GUIDELINES FOR CONCRETE ROUNDABOUTS; THE DUTCH PRACTICE

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1 GUIDELINES FOR CONCRETE ROUNDABOUTS; THE DUTCH PRACTICE Marc J.A. Stet, B.Sc. Civ. Eng, Senior consultant KOAC WMD (Dutch Road Research Laboratories), Schumanpark 43, NL 7336 AS APELDOORN, The Netherlands, Phone: , Fax: , Adrian J. van Leest, M.Sc. Civ. Eng., Project manager CROW (Information and Technology Centre for Transport and Infrastructure), P.O. Box 37, NL 6710 BA EDE, The Netherlands, Phone: , Fax: , George Jurriaans, B.Sc. Civ. Eng. Senior consultant ECCRA (European Concrete Consultants Roads and Airfields), P.O. Box 10, NL 7090 AA DINXPERLO The Netherlands, Phone: , Fax: , SYNOPSES Continuously reinforced concrete pavement (CRCP) structures are becoming more popular for roundabouts due to the durable and sustainable character of this pavement type. This led to a need for practical guidelines, recommendations and standardisation with respect to design, details and the preparation of tender documents. The paper summarises a recent publication of CROW and focuses on the design of concrete roundabouts. Various aspects with respect to geometrical and structural pavement design are addressed. Pavement solutions such as jointed and continuously reinforced pavements are given. The design is simplified by means of a standardised pavement design. Examples of drawings for pavement lay-out with details for cross sections, reinforcement patterns, pavement ends and joints etc. have been given. Last but not least, attention has been paid to the execution and construction of these pavements. The paper addresses the current engineering practice of concrete roundabouts in the Netherlands and several examples have been given. 1. INTRODUCTION Busy junctions are being increasingly laid out as roundabouts, which ensures a traffic flow that is both smoother and safer. As a result of turning traffic and relatively tight bends, roundabout pavements are often subject to excessive twisting force. Cement concrete offers excellent resistance to these heavy forces, which is why provincial and municipal administrators and designers show a great deal of interest in roundabouts constructed with cement concrete. Especially continuous reinforced concrete pavements, having no transverse joints, are very suitable for use in roundabouts. However, designers still have limited knowledge regarding this technology. Various CROW publications offer assistance and recommendations for work guidelines Sustainable and Safe In the Netherlands, the single-lane roundabout has been a much-used and safe type of junction for a number of years now. The old-style roundabouts never achieved the same popularity, because traffic came to a standstill at maximum capacity as a result of the rule giving priority to traffic approaching from the right. To prevent this, roundabouts were given larger slip roads (and therefore increased in size) to create a buffer. The principle of the single-lane roundabout is based on a radial connection of the approach roads and a priority rule where traffic approaching the roundabout gives way to circulating traffic. This results in a relatively high capacity being achieved on a small surface. Moreover, a safer situation is created as approaching traffic is made more aware of the priority rule and is forced to slow down. With the frequent construction of roundabouts, the applications, appearances and priority situations are becoming increasingly varied. This does not fit in with the Sustainable Safety concept, which aims for clarity and uniformity with respect to the priority rules and appearance of traffic provisions. Within built-up areas, cyclists have priority and pedestrians preferably have free passage. Outside built-up areas, cyclists no longer have priority and pedestrians do not have free passage. CROW (Information and Technology Centre for Transport and Infrastructure) publication no.126 Uniformity for Roundabouts makes unambiguous recommendations for the construction of one- and two-lane roundabouts, also referred to as capacity roundabouts. The new-style roundabout, when constructed according to the recommendations in the publication, generally offers an adequate and safe solution that meets the principles of Sustainable Safety.

2 1.2. Choosing for concrete In each project, a pavement solution is to be selected, and choices between bituminous and concrete pavements are often based on initial pavement costs. A continuous reinforced cement concrete pavement is usually somewhat more expensive than a jointed concrete or an asphalt pavement. On the other hand, it requires less maintenance. Since maintaining roundabouts requires traffic measures (diversions, closing off), which is regarded unfriendly to the user, a CRCP pavement is often preferred. Furthermore, it must be mentioned that the costs of the pavement is marginal compared to the total costs of a reconstruction project Design elements of roundabouts transition curve exit width exit width entry transition curve exit Outer diameter Inner diameter Wide swing strip lane width Figure 2. Design elements of a roundabout. 2. CRCP ROUNDABOUTS 2.1. Jointed or Continuous Reinforced Concrete Figure 1. Damage due to too tight dimensions. If a roundabout is too tight, frequent repairs are necessary. CROW s Publication 126 Uniformity for Roundabouts presents guidelines for traffic engineering and for determining if a roundabout is desirable considering the level of service and volume of traffic. In order to avoid damage, certain minimum dimensions are advised (see table 1). The design elements are given in figure 2. Table 1. Dimensions of roundabouts [2] Standard dimensions Design element (meters) urban rural outer diameter 16,00 18,00 inner diameter 10,50 12,75 lane width 5,50 5,25 transition curve entry 8,00/12,00 8,00/12,00 transition curve exit 12,00/15,00 12,00/15,00 width entry 4,00 (3,50) 4,00 (3,50) width exit 4,50 (4,00) 4,50 (4,00) width swing strip 1,50 1,50 Until recently, non-reinforced concrete pavements were predominant in the Netherlands. Such pavements consist of rectangular slabs with connected longitudinal joints and doweled transverse joints. In the case of non-reinforced concrete pavements for roundabouts, it is often difficult to design an ingenious joint pattern without sharp angular edges. It is particularly these sharp angles can soon lead to damage from heavy traffic. This can be prevented by constructing the concrete pavement from continuous reinforced concrete. Shrinking transverse joints are not necessary in continuous reinforced concrete pavements and any longitudinal construction joints can be provided with a load-transferring reinforcement. In the past few years, there has been an increase in the number of continuous reinforced concrete pavements. Such a pavement for a roundabout has longitudinal and transverse reinforcement and no transverse joints. The reinforcement assumes the function of the transverse joints in a nonreinforced concrete pavement. In the case of continuous reinforced concrete pavements, the reinforcement is intended to distribute any cracks, rather than for absorbing flexural tensions. A good starting point for continuous reinforced concrete pavements is a reinforcement percentage of 0.7 percent of the concrete cross-section for a concrete strength classification of B45. For that matter, the thickness of the concrete slab is equal to that of a non-reinforced concrete pavement.

3 A characteristic of a roundabout with continuous reinforcement is the bituminous binder course; this is absent in the non-reinforced variant. application, finishing and curing. In the event of smaller surfaces and many variations in width as is the case with roundabouts, the site agent usually decides to apply the pavement manually. Only in the case of a large outer radius (approximately 45 metres) is it possible to construct the carriageway mechanically with a slipformpaver. Figure 3. Construction of CRCP with 0.67% steel Pavement structure The function of the pavement structure is to bear traffic loads and transfer them to the natural ground in such a way that the road allows safe and comfortable use. The soil properties of the underlying sub-grade or natural ground affect the durability of a road pavement and are influencing the choice of pavement structure and have a profound affect on the pavement performance. Homogeneous load-bearing capacity, adequate resistance to erosion of materials used and limited susceptibility to settlement are the most important parameters to consider. In figure 4 the components of a cement concrete pavement structure are indicated. Figure 5. Rolling finisher results in smooth pavement. For manual application, the minimum concrete quality for roundabouts is B45, consistency area 2. This consistency is achieved by adding a superplasticizer to the usual dry concrete mortar for mechanical road construction with consistency area 1, so that the mortar is suitable for manual processing. With consistency area 2, it is also possible to apply the necessary superelevations in the pavement. The B45 quality ensures a high surface wear resistance and sufficient frost and thawing salt resistance. 3. WIDE-SWING STRIP Figure 4. Structure of a CRCP pavement Concrete quality The quality of a concrete pavement is largely determined by its construction, which in turn depends on the composition of the concrete and the manner of One of the most critical parts of roundabouts is the wide-swing strip. In order to allow long trucks to pass a roundabout, a large roundabout diameter is required, but this results in part of the desired effect being lost, as it slows down all traffic. By applying a wide-swing strip, basically intended just for heavy goods vehicles, the desired effect can be achieved. By applying a raised strip with deviating surface characteristics, car drivers can be discouraged from using the wide-swing strip. If the wide-swing strip is constructed in concrete, it can be provided with a print texture, for instance, using basalt stones. This is done by printing the unhardened concrete with special moulds intended for this purpose. When the concrete surface is coloured using special colouring agents, it is necessary to apply the colouring agent before printing commences. Although the surface may look like a modular pavement, the advantage

4 is that there are no real joints between the elements, preventing the growth of weeds and grass in the joints. The concrete is always applied manually because of its composition. When applying non-reinforced concrete, it is advisable to include shrinkage or expansion joints every 5 metres in order to absorb tensions due to shrinkage or expansion. Figure 7. Slipform paved heavy roundabout block. 4. CONSTRUCTION DETAILS 4.1. Slab thickness and reinforcement Figure 6. Printing a block pattern in the fresh concrete. The carriageway pavement(s) and the wide-swing strip are separated by concrete kerbs. There are various possible solutions for concrete kerbs: small prefab concrete elements (concrete kerbs and/or roundabout blocks approved by the RWS (Directorate-General for Public Works and Water Management), large prefab roundabout block elements (approximately 6 metres in length) and cast-in-place roundabout blocks. In principle, there are three possibilities for avoiding damage to critical components such as the roundabout blocks. For instance, instead of gluing kerbs, it is possible to place small prefab elements in openings in a pre-cast concrete slab. A second solution is placing kerbs and roundabout blocks in mortar with a herringbone steel concrete backing as a permanent form work for feeding through reinforcement; the concrete is only applied later. A third solution is the mechanical application of a heavy roundabout block cast on site on the foundations. The concrete is applied following the placing of the RWS kerbs in mortar with a concrete backing. Roundabout blocks are preferably cast directly on the foundations using a slipformpaver. It is also possible to position the elements prior to casting the concrete, so that they are actually included and embedded in the concrete pavement. This solution means that the replacement of damaged elements is difficult. Mechanical casting not only prevents traffic damage, it also avoids heavy lifting. The thickness of the concrete slab may either be calculated or assessed form a table resulting in standardised thickness of 23 or 25 cm. The amount of steel is based on the concrete tensile strength and the steel stress in the bars not exceeding the allowable 0.2% elastic limit (yield point). Unlike roads, the amount of steel in the transverse direction is equal to the longitudinal direction. Table 2 gives guidelines on the minimum required steel for two thickness and for concrete qualities B35 ad B45. Table 2. Reinforcement & slab thickness Standard thickness Reinforcement (mm) details concrete quality B35 B45 B35 B45 Required minimum percentage min. steel mm 2 /m bars ø16 distance (mm) overlap (mm) steel mm 2 /mm bars ø20 distance (mm) overlap (mm) steel mm 2 /mm The reinforcement is positioned on an even, clean asphalt working platform. The placing and positioning of the reinforcement is done manually. The transverse reinforcement rods are placed at an angle of 90 to the

5 longitudinal axis. The longitudinal reinforcement has an certain overlapping length for assuring bond. The longitudinal reinforcement is fastened to truss shaped interval spacers using wire ties. When the connecting pavement is an asphaltic one, a but joint is made. In the case the adjacent pavement is a concrete pavement, this pavement is continued over the roundabout in order prevent expansion movement from the concrete road Opening to traffic Figure 8. Overview of reinforcement works. Bars come in 14 meter lengths and can easily be put in position. Points for attention to be heeded when positioning reinforcement are: the dimensioning and height at which reinforcement is positioned the number of ties must be sufficient to prevent horizontal movement [5] the bars should not be weaved in an ordered pattern to avoid crack clusters the working platform should be clean prior to pouring concrete Additional reinforcement is placed at the edge of a roundabout over a length of 1,0 meter. Since the exit and entry of a roundabout normally comprise of jointed concrete, these slabs are reinforced End structures voeg met rubber profiel (50mm) ls Ø25-500mm lang h.o.h. 250mm ls Ø n 16 stuks Ø12 abele lengte 5180 naar 7666 naar 6140 mm van 5180 naar 7150 naar 6140 mm Ø ,75mtr Ø Ø van 5180 naar 7607 naar 6140 m Ø (L= va Ø16 Once the concrete has hardened sufficiently, the concrete pavement can be open for use. However, the Dutch Standards offer no definitive requirements. As a guide the following timeframes can be maintained: No traffic or pedestrians should be allowed on the pavement during the first 24 hours; Pedestrians and cyclists may be granted access after 24 hours; After 48 hours private cars and light motor vehicles with only two axles and having a maximum weight of 1500 kg may be allowed on the new pavement; After 7 days or at a compressive strength of 70% of the required characteristic compressive strength other traffic may then be allowed onto the new pavement. These timeframes must be regarded as minimum requirements and must extended when there are unfavourable weather conditions. In addition it is recommend to carefully apply salt and that no thawing agents are deployed during the first winter. 5. CLOSURE Despite the rediscovery and the great advantages of continuous reinforced concrete in roundabouts in the Netherlands, very little has been written about this in recent publications and manuals. As a result, designers continue to draw up separate specifications for each project. They also continue to describe new detailed solutions for each project, and sometimes even for each case. This requires unnecessary effort. Moreover, the lack of general descriptions and guidelines means that a great deal of knowledge and experience is lost. By presenting a publication on the design and construction of concrete roundabouts CROW hopes to fill this lacuna. Ø16-62,5 Ø (L= 5180) Varia van 5 6. REFERENCES van 6140 naar 7330 naar 5180 mm Figure 9. Roundabout with support slabs. Ø16-62,5 Ø (L=6140) van 6140 naar 70 Uitzetvoeg met r Deuvels Ø25-5 Beugels Ø12-15 Staven 16 stuks Ø 1. Standaard RAW Bepalingen 2000, CROW, Ede 2. Eenheid in rotondes. Publ.126, CROW, CRCP: A long lasting Pavement solution for todays motorways, M.J.A. Stet and A.J. van Leest. Proc. 9 th. Int l. Symposium on Concrete Roads. Istanbul, Turkey, 2003.