STATE OF THE ART OF RCC PAVEMENTS IN AUSTRALIA

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1 STATE OF THE ART OF RCC PAVEMENTS IN AUSTRALIA George Vorobieff Head to Head International Pty Ltd Craig Whitaker Pioneer Construction Materials Pty Ltd ABSTRACT A simple material like concrete is suitable for large-scale production of concrete pavements in either slipformed and roller compacted construction techniques. While slipformed concrete is the preferred option for road authorities, the high production levels of roller compacted concrete (RCC) makes it a viable alternative for contractors, especially where composite pavement construction exists. Better design techniques over the last few years along with more suitable QA specifications, has minimised construction risks and led to less conservative pavement design. The use of better jointing techniques has also improved pavement performance.. RCC has applications for base and subbase pavement layers, for concrete inlays and at intersections for composite pavement construction. This paper looks at some current aspects of design methods, specifications, manufacturing and construction techniques used for RCC pavement layers in Australia. Keywords: roller compacted concrete, pugmill, pavement design, specifications State of the art of RCC pavements in Australia Page 1 of 9

2 INTRODUCTION Roller compacted concrete (RCC) takes its name from the construction method used to place the concrete. Unlike slipformed concrete laid with paving equipment, RCC is laid using either granular or asphalt paving equipment, or graders and is then compacted with steel drum rollers as shown in Figures 1 and 2. Where ride quality requirements are sought the paver using string lines will achieve better results than a grader. Figure 1 ABG granular paver laying roller compacted concrete mix. Figure 2 Conventional graders used for spreading roller compacted concrete mix. RCC has been in use in the USA and Europe for some years and was introduced to Australia in It is extensively used for dam wall construction but the techniques are different to those used in pavements. The technology is not complex, however like base concrete, the concrete pavement requires joints and careful planning of the transverse and longitudinal joints. Work by Petrie and Matthews lists many of the pavements constructed in Australia using RCC (Petrie, 1990). Very little work on RCC has been published in Australia since the early 1990s and this paper is therefore a timely update on the use of RCC. RCC should not be confused with cement-treated bases (CTB) or subbase, as the cement content for CTBs is about 3 to 5% whereas RCC has a cement content in the range of 10 to 14% by mass. However, RCC is applicable for both subbase and base layers according to the specific requirements required by the owner. For instance, if a tight, transverse tined surface is required for highway pavements, an RCC base is not suitable. If a composite pavement configuration is economical for a heavy duty urban road, RCC could be an economical option for the subbase construction. State of the art of RCC pavements in Australia Page 2 of 9

3 The applications of RCC are: Road pavements External industrial hardstands Auxiliary and mine haul roads Semi-permanent military roads Table 1 Benefits and limitations of RCC for pavements. Benefits Can be placed using a grader rather than specialist paver. Relatively inexpensive for subbase concrete. Does not need side forms and applicable for inlays in existing asphalt pavements. RCC subbase with asphalt overlay permits early opening of pavements. Limitations Requires paver to meet ride quality limits in SRA specification. Porous looking surface finish (ie low aesthetics). PAVEMENT DESIGN A number of methods are available to the design of RCC pavements, such as the Austroads Pavement Design Guide (Austroads, 1992) or other publications from the Portland Cement Association (PCA, 1987, Rollings and C&CANZ, 1993). If designers intend to use the Austroads Pavement Design Guide for a base concrete layer, the following design model assumptions must be considered: Channelised or road lane flow for traffic loading. Base and subbase layers should be debonded. The base should consist of discrete slabs surrounded by joints that either contract and/or expand. In the latter point, if contraction joints are sawn and aggregate interlock is the assumed mechanism of load transfer then the width of the joint must be limited to ensure load transfer during the pavement life, otherwise the loading becomes an edge type loading and stresses in the base concrete increase. Where longitudinal joints are proposed for base concrete, consideration should be given to whether the joint is tied and the joint is formed or induced. In situations where paving exceeds 5 m in width, an induced longitudinal joint is required to avoid the potential for a crack along the middle of the paved section which subsequently opens over time. If a tied sawn longitudinal joint is planned, some consideration is required for the installation of the tied bars in the longitudinal joints. A trial pavement was carried out by the QCL Group for an access road at one of their quarries and Queensland Department of Main Roads (QDMR) were invited to carry out an assessment of the trial. This pavement consisted of RCC in two layers and before the second layer is placed, tie bars are laid on top of the first compacted RCC (Carse, 2000). Using this approach requires careful attention on site to the formation of a suitable bond between the two RCC layers to ensure a fully bonded base layer. State of the art of RCC pavements in Australia Page 3 of 9

4 In NSW, where RCC is used in composite pavement construction, a layered elastic analysis approach is carried out for the thickness design process. In this instance, the following assumptions could be considered in the analysis: The subbase and base interface may be rough provided bitumen emulsion is used as the curing compound. The RCC is an isotropic material with a Poisson s Ratio of 0.20 and an elastic modulus of 10,000 MPa (RTA, 1997). The fatigue equation for cemented materials is (260/µε) 12 (RTA, 1997). CONCRETE MIX As noted in the introduction, there are several critical factors in the concrete mix design for RCC and these are summarised in Table 2. The typical mix proportions of cement and aggregates for subbase and base roller compacted concrete mixes are also listed in Table 3. Factor Overall grading envelope Particle shape Cementitious content Compactibility Index 1 to AS Working Time Table 2 Critical factors in the concrete mix design for RCC. Explanation Unlike conventional concrete, gap grading of the overall mix design is undesirable as it may initiate aggregate segregation of the mix during transport, handling and placement. Particle shape of coarse aggregate may affect the desired compaction of the RCC in that flat and elongated aggregates will not achieve dense RCC. The flat and elongated shape particles may cause segregation of the mix irrespective of the mixing. Due to the low cementitious content of the RCC used in subbase the mix may segregate if either of the above are in question. This generally ranges in the order of 60 to 80 and requires the index to be established in the trial mix and on site, as mixes will vary subject to overall grading, material properties, working time, and the method of placement and compaction equipment. If the RCC is too wet when rolling commences waves will form at the surface of the pavement due to the displacement of the concrete under the roller. If the RCC is too dry, the pavement will generally have inadequate compaction and excessive voids in the finished surface. This will vary depending on the location of work in relation to mixing plant site, method of placement and compaction, and the time of year. Admixtures may be used to achieve a longer working time. The working time, (i.e. from the time of mixing to deliver, spread and compact RCC), is typically up to 3 hours. In hot weather, retarding admixtures may be required to achieve an extended working time to adequately compact the concrete. NOTE: 1. Compactibility Index rather than the slump test is used for a measure of workability of RCC mixes. Roller compacted concrete is typically manufactured in pugmills rather than premix plants to allow higher production outputs and efficient mixing techniques (see Figure 3). Pugmills maybe mobile (see Figure 4) and based onsite or fixed in a quarry. Transit mixers are commonly used for small projects and where there is limited access for discharging the concrete. State of the art of RCC pavements in Australia Page 4 of 9

5 Component Table 3 Typical concrete mix proportions for base and subbase concrete. Base (Nominally 32 to 35 MPa) (kg/m 3 ) Subbase (Nominally 5 to 10 MPa) (kg/m 3 ) Cement (Type SL) Fly ash Fine aggregates Coarse aggregates Water Compactability Index Working 21 C 2 to 3 Hours 3 Hours Admixtures Nil Nil Figure 3 RCC manufactured in a pugmill and discharged in tipper truck. Figure 4 A typical rural set up for RCC manufacturing. SPECIFICATIONS The RCC specification appears to fall between a cement-treated material and concrete road specifications. As previously described, unlike concrete that requires a form and internal vibration to compact, RCC is compacted with the use of rollers. Compliance in most specifications is confined to relative density, compressive strength and dimension limitations. State of the art of RCC pavements in Australia Page 5 of 9

6 The authors note that specifications for RCC have not been updated over the last 10 years and both VicRoads and the RTA are currently upgrading their existing specification. The RTA specification for RCC is entitled Lean rolled concrete subbase (RTA, 2003) and as the title indicates, its scope is as follows: For lean rolled concrete subbase to be used under asphaltic concrete. The work to be executed under this Specification consists of: (a) manufacture of lean rolled concrete (b) transportation (c) placement (d) compaction, trimming and curing of lean rolled concrete (e) provision of a bitumen emulsion curing membrane, prime or primer seal. A paving machine shall carry out placement unless otherwise approved by the Superintendent. Compliance in the RTA specification is by: Cement and fly ash content Compressive strength Compaction Compactibility Index Finished surface and levels Layer thickness Pavement width The VicRoads specification is entitled Construction of roller compacted concrete base courses and was last updated in Unlike the RTA specification, this may be used for base layer construction and relies on VicRoads Specification 501 Materials and construction plant for concrete base and subbase pavement courses for the material requirements. This approach is unusual and could lead to contractual disputes and make it difficult for a contractor to successfully supply concrete for the construction of the RCC. CONSTRUCTION There are no known instances around the world where steel reinforcement has been used for RCC pavements. Whilst pavements could be reinforced, a paver with a side placement device would be required. However the drier concrete mix would limit the amount of reinforcement which could be placed to ensure the concrete is compacted under the layer or mat of reinforcement. The initial level of RCC prior to rolling is most commonly achieved by spreading the RCC with either: - Laser sensors on the grader. A paver with sensors guided by wire ropes preset along the length of the pavement. After spreading, the RCC is rolled (i.e. to compact) until both the required density and final thickness are achieved. The density is determined by the use of a calibrated Nuclear Densometer and thickness is measured by survey levels. The density is achieved by external, not internal, vibration generated by the use of a vibrating roller. Therefore, if segregation occurs in the mix during the transport and spreading phases of construction, this results in the concrete becoming non-conforming. The concrete mix needs to be well designed to minimise segregation especially when using tipper trucks. State of the art of RCC pavements in Australia Page 6 of 9

7 A typical construction timetable for RCC subbase and asphalt surface is within three hours and the following steps would occur: Mix & transport RCC to site. Place RCC in front of spreader, either by grader or paver. Spread to predetermined level. Compact with vibrating roller. After the nominated working time: Typically two to four hours after rolling lay asphalt. Place extruded kerb & gutter concrete either prior to opening or after opening to traffic as required. Open to traffic as soon as asphalt is completed. Typical construction timetable for RCC base is the same as for subbase except that no asphalt is laid and the pavement is not opened until the base course has achieved the required insitu compressive strength for opening. To achieve good ride quality from a RCC base layer, a paver using string lines and precompaction of the concrete is essential. Where the pavement is boxed out, such as in a subbase condition and inlays, it is preferable to use graders for spreading and rollers for compaction. This requires little set up time (i.e. no edge forms to be constructed) and the potential to open the pavement to traffic in a short time period. PERFORMANCE DATA The only known performance data on RCC base concrete is work by QDMR (Carse, 2000) on a heavily trafficked quarry road at the Glasshouse Mountains. The 180 mm thick pavement supported on 125 mm cement-treated subbase, was constructed in December, After 12 months the following comments were reported by QDMR: In general the pavement was in good condition with only limited amount of surface defects. The most significant recorded affect was the abrasion occurring at slabs 22 to 24 adjacent to the intersection. The extent of wear is consistent with the petrographic analysis and abrasion test results determining the softer nature of RCC top surface in relation to slipformed concrete pavement. It should be noted that the pavement was trafficked some 16 hours after construction with loaded quarry trucks. State of the art of RCC pavements in Australia Page 7 of 9

8 Figure 5 This 170 mm thick RCC base layer is Rocky Creek Road, Tarong Power Station. After 12 months, concrete cores were taken from the road and tested for strength and shear capacity at the interface of the two RCC layers (Carse, 2001). The results from testing four cores indicated that the shear bond at the interface was lower than desired by the designer. It is noted that concrete slurry was applied after paving of the first layer and due to side access restrictions, tipper trucks were required to drive over the slurry, possibly damaging the slurry surface. Overseas, the PCA has published a report on long term performance of RCC pavements (Piggott, 1999). The study was carried out in 1998 and only those projects exceeding three years (a total of 18 pavements) were included in the study. The major conclusions that may be drawn are: The quality of RCC pavement construction in terms of smoothness, jointing and durability has greatly improved since the first projects were built in the 1970's. The introduction of high density asphalt paving machines to place and compact the RCC mixture, has been the single most significant factor to influence RCC pavement construction. There was very little evidence of structural failure in the pavements studied. This may be due in part to the high strength achieved as RCC ages. Core samples taken eight years after construction at a log sorting operation on Vancouver Island showed compressive strengths of 40 MPa. It is also possible that some pavements did not receive the loadings anticipated in the original design. The virtual absence of faulting at both transverse and longitudinal cracks is a surprising observation to came out of the study. Even for cracks of up to 4 mm wide, where there was unlikely to be any load transfer, no faulting was evident. Where crack width was less than 2 mm, some load transfer through aggregate interlock may occur. In some cases, it is probable that the magnitude and frequency of loading was below design values, resulting in reduced fatigue of the RCC pavement slab. CONCLUSION Roller compacted concrete is old technology being revitalised due to the better application of concrete technology and experience gained in slipformed concrete paving in NSW. Unlike hand placed and slipformed concrete, RCC is a dry high-strength 1 concrete mix which is compacted by external rolling rather than internal vibration. This mix provides the opportunity for early strength development when used for composite construction to allow traffic onto the road and ensure that road user demands for 24- hour access to the road network can be met. 1 For base concrete layer. State of the art of RCC pavements in Australia Page 8 of 9

9 Where RCC is used for composite pavements with the subbase layer constructed in RCC, the strong subbase, and lack of forming sides and jointing, especially at intersections, reduce construction time. Unfortunately the lack of local performance data and general expertise in the design and construction of RCC hinders its use in road and general pavement construction. There is growing evidence from North America that RCC for base layers meets design performance levels, and in time, projects constructed over the last ten years will also show that this form of construction is viable for rigid and composite pavement construction. REFERENCES ACI (1985) Roller-Compacted Concrete ACI Standard 207.5R-80, ACI manual of Concrete Practice, Detroit, Michigan, USA. Austroads (1992) Guide to the structural design of road pavements Sydney, NSW. C&CAA (2002) Slipformed Industrial Floors Road Note 63, Cement & Concrete Association of Australia, St Leonards, Australia. C&CANZ Design, Specification and construction of roller compacted concrete pavements Report TR 07, Porirua, NZ Carse, A (2000) Report No.1 on the assessment of roller compacted concrete at QCL Excel Quarry Qld Main Roads, Transport Technology Division, Report No. R99200/001C, Herston, Queensland. Carse, A (2001) Report No.2 on the assessment of roller compacted concrete at QCL Excel Quarry Qld Main Roads, Transport Technology Division, Report No. P51200/001C, Herston, Queensland. Jameson, GW, Sharp, KG, Bailey, DP and Warwick, RA (1990) An investigation of the suitability of roller compacted concrete for high-speed traffic pavements Research Report ARR No. 181, Australian Road Research Board, South Vermont, Victoria. PCA (1987) Structural design of roller-compacted concrete for industrial pavements Concrete Information, Portland Cement Association, Chicago, Illinois, USA. Petrie, RE and Matthews, SM (1990) Roller compacted concrete pavements, recent Australian developments and prospects for the 90 s Proceedings, 15th ARRB Conference, Darwin, Australian Road Research Board, South Vermont, Victoria. Piggott, RW (1999) Roller compacted concrete pavements A study of long term performance Portland Cement Association, Illinois, USA. Rollings, R S (2001) Design and construction of roller compacted concrete pavements International Workshop on roller compacted concrete pavements, USA. RTA (1997) Amendments to RTA Form 76 File: TP3008:1, Sydney, NSW. RTA (2003) R90 Specification - Lean rolled concrete subbase Sydney, NSW. Standards Australia AS Methods of Testing Concrete - Compactibility Index. ACKNOWLEDGEMENTS The authors would like to thank Edwin Petrie of QCL for access to several published documents and some photographs. State of the art of RCC pavements in Australia Page 9 of 9