Concrete is one of the oldest and most widely used building materials in the world. In one form or another various types of concrete have been used for construction purpose for around 9,000 years by now. Concrete platforms dating back to 7,000 B.C. have been unearthed in West Asia and concrete structures have been found in a 7,000 years old sunken city, discovered off the coast of Gujarat. These are just two examples, taken at random, from hundreds of concrete structures built throughout known human history.
One of the enduring mysteries of all times, is the answer to the question as to how did the ancient Egyptians, who had no machines worth the name, haul up huge limestone blocks weighing over fifteen tonnes, to construct their massive pyramids. This question has for centuries been very widely debated by archeologists, historians and engineers, and several possible answers arrived at. Leaving aside improbable conjectures like the one that the pyramids were constructed by alien beings who visited our planet from outer space, most other theories focus on methods used to quarry the gigantic blocks, transport them to the building sites, shape and polish them so finely—even though no mortar has been used to join the blocks together, they fit so nicely that even a knife blade cannot be slipped between adjacent stones–and finally haul them into position. Most aver that the blocks were chiselled out of hillside rock formations, floated down the Nile on boats or rafts, moved across land using wooden rollers placed below them, and positioned using long sloping ramps. Human slaves, along with elephants, formed the motive power. Although, eminently feasible, this method would have been painfully laborious and slow. Some years back a new theory of pyramid construction was put forth. According to it, there was no question of quarrying, transporting, shaping and polishing of blocks; nor of hauling them into position. This is because, according to this revolutionary theory, there were no limestone blocks to start with. They were, in actual fact, poured in-situ lime concrete blocks. This process, it is argued, would have saved the almost impossible effort required to construct the 4,500 years old pyramids, especially as the Egyptians of that time apparently had no iron tools, now were aware of the invention of the wheel.
As concrete evolved over the ages, it has become quite clear from recent discoveries, that several ‘modern’ varieties of concrete, may not be so modern after all. Take for instance, lightweight concrete. As far back as 83 B.C. Roman architects used lightweight aggregates formed by the cooling of lava, from volcanoes like Etna, Stromboli and Vesuvius, to build the Temple of Fortune in Palestrina, Italy, whose ruins were discovered some time back. Excavations in Italy have also revealed the remanants of large number of other residential and official buildings, made with lightweight concrete, dating back to between the 1st Century B.C. to the 2nd Century A.D.
Another example of a supposedly modern form of concrete which in actual fact was fairly widely used by the ancients, is fireproof concrete. Sometime during the 3rd Century B.C., the buildings of almost the entire city of Rome, were re-built with fireproof concrete. Then, to give them an aesthetic look, they were given a facing of bricks. When Rome’s first Emperor, Augustus Caesar (after whom the month of August is named), nephew of Julius Caesar, took independent charge of the Roman Empire in 32 B.C., he decided that his capital did not look grand enough. So he had marble facades put on every building, so that Rome literally glittered in the sunshine. Four hundred years later, when the Goths under Alaric looted Rome, they set the entire city on fire. The marble facades and brick burned off, but the basic concrete structures of Rome almost all survived
InnovationsAnother innovation that originated over 2000 years ago in Rome, was the blending of reddish volcanic earth with lime. This resulted in a fairly unique product–concrete that set under water. Undersea structures built at that time, are still existing today, though most of them are damaged or broken.
Coming to more modern times, concrete was used for making boats, yes you read that right, it is boats, since 1850s, in France. These boats were made by plastering concrete over an iron mesh boatshaped framework. This composite was named as Fericement in early days and Ferrocement later on. It is still being used for making boats, water tanks, house components, irrigation & sanitation item etc. Such boats had many advantages since they were waterproof and leak proof, did not rot and were also almost damage proof. Right till the 1920s, concrete boats and ships, some as big as 132 metres long and 17 metres wide, weighing over 7,500 tonnes, were plying on ocean going routes. Even today, many colleges in USA, organize regular concrete boat races, which are extremely popular with students.
Next on our list, is fibre reinforced concrete. This product too, is not all that modern as, according to available records, the first fibre-reinforced concrete products were bricks, reinforced with straw fibres, which were in wide use some 3000 years ago. And concrete roofs, reinforced with horse-hair, were all the rage around the 3rd Century A.D. Steelfibre reinforced concrete is a more recent product, since the know-how for the manufacture of steel fibres was not available earlier on. Hence steel fibre reinforced concrete only made its appearance in 1874, a mere 132 years ago.
And do you know when Ready Mixed Concrete (RMC) i.e. concrete mixed in a central batching plant and transported to different work sites, first made its appearance. It was in 1903. Unfortunately, suitable motorized transport for its conveyance, from batching plant to work site, was not available in those days, since the automobile industry was still in its infancy. So, when centrally batched concrete was carried by horse-drawn container vehicle, it often set on the way, as there was almost no knowledge regarding retarding chemicals available at that time. The manufacture and use of RMC was therefore somewhat slow, till about 1914, when the first petrol engine driven RMC trucks made an appearance. Incidentally, 1914 was also the year when the first concrete road was constructed in our country.
Despite all that is mentioned above, modern-day concrete technologists need not feel that “there is nothing new under the sun.” Today, we have a number of innovative usage for concrete, most of which–as far as current knowledge goes–were unknown or even undreamt of, till just a few decades ago. These include flexible concrete, spun concrete, whisper concrete, ultra-thin concrete and even cementless concrete. Some details of these products and processes are given in the succeeding paragraphs.
Eventually, after dozens of hits and misses, they produced a fairly satisfactory mix which, after setting, resulted in a concrete that, when overloaded, bent/sagged but did not crack. The mix was similar to a normal concrete mix, except that there were no coarse aggregates in it. Also it contained around two percent fibres, compared to the normal half percent contained in ordinary fibre-reinforced concrete. Additionally, the fibres incorporated in ECC were specially coated ones; this coating allowed the fibres to slide within the concrete, thus imparting flexibility to it.
Concrete produced by ECC techniques, has already been used in projects in several countries, including Australia, Japan, Korea, Switzerland and USA. The latest formula has given an end product that is 40 percent lighter in weight and 500 times more resistant to cracking, than normal concrete. This latest composite concrete has been used for a 5cm ultra-thin deck on a bridge in Japan. The 40 percent saving in weight has led to significant economies in construction cost especially in the understructure on which the dead load came. An additional bonus that the deck’s flexibility gave, was that there was no requirement for expansion joints–the entire deck slab was a continuous one. This not only provided a smoother ride for motorists using the bridge, but also saved on the bother and cost of joint filler maintenance/ replacement.
Spun ConcreteColumns are vital part of most buildings. Load bearing columns, unfortunately, tend to be large in size. Though large columns can be fashioned and designed artistically, thus giving a pleasing appearance, they often take up vital space and obstruct free movement as well as vital viewability. Pre-stressing columns imparts additional load bearing capacity to them, thus allowing them to be made slimmer in size and permitting larger spacing between them but even then, their size can create problems.
To provide columns with even more load bearing strength, so that their diameter could be further reduced, a new technique has been conceived, which is basically German in origin. This technique results in the production of what is known as spun concrete. The procedure for making columns of spun concrete is roughly as follows. A steel mould in the shape of the column is made, in two halves. The reinforcement cage for the column is also made in two parts. One part is placed in each half of the mould, anchored to fixing devices, which are a part of the mould, and pretensioned. High strength concrete, up to M-100, is then poured into the mould halves. After that the halves are bolted together and placed in a centrifuge.
The mould, with the concrete in it, is then spun for approximately 10 minutes, at 600 rpm. After that, the concrete is left to set, for between 12 to 16 hours, depending on various factors, such as strength required, column size, ambient conditions and so on. The mould is then removed and the column cured, then transported to the construction site. This process produces a very dense, high strength concrete structure. Heavy reinforcement ratios up to 15 percent, have enabled production of 28 metre high columns, having a diameter of only 70 cm, capable of taking loads up to 360 tonnes, by the use of spun concrete.
And that is how ‘Whisper Concrete’ came into being, although it was partly by accident.
In late 70’s and early 80’s, despite predictions that the then quantum jump in oil prices would drastically reduce individual usage of vehicles, traffic on concreted European roads increased by leaps and bounds. Simultaneously, there was an increase in vehicular speed, particularly on inter-city highways. This caused a greater wearing action on road surfaces and also an almost unbearable increase in the level of sound being produced. Smoothened pavements, worn down due to excessive wear and tear, led to skidding of vehicles, causing accidents; and noise pollution gave rise to headaches and other soundrelated psychological problems. These troubles were particularly noticeable near and on autobahns, motorways and freeways, where speeds generally exceeded 120 kmph, and between 75,000 to 1,00,000 vehicles traversed the facility every day.
Among the first countries to take cognizance of motorists complaints was Belgium. Since accidents due to skidding (which led to a large number of deaths and serious injuries) caused much more damage than mere noise pollution, priority was given for mitigating causative factors for the former.
Investigations into the causes of skidding, showed that when concrete pavements were initially laid, they were invariably given a non-skid surface by brooming; a method in which the surface of the road had grooves etched into it, by dragging steel-wire brooms across the top of the concrete pavement, before it had hardened fully. These grooves, which were generally made two millimeters deep, imparted excellent anti-skid properties to the road. However, continuous heavy traffic over on extended period of time, caused the ridges between the grooves to get worn down, thus flattening the surface of the pavement. In those days, re-grooving of the road surface, though eminently feasible, was a somewhat costly and laborious process (new techniques have made it easier to re-groove the top of a concrete pavement today). Hence road maintenance authorities tended to delay the operations, or even give it the complete go-by. So when it next rained, interaction between vehicle tyres and the wet, smooth road surface, produced a phenomenon known as ‘hydroplaning.’ Hydroplaning is a particularly nasty form of skidding, and normally leads to total loss of control of vehicles by drivers.
An accident rates in the country went up and criticism of official apathy mounted, the Belgian authorities started to take action. They began to look for ways and means to restore the anti-skid surface of concrete roads economically and speedily.
Initially, trial lengths of smoothened road surface, were overlaid with 40-50 mm of concrete having a maximum aggregate size of 6-8 mm. The surface of the new concrete, while still wet, was sprayed with a retarder consisting of glucose, water and alcohol; it was then immediately covered securely with polythene sheeting, to prevent evaporation. This particular retarder, as tests had shown, affected only the top 2 mm of the concrete.
Once partial curing of the remaining concrete had taken place (anything between 8 to 36 hours later, depending on the ambient conditions), the polythene sheeting was removed, and the surface of the road was swept with a machine, which had stiff, rotating wire bristle brushes. These rotating brushes removed the cement mortar from the top 1.5 mm of the pavement, thus exposing the aggregate and making the surface rough enough for safe high-speed driving in wet weather.
Further trials were then carried out, with the emphasis now on reduction of the amount of noise pollution being created. These only served to confirm the earlier findings, that the new type of surface was much quieter than any of the other pavements in service. The delighted public works authorities–who had got two benefits for the price of one–soon labeled the exposed-aggregate pavement as ‘whisper’ concrete, and decided to go in for it in a big way.
However, the Belgians soon discovered that along with its advantages, whisper concrete had one fairly serious drawback. Where overlaying of old smoothened concrete pavements was involved, the cost of using whisper concrete was more or less the same as regrooving, but involved much less effort; and where it was laid on an existing worn-out bitumen pavement, whisper concrete costs matched those of white-topping (re-surfacing of an old blacktopped pavement with thin concrete slabs). But where new roads had to be built, it was found that the pavement had to be constructed in two layers. A lower layer of 200 mm of ‘normal’ concrete, having a maximum aggregate size of 30 mm; and an upper layer of 40-50 mm of whisper concrete, having a maximum aggregate size of 6-8 mm. This double operation increased both time and cost of construction. Nevertheless, Belgian authorities decided that the advantages of whisper concrete far out-weighted its disadvantages. They therefore went on constructing fresh roads and topping existing ones, with the new material. Between 1981 and 1994 eight million cubic metres of whisper concrete was laid down on the country’s roads. Today, CRCP (Continuously Reinforced Concrete Pavement) with an exposed aggregate surface, is the standard form of road construction in Belgium, for all inter–city highways.
After Belgium, whisper concrete was taken up in a big way by neighboring Netherlands. Extremely happy with its performance, but not too pleased in having to build it in two layers, the Dutch carried out some trials of their own. They soon discovered that if the maximum aggregate size in the entire concrete mass was reduced to 20 mm, and a good percentage of small stone chippings added to the mix, whisper concrete pavements could be laid in a single pass. Though driving on such pavements was not as ‘comfortable’ as on two-layer whisper concrete, the noise produced was somewhat less, apart from the considerable saving in time and money since only a single laying operation was involved.
The next European nation to take up the new road building method was Austria. Austria is by and large a mountainous country, with many of its roads running along the lower portions of valleys. Increasing traffic at greater speeds along these arteries, caused noise to roll up the hillsides in waves. This phenomenon started causing ‘adverse political fall-out.’ Fearing loss of votes, worried Government officials, scanned literature, organized conferences and toured Europe, looking for solutions to the problem. It was not long before they discovered whisper concrete. After due trials and deliberations, the Austrians decided to adopt the Belgian two-layer technique of construction, rather than the Dutch single-layer method. This is because in Austria, despite the country’s middle–of–the–Alps location, suitably tough and hard aggregates are extremely costly. Such aggregates are essentially required for that country’s roads because the heavy snowfalls it experiences, means that most vehicles use studded tyres; and such tyres wear out soft aggregates very fast. Hence the Austrians used soft aggregates for the thicker lower layer of their concrete roads, and hard tough aggregates for the thinner, upper whisper concrete layer. Austria’s selection of the twolayer method of construction, proved to be a wise one, because even five years after the initial whisper concrete roads were built, their surfaces showed no signs of wear and tear, despite their regular use by studded tyre traffic.
The British, traditionalists as usual, waited to see the experience of others and then took up the construction of whisper concrete pavements only in 1995. The guidelines provisionally enunciated by them, are probably the most suitable ones for use by those building whisper concrete roads for the first time. These include:
- Under standard highway conditions, a concrete road should consist of a cement-bound sub-base, between 150-200 mm thick. On top of this, there should be 200 mm of CRCP, followed by 50 mm of whisper concrete surfacing.
- Existing concrete paving trains should be modified to lay the lower CRCP and the upper whisper concrete surface in the same pass.
- Full pavement width (even for double-lane roads) on each side of dual carriageway roads, should be laid in a single operation.
- Normally, 8 mm size coarse aggregate should be used in the surface layer. Not more than 3 percent of these should be oversized and 10 percent undersized.
- These aggregates should posses a polished stone value greater than 60; this will ensure sufficient hardness to combat wear and tear. The aggregates should also have a ‘flakiness’ index less than 25% which will ensure that they have a fairly uniform shape.
- Coarse aggregate should form around 60% of the whisper concrete, which should be airentrained. Sand used should be very fine. The cement used should be OPC (Ordinary Portland Cement).
- The whisper concrete layer should be initially levelled by a conventional mechanical float with oscillating beams. This should be followed by further levelling by a ‘super smooth’ float, set longitudinally down the carriageway, at right angles to the first float, which should remove any remaining imperfections or ridges.
- Spray the smooth finished surface immediately with a retarder consisting of glucose, water and alcohol. Then cover the surface with a polyethylene ‘cling’ film.
- Between 8 and 36 hours later (depending on ambient conditions), remove the polyethylene film and brush the surface with mechanically rotating, stiff bristles; to remove cement mortar from the top 1.5 mm.
- Properly planned operations should enable construction of about 3000 linear metres of whisper concrete per day.
Ultra-Thin White- Topping
However, a new technology emerged in the early 1990’s, which has dramatically expanded white topping technology and its use. This rehabilitation technique purposely seeks to bond the concrete overlay to the existing bitumen. As a result, the concrete overlay and the underlying bitumen act as a composite section rather than two independent layers. This composite action significantly reduces the load-induced stresses in the concrete overlay. Therefore, the concrete overlay can be considerably thinner for the same loading as compared to a white topping section with no bond to the underlying bitumen.
When describing pavement thickness, terms such as “thick” and “thin” are relative and depend on the viewpoint and experience of the user. For Ultra-Thin White (UTW) topping, a more definitive description is needed. Based on the international experience, ultrathin white topping can be defined as: “A concrete overlay 50 mm to 100 mm thick with closely spaced joints bonded to an existing bitumen pavement.”
There are three basic requirements for UTW overlays to perform properly. These are:
- Availability of an appropriately thick existing bitumen layer.
- Achievement of a bond between the existing bitumen pavement and the UTW.
- Provision of short joint spacing.
The composite section has opposing effects on corner stresses. There is a decrease in the concrete stresses because the whole pavement section is thicker. However, if the neutral axis shifts low enough in the concrete, the critical load location may move from the edge to the corner depending on the materials and layer characteristics. Essentially, the corner stresses decrease because the bonding action creates a thicker section, but increase because the neutral axis shifts down and away from the top surface.
To combat this effect, close joint spacing is critical. All pavement types must absorb the energy of the applied load by either bending or deflecting. Traditional concrete pavements are designed to absorb energy by bending and thus are made thick enough to resist stresses induced by bending. With UTW, short joint spacings are used so that energy is absorbed by deflection instead of bending. The short joint spacing also minimizes stresses due to curling and warping by decreasing the amount of slab that can curl or warp.
For the UTW overlays, the short joint spacing in effect forms a minipaver block system, which transfers loads to the flexible pavement through deflection rather than bending. Typical joint spacings that have performed well on UTW projects are somewhere between 0.6 and 1.5 m. It is recommended that the maximum joint spacing for UTW be between 12-15 times the slab thickness in each direction.
The construction of a UTW consists of three basic steps:
- Prepare the existing surface by milling and cleaning, or blasting with water or abrasive material.
- Place, finish, and cure the concrete overlay using conventional techniques and materials.
- Cut saw joints early at prescribed spacings.
Paving a UTW is no different from paving any other concrete pavement. Conventional slip-form and fixed-form pavers, as well as hand-held equipment–such as vibrating screeds–have all been used successfully without major modifications. The only real change is that the concrete layer is thinner than normal. Normal finishing and texturing procedures are applied to the surface.
Proper curing is critical to avoid shrinkage cracking and debonding between the bitumen and concrete pavements. Curing compound should be applied at twice the normal rate, because the overlay being a thin concrete slab, has high surface area to volume ratio, and can thus lose water rapidly due to evaporation. Care must also be used during application, to avoid spraying curing compound on adjacent uncovered prepared bitumen surfaces, since that would decrease bonding.
Joint sawing should be carried out with lightweight saws, as early as possible, to control cracking. Saw depth should be approximately one–fourth to one third of the total depth of the overlay. Typically, UTW joints are not sealed. Test studies have shown that UTW pavements perform well without sealants because the compactness of the slabs minimizes joint movement.
The concrete mix selected for particular project is matched to the traffic conditions and opened-fortraffic requirements. Synthetic fibers are often added to increase the post-crack integrity of the panels.
Ultra-thin White-topping projects have been carried out in several countries including USA, Brazil and Canada. However, the technique is still regarded to be in its infancy and requires considerable research to streamline and standardize it.
The American Concrete Institute issued ‘Supplement Specification 852’ on 11th July 2000, which laid down specifications for ‘Ultra-thin White-topping Overlay with Steel Fiber Reinforced Concrete.’ As far as is known this is the only existing specification on the subject.
Cementless ConcreteIn the late 1980s, Austria was facing a shortage of cement, due to several factors. Shortage of suitable quality limestone was one of them. Another was the extremely stringent emission standards for cement manufacturing plants set by the country’s Government, due to concern about the steadily deteriorating environment.
Both the cement and construction industries were worried, and decided to do something to sort out the problem. Discussions, experiments, laboratory and field trials became the order of the day. Eventually, an absolutely new, novel and unique product was developed, after 15 years of intense effort.
The scientists, technologists and others involved in the project, started off by thinking ‘outside the box.’ They decided that they did not want to produce a modified cement, or even an improved version of OPC. They resolved to create an alternative to cement. This was a tall order indeed, but the experiment team was determined to succeed; and succeed they did. Their basic premise was, that although they did not want cement, their alternative binding material, had to have cementitious properties, if they wanted it to take over cement’s role.
By trial and error, they narrowed down their choice of the base material to slag. Austria, located right in the heart of Europe’s biggest steel producing zone, was ideally situated to procure massive quantities of slag, easily and economically. And blast furnace steel slag is a highly cementitious material.
Once the base element had been identified further experiments and trials were carried out to find ways and means to convert it into a suitable, easy-to-use and economical binding agent. Finally it was determined that by blending gypsum, certain alkaline products and a few other additives with slag, they could obtain a substance that had all the binding properties of cement, yet was superior to it in many ways.
The advantages that this new slag-based binder had included:
- No burning process was involved in its production. Hence emission of carbon-dioxide and nitrous oxides was reduced to almost zero, making it extremely friendly to the environment.
- It has a very low heat of hydration. Hence it is ideal for mass concrete applications such as dams and foundations. Also, low heat of hydration means almost no cracks in the finished product, hence eminently suitable for water-retaining structures.
- High resistance of concrete products made from it, to sulphate and acid attack, as well as damage by alkali-reactive aggregates. Thus can be used with great advantage in aggressive environment.
- Energy saving of up to 80 percent in its manufacture, since this involves only grinding.
The author is grateful to the International Cement Review, BFT International and the Indian Cement Review for some of the information contained in the above article.