High Performance Concrete And Higher!
Dr. A. K. Mullick, Former Director General, NCBM
The concept of High Performance Concrete (HPC) as
something different from the ordinary, run-of-the mill
concrete was evolved nearly five decades ago. HPC was
designed to meet special requirements of strength, durability and
fluidity of individual projects, by choice of appropriate materials,
workmanship and quality control. In India, high performance
concrete has been widely adopted in Nuclear power generation,
infrastructure and water resources projects. Meanwhile,
development of concrete with greater characteristics, called
‘Ultra High Performance Concrete’ (UHPC) has been going on
in many countries. This paper summarises the trend and the
steps required to make wide application of UHPC in structural
High Performance Concrete
The advent of superplasticizers and silica fume are among the foremost developments in concrete technology in the last few decades; which led to high strength and high performance concrete. Superplasticizers allowed the workability of concrete to increase greatly, or to lower the water/cement ratio, resulting in lower capillary porosity and high compressive strength. Addition of silica fume aided high strength by pozzolanic reaction and denser packing of solid particles by fine powder effect. This resulted in higher compressive strength and enhanced durability. High performance concrete is defined as ‘Concrete, which meets special performance requirements that cannot be always achieved routinely by using only conventional materials and normal mixing, placing and curing practices’. The requirements may involve high strength; greater durability and increased service life in severe environments; high ductility, toughness and blast resistance etc. (1,2). Another requirement can be very high workability without segregation; this is the main characteristic of self-compacting concrete (SCC).
The basic considerations of high performance are the relationships linking water-cement ratio to the strength and durability of concrete. The requirements of high strength and low permeability (high durability) are achieved by (3); – Low water-cement ratio for high strength (Fig. 1),
– Low water-cement ratio for low permeability (Fig. 1),
– Pore blocking by fine powders
– Partial replacement of cement by silica fume, fly ash or slag to reduce cement content
– Increased fine powder and low water content require the use of superplasticizers.
Another aspect requiring attention is the brittle nature of concrete. Concrete is a quasi-brittle material, prone to cracking. The brittleness increases with high strength (Fig.3). Another related development, therefore, is the use of fibre reinforcement, to improve ductility, crack resistance and toughness.
With the above information, it is easy to list the necessary ingredients of high performance concrete. Cement, aggregates and water are the usual ingredients of concrete; additionally, for high performance concrete, the following is required (3);
– Silica Fume,
– Fly ash, Granulated Slag – optional,
– Viscosity modifying agents (VMA) for very high workability concrete, self-compacting concrete (SCC).
– Fibre reinforcement for ductility, toughness, abrasion resistance.
Typical Applications in India
High strength and high performance concretes have been widely used in India during the last three decades for construction of nuclear power projects, long span bridges, high-rise buildings and water resources projects. As such, only a few illustrative applications are mentioned below. High strength concrete is used where strength is the basic consideration, e.g. in buildings, industrial structures. It is noteworthy that IS: 456 (draft revision) defines ‘High Strength Concrete’ from Grades M65 to M100, but does not mention high performance concrete. On the other hand, where durability is added consideration, e.g. in river bridges, high performance concrete is used. IRC Concrete Bridge Code IRC: 112 – 2011 defines ‘High Performance Concrete’ from Grades M30 to M90. It should be kept in mind that high performance concrete is not always the same thing as high strength concrete. Among the application of high concrete strength, mention may be made of JJ Flyover in Mumbai. The superstructure of the viaduct used M75 Grade concrete, which was manufactured in RMC plants (4).
Target compressive strength of concrete at 28 days was 83.2 MPa. The mix composition per m3 of concrete was as follows;
Cement - 500 kg
Silica Fume - 50 kg
Fine aggregate - 682 kg
Coarse aggregate - 1156 kg
Water - 148 litres (w/b ratio = 0.269)
Superplasticizer - 8.25 litres
Properties of concrete achieved in the field were;
Slump- 130 to 180 mm at RMC plant,
80 to 120mm at placement, after 150 minutes,
Field strength obtained was 79.6 MPa at 28 days (average) and 94 MPa at 365 days.
CoV - 2.64%.
Low variability was testimony to strict quality control Coming to high performance concrete, domes of Reactor buildings in nuclear power projects at Kaiga, Tarapur and RAPP were among the first to use high performance concrete (5). The performance characteristics required were; moderate compressive and high tensile strength; very high durability; low creep and shrinkage; low permeability and good workability. Relatively crack-free performance was required to prevent leakage of radioactivity.
Typical mix proportions (per m3 of concrete) were (Table 1)
The average 28–day compressive strength was 77 MPa, and CoV was 5%. The cement content was reduced subsequently, as experience was gained. Most of the highway bridges use high performance concrete. The mix proportions for M60 grade pile caps for Bandra – Worli Sea link project are given in Table 2 below.
This project was among the earliest to use ternary blend for binder system (OPC + fly ash + silica fume), now becoming common. This practice of ternary cement blends allows optimum packing of fine materials; a trend, which extends to all solid materials including aggregates in further developments of high performance concrete (6). It is to be noted that M60 Grade concrete was achieved with 320 kg cement per m3. In the Waterways Sector, high performance concrete is used for locations subjected to abrasion and impact, e.g. intake; overflow sections, sluices and stilling basins in spillways. M60 concrete is routinely specified for such applications (6). Another area of application is for immediate and permanent roof support for underground caverns like Powerhouse, Transformer Hall, Desilting chambers and Sedimentation chambers, or for the lining of tunnels, where fibre-reinforced shotcrete is used.
Another type of high performance concrete is Self Compacting Concrete (SCC). Self-Compacting Concrete is a concrete that fills uniformly and completely every corner of formwork by its own weight and fluidity without application of any vibration, without segregation, whilst maintaining homogeneity. It is suitable in situations where;
– Reinforcement is very congested,
– Access to allow vibration is not available,
– Complicated geometry of the formwork,
– Pouring is possible only from a single point,
– Speedy placement is required
It also has the advantage of no noise due to vibration and no requirement of finishing.
Because of ease of placing, SCC is now widely used in many constructions in India. Its applications started with concrete of moderate strength grades (M35 or so), where congestion of reinforcement or difficulty in placing were the primary reasons. Its application to high strength concrete (M60 Grade) was extended to bridge piers in Signature Bridge in Delhi (6), and later on to many high-rise buildings, where M80 or M90 grade concrete is being used (7). In the absence of any Code for mix design, comprehensive guidelines of EFNARC are widely used (8). It also prescribes necessary tests for fluidity, passing ability and cohesiveness of concrete and suggests appropriate values of test results for different placing conditions.
Further Developments - UHPC
With appropriate mix design, high strength and high performance concretes of compressive strength approaching 100 MPa at 28 days became common, including in India. Simultaneously, the development of very high strength concrete following ‘Reactive powder concrete (RPC), Macro defect free cement (MDF) or Dense silica particle cement (DSP) routes were pursued. New generations of high efficiency superplasticizers rendered high fluidity; and fibre reinforcement was used for ductility and toughness. This led to the recent trend in many countries to develop ultra high performance concrete (UHPC) and their industrial applications. Compared to the common strength level in HPC of around 60 - 100 MPa, the ultra high performance concrete has strength levels of up to 200 MPa or more. A common classification on the basis of compressive strength is given below:
– High strength concrete (HSC) – 50 to 100 MPa,
– Very high strength concrete (VHSC) – 100 – 150 MPa,
– Ultra high strength concrete (UHSC) – 150 – 200 MPa, and
– Super high strength concrete (SHPC) – 200 – 250 MPa.
As mentioned above, these are achieved by with the use of improved materials, very high amount of cement and silica fume, (called ‘reactive powders’), low water/binder ratio of the order of 0.11 – 0.22 made possible by the use of higher dosage of high efficiency superplasticizers. Addition of fibres results in high flexural strength and improved ductility. Conventional sized coarse and fine aggregates are omitted. Some of the formulations are commercially available in trade names of DUCTAL or CERACEM. Reactive Powder Concrete (RPC) Ductal– High cement content @@ 1000 kg/m3, silica fume – 230 kg/m3 and water binder ratio – 0.11 – 0.15, Steel fibres @@ 200 kg/m3 are added to render ductility. RPC does not contain coarse aggregate and maximum size of fine aggregate is 0.4 – 0.6 mm. Typical compressive strengths of 170 – 230 MPa or higher have been obtained. Special Industrial Concrete (BSI) Ceracem – Mix proportions are similar to Ductal, except fine aggregate of size up to 6 mm are used. Typical strength levels are 150 – 175 MPa. ACI Committee 239 has given definition for Ultra High Performance Concrete (UHPC) as ‘Concrete that has minimum specified compressive strength of 150 MPa with specified durability, tensile ductility and toughness requirements; fibres are generally included to achieve specified requirements’ (9). Early Applications A recent (October 2015) Symposium on ‘Ultra High Performance Concrete’ held in Kolkata contains state-of–theart information on development, applications and challenges on the use of UHPC (10). Much of the information given below is gathered from the above publication, which lists pioneering applications of UHPC as:
– 130 MPa UHPC for an 88 story building in Chicago (1987),
– 300 MPa UHPC used for 60m long Sherbrook bridge in Canada (1997), and
– 200 MPa UHPC used for 120 m long pedestrian bridge in Seoul, South Korea (2002).
Sherbrooke Bridge (Fig. 6) has a 60 m clear span, 3.3 m wide. It was constructed with six precast UHPC elements. The precast elements were 3-dimensional space truss, which were posttensioned at the site. 300 MPa Ductal concrete was used. It is used as a footbridge.
Bridge of Peace in Seoul, South Korea (Fig. 7) has a 120 m span, The structure consists of six precast units made with 200 MPa Ductal. In longitudinal directions, the segments are posttensioned by six tendons. In transverse direction, single strands are used along with small, specially produced anchors.
Opportunities And Challenges On Large Scale Application:
Ultra high performance concrete not only offers extremely high compressive strength, but outstanding durability. Extremely high compressive strength of concrete will allow a high degree of precompression and high level of tensile stresses can be compensated (10). Addition of fibres provides major enhancements of ductility and tensile capacity. The most notable characteristic is extremely dense microstructure of the matrix due to the high amount of fines with an optimum packing density and low water/binder ratio. Consequently, long span structures or very high buildings with reinforced and prestressed concrete are potential areas of application. However, the wide scale application has been restricted due to the fact that UHPC-specific structural design rules are not available (9, 13). Framing of design rules and testing standards have been identified as a major task for ASTM and ACI (9). All the earlier applications were designed with conservative approaches, fully prototyped, and load tested to failure. In the early implementation of such break-through technology, the risks of owner, designer, concrete suppliers and contractors add to the costs. (13). Reference 10 gives a good insight into the current research activities on material- and structural design aspects being carried out in Europe, USA and some Asian countries like South Korea and China.
Ultra High Performance Fibre-Reinforced Concrete (UHPFRC)
Meanwhile, more applications are reported for repair and capacity enhancement of existing concrete structures, which require high strength, improved durability and multiple fine cracking phenomena. The latter property is owing to ‘strainhardening’ nature of the composite. Such a composite will have very high tensile strain capacity; about 4 per cent i.e. 400 times that in normal concrete.
The concept of strain-hardening is explained in Figure 8 (14). In conventional FRC, peak stress will be followed by a falling softening branch, with not much increase in tensile strain and opening of large crack (Fig. 8 (a)). On the other hand, the stress-strain curve of a strain-hardening composite starts with a steep initial ascending portion up to first structural cracking (Fig. 8 (b) - part I), followed by a strain-hardening branch where multiple micro cracks develop (part II) (14). The peak point at the end of strain-hardening branch, B in Fig. 8(b) corresponds to the maximum post-cracking stress and strain. At the peak point, one crack becomes critical; onset of crack localization takes place, and decrease in the resistance (Fig. 8 (b) - part III). Multiple crack formation, of 50 µ to 70 µ width, rather than a single crack, is important for strain-hardening response.
Applications in Repair and Strengthening
Pioneering applications of UHPC for strengthening and repair of structural members are reported in Ref. 15. Typical examples include;
– Widening of existing bridge (Figure 9).
– UHPFRC protection to a crash barrier wall.
– Rehabilitation of bridge pier using precast UHPFRC shell elements (Fig. 10), and
– Strengthening an industrial floor.
Typical composition of UHPC in such applications will be as given in Table 3 (10, 15). Table 3. Typical composition of UHPC for repair applications
Fine powders are batched premixed, which are then mixed with the rest in a high shear mixer. The mixing can be at the site or transported after mixing in a central plant (13). The workability is high; around 200 mm slump. After 24 hours curing at room temperature, the mix may have to be steam-cured at 90OC for 48 to 72 hours. In such cases, precast elements are used. The design compressive strength is 180 MPa and design tensile strength 13.0 MPa.
The use of strain-hardening, ductile concrete of normal strength grades (M40 or M50) have been proposed for joint less bridge decks in NHAI project. Because of high tensile strain capacity and controlled crack behaviour, expansion joints are proposed to be eliminated. Full details are in Ref. 16. This is in line with what is practised in the USA. IIT’s and SERC Chennai have reported investigations on high and ultra-high performance fibre-reinforced concretes. In SERC investigations, the 28 – days compressive strength of the composites ranged from 81MPa (high performance concrete) to 188 MPa (Reactive powder concrete). Uses of different types of fibres – both steel and polymer, like PVA or PP, to obtain strainhardening behaviour has been investigated by different academic institutions. Use of UHPC overlay for repair of damaged RCC beams have been reported from SERC Chennai. It is apparent that the design and construction of high-rise structures or long span bridges with UHPC will have to wait, till design rules are framed abroad and then in India. Enterprising engineers can opt for innovative designs on the basis of prototype testing. However, use of UHPC for repair, strengthening and retrofitting applications can start straight away. (Dr. A. K. Mullick, the former Director General of National Council for Cement and Building Materials in India, has spent more than 45 years in research, teaching, design and consultancy, devoted to propagation of sustainable concrete practices. He has authored 170 papers, two book chapters and is the co-inventor of six patents.) References
1. Strategic Highway Research Program, SHRP-C/FR-91-103, High Performance Concretes: A State-of-the-Art Report, 1991, NRC, Washington D.C., p. 233.
2. Neville, A.M., ‘Properties of Concrete’, 4th ed., Pearson Education Asia, Essex, England, Indian Reprint, 2000.
3. Mullick, A.K., High Performance Concrete in India – Development, Practices and Standardisation, Journal, Indian Concrete Institute (ICI), Vol. 6 (2), 2005, pp. 7 – 14.
4. Saini, S., Dhuri, S. S., Kanhere, D.K. and Momin, S. S., High Performance Concrete for an Urban Viaduct in Mumbai, Indian Concrete Journal, October 2001, pp. 634-640.
5. Basu, P.C., NPP Containment Structures: Indian Experience in Silica Fume based HPC, ibid, pp. 656-664.
6. Mullick, A. K., ‘Sustainable binder combinations for durable infrastructure Projects’, Proceedings., 3rd Asian Conference on ‘Ecstasy in Concrete’, ACECON 2010, Indian Concrete Institute, Chennai, December, 2010, pp. 395 – 404.
7. Reddi, S., A., ‘Self compacting concrete for the tallest building in India’, ibid, 815 – 826.
8. European Federation for Specialist Construction Chemicals and Concrete Systems (EFNARC), The European Guidelines for Self Compacting Concrete; Specification, Production and Use, May 2005, 63 p. Website - www.efnarc.org.
9. Ahlborn, Theresa M., Advancing UHPC in the United States concrete construction market, 4th Asian Conference on ‘Ecstasy in Concrete’, ACECON 2015, Indian Concrete Institute, Kolkata, October, 2015, pp. 1 - 8.
10. Proceedings of the 1st International Symposium of Asian Concrete Federation on ’Ultra High Performance Concrete, (ACF 2015), October, 2015, Kolkata, Indian Concrete Institute, 107p.
11. Aitcin, P. C., and Richard, P., The pedestrian/bikeway bridge of Sherbrooke, in: Proc., 4th International symposium on ‘Utilisation of High Strength/High Performance Concrete’, Paris, 1996, pp. 1399 – 1406.
12. Behloul, M., and Lee, K. C., Ductal Seonyu footbridge, Structural Concrete, Vol. 4, 203, Telford, London, pp. 195 – 201.
13. Perry, V. H., Case studies on innovative applications and challenges of introducing breakthrough technologies (UHPC) in the construction industry, in: Ref.10, pp. 33 – 41.
14. Naaman, A. E., Development and Evolution of Tensile Strain-hardening FRC Composites, Proceedings, Seventh International RILEM Symposium on Fibre Reinforced Concrete: Design and Applications, (BEFIB 2008), Chennai, September, pp.1 – 28.
15. Denarie, E., and Bruhwiler, E., Structural Rehabilitations with Ultra High Performance Fibre Reinforced Concretes, International Journal for Restoration of Buildings and Monuments, Aedificatio, Vol. 12, No. 5 and 6, 2006, pp. 453 – 467.
16. Viswanathan, T., and Mullick, A. K., Design and construction of Link slabs for jointless bridge decks with high performance fibre reinforced concrete (HPFRC), Highway Research Journal, Highway Research Board, Indian Roads Congress, New Delhi, Vol. 4, No. 2, July – December 2011, pp.25 – 40.
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