CONCRETE ROUNDABOUTS n WHY CHOOSE A CONCRETE PAVEMENT FOR A ROUNDABOUT ? n DESIGN OF ROUNDABOUTS IN JOINTED AND CONTINUOUSLY REINFORCED CONCRETE n CONSTRUCTION ASPECTS Herselt, Belgium (Ref 2) A. Nullens CONTENTS 1. Introduction 3 2. Why choose a concrete pavement for a roundabout ? 6 3. Design of roundabouts in jointed concrete pavement 8 4. Design of roundabouts in continuously reinforced concrete 14 5. Construction aspects 17 6. Conclusions 26 7. References 27 INTRODUCTION Examples of a typical layout of a roundabout and its access roads (right : Flemish Agency for Roads and Traffic AWV, Belgium – left : the Netherlands) In many European countries, traffic at ma- pavement, both as a result of centrifugal jor junctions is increasingly managed by forces as well as the overloading exerted constructing right-of-way roundabouts, by the offside wheels of tilting vehicles. which allow safer and smoother traffic The effects of such stresses include rut flow. An example of typical layout and formation, deformation of the wearing cross section of a roundabout are shown in course, the loss of surface aggregate, and figure 1. cracking as a result of insufficient bearing capacity in the road structure beneath the Intense and heavy traffic at these round- offside wheels. abouts induces extreme stresses in the Central island 2% 1% Truck apron Roundabout pavement Kerb Figure 1 : General layout and cross section of a roundabout (Ref 11) Connection curb of the exit road Outer radius Lane width of the exit road Inner radius Truck apron Median traffic island Lane width of the entry road Connection curb of the entry road Lane width 3 CONCRETE ROUNDABOUTS Figure 2 : change of axle load distribution of a heavy goods vehicle on a roundabout (Ref 1) These findings encourage designers to use concrete for the construction of round- 13 t 4,5 t abouts, either as a jointed plain or a joint- 13 t ed reinforced concrete pavement (JPCP or JRCP), or as a continuously reinforced 1,5 m concrete pavement (CRCP). This publica- 6,5 t 6,5 t 3 t 10 t tion deals with design and construction 2,0 m 4,5 t aspects for both solutions. JPCP (or JRCP) has been used already for many years for 10 t roundabouts and is still the standard op- tion in most countries (Austria, Germany, 6,5 t 6,5 t Switzerland,…). CRCP was only used for 3 t the first time in 1995 in Belgium. The first 4,5 t examples are now almost 20 years old and are still in excellent condition today. Many others have been built in the meantime in Belgium, France and the Netherlands, dem- onstrating the suitability of this technique for application on a larger scale. 4 CONCRETE ROUNDABOUTS > WHAT LIFETIME CAN BE EXPECTED FROM A CONCRETE ROUNDABOUT ? Many examples can be found of concrete design, combining CRCP and JRCP by the pavements older than 50 years, even use of longitudinal steel reinforcement when they are subjected to heavy traffic together with transverse joints. loading. Roundabouts, however, are only built since the 1990’s and the oldest ones Today, after 18 years of service, the pave- are now about 20 years old. ment is in excellent condition and no damage at all is visible. The theoreti- In the Netherlands, a nice example can cal design criteria for this project were be found at the intersection of Reeker- chosen for a 40 year design life. Taking molen in the Province of North-Brabant. into account this concept and the expe- In 1995, traffic volumes were situated rience with other pavements in continu- between 13.000 and 20.000 of which ously reinforced concrete, a 40 to 50 year 15% heavy vehicles. A high accident rate service life can be easily achieved. Such and frequently needed repairs of the a service life, linked to a absolute mini- asphalt surface due to deformation and mum of necessary maintenance, is of rutting were the reasons for turning the significant importance in economic and intersection into a concrete roundabout. environmental assessments, turning this It was built in 1995 following a peculiar solution into the best possible choice. 5 CONCRETE ROUNDABOUTS 2. WHY CHOOSE A CONCRETE PAVEMENT FOR A ROUNDABOUT? Light coloured and aesthetically pleasing roundabouts at Saint Jean de Cardonnay, France (Ref 9) and in Austria (Ref 5) Choosing for concrete offers a number of Two reasons in particular justify the choice benefits that are typical for concrete pave- of a concrete pavement for roundabouts, ments in general : namely : • durability and robustness; • the elimination of rutting due to the loads imposed by heavy goods vehicles • long service life (30 to 40+ years); moving at a moderate speed; • low maintenance requirement; • the elimination of deformation of the wearing course as a result of loads • economical over long term; caused by centrifugal forces. • light coloured surface; • good and durable skid resistance; • aesthetic appearance. Illustration of the intense traffic loadings on a roundabout pavement (Ref 1) 6 CONCRETE ROUNDABOUTS Example of a roundabout in CRCP in Airvault – Deux Sèvres, France (Ref 9) One of the main advantages of continu- ously reinforced concrete is the elimination of the conventional contraction joints re- quired in unreinforced concrete pavement and thus to avoid loading of the slab cor- ners. Reinforcement by a steel mesh or by the use of structural fibres in the concrete (steel or synthetic fibres) also makes it possible to reduce the number of transverse joints. Steel mesh reinforcement (Febelcem) 7 CONCRETE ROUNDABOUTS 3. DESIGN OF ROUNDABOUTS IN JOINTED CONCRETE PAVEMENT For the design and construction of a round- x 150 mm, placed in the upper third of the about with a pavement of in situ cast con- slab and cut through in the contraction crete slabs, there are the following options: joints in order to allow the joint to func- tion. • jointed plain concrete with dowelled or non-dowelled joints; If steel fibres are added to the concrete mix, typical dosages are between 30 and • jointed steel mesh reinforced concrete 50 kg/m³. The steel quality and the shape with dowelled or non-dowelled joints; of the fibres also play an important role in the final characteristics of the pavement • jointed steel fibre reinforced concrete concrete. Steel fibres also enhance the with dowelled or non-dowelled joints; post-cracking behaviour of the pavement by keeping the cracks closed. Another ben- • jointed synthetic fibre reinforced efit is the increased flexural strength of the concrete with dowelled or non-dowelled concrete. joints. Even for reinforced concrete, the stresses The use of dowels contributes largely to induced at the edge of the pavement are the load transfer between adjacent slabs still large and the dimensions of the struc- and helps prevent pumping, curling of the ture and reinforcing must take this into slab and faulting of the joints. Dowels al- account. One solution might be the provi- low a thinner pavement design for a same sion of extra thickness in the base layer on traffic or a higher traffic volume for the the outer perimeter of the roundabout, same thickness. When it is predicted that which makes it possible to strengthen the the roundabout will have to bear higher road structure in the most heavily loaded volumes of heavy traffic, the joints need to area. Another way of preventing the load- be dowelled. The correct and stable posi- ing of the edges consists of careful placing tioning of the dowels during the construc- of the road markings or the installation of tion work is vital if the joints are not to surface-mounted kerbs at a distance from become locked up. the outer radius of the roundabout (see also § 5.3 about kerbs and gutters). The reinforcement of the concrete (steel mesh/steel fibres/synthetic fibres) allows a The general design rules for jointed con- reduction of the pavement thickness or an crete pavements are applicable. Therefore increase of the slab length. In the case of consideration is required of the following a steel mesh reinforcement, a typical mesh issues: Roundabout in coloured is: diam. 10 mm x diam. 10 mm x 150 mm steel fibre reinforced concrete (Ref 1) Detail of a steel fibre concrete mix 8 CONCRETE ROUNDABOUTS • maximum joint spacing should be When designing a concrete slab pavement Example of a roundabout limited to 6 m for a thickness ≥ 25 cm; for a roundabout, special attention must with well-constructed 5m for a thickness between 20 and 25 be given to the joint design, specifically the matching joints near cm and even 4 m for a thickness < 20 positioning of the contraction, isolation Madison, Wisconsin ,U.S cm. Reinforced slabs can be made about and construction joints. The project de- 25 to 50% longer, depending on the signer must therefore carefully define the amount of reinforcement; location of all the joints during the project design phase. The jointing for concrete • maximum slab width should be limited roundabouts is different from a normal in- to 5m; if necessary a longitudinal joint tersection (square plan layout) and is based (construction or contraction joint) is on the idea that the circle is isolated from needed; the legs. The “transverse” joints in the cir- cular portion radiate from the centre while • length/width ratio of the slab should be an eventual “longitudinal” joint follows a limited to 1,5. This means slabs should concentric circle. have a rather square shape (or pie shaped in the circular part of a round- Basically, two solutions can be adopted for about); the radial joint pattern: • slab surface should be limited to 30 m² • Solution A. Make the radial joints of the for plain concrete; circle match with the longitudinal joints of the legs; • joint angles should be greater than 75°.