or

·    through mixture designs, lower modulus

·    sharper edges,

·    greater curing predictability,

·    higher performance,

·    improves contact zone,

·    does not adversely affect finishability,

·    does not adversely affect pumpability,

·    reduces effect of insufficient external curing.

 

CONCLUSION:

             The internal curing (IC) by the addition of saturated lightweight fine aggregates is an effective means of drastically reducing autogenous shrinkage. Since autogenous shrinkage is a main contributor to early-age cracking, it is expected that IC would also reduce such cracking. An additional benefit of IC beyond autogenous shrinkage reduction is increase in compressive strength. As internal curing maintains saturated conditions within the hydrating cement paste, the magnitude of internal self-desiccation stresses are reduced and long term hydration is increased.

               Prime applications of IC could be: concrete pavements. precast concrete operations, parking structures, bridges, HPC projects, and architectural concretes. Concrete, in the 21st century, needs to be more controlled by the choice of ingredients rather than by the uncertainties of construction practices and the weather. Instead of curing through external applications of water, concrete quality will be engineered through the incorporation of water absorbed within the internal curing agent.