Self Compacting Cocrete

Dr. S.C. Maiti, Ex–Joint Director, National Council for Cement & Building Materials, New Delhi. Raj K Agarwal, Managing Director, Marketing & Transit (India) Pvt. Ltd, New Delhi.

Introduction

Concrete mixtures having high workability and high cohesiveness will be self–compacting concrete. The self–compacting concrete (SCC) is defined as a flowing concrete that can be transported without any segregation and placed without the use of vibrators to construct concrete structures free of honeycombs. Initially such concrete was developed by Japanese researchers. For such concrete which is specially required for heavily reinforced sections, a viscosity modifying agent (VMA) is required along with a polycarboxylic ether (PCE) based superplastisizer. Because of high fluidity, SCC requires higher fines content, in order to resist bleeding and segregation. Natural fine aggregate together with manufactured sand and mineral admixture {flyash or ground granulated blast furnace slag (ggbs) or silica fume} provide higher fines contents in the concrete mix. A cohesive SCC is thus produced in order to flow steadily in the heavily reinforced concrete sections, without any segregation & bleeding.

Materials and Mix Proportions

Besides cement, water and aggregates, the necessary ingredients for producing SCC are superplasticizers (PCE based), viscosity–modifying agents and mineral admixtures e.g. flyash, ground granulated blast furnace slag & silica fume. The proportion of fine aggregates required is higher, may be around 55% and the corresponding proportion of coarse aggregate (generally of smaller size, say 10 or 12 mm maximum size) will be around 45%. The mineral admixtures and fine sand (manufactured sand) are required to make the highworkability concrete mix cohesive.

Typical concrete mix proportions for high strength (74.5 MPa at 28 days) SCC used by Gettu & others (from Spain) (1) are as follows:

• Cement (OPC-53 grade) = 428 Kg/ m3
• Water = 188 l/ m3
• Flyash (2935 cm2 / gm) = 257 Kg / m3
• Superplasticizer (vinyl copolymer) = 7.9 Kg / m3
• Sand (crushed limestone ) (0-5mm) = 788 Kg / m3
• Coarse aggregate (gravel) (5-12mm) = 736 Kg / m3

The water / binder ratio of the concrete mix is 0.27. A look at the materials & mix proportions indicate use of smaller size coarse aggregate (12mm maximum size) & the shape is rounded, being gravel aggregate. In fact, crushed gravel will be a better option in order to obtain high- workability and highstrength SCC.

The workability measured for the above mix is “ slump flow” of 48cm. In our country, still we are carrying out the usual slump test even for high–workability concrete mix. The “flow test” as specified in IS 9103 (2) can be conducted for testing such high–workability concrete mix, but the “slump flow test” will be better than the “flow test,” as no lifting (15 times in 15 seconds) of concrete is necessary, as the SCC is a flowing concrete mixture.

Vachhani and others (3) used SCC in the prestigious Delhi Metro construction. Concrete mix proportions for M-35 grade of SCC are as follows :

• Cement = 330 Kg / m3
• Water = 163 l / m3
• Flyash = 150 Kg / m3
• Superplasticizer = 3.12 l / m3
• VMA ( glenium stream 2 ) = 1.3 l / m3
• Retarder ( Pozzolith 300 R) = 0.99 l / m3
• Sand = 917 Kg / m3
• Coarse aggregate:
• 20mm maximum size = 455 Kg / m3
• 10 mm maximum size = 309 Kg / m3

Vachhani and others (3) highlighted the mechanism of self compaction, which is based on :

1. Large quantity of “fines” (500 to 650 Kg / m3),
2. Use of high –range water – reducing superplasticizers (with water- reduction of 25%), and
3. The use of Viscosity Modifying Admixtures.

“Fines” includes cement, flyash and the part of sand of size less than 0.125 mm. This together with water & chemical admixtures constitute the paste in the concrete mix. The paste makes the concrete mix cohesive and controls the segregation–resistance of the mix. The polycarboxylic ether–based superplasticizer ( presently being imported) generally provides water–reduction of the order of 30-40 % in the concrete mix. The VMA improves the segregation–resistance of the mix without changing the fluidity or workability. The retarder in the concrete mix controls the workability–retention, which is specially important in hot climate.

Fresh Self–Compacting Concrete

The characteristics of fresh SCC are fully described by the following properties :

1. Filling ability–ability to completely fill all the spaces in the formwork,
2. Passing ability–ability to flow around reinforcement, and
3. Segregation resistance–ability to resist segregation of materials during transportation and placing.

Consequently new test methods have been developed to test SCC in the fresh state. The “filling ability” is tested by “ slump flow” and “ V funnel,“ the “passing ability” is tested by “L- Box“ and“ U–Box“ and the segregation–resistance is tested by “ V- funnel”

Hardened Self–Compacting Concrete

The properties and characteristics of hardened SCC do not greatly differ from those of normal concrete, except that SCC can not be used in mass concrete construction using bigger size aggregates, say 75mm or 150mm sizes. Because such concrete always needs to be compacted with needle vibrators, in order to compact thoroughly in the forms.

Any required compressive strength of SCC can be achieved. Vachhani & others (3) obtained 28- day compressive strength of 44-49 MPa in the above–mentioned concrete mix proportions for M-35 grade concrete, for the Delhi Metro construction.

The high – strength SCC can be called “ High–performance concrete,” as such concrete has denser microstructure with lower inherent “porosity” and “permeability,” because of lower water- cementitious materials ratios and use of mineral admixtures in concrete.

Concrete Mix Proportioning Approach

The Self–Compacting Concrete, because of its high–workability and cohesiveness, generally needs higher fines content and lower size (10 or 12 mm maximum size) of coarse aggregate. Smoother and rounded or semi- rounded (may be crushed gravel) coarse aggregate will develop cohesiveness in the concrete mix. Bapat’s (4) suggestion is good. Flakiness & elongation indices of coarse aggregate should be less than 15% each. Large quantity of fines is also required–500 to 650 Kg/m3 of concrete, & therefore crushed stone fine aggregate is also required along with natural fine aggregate. Flyash has also been used as an essential ingredient of SCC. In India, 30 to 50% flyash has been used in SCC. Originally Japanese people (5) suggested water–powder ratio between 0.90 & 1.1 (by volume). But it is the paste that controls the segregation of the concrete mix. The powder & the paste includes finer ( less than 0.125mm) part of the fine aggregate. Vachhani (3) & Bapat (4) used about 35 to 36% paste to produce self compacting concrete. The viscosity modifying agent also controls the segregation– resistance of the concrete mix.They are generally starch, cellulose & gum–based. Preferable & satisfactory VMA is “Welan Gum.” The quantity of such VMA required in SCC is very less, about 0.1% by weight of cementitious materials.

Prof P.K. Mehta (6) included “Welangum,” silica fumes & ultrafine colloidal silica under the list of VMA. Gum or cellulose– based material is capable of modifying the viscosity of SCC, but the silica fume may not be able to modify the viscosity of concrete. Subramanian and Chattopadhyay (7) observed that micro silica at an appropriate dosage may be beneficial in reducing the dosage of “Welan gum.”

The following mix proportioning steps for SCC can be followed.

• The target 28-day compressive strength of concrete can be calculated first based on standard deviation value used for the specified grade of concrete.
• The water–cementitious materials ratio can be decided based on the target 28–day compressive strength of concrete. This can be in the range of 0.30 – 0.50, 0.30 for a 28 day compressive strength of about 90 MPa, while 0.50 for a 28 day compressive strength of about 30 MPa .
• For the high – workability concrete mix, the water content of concrete will be in the range of 180 – 190 l/m3 of concrete.
• The maximum size of aggregate for SCC is more or less fixed at 10 or 12 or 16 mm.
• The sand (natural + manufactured) content can be kept at about 55% & the coarse aggregate content can be about 45%, by weight of total aggregate.
• The superplasticizer required is PCE–based and about 1% by weight of total cementitious material. The cementitious material includes ordinary Portland cement, flyash /ggbs & silica fume (in case of high strength concrete). For normal strength concrete (say from M-25 to M-50), no silica fume will be required, but about 20 to 30 % good quality flyash will be required. If ggbs is used in place of flyash, its percentage can be 40 to 50 %, by weight of total cementitious material. For high strength concrete of M-60 to M-80, about 10% silica fume will be required instead of flyash or ggbs. The dosage of super plsticizer & the viscosity modifying agent can be fixed based on one or two trial mixes in a laboratory.
• With the above details in hand, concrete mix proportions for any grade of SCC can be arrived at.

Conclusions

The self compacting concrete, a high workability cohesive concrete mix needs polycarboxylic ether–based superplasticizer and a viscosity modifying agent.

The proportion of fine materials in the concrete mix is also higher than that of normal concrete mixes. Therefore, in addition to natural fine aggregate, manufactured sand and mineral admixture eg flyash, ggbs or silica fume is also to be used.The percentage of fine aggregate is around 55%, while that of coarse aggregate is around 45%, by weight of total aggregate. Smaller size of coarse aggregate (10,12 or 16mm maximum size) having soother surface texture (rounded or crushed gravel) is required for concrete to flow smoothly in the formwork. For normal “standard” concrete grades of M-25 to M-50, about 20 to 30 % flyash or 40 to 50 % ggbs can be used, whereas for high strength self – compacting concrete of grades M-60 to M-80, 10 % silica fume will be required.

References

• Gettu,R, Izquierdo, J, Gomes, P.C.C & Josa, A. Development of high – strength self- compacting concrete with flyash: a four – step experimental methodology. 27th conference on OUR WORLD IN CONCRETE & STRUCTURES : 29 – 30 August 2002, Singapore, pp.217 – 224.
• IS 9103. Specification for concrete admixtures. Bureau of Indian Standards, New Delhi.
• Vachhani, S.R, Chaudary, R & Jha, S.M. Innovative use of self compacting concrete in Metro construction. I.C I Journal, Vol. 5, No 3, Oct – Dec 2004, pp.27 -32.
• Bapat, S.G, Kulkarni, S.B & Bandekar, K.S. Self- compacting concrete in nuclear power plant construction. I.C.I Journal, Vol-6, No 3, Oct- Dec 2005, pp- 37- 40.
• Okamura,H, Ozawa,K & Ouchi,M. Selfcompacting concrete. Structural Concrete, Vol-1, No1, March 2000.
• Mehta,P.K & Monteiro,P.J.M. Concrete-Microstructure, Properties & Materials. Third edition, 2006, Tata McGraw –Hill Publishing Co Ltd, New Delhi, p.478.
• Subramanian, S & Chattopadhyay, D. Experiments for mix proportioning of Self – compacting concrete. The Indian Concrete Journal, Jan 2002, pp. 13 – 20.