Trends and Challenges in Structural Engineering and Construction Technology
INTRODUCTION
Self compacting concrete (SCC) does not require mechanical means to compact it, when placed in reinforced concrete members. It flows and fills in all the corners with self weight only and does not segregate. The method for achieving self compactability involves not only high deformability of the paste or mortar but also resistance to segregation between coarse aggregate and mortar when the concrete flows through the confined zones of inforcing bars. A highly viscous paste is required to avoid the blockage of coarse aggregate when concrete flows through obstacles. The materials used in SCC are the same as in conventional concrete except that an excess of fine material is used. Median particle size of this extra ‘powder’ material in SCC affects both yield stress and viscosity (higher values for lower median particle size, Pedersen and Mortsell have illustrated that the particle size distribution and the specific surface area of the fillers are of great importance for the rheological properties of the matrix in SCC.
This paper reviews test results reported in literature with alternative powder materials, mainly, limestone powder and flyash. A comparative evaluation is
presented. The test results of an experimental study performed by the authors exploiting the characteristic properties of flyash are also included.
POWDER REQUIREMENTS OF SCC MIX AND OPTIONS AVAILABLE
To limit the segregation and bleeding in self compacting concrete, fine powder/s with particles less than 0.125 mm are required in the range of 400-600 kg per cubic metre of concrete15. The ordinary portland cement (OPC) has ninety nine percent of its particles less than 0.090 mm size and almost hundred percent smaller than 0.125 mm. Hence, OPC qualifies as the primary “powder” material for SCC. Other supplementary powder materials useful in SCC includes flyash, ground granulated blast furnace slag (GGBFS) powder and limestone (LS) powder etc.
Petersson (2002) has reported that the compressive strength is significantly higher using the limestone filler for SCC than with fine aggregate as filler (0-2 mm size)17. The average particle size of the material should be in the range of 100 μm-30 μm to yield a desirable plastic viscosity of more than 40 Pa
Blast furnace slag in fine powder form is obtained by pulverizing the granulated slag to fine powder and increasing its hydration reactivity. With the advancements in grinding and separating technologies in recent years, it has become possible to manufacture much finer pulverized powder with a higher fineness (greater than 1,000 m2/kg) compared to the blast furnace slag in powder form with a fineness 340 m2/kg to 440 m2/kg used in conventional portland slag cements. High fineness ground blast furnace slag powder does not initiate complete hydration immediately after contact with water, as in the case of clinker particles. Moreover, since the surface of the particles is glassy and smooth, even if high fineness blast furnace slag powder is used as the concrete material, adverse effects such as increase in the unit water content, do not occur. It has been reported from rheological studies on paste using a rotational viscometer that the thixotropicity of paste incorporating finefineness blast furnace slag powder decreases compared to ordinary portland cement.
So, it has been used as a supplementary powder material in SCC. Dry flyash collected from electrostatic precipitators in coal based thermal power plants has its intrinsic particle size below 0.125mm. Granulometry, chemical and mineralogical characteristic of flyash help in the strength development and durability of normal cement concrete (Mehta, 1985, Malhotra and Ramezanianpour, 1994)10,11. This applies to SCC too. In last one decade, this area of application of flyash has attracted attention of many researchers and construction industries (Bouzoubaa and Lachemi, 2001, Raghavan et al, 2002, Vengala et al, 2003, Kumar et al, 2004 and Bapat et al, 2004). Among the three options of the powder materials described above, flyash has its intrinsic particle size in the desired range, whereas the other two materials need pulverisation of the parent material for transformation into the specified particle size range. This ‘extra’ process required in the case of GGBFS and LS enhances the cost of these materials even up-to the cost of cement. On the other hand, flyash is available at comparatively lesser price, particularly in India.
There are more than one hundred coal based thermal power stations in India, out of which more than fifty percent have installed facility of delivery of ‘dry flyash’ collected in electrostatic precipitators. This ‘dry flyash’ is available in ‘raw form’ in a distance range of one hundred kilometers around the thermal power stations from contractors and transporters operating at different locations in the country at a price ranging from one sixth to one fourth to that of cement. IS 3812-2003 has specified the general physical and chemical property specifications for flyash for use in cement concrete as an admixture7. This requires a minimum fineness equal to 200 m2/kg and maximum residue on the 45 μm sieve equal to 50. Generally, the ‘dry flyash’ available from the thirty thermal power stations spread across the country, conforms to these physical requirements. The flyash from captive thermal power stations is in general found to contain larger content of un-burnt carbon than allowed, i.e. the value of the ‘loss on ignition’ is greater than 5.0. Particles of the ‘un-burnt carbon’ absorb some of the super-plasticiser (SP) and hence less SP remains available for reduction of water demand/imparting flow-ability in SCC.
Beneficiated/Processed/Segregated flyash is recommended for use in case where the ‘raw flyash’ does not meet the specifications of IS 38127. Processed flyash is available commercially in many countries including India.
EFFECT OF FLYASH ADDITION IN CONCRETE
Flyash when added in cement concrete plays both active and passive roles in the concrete mass. The main effects are summarized below:
1. Proper quality flyash addition up-to some proportion helps in reducing the water demand of the concrete. The value of the ‘limiting proportion’ depends upon the quality of the flyash.
2. The flyash addition helps concrete to behave as a cohesive mass in fresh state and reduces the risk of segregation and bleeding of concrete.
3. Some part of it reacts with calcium hydroxide (which is produced during the hydration reaction of the cement), and forms compounds which exhibit similar mechanical properties as those exhibited by hydration products of the cement.
4. Smaller un-reacted particles fill the voids and densify the matrix through decreasing the amount of bigger pores.
5. Presence of flyash in concrete promotes the hydration of cement.
The first four effects as enumerated above are well described by Neville, Papadakis, Malhotra and Ramezanianpour, Mehta and Monterio, Montgomery et al and many other researchers. Gopalan suggested that flyash particles act as nuclei for growth of hydration products and pozzolanic reactions occurs thereafter6. The ‘microaggregate’ effect of flyash particle has been described amply in the literature of CANMET and other researchers involved in high volume flyash concrete10,25.
Wang et al published test data which support the hypothesis of enhancement of cement hydration due to the presence of flyash particles24. With the increase of the content of flyash, the hydration degree of cement increases, but the pozzolanic degree of flyash reduces24.
Most of the investigations utilized mix proportioning guidelines as suggested by Okamura and his co-workers to obtain a SCC mix through keeping water to powder (w/p) ratio in the range of 0.90 to 1.10. This limit of w/p does not consider powder material characteristics, i.e. whether limestone powder or flyash has been used.
When proper quality flyash is used, it reduces water demand of concrete10. This property of flyash can be of specific use in SCC.
Cement content required to develop a target compressive strength can become quite low, in case ‘processed’ flyash is used. It is illustrated from the test results of Bapat et al provided in Table 1. Due to high cost associated with the ‘processed’ flyash,
mostly ‘raw’ flyash is used. Use of a small quantity of micro-silica in association
with such flyashes can form a good option in SCC. Mittal et al used such ternary
combination for SCC in the construction of an atomic power plant12.
Table 1 provides details of some other typical SCC mixes reported in literature with
flyash as well as limestone powder. Also shown is a typical mix developed by the
authors for high strength self compacting concrete using flyash as about 40 % of the
binder material content. The flyash used in the M6 mix was ‘raw flyash’, and
obtained from a thermal power station situated in North India8.
A comparison of the mixes M1 and M2 show that replacement of limestone powder
by flyash is beneficial in SCC. M3 mix contains a substantial content of micro-silica,
which is known to accelerate early age strength gain, but is a lot costlier than flyash.
Present cost of the micro-silica is about seven- eight times that of cement in India.
Authors investigated use of a lower content of micro-silica in association with ‘raw’
flyash to produce high strength SCC. M6 provides a typical mix proportion
developed. It may be mentioned here, that the compressive strength goes on
increasing further beyond 28 days in the case of flyash based mix, and was observed
to be about 76, 83 and 88 MPa at the 90, 135 and 180 days ages respectively7.
A comparison of mixes M4 and M5 in Table 1 illustrate that with an increase in the
flyash proportion, early age strength reduces, but the strengths attained at 28 days
age is almost similar. It may be noted that the full service load generally come on a
structure not before 28 days age, hence the higher proportion of the flyash, as used
in the mix M5 may be used in majority of the cases.
DURABILITY ENHANCEMENT OF SCC DUE TO FLYASH
All effects of flyash put together makes resulting concrete less permeable and more
durable than without it12. The interfacial transition zone is the ‘weakest link’ in
concrete affecting its strength and to some extent its durability as well12. In a
concrete made with cement as the only binder material, the transition zone consists
of more porosity than bulk of the hydrated concrete mass. A lot of calcium
hydroxide crystals remain present adjacent to the aggregate surface. These can
dissolve in case some water finds its way inside in the hardened concrete mass,
which leads to loss of strength and loss of durability12. The use of flyash in the SCC
is observed to reduce pores of the transition zone and consumption of the calcium
hydroxide9. Fig. 2 provides a typical microstructure of the transition zone observed
by the first author in case of SCC containing high volume flyash8. It shows ample
number of flyash particles embedded in the zone near to the aggregate, acting as the
‘micro-aggregates’. This densification of the transition zone leads to better
mechanical and durability properties.8.
Nehdi et al investigated chloride penetrability and sulfate expansion of SCC made
with cement alone as the powder material, as well as that with cement and flyash as
the powder13. The SCC with 50 % OPC and 50 % flyash is shown to exhibit considerably lower chloride penetration (about one fourth) than the concrete with
OPC alone. Sulfate expansion value of about half was recorded by the investigators
in the case of SCC made with 50 % cement + 50 % flyash.
The microstructure studies by the authors and the chemical penetration study by
Nehdi et al put together illustrate better durability attributes in the SCC made using
flyash in large proportion (about 35 % or even more of the total powder material in
the SCC).
CONCLUDING REMARKS
Limestone powder, GGBFS and flyash may constitute as the supplementary powder material in SCC. Flyash has emerged as a preferred material due to its various associated advantages in SCC mix proportioning
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Table 1 Comparative Performance of SCC Mixes with Limestone Powder / Flyash
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