Concrete

Dr S C Maiti, Ex-Joint Director, National Council for Cement and Building Materials, New Delhi

Raj K. Agarwal, Managing Director, Marketing and Transit (India) Pvt. Ltd., New Delhi.

This paper discusses the quality of concrete vis-a-vis the life span of the present-day concrete structures in India. The paper's main focus is on durability of concrete. How to produce good quality durable concrete structures, with the available concrete - making materials! The focus is on materials like aggregates, mineral admixtures like flyash, g.g.b.s., silica fume and on mixing, placing, compaction, and curing of concrete. Use of blended cements to produce durable concrete structures, and details on concrete mixes used for M 70 and M 80 grades of concrete using such cements in developed countries have also been highlighted.

Introduction

Concrete
'Concrete' is a mixture of Cement, water, aggregates and admixtures. The chemical admixtures change the initial characteristics of concrete e.g. setting time, workability, cohesiveness, fluidity etc. The mineral admixtures e.g. flyash and ground granulated blast furnace slag (g.g.b.s.) modifies the resisting capacity of concrete in aggressive environments, and they, being fine materials, help in producing a cohesive, non-bleeding concrete mix. Thus, concrete is a versatile construction material. Its characteristics can be made as desired, using suitable concrete-making materials and their right proportions. The basic characteristics of concrete i.e. workability (fluidity and cohesiveness) and 28-day compressive strength can be obtained as desired, using suitable proportions of the constituent materials. Cement is the binding material in concrete. The 28-day compressive or flexural strength of concrete mostly depends on the water-cement ratio or water-binder ratio and the 28 days compressive strength of cement. The water-reducing admixtures reduce the water content of concrete and thereby, the compressive strength of concrete can be increased by reducing the water-binder ratio. Thus, different grades of concrete i.e. M20, M30, M70 etc. can be produced. Without altering the water content or water binder ratio for a particular grade of concrete, the workability of concrete can be increased with the use of superplasticizers. Thus, in heavily reinforced concrete sections, high-workability concrete can be placed without much compacting effort.

Concrete
In mass concrete structures, bigger sizes of coarse aggregate are used. Air-entraining admixtures are used to produce cohesive concrete mix in such cases. The mineral admixtures like flyash or g.g.b.s. reduces the heat inside the concrete. Thus, PPC (containing flyash) or PSC (containing g.g.b.s.) can be used as low heat cement. For producing very high strength concrete (say M 80), efficient superplasticizer reducing 30-35% of the mixing water will be required. Along with high-strength OPC, mineral admixture silica fume (8-10% by weight of cement) will also be required to produce such high-strength concrete. Silica fume concrete is also abrasion-resistant, and therefore, is being used in construction of spillways of concrete dams.

The long-term strength and durability of concrete are important characteristics of concrete. The concrete structures must provide the designed service life. The timely maintenance of structures is essential to obtain the desired service life.

In spite of good quality cement being manufactured these days, the service life of concrete structures is not increasing. Indian construction industry should rise above certain level, and produce concrete structures, which will have long service life.

Use of Mineral Admixtures in Concrete

Flyash, ground granulated blast furnace slag and silica fume have been recommended to be used in concrete as mineral admixtures1. Flyash and silica fume are good pozzolanas, whereas g.g.b.s. is a latent hydraulic material, which also reacts with the lime liberated due to the hydration of OPC in concrete.

Silica fume, is a very fine non-crystalline silicon dioxide, a by-product of ferrosilicon industries. It is presently being imported in India from Australia, Norway and China. Generally 5-10% silica fume by weight of the cement is sufficient to produce high-strength concrete (M60 grade and above). Being a highly reactive pozzolana, it develops also the early strength of concrete. silica fume (about 8% by weight of cement) has been used in the spillway of Tehri dam, for abrasion resistance. In the spillway of Kol dam (Himachal Pradesh), it is proposed to use about 10% silica fume for M 80 grade of concrete2.

Good quality flyash is available from the electro-static precipitators of our super thermal power stations. But the quantity of Grade 1 flyash is not sufficient to be used to produce portland pozzolana cement (PPC) by the cement manufacturers. The requirement of such flyash by our ready mixed concrete (RMC) plants is also very high.

It is observed that, because of insufficient quantity of good quality flyash, the cement manufacturers are grinding the coarse flyash to the required fineness to produce PPC of minimum fineness of 300m2/kg. By this process, the "ball-bearing effect" of the spherical shape of the flyash particles is lost, and consequently, the beneficial properties of flyash e.g. lower water demand and increased workability of concrete are also lost.

Concrete
We use flyash in concrete, because as a pozzolana, it has good effect in concrete and we get a cohesive concrete mix. In mass concrete, heat development inside the concrete is less. Sometimes, it substitutes part of cement and sometimes, part of the fine aggregate also. When the quality of flyash is not good, it still has beneficial effect. The reactive silica of such flyash may be in less quantity, but in the long run, the part of such flyash is going to provide a denser microstructure. In such case, use of chemical admixtures i.e. superplasticizer has a major role to play. It should reduce the water-binder ratio (w/b ratio) as low as practicable. With reduced w/b ratio, the permeability of concrete will get reduced, and will result in increased compressive strength in the long run. Therefore, it is expected that low-quality flyash in conjunction with the use of compatible and efficient superplasticizer will provide long service-life for concrete structures. Proper mixing of concrete, adequate compaction and longer curing period will definitely provide durable concrete structures is such cases, also.

The mineral admixtures i.e. flyash, g.g.b.s. and silica fume being very fine particles, their uniform blending with cement and aggregates in ordinary drum mixers can not be ensured. In most of our construction sites, batching and mixing plants are not available. Therefore, it is suggested that, slurry can be made with the mineral admixture and part of the mixing water, in the drum mixer, before placing cement and aggregates.

IS 456 stipulates that the mixing of concrete in the concrete mixer shall be continued untill there is an uniform distribution of the materials, and the mass is uniform in color and consistency. If there is segregation after unloading from the mixer, the concrete should be remixed. The mixing time shall be at least 2 minutes for drum mixers. For efficient concrete mixers as in RMC plants, the manufacturer's recommendations shall be followed, and trials may be carried out to produce cohesive concrete mix, using proper combined grading of coarse aggregate fractions.

In our construction sites, generally drum mixers are used. Although, the minimum mixing time for concrete has been fixed as 2 minutes, in many cases, it has been observed that the concrete mix is not a cohesive mix, and is not uniform in color and consistency. The cohesive (not segregated) concrete mix can only be placed properly in the formwork and compacted well. This process of placing a cohesive concrete mass around the reinforcement, and fully compacted and finished well, has got considerable influence on the durability of concrete. The adequate cover to reinforcements, dense concrete cover and well-compacted concrete will not get carbonated during its service life.

To get maximum benefit out of mixing of concrete, it is suggested a two stage mixing : firstly dry mixing of materials (with covered mixer drums) and then wet mixing process. The Japanese researcher used "neiling" of concrete mass (with small quantity of water) and then wet mixing process. The laboratory data of such two stage mixing indicate higher compressive strength and lower permeability of concrete.

In the mixing process, part of the mixing water should be placed in the concrete mixer in the beginning and the chemical admixture along with rest of the mixing water should be placed towards the end of mixing. This procedure provides a concrete of uniform colour and consistency and a cohesive concrete mix.

Concreting in Rural India

Our rural roads and housing are suffering a great set back, because of improper mixing and compaction of concrete. How can we expect durable concrete structures when the quality of concrete placed is not 'good'. The normal drum mixers are not available in rural India. The concrete vibrators are not used, because of non-availability of electricity. In such situation, although rural concrete roads are supposed to be of M 30 grade, and the RCC slabs in houses are supposed to be of M 20 grade, the real scinario is terrible. With hand mixing of concrete, and with rodding of concrete in the formwork, no durable concrete can be produced. The result is shorter life - span of concrete structures.

In rural india, diesel-operated concrete mixers and vibrators can be used to produce cohesive concrete mix and placed and compacted efficiently. However, if hand mixing is the only alternative, 10% extra cement is to be added to the concrete mix, and the mixing time is to be increased, in order to obtain a cohesive concrete mix.

Life Span of Concrete Structures

In India, we design buildings for a life span of about 50-60 years. Bridges are designed for about 100 years, while the concrete dams are designed for about 150 years. Delhi Metro structures have been designed for 120 years. The Euro tunnel connecting England and France under the sea has also been designed for 120 years.

Concrete
Although, our buildings are designed for 50-60 years, it has been observed that the buildings have longer life span. Old buildings in Mumbai and Kolkata developed distress at about 100 years age. With the availability of good construction materials in the present days, our structures should provide longer service life, provided we build them with adequate care, using proper method if mixing, placing, compaction and curing of concrete.

In addition to mineral admixtures like flyash, g.g.b.s. and silica fume, the chemical admixtures e.g. superplasticizers provide the required workability and compressive strength of concrete. But because, both cement and the chemical admixtures are chemicals, their compatibility must be ensured before use.

Fortunately, a large number of chemical admixture companies are operating in our country, specially in cities, and the materials like plasticizers, superplasticizers (normal and retarding type), accelerating and retarding admixtures are available in good quality and quantity. Only their proper dosage and compatibility with different types of cement, if ensured, shall provide cohesive, workable concrete mix, which can be placed comfortably in the formwork and compacted well.

Thus, production of good quality concrete, their proper placing, compaction and curing shall ensure longer life span of structure. In normal "mild" exposure condition, the structures shall have required service life. But in aggressive environmental conditions, they require special care and protection. For example, in marine environment or in heavy rainfall areas, or where aggressive chemical environment exists, the concrete structures should be built with the required materials, and protected well. In sea environment, chloride and sulphate will reduce the service life : the sulphate will attack the concrete, and the chloride will corrode the reinforcements. In such situations, codal provisions must be followed. With the use of superplasticizers, water-cement ratio can be reduced, as low as possible, may be in the range of 0.35 to 0.40, so that the permeability of concrete gets reduced. Portland slag cement with more than 50% slag content is suitable in case both sulphate and chloride are encountered in the environment.

For 'Severe' conditions, such as thin sections under hydrostatic pressure on one side only, and sections partly immersed, considerations should be given to use a water-cement ratio or water-binder ratio (in the case of use of mineral admixtures), as low as possible. IS 456 states that portland slag cement conforming to IS 455 with slag content more than 50% exhibits better sulphate-resisting properties. For thin structures, the life can be increased by providing extra cover to steel reinforcements, by chamfering the corners or by using circular cross sections or by using surface coatings which prevent or reduce the ingress of water, carbon dioxide or aggressive chemicals1.

For high sulphate concentrations i.e. more than 2% in soil or more than 5% in ground water, IS 456 stipulates use of protective coating on the well-made (with lower w/c ratio and with proper cement) concrete surfaces. The coating can be based on asphalt, chlorinated rubber, epoxy or polyerethene materials.

In 'severe' exposure conditions i.e. in marine environment, in many places, fusion-bonded epoxy coatings have been used on steel reinforcements. The TMT bars, and the metallurgically changed corrosion-resistant steel bars are definitely going to increase the service life of concrete structures near the sea. Polymer-based coatings to steel reinforcements have also been found useful.

Concrete Aggregates

Use of good quality aggregates will definitely increase the service life of concrete structures. In many places in India, good quality river bed sand is not available. Sometime, only very fine sand is available. Sand and coarse aggregate must be strong and good, in order to produce durable concrete structures. Well-graded aggregates provide dense concrete mix. Crushed rock coarse aggregate from strong rocks like basalt, quartzite, granite etc. are angular and they provide good bond in concrete and develop strong concrete. Flaky and elongated coarse aggregates should be avoided, if possible. The crushing, impact and abrasion values of coarse aggregates shall not exceed the limits specified in IS 3833.

The fine aggregate should generally be of grading zone II or grading zone III. For very high strength concrete, coarse graded fine aggregate (Zone I) is suitable. Sometimes, river bed sand is not available, so we use crushed stone fine aggregate. They can also produce good quality dense concrete, provided their grading is satisfactory as per IS 383. The deleterious materials like clay lump, coal, lignite, shale and soft fragments should be avoided. Use of good quality well-graded coarse and fine aggregate will definitely increase the service-life of concrete structures.

Use of Blended Cements

Our cement manufacturers are producing more blended cements. The portland pozzolana cement (PPC) is about 50% of the total cement production. The PPC is with about 20% flyash. The portland slag cement (PSC) is a good quality cement, with slag content of about 40-50%. But this cement is limited, about 15% of the cement production. This has become a special cement, because of its availability near the steel plants. The slag (blast furnace slag) is a consistent material, and therefore is of low variability. The use of PSC has become important because this cement in concrete can resist very aggressive environments, and hence increases the life span of concrete structures. It has been prescribed to be used in sea environments and also in concrete dams. The PSC in concrete can resist the alkali-silica reaction in concrete. In aggressive environments, e.g. in concrete piles in sea environment in Gujarat, PSC with 70% slag has been used to produce high-workability M40 grade concrete. Typical concrete mix proportions used are as follows:-

OPC (53-grade) - 30% (141 kg.)

g.g.b.s. - 70% (329 kg.)

sand - 41% (720 kg.)

Coarse aggregate (20mm MSA)) - 1044 Kg.

Retarding superplasticizer - 0.8% (by wt. of OPC + ggbs)

In mass concrete, both PPC and PSC are advantageous, because of low heat of hydration, and also because their alkali is not fully effective. Typically, about 1/6 of the alkali of flyash is potentially reactive, and about 50% of the alkali of ggbs is effective4.

For the underground structural elements of cut and cover tunnels and station boxes, 30% flyash has been used for M35 grade of concrete in Delhi Metro projects5. Delhi Metro structures have been designed for 120 years. The water /(cement + flyash) ratio was 0.40, and the slump of concrete was 120mm.

In the Salhus mono tower cable stayed bridge in Norway, PSC was used. The bridge superstructure main span is 163m. the following mix proportions were used for M70 grade of concrete.

PSC - 450 Kg.

Silica fume - 35 Kg.

Light weight aggregate - 470 Kg.

Water - 195 Kg.

At kualalumpur city centre, there are 450m high twin towers, made of RCC core and columns and composite steel /concrete deck floors. Flyash was used in concrete for a grade of M80. Concrete admixture was used to produce 200mm slump of concrete. The elastic modulus for such concrete at 56 days was 35.5 Gpa. The total alkali in concrete was less than 3kg/m3 of concrete. This satisfies the BRE recommendations6 for resisting alkali-silica reaction in concrete.

The construction of Euro-tunnel under the sea (50-250m below sea level) is designed for 120 years. Each tube is composed of rings made of 5 segments. There are total 2,26,000 segments and 5,00,000 m3 of concrete is used. The length of the tunnel is 51km, of which 37 km is under the sea, 4km under land on French side and 10km under land on British side. The tunnel lining concrete (precast) is of M45 grade, and the cast in place concrete is of M 30 grade.

For the tunnel lining concrete, low C3A cement was not recommended. In marine environment, C3A is important to trap the chloride ions entering the concrete. OPC with 5-8% C3A was used.

For the cast - in - place M30 grade, durability of concrete was of more concern than the strength. Three blended cements were used:
  1. One blend containing 51% OPC, 25% slag and 24% flyash.
  2. One blend of 70% PSC (with 82% slag) and 30% flyash for RCC.
  3. One blend of 72% PSC and 28% flyash for unreinforced concrete.

Conclusion

The paper provides the 'definition', 'durability' and 'applications' of concrete of various grades and various constituents. Use of chemical and mineral admixtures along with good quality cement can produce concrete of any desired grade and if properly constructed, the service life of concrete structures can be enhanced. the mixing procedure and mixing time have considerable influence on the quality of concrete in terms of its cohesiveness and workability. Concreting in rural India has to be given special importance as producing and placing good quality concrete have become difficult, in the absence of electricity and proper mixing and placing equipments. The paper further discusses use of blended cements to enhance durability of concrete structures, and provides details on concrete mixes for M70 and M 80 grades of concrete used in developed countries. Finally the paper provides the details on the selection of concrete making materials (specially on blended cements) for the Euro tunnel, which has been designed for 120 years.

References

  • Indian standard code of practice for plain and reinforced concrete. IS 456:2000, Bureau of Indian Standards, New delhi.
  • Nanda R.L., S. Ratanaramig, P and Boonsiri S. Developing 'High performance concrete for hydraulic structure at Koldam.' the Indian Concrete Journal, Vol. 84, No.1, January 2010, pp. 21-33.
  • Indian standard specification for coarse and fine aggregates from natural sources for concrete. IS 383, Bureau of Indian standards, New Delhi.
  • Neville, A.M. 'Properties of concrete, 4th Edition,' Pearson Education Ltd. 1995.
  • Shetty, M.S., Muenz, K and Gall, N. Delhi Metro : 'Quality control of concrete for underground section.' The Indian Concrete Journal, April 2005, Vol. 79, No. 4, pp. 11-21.
  • BRE. Alkali aggregate reactions in concrete. Building Research Establishment Digest 330, March 1988.
  • Moranville-Regourd, M. 'Selection of Concrete materials for the Euro-Tunnel.' Proceedings, P.K. Mehta Symposium on Durability of concrete, Nice, France, May 23, 1994, pp. 147-159.
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