Effect of Dosing Sequence of Micro Silica on Slump & Compressive Strength of Concrete

Silica fume has been successfully used in production of high performance and high strength concrete for the past couple of years. The benefits of using silica fume for enhancement of durability and improvement in the microstructure of concrete are already well established through various researches and other developments. Few years back, it was a normal practice to introduce silica fume in the concrete mix in slurry form. However, currently silica fume is dosed in dry form at most of the batching plants. In this paper, an effort has been made to study the effect of dosing sequence of silica fume in the concrete mix in dry form, on the slump and strength characteristics of concrete.

S.B.Kulkarni - AVP, Technical Services & Hemendra S, Manager, Technical Services. UltraTech Cement Ltd. Mumbai.

Introduction

Micro Silica is a mineral composed of ultrafine (fineness in the range of 15,000, m²/kg) amorphous, glassy spherical particles of silicon dioxide produced during manufacture of silicon or ferro silicon. The average particle size is less than 0.5µ meaning that each microsphere of silica fume is about 50 times smaller than an average cement grain particle. The ultrafine particles of micro silica fill the gaps between the cement particles thereby helping to fill the micro voids in the fresh concrete. These particles also act like ball bearings and make the concrete more cohesive. Earlier it was a common practice to use micro silica in slurry form in order to ensure uniform distribution of micro silica in the matrix. However, there were many issues faced at site in introducing silica fume in slurry form.

Issues faced earlier while using silica fume (SF) in slurry form:

• Separate plant has to be set up for converting silica fume into slurry form.
• Setting of plant involved an additional capital investment.
• The pot life of SF slurry is in the range of 30 to 40 minute, so it had to be used within short span of time.
• Traffic Jams delayed the movement of transit mixer and sometimes led to dosing of slurry even after the lapse of pot life period.
• If slurry is not used within pot life period, the interaction between water and silica fume particles brings down the effectiveness of silica fume.
• In Metro cities, it might be difficult to allocate an additional land to set up separate unit for silica fume slurry production.

As such use of silica fume in dry form is considered the most suitable and convenient system. However, the effect of sequencing of dosing of micro silica on concrete slump and strength properties (at 7 and 28 days) need to be studied. To study the impact of dosing of dry silica fume, experiments were conducted in the laboratory with two different dosing sequences and the results were compiled.

Sequence of Micro silica dosing in dry form in the concrete mix:

A. General Practice:

Mixing of ingredients in the process

It has been observed that at most of the batching plants, the various ingredients of concrete and micro silica are loaded in mixer in the following sequence:

1) Coarse Aggregate and Fine Aggregate

2) Cement & Micro silica + Additives (Fly ash / Slag)

3) Mixing of (1) & (2) for approx 30 sec

4) Water & Chemical Admixture

5) Mixing for another, approx 30 sec

6) Unloading the mix

B. New Practice explored for experiment.

It was decided to explore possibility of changing the dosing system of micro silica while keeping the mix design proportion same for all the ingredients and to study its effect on slump and compressive strength of concrete.

For this experiment various materials used were:

• Cement – UltraTech Slag cement conforming to IS 455.
• Admixture – Reobuild R620 of BASF.
• Micro Silica – Elkem.

Grade of Concrete – M35.

Trials were conducted in the laboratory in Pan Mixer: (40 litre Capacity)

Trial A- In this first Trial, following sequence was followed for introducing silica fume in the mix

1. Coarse Aggregates
2. Fine Aggregates
3. Cement
4. Micro Silica
5. Water
6. Plasticizer
7. Mix all together for 2 minutes.

Trial B- The following sequence was followed in second trial, as given below:

1. Coarse Aggregates + Micro Silica followed by
2. Dry mixing for 90sec
3. Fine Aggregate
4. Cement
5. Water
6. Plasticizer
7. Mix the ingredients for 90 seconds.

From the above two trials, it can be seen that in Trial A silica fume is added after all Coarse Aggregates + Fine Aggregates + Cement have been put in the mixer, whereas in Trial B silica fume is added after Coarse Aggregates and mixed together for 90 seconds and thereafter balance ingredients namely Fine Aggregates, Cement etc. are added. In both the cases, initial slump was measured followed by slump measurement at 30 minutes & 60 minutes. Total 12 cubes were casted for each trial for measuring the compressive strength by

1. Accelerated Curing Test
2. At 3 days
3. At 7 days
4. At 28 days

The results as referred above were compiled as given below:

Location: UltraTech RMC Lab, Mumbai.

Discussion on Test Results

i. Slump test results (as per IS 1199 — 1959)

• Initial Slump — In Trial A slump was 115mm whereas in Trial B initial slump was 170mm indicating that there was increase in slump by 55mm (47.8% increase).
• Slump at 30 minutes — In Trial A slump was 80mm whereas in Trial B slump was 140mm indicating that there was increase in slump by 60mm (75% increase).
• Slump at 60 minutes — In Trial A slump was 70mm whereas in Trial B slump was 125mm indicating that there was increase in slump by 55mm (78.5% increase).

View of Cube Casting under progress

It is interesting to see that in Trial A & B the quantity of various ingredients is same, that includes quantity of water & plasticizer used. Normally, the slump increase happens either by increase in the quantity of water or plasticizer or both. But in this case, it is seen that without changing the mix design proportion or adding extra quantity of water / plasticizer the slump for trial B has increased between 47% and 78% just by changing the sequence of dosing of silica fume (as given in Table 2).

ii. Compressive Strength test result:

a) Accelerated Curing Test cube results (Conforming to IS 9013 – 1978 - Reaffirmed 1999)

As mentioned above in the article, concrete cubes were tested by using Accelerated curing test method and results were tabulated as mentioned in Table 3. It is seen that for Trial A, the anticipated 28 days strength was 43MPa, whereas for Trial B it was 45.56MPa. In this case also, the change in sequence of micro silica dosing has resulted into increase in accelerated curing test result value for Trial B by 5.95%.

b) Concrete cube results (as per IS 516 – 1959 - Reaffirmed 1999)

Concrete cubes (150x150x150mm) were casted, cured and tested for 3,7 & 28 days compressive strength as per standard procedure. From Table 3, it is seen that for Trial A the 3, 7 & 28 days strength was noted as 24.58MPa, 31.76MPa & 40.16MPa respectively, whereas for Trial B the compressive strength was noted as 27.87MPa, 37.29MPa & 45.66MPa respectively. It is again interesting to note here also that the compressive strength for 3, 7 & 28 days for Trial B have increased by 13.38%, 17.41% & 13.70% respectively as compared to test results of Trial A. Here also except for change in dosing sequence of micro silica no other changes were made in the mix.

Conclusion

• It can be seen that introduction of micro silica in the mix immediately after adding coarse aggregates and then mixing the same for 90 sec, has changed the test results considerably both in terms of slump and compressive strength.
• The test results show that without adding additional water or plasticizers the slump values have enhanced by 47% to 78% and the compressive strength values have enhanced by 13% to 17%.
• The results are very encouraging as increase in slump values can facilitate reduction of plasticizer dose & enhancement in compressive strength can facilitate reduction in the cement content thereby leading to optimisation in the cost of concrete mix.
• It is felt that at construction site the Quality Managers should conduct similar trials based on the findings of the above referred article and try to get best benefit in terms of additional retention of slump or reduction in the plasticizer dose resulting in ease in placing, compaction & finishing of concrete, reduced honey comb and also optimization of the concrete mix design cost without spending extra penny for the same.
• It is hoped that the contents of the article would give further guidance to Quality Managers for exploring the benefits of changing the dosing sequence of micro silica, to their full advantage.

Acknowledgment

Authors acknowledge the experience shared by Mr. S.C. Verma Ex HCC, Q/C Head, on the similar trials conducted earlier.

References

• IS 456 – 2000 – Plain and Reinforced Concrete – Code of Practice.
• IS 455 —1989 — Portland Blast Furnace Slag Cement.
• IS 10262—2009 – Concrete Mix Proportioning – Guidelines.
• Neville on Concrete by Adam Neville.
• Construction Technology by Prof. M.S. Shetty.
• Mineral Admixtures in Cement & Concrete Vol 4 by S.N. Gosh & S. Savkar.
• Concrete Micro structures, Properties & Material by P.K.Mehta & P.J.M. Monteiro.
• IS 1199 – 1959 (Reaffirmed 1999) - Methods of Sampling & Analysis of Concrete.
• IS 9013 – 1978 (Reaffirmed 1999) – Method of making, curing and determining compressive strength of Accelerated cured concrete test specimens.
• IS 516 – 1959 (Reaffirmed 1999) – Methods of Test for Strength of Concrete.