03-11-2016, 11:11 AM
1463835478-Tech.NoteonNACMay2013.pdf (Size: 663 KB / Downloads: 10)
The aggregates (sand and stone) in concrete are indispensable inputs for three reasons:
Without aggregates a cement cast element is liable to get shrinkage, leading to
cracks.
Strength of neat cement is around 70-90 MPa against popularly used concrete with
strength of 20-25 MPa. Thus aggregates help to moderate strength of concrete to
required grade.
Cost of aggregates is certainly lesser than that of cement, thus bringing down the
cost of concrete.
If there is a cementitious paste which can overcome shrinkage and cost-issues, despite
avoiding aggregate, why not accept such concrete? Moreover, if such concrete is lighter in
weight and higher in strength, increasing the factors of safety, is it not more desirable? Over
and above, if such concrete is made of industrial byproducts using lesser cement it is all the
more a welcome development.
This is exactly what the founder directors of this institute, Dr N Bhanumathidas and N Kalidas
have invented and patented in 2010. This wonder product is called No-Aggregate Concrete
(NAC), which means a concrete without sand and stone.
Background to the Invention:
Urge to use complementary cement materials out of industrial wastes and thrust to conserve
natural materials have given rise to NAC. As popularly known in building material research,
the inventors have developed FaL-G technology in 1989 introducing FaL-G as the
cementitious material by using all the inputs, fly ash, lime and gypsum out of industrial byproducts.
To prove its efficacy they did cast 2000 sft of slab way back in 1991 for their house that
consists of 15-18 ft long beams, where a couple of them are tie beams too. This has helped to demonstrate the potential of FaL-G as structural cement and inspiring confidence to its
use for bricks and blocks.
In their further studies for optimising FaL-G they developed various mixes using 4th
generation admixtures in result of which they encountered with a mix showing up absolute
workability, compaction and cohesiveness, all at 0.15 WCF (factor of Water/cementatious
material) with grade strengths of 55 MPa to 65 MPa. When they have looked at the broken
specimens the matrix was resembling somewhat close to that of ceramic with absolute porerefinement.
This has caught up their imagination that, using such material as structural media
would address the issues of durability in a single go in a holistic manner.
The inventors emphasize FaL-G as the ‘Ayurveda’ of cement-concrete formulation. More the
attrition greater is the potency. The edge runner serves this basic principle by tapping the
holistic performance of fly ash. There upon the special admixture does the wonder. The role
of anhydrite too cannot be ignored. All this collectively account for the high workability at the
least WCF in FaL-G as NAC.
Immediately they conducted some basic engineering studies applicable to concrete, patented
and declared the invention as No-Aggregate Concrete (NAC). In convention to their practice
of taking the lab work to field without delay, they did cast the dome of 10.5 ft dia with NAC
over the 2
nd floor of FaL-G Mansion, which was due for face-lift at that time, in addition to
other applications such as cantilever beam and shear wall. It is interesting to note that the 2-
year specimen has shown up a compressive strength of 110 MPa, almost double the
strength over its 28-day strength.
They have also christened their product as Nano Concrete. In a given material and matrix,
smaller the particles higher the bond, so much so the energy required to snap such bond.
Nano science works on this premise, and Nano Concrete performs very much within this
frame. Micron and sub-micron particles of fly ash that develop adhesive bond at early ages
do attain cohesive bond with progress of age due to reaction of lime and other mineralogical
hydrates on surface of particles. This is manifested in Nano Concrete developing over 100
MPa strength over an year as against 55 MPa at 28-day. Such quantum jump in strength is
uncommon in cement concrete, whatever be the technique of preparation.
Issues with Transition Zone:
Transition zone is the interface between coarse aggregate and cement paste. Generally
concrete do fail at transition zone, when subjected to stress, because of adverse effects
caused by differential thermal stresses and weak crystallography at this zone. When OPC is
used, belching out high surplus lime at early ages, such surplus lime gets dissolved in water
tending to settle at transition zone. In high performance concretes aggregate size is
rationalised in order to minimise differential stresses at transition zone, upon which the
strengths are attributable to the strength of cement matrix associated with sand.
In NAC, first of all, there is no scope for transition zone for having avoided coarse aggregate.
Even the inert fly ash particles do develop cohesive bond with cement matrix making the
ultimate NAC-matrix close to monolithic. This is manifested in NAC by high strength (55-70
MPa) and lowest permeability at 27 coulombs.
There are a good number of constraints to use NAC in commercial way:
Firstly, the WCF to admixture dose is very critical. During some portion of the studies, top 10-
15 mm layer of cast elements were observed to be incohesive and soft for violating the
discipline on WCF. It took almost six months to identify the flash point to avoid such mishap.
Secondly, mixing regime is crucial with respect to specific duration to tap the workability. If
one is in hurry and keeps adding water, the mix attains more fluidity causing plastic
shrinkage with visible cracks on the surface of cast element leading to dilution of strength.
Third dimension is brittleness due to ultra high strengths. A well prepared NAC has the ability
to attain 35 MPa strength at 3-day; 52 MPa at 7-day and over 80 MPa by 28-day. Though
brittleness is not uncommon to high strength concretes, NAC with its ceramic-like matrix
appear to behave distinctly different against cement concrete, more so as RCC member.
This aspect gives thrust for structural engineering research to identify the limitations or
benefits of this wonder-product, keeping in view its application for high rise buildings, bridges
and high profile structures.
NAC makes good sense for precast elements because:
Accuracy of inputs is possible due to dosing mechanisms governed by
instrumentation techniques.
Butter-like mix of NAC facilitates casting with absolute finish associated with detailing
of mould profiles, if any, making it amenable for ornamental concrete too.
Density of NAC is around 1800-1900 kg/cu.m; almost 20-25% reduction over that of
control concrete.
High early strengths of NAC, which can also be augmented within 36 hours through
accelerated curing, prove conducive for handling precast elements for immediate
despatch to market bringing down the inventory costs.
As the construction market is gearing up to produce and use precast elements, we
believe that NAC is the timely boon to precast industry.