IMPORTANCE OF PARTICLE PACKING DENSITY FOR PRECAST CONCRETE PRODUCTION

For precast concrete construction, shortest de-moulding time has a big significance. This needs concrete with high early strength rather than high early set. There are ways to achieve high early strength.

One Grade Higher:

Conventionally, increasing the OPC content per cubic meter was resorted to get high early strength. This increases the early strength and consecutively later (28 days) strength. In other words, M50 grade of concrete is used for M40 grade requirement. It imparts increase of heat of hydration, shrinkage and cost of the materials.

Low W/C Ratio:

Another approach is reducing the water to cement ratio to achieve high strength of concrete. This reduces the workability of the mix or increases the demand for high range water reducing admixtures. High dosage of powerful admixtures attracts the risk of bleeding in the concrete. Concrete mixes with low W/C and manufactured sand with high amount of 150 micron passing particles pose higher challenges of shrinkage cracks and poor slump retention.

Chemical Accelerators:

These additives do accelerate the initial & final setting of the cement paste. Most popular is calcium chloride based accelerators but it poses the risk of induced corrosion of steel reinforcement in the precast elements. Sometimes, accelerating additives tend to arrest the flow or slump of the concrete resulting in poor compaction & surface finish of precast elements.

Particle Packing Density Concept:

Significance of “all in grading curve” & well graded aggregates for good quality concrete is a well-known concept.

Similar concept is applicable for the paste component of the concrete. If we can blend the cement with micro fine particles with right particle size distribution; one can achieve optimum density of cement paste. Average particle size (d50) of OPC is 20 to 30 microns whereas d50 of micro fine particles is 4 to 5 microns. Some types of micro fine particles fill up the voids between

cement particles pushing the entrapped water out. This “released” water helps in improving the slump or facilitating further reduction in water content of the mix and hence improvement in the compressive strength.

Majority of micro fine materials like silica fume, ultra-fine fly ash, rice husk ash, meta-kaolin, etc. works pozzolanically unlike hydraulic action of OPC.

Higher specific surface area, results in high water demand or super plasticiser dosage in concrete mix. Recent technological advancements have resulted in advent of Alccofine kind of micro fine materials which works both pozzolanically & hydraulically. Also due granulometry & surface texture of these particles; water demand is reduced. Hydraulic nature of Alccofine particles facilitates quick hardening of concrete and improvement of compressive strength at early age. This results in overall cost saving for precast manufacturer maintaining the enhanced quality level of concrete elements.