11th International Symposium on Concrete Roads, 2010
Conclusions by Mr Carlos Jofré, President of the ITPC
"This eleventh edition of the International Symposium on Concrete Roads has strengthened its character as an international forum to exchange experiences and innovations. 112 papers from 27 countries of all the parts of the world have been received. Interestingly, some of them deal with the results obtained when applying a certain technique in different countries on both sides of the Atlantic. A remarkable development of concrete pavements in some Latin American countries has also been reported.
All accepted papers contained useful and comprehensive information. Therefore it is not possible to summarize them in this limited space, but some general conclusions can be drawn:
Once again, it has been confirmed that concrete pavements, when properly designed and constructed, have a long service life as well as a minimum maintenance. Several papers dealing with real cases were presented during the Symposium, and participants had occasion of circulating on a motorway close to 40 years under traffic.
three types of concrete pavements are predominant: undowelled plain concrete pavements, dowelled plain concrete pavements and continuously reinforced concrete pavements. All of them have shown good results in the long term.
This durability is a clear evidence that, concerning thickness design, materials and construction, concrete pavements constitute a very consolidated technology. However, joint spacing is an issue still under debate. Shortening of slabs diminishes stresses due to both temperature and moisture changes, and could result in an eventual reduction of thickness. Several Latin American highways have been provided with relatively thin slabs not longer than 2,4 m; but not enough time has elapsed to accurately evaluate the performance of this approach.
Durability must be obtained by meeting also the criteria of sustainable construction. Some factors favouring the sustainability of concrete pavements are:
** Use of cements with active additions;
** Use of recycled materials coming from existing pavements, either rigid or flexible;
** Use of materials with very demanding technical specifications, and therefore scarce, just where they are really needed (e.g. polish-resistant aggregates only in the upper part of the pavement).
However, it has been stressed that, along a service life of 30 years, the potential environmental impact due to traffic load is 50 to 100 times more than that due to construction and maintenance together. Therefore, the largest and most effective reduction in impact is associated to a decrease in fuel consumption. Numerous studies have already shown the positive effect of concrete pavements in this regard. In some tests conducted in Sweden, differences of 1,1 % with a car and 6,7 % with a truck were recorded. These values are similar to those obtained previously in Canada and other countries. Higher fuel consumption on asphalt pavements is attributed to their higher rolling resistance, which has been confirmed in a study performed in Japan.
Anyhow, sustainability is a very complex issue, where many factors should be considered. Computer programmes specially written to assist both designers and decision - makers in the use of sustainability models can be a valuable tool. One of these models and its associated software, developed in the Netherlands, were presented during the Symposium.
In regions with severe winters, use of air - entraining agents is the measure most frequently adopted to achieve an adequate freeze - thaw resistance. However, an excessive air content due to an inadequate mixing can result in a concrete with a low - mechanical strength. Following a research study performed in Germany on the working mechanisms of different air - entraining agents, some practical recommendations have been formulated to avoid this problem. In Russia, a new method has been developed to estimate the so called spacing factor, another key parameter to obtain frost - resistant concretes. Authors state that it can be easily carried out with the use of regular equipment that is available in all road construction laboratories A number of methods are also available to predict freeze - thaw resistance based on tests performed on specimens. However, a comparative study conducted in Belgium concludes that some of them require a more precise wording to avoid discrepancies in their interpretation. In addition, dispersion of the results was significant.
It is possible to obtain reduced noise levels in concrete pavements without sacrificing their antiskidding characteristics. Several options are available. Currently the most effective one is the use of exposed aggregate textures with a small maximum size of aggregate (6 to 8 mm) and a high quality concrete. A long, positive experience in this technique has been obtained in countries like Austria and Belgium.
Exposed aggregate textures are frequently associated with the construction of the pavements in two layers. Aggregates not so demanding as those of the upper layer can be used in the lower one, even resulting from the demolition of old pavements. Two - layer construction is a well mastered technique that can be used even at jobsites with space restrictions, as it is the case in tunnels.
In motorways and main highways, pavements must be periodically monitored to program maintenance operations. The Falling Weight Deflectometer (FWD) is frequently used to evaluate the bearing capacity condition, in addition with some devices specially intended for concrete pavements, as the so-called Faultimeter. Special attention must be paid to the influence of thermal and/or moisture gradients when analyzing the results of this type of measurements.
A long-life pavement requires to be uniformly supported by a non-erodible subbase. In this regard, positive experience has been obtained in Spain with the use of vibrated lean concrete. Other countries where deicing agents are extensively applied favours the use of cement - treated materials protected either with a thin asphalt concrete layer or a non-woven, alkali-resistant geotextile.
All pavements need from time to time some repair or rehabilitation operations to be conducted on them, although their frequency in properly designed, constructed and managed concrete pavements is very low. Effective repair techniques are available. In this regard, it has been reported that repairs of longitudinal cracks due to moisture variations in the subgrade can last more than twenty years.
Ultra-fast cements are available to allow repaired slabs to be opened to traffic just in a few hours. Precast slabs can also be used for intermittent repairs.
If a structural rehabilitation is needed, it is possible to overlay an existing concrete pavement with another concrete pavement. It should be stressed that concrete overlays are also more and more placed on asphalt pavements, a technique usually referred as whitetopping. There are several possibilities: thick overlays or, if the existing pavement is in good condition, bonded ultra-thin overlays. For new asphalt pavements, tests are being performed on the suitability of ultra-thin wearing courses of high and ultra-high performance mortars, in which hard, non-polishing aggregates can be embedded. Some case studies dealing with whitetopping have been presented, in many cases built to avoid the more and more frequent problems posed by aging bituminous pavements. If there are problems to match the levels of adjoining elements, another possibility is to demolish the existing pavement and to replace it with a concrete one. A remarkable example is that of the Mexico City Interior Loop, a facility 42 km long and 35 to 49 m wide, where more than 110,000 cars transit daily.
Road widening is one of the main options to improve mobility. A sound technical and economic solution for widening asphalt pavements is to add a new concrete right-hand lane and hard shoulder. Experience in countries like Germany, Belgium and the Netherlands is positive and shows that no special measures are required when connecting concrete pavements to bituminous ones.
Conversion of a dark pavement into a brighter one has a number of benefits. There is a growing trend towards the use in cities of light reflective surfaces both in roofs and pavements to reduce the amount of energy needed to cool urban environments associated with the urban heat island effect. In this respect, the high solar reflectance or albedo that concrete pavement can provide is advantageous. Moreover, they significantly reduce the amount of energy needed for artificial roadway illumination during night-time, mitigate the greenhouse effect and contribute to global cooling by reducing the amount of solar radiation absorbed by the earth's surface. Bright surfaces can be obtained even if dark aggregates are used.
A new generation of cements with titanium dioxide TiO2 has made it possible to build depolluting pavements. The most harmful nitrogen oxides, notably NO2, can be reduced by means of an oxidoreduction phenomenon that generates photocatalysis. Encouraging results have been obtained both with cast in situ slabs and with precast blocks.
Cities worldwide are facing problems of traffic congestion in their centres and suburban areas as well as on access roads. In densely populated countries this phenomenon also occurs on roads linking cities. Dedicated lanes for public transport can guarantee the travelling time and time of arrival for the traveller. Rutting associated with channelised traffic is a particular issue in these facilities, since in addition to general problems as stability of vehicles or risk of aquaplaning in rainy periods, optical guidance systems can be impaired and access of passengers hindered, especially when using wheelchairs. Concrete pavements are the safest way to avoid ruts. In addition, due to their durability, restrictions to bus operations caused by maintenance works are reduced to a minimum. Some examples of dedicated bus lanes in the Netherlands, Belgium, France and Spain were presented during the Symposium.
Roundabouts are other particular places in highway and street networks where pavement distresses are likely. In many European countries, traffic at major junctions is managed increasingly by constructing roundabouts, which allow safer and smoother traffic flow than other alternatives. Heavy traffic at these roundabouts induces severe stresses in the pavement, both as a result of centrifugal forces as well as the overloading exerted by the offside wheels of tilting vehicles. Effects of such stresses such as rut formation, sideways displacement of the wearing course, loss of surface aggregate and cracking can be avoided if a concrete pavement is selected, as well as the associated repair works, usually very difficult to conduct due to the intense traffic. Switzerland, Austria, The Netherlands, France and Belgium are some of the countries where this solution is frequently used. In the latter, the first roundabout with a concrete pavement was opened to traffic in 1995 and is still in service.
Concrete pavements also have a significant contribution to safety when used in tunnels. It is essential that, in the event of fire, the pavement does not aggravate its consequences. Concrete is an incombustible material, which does not release smoke or toxic gases nor increase the fire load. Therefore, in some countries, like Austria and Spain, concrete pavements are compulsory for tunnels over 1 km long.
Concrete barriers are a sustainable and reliable option to improve occupants' safety. Different types, either precast or in situ slipformed, are available. On the United Kingdom's road network the road authorities have chosen to specify them as the default requirement for central reserve barrier on high capacity motorways or trunk roads. Designs for new uses have been developed. These include, amongst others, flood barriers, anti-terrorism security barriers, special precast barrier types with no deflection at heavy impacts to safeguard obstacles and bridge piers and integrated noise barriers.
One of the reasons to explain the use of concrete pavements in low volume roads is their reduced maintenance costs. However, studies conducted in several countries, as Poland and Spain, prove that even their construction costs can be lower than those of other options. Moreover, their ability to withstand concentrated loads makes them an appropriate solution for municipal facilities such as recycling centres or fairgrounds.
Concerning industrial and harbour pavements, concrete is the preferred option. Frequently, high, concentrated loads are applied on them, as those due to reach stackers, fork lift trucks and other special purpose vehicles in harbour areas. Specific thickness design methods, as one developed in Germany, are needed to take into account factors such as dimensions of the slabs, size and position of loaded areas, etc. An interesting innovation in this type of works is the simultaneous use of plastic and steel fibres, combining the reduced shrinkage provided by the former with the higher resistance to fatigue obtained with the latter. From about the year 2000 a number of concrete pavements (up to 4000 m2) using this approach in gas stations and harbours without any joints have been built in the Netherlands and almost all of them are still without any cracks. In this regard, recycled steel fibres from post-consumer tyres seem to be a promising alternative. From some tests carried out in the United Kingdom it was concluded that, when used at higher content, they can improve the performance of concrete in terms of compressive and flexural strength to the same extent found with industrial fibres.
Treatment of soils and recycling of existing pavements are techniques which provide excellent results in terms of technical, economic and environmental performances. They are on a constant increase in terms of market.
From the previous paragraphs it is clear that for all the questions posed nowadays by the different stakeholders in pavement techniques (decision makers, administrations, designers, contractors, users ...), concrete has a positive contribution, that justifies the motto of the Symposium: "the answer to the new challenges"."
Read the short summary of this 11th edition on our Symposium 2010 web page.