INTRODUCTION
From the ancient relicts it appears that lime mortars and pozzolanic concretes have been in the service of mankind for at least a few thousand years. However, after the invention of Portland cement in 1824, there was the appearance of modern concrete that extended the range of technically achievable goals. For most of the second half of the 19th century, the use of concrete was by and large governed by the individual skill, experience and innovativeness of the architects and builders. The lack of any effort to organise the knowledge gained through experience and research resulted in the concetration of the know-how in a few hands, widespread failures, inefficient and uneconomical use of the materials. However, at the beginning of this century, in the aftermath of industrial revolution, there was a boom in construction activities which led to an increased attention to concrete technology and improved application practices. This was a direct outcome of the public perception of high risk to the life and property involved in the large-scale construction projects using this material and technology. Consequently, at the beginning of this century, both in Europe as well as in the United States, considerable efforts were made to organise the available knowledge on structural engineering and concrete technology so as to regulate the growth of construction activities towards human safety and durability.

As a result of these efforts concrete turned out to be the most widely used man-made product. It is second only to water as the world's most heavily consumed substance. During the 20th century there has been phenomenal growth of the Portland Cement Concrete Industry. The world wide annual consumption of concrete today is approximately eight billion tonnes and due to increasing urbanisation, it is expected to grow steadily during the coming century. However, a general concern for lack of durability is perceived as the prime threat to the future of concrete. The visible deterioration of concrete structures and the need of repair and rehabilitation are progressively compelling us to take the durability of concrete more seriously now than ever before. There is a growing interest in building more durable structure with extended service life because of rising costs of repair and rehabilitation.
The present article is aimed at culling out a comparative scenario of concrete that was made in the past, what is made now and what would be made in near future.

CONCRETE OF THE PAST
The archeological remnants indicate that the use of building materials from a clay-based mixture was established in the bronze age (about 3000 BC). The same age is also taken as the period when the use of air-hardening lime started. This obviously was linked with the discovery of fire and chanced calcination of limestone in fire rooms. It appears from the excavations of Mohenjo-daro and Harappa in the Indian subcontinent that the art of brickmaking and properties of lime mortars were also known to the Indians at that era. More or else around the same time (2500 BC), as the researches on the Pyramid of Cheops revealed, the Egyptians discovered gypsum as a binder. It is believed that the Phoenicians systematised the practice of mortar preparation with burnt lime and volcanic ash. From the Phoenicians the use of lime was transmitted to the Greeks, Romans and other civilisations down to modern times. A sample of concrete (based on calcined limestone, volcanic ash etc. of Greek and Roman invention), which had been underwater in the harbour of Puzzuoli (Italy) over 2000 years, had been physically retrieved and studied. The concrete walls of the cistern at Kameiros on the Greek Island of Rhodes, built about 500 BC show that this type of material can last at least 2500 years.

There is a controversial theory that the Pyramid rocks were then cast in wooden moulds in their final location. Using a variation of what we call match casting, one or more sides of the blocks were cast directly, against neighbouring blocks producing the perfect fit as close as 1/500th of an inch. How else could massive blocks weighing as much as 30 tonnes be built at thirty stories height, involving placement of two and half million blocks in a span of 20 years reign of the Pharaoh Cheops ? To-day, even the head of the Sphinx is suspected to be made of ancient concrete.

It is interesting to note that "concrete" or "opus caementium" had been elaborately documented by Vitruvius, a first century BC Roman architect and engineer. Classification of volcanic ashes suitable for the preparation of lime mortars, mix ratios required for walls, plasters and other applications, procedures for making mortars and concrete, etc. had been reportedly covered in those documents. From those records and from the relicts of Roman aqueducts, it is evident that the lime-pozzolana concrete making in the period from the 19th to 2nd centuries BC had reached a high level of perfection.

Many observers of ancient architecture are struck by the vast difference in quality between original structures and more recent repairs. Modern restoration of the sphinx done between the last ten and fifty years is already damaged, while repairs made a few centuries ago with gypsum mortar prevented damage by causing a protective crust on the surface. Similarly within the last half a century the concrete blocks used for the restorations on the Step Pyramid at Saqqara have already cracked. All these observations raise a basic question was the ancient concrete more durable ? If so, why ?

CONCRETE IN TRANSITION
With the fall of the Western Roman empire, the building materials technology suffered a setback with decadence of constructions. While lime and lime pozzolana binders continued to retain their practical importance, at the end of the 18th century there was much activity in Europe to discover an inorganic binder of improved properties. All these activities culminated in Joseph Aspdin inventing Portland cement in 1824 and I C Johnson on perfecting the production technology and hardware for Portland cement in 1847.
The rebirth of to-day's concrete dates back to these inventions and innovations. Subsequently in 1880 Hennebique introduced the concept of reinforced concrete building. Since then structural portland cement concrete has been routinely produced. Its use gained momentum at the turn of the last century and increased sharply after the World War.

CONCRETE MAKING
AS SEEN TODAY
Typical Portland cement concretes have volume fractions of aggregate that range approximately from 0.7 to 0.8. The remaining volume is occupied initially by a matrix of fresh cement paste consisting of water, cement air voids and often admixtures. While the aggregates occupy most of the volume, they are relatively inert and intended to be stable. It is the cement paste matrix that undergoes the remarkable transformation from nearly fluid paste to rock-hard solid, transforms plastic concrete into an apparent monolith, and controls many important engineering properties of hardened concretes Successful use, therefore depends on understanding the nature of concrete. The steps to obtain concrete performance are conceptually illustrated in Fig 1. The process starts with a mix design and specification developed for a particular application. It is followed by selection and acquisition of constituent materials and processing of these materials in accordance with the specifications. Presumably if the design, selection and implementation steps are properly conducted, the concrete properties and performance will meet job requirements. However, it is nalve to assume that the steps to obtaining properties and performance can be achieved without accommodating variations. But the question remains as to what level of variations can be accepted without any detrimental impact on programme.
 
 

Fig. 1. Consistency of concrete properties as related to its production process

Further, with improvements in concrete technology, concrete has become more versatile, but also more complex in that the number of mix constituents has increased. It is rare to encounter concrete that consists only of cement, fine aggregates, coarse aggregates and water. Today most mixes also contain chemical or mineral admixtures or both. To minimise the variability of concrete, it is necessary to maintain the uniformity of constituent materials.

Present Status of Concrete Quality
It is a common knowledge that the normal concrete with compressive strength ranging from 20 to 40 MPa is adequate for most structural applications. The tensile and flexural strengths of such concrete are relatively low and, therefore, the material tends to crack easily under tensile stress. A concrete with a few visible cracks shall continue to perform satisfactorily under compressive loading, however; if the cracks are interlinked with the micro cracks, the susceptibility of concrete to water penetration and deterioration is greatly increased.

The use of high strength concrete (HSC) (more than, say 60 MPa) during the recent past has proved to be advantageous. It has enabled the construction industry to design taller, longer, slimmer and lighter structures. One major disadvantage of HSC is that the material is more brittle than normal concrete and there is no load-carrying capacity beyond ultimate stress. Therefore, while using HSC in structural members, it is essential to provide ductility in the members by some means. Thus, the use of HSC, per se, did not offer any significant relief to the issue of non-durability of concrete.

Notwithstanding this shortcoming of HSC, the strength property of concrete has been continuously improved. In reality the concrete strength has increased almost four times in the last two decades from about 30-40 MPa to 130-140 MPa. In fact, ultrahigh strengths exceeding 300 MPa have been achieved in the laboratory. However from the point of view of long-term durability of structures, what is more important is the impermeability of concrete and not strength alone. This consideration has led to the development of High Performance Concrete (HPC),- the concrete of tomorrow.

REQUIREMENTS AND PROPERTIES OF HPC-THE FUTURISTIC MATERIAL
HPC has been defined as concrete with improved constructability, improved durability and improved mechanical properties. Compared with conventional concrete, HPC meets one or more of these requirements :

• Places and compacts easier
• Achieves high strengths at early ages
• Exhibits superior long term mechanical properties such as strength, resistance to abrasion or impact loading, and lowpermeability.
• Exhibits volume stability and thus deforms less or cracks less
• Lasts longer when subjected to chemical attack, freezing and thawing, at higher temperatures.
• ¨Demonstrates enhanced durability

Barrier to Proliferation of High Performance Concrete
The most critical barrier preventing the rapid introduction of high-performance concrete to the market is the lack of enabling technologies to support cost-effective commercial-scale production. It is felt that the enabling technologies should directly support the following classes of potential commercial applications :

• Construction with low maintenance needs
• Construction enabling rapid progress to completion
• Earthquake resistant construction
• Infrastructure repair, retrofit and renovation

* Innovative Materials Systems and Processing Technologies

•  Chemical and mineral admixtures
•  Corrosion-resistant steel reinforcement systems
•  Fibre reinforcement including carbon fibres
•  DSP, MDF and Reactive Power Concrete
•  Recycling of concrete materials

* Innovative Structural Design Concepts and Methodologies

•  Integrated design methodologies (to establish limit states of design)
•  HPC structural elements and systems
•  Strengthening and retrofitting of structures
•  Fire resistance

* Commercially upscalable Cost-Effective Construction Systems

•  Extrusion technology for fibre reinforced cement-matrix composites
•  Microwave curing technology
•  Rapid forming technology
•  Rapid mixing, placing and curing facilities
•  Autoclaved and aerated products
•  Rapid repair and renovation of underground pipes

* Performance Prediction and Monitoring Technologies

•  Measurement, recording and analysis of fracture and fracture growth in the submicron

 range(interferometers)
•  Embedded network of fibre optic sensor
•  Computational models for microstructural engineering and service life prediction

The challenge for high-performance construction materials and systems is translating this knowledge to our infrastructure that will be characterised by :

 
• Superior strength, toughness and ductility
• Enhanced durability/service life
• Increased resistance to abrasion, corrosion, chemicals and fatigue
• Initial and life-cycle cost efficiencies
• Improved response to natural disasters and fire
• Aesthetics and environmental compatibility

There has been a rapid rise in the consumption of chemical admixtures throughout the world during the past two to three decades. In the UK, the proportion of concrete containing chemical admixtures has increased from 10 percent to over 50 percent during the past two decades and the trend in other developed countries would be more or less on the similar lines.

Interesting developments are occurring in the field of super-plasticisers, air-entraining agents, anti wash-out admixtures and waterproofing materials. Some promising new superplasticisers which can be independent of the addition procedure and are able to maintain slump levels for at least 1-2 hours have been investigated to replace the traditional products. Similarly, preformed bubble reservoirs in the form of porous particles or hollow plastic microspheres of size 10 to 50 micron - have found to possess a potential to replace air entrainment method to produce frost-resistant concrete. In Japan, anti wash-out admixtures, water-soluble cellulose-type or polyacrylamide-type polymers have been used for effective underwater placement of concrete, the need for which is incidentally bound to grow in the future. Also in Japan, polymer-modified paste or slurry with a very high polymer-cement ratio of 50 percent or more is applied over million m2 of reinforced concrete substrate each year as a liquid applied waterproofing membrane with thickness ranging between 2 to 4 mm. Recently, an interesting waterproofing material which solidifies in water has been developed in Japan and it has a potential as a shock-absorbing, waterproof, backfill material for tunnels and drains.

The performance of concrete can be improved with the addition of fibres and polymers and a great deal of work has already been done with some promising results in both these areas. Fibre-reinforced concrete (FRC) is generally produced by adding steel, polymer, glass and carbon fibres and such addition leads to improvements in flexural strength, toughness and impact resistance. Recently, a strong interest is evinced in carbon fibres. In addition a new generation of micro-fibres, which can be used at higher addition rates to bring about major improvements in the mechanical properties of FRC, is being developed. Slurry-infiltrated fibre concrete (SIFCON) with micro-fibre contents upto 20 percent and compact reinforced composites (CRC) having very high compressive and flexural strengths are the promising developments in this area.
Polymers have already made a considerable headway in the field of concrete and their use enhances certain properties of concrete, mainly the tensile and flexural strengths, chemical resistance, toughness, bond, impermeability, etc. 
They are used in concrete for varied purposes such as improved bonding, pore-sealing, reducing permeability to water and aggressive chemicals, self-levelling, dust proofing and a host of other functions. Currently, considerable interest is oriented towards the use of polymer-modified mortar and concrete for repair and rehabilitation of deteriorated concrete structures.

While the above-mentioned advances in the sphere of cencrete technology have indeed been impressive and immensely beneficial, an almost entirely new set of exotic materials is now emerging. These new advances have occurred as a result of "manipulating of microstructure and controlling the chemistry, or both, of cements" and have led to the development of more advanced cement matrix composites, with some materials crossing even the boundaries of the traditional cementitious materials. These include, the materials formed by densification through pressure and heat with properties approximating those of fired ceramics, strong macrodefect-free (MDF) cements produced by special processing; materials termed as DSP (densified systems containing homogeneously arranged, ultra fine particles) which are ceramic-like materials formed as a result of chemical reactions occurring at or near ambient temperatures and are known as chemically bonded ceramics (CBC).

GOAL SETTING FOR HIGH PERFORMANCE CONCRETE
Many natural disasters including earthquakes have focussed attention on the fragility of any country's built environment. Looking from another perspective, it has been realised that the costs of an inadequate infrastructure are enormous. It is therefore, felt that the HPC-related technologies are needed to achieve the following technical goals for large-scale concrete constructed facilities :

• 100% increase in durability (service life) with a commensurate reduction in maintenance costs.
• 50% reduction in time required for construction
• 100 to 1000-fold increase in energy-absorbing capacity without a compromise in strength or stability for earthquake resistant structures.
• 50% increase in service life of infrastructure after repair,retrofit and renovation

Further there is gross shortage of over 20 million dwelling units and the existing infrastructure is in a stage of disrepair.
With this kind of an outlook for the potential of the construction industry, it is high time that we look at upgrading the technological plane of this construction industry. In this context the emerging world of high performance materials and systems becomes relevant. The need to implement a research, development and deployment programme to exploit the potential of high performance materials and systems is evident and timely.

CONCLUSION
In the evolutionary history of mankind, it is now believed that construction as a human activity would have started almost ten thousand years ago but apparently the first five thousand years saw only construction through juxtaposition of stoneblocks without any binders. The history of cement relates to the subsequent five thousand years, when clay, lime, gypsum and natural polymers were used as binders. In this long history of cements, the story of Portland cement and modern concrete does not span even two hundred years.

It is generally surprising that the structures built 2500 years ago are still surviving but it is not so surprising when seen scientifically to explain "why". It seems that extensive use of pozzolanic materials, non-use of steel in the structures, placement of thick structural members and adoption of very rigid QC measures were the major factors contributing to the durability of ancient buildings. With the advent of Portland cement and RCC, there has been a significant shift in the mode and techniques of construction, leading to faster onset of structural distress in a very extensive manner. Search for solutions to these problems has now led us to realise that the design and placement of concrete cannot rely on "strength'alone, instead the concept of "durability" needs to be emphasized by producing more impermeable and easily constructable concrete. Thus lately, high performance concrete has made its appearance in the field of construction with promise of durability and service ability of structures. Interestingly, the use of mineral and chemical admixtures in HPC resembles similar approaches adopted in ancient concretes. Does "science", like history, also repeat itself?