Australian Society for Concrete Pavements 4 th Concrete Pavements Conference. Two-Lift Concrete Paving

Transcription

1 Australian Society for Concrete Pavements 4 th Concrete Pavements Conference Two-Lift Concrete Paving Kurt D. Smith, P.E. Program Director Applied Pavement Technology, Inc. ABSTRACT Two-lift concrete pavements use two separate lifts of concrete that are placed in a weton-wet process to produce a monolithic structure. Although not new, two-lift concrete pavements are an innovative approach to optimizing the characteristics of each layer and, hence, the overall pavement structure. This paper provides an overview of two-lift concrete pavement technology in the United States, including its background, benefits, and design and construction considerations, and also describes several recent two-lift paving projects.

2 Introduction Two-lift concrete pavements use two separate lifts of concrete that are placed in a wet-on-wet process to 2 3 inches (50 75mm) (typical) produce a monolithic structure (see figure 1). Although not new, two-lift concrete Figure 1. Conceptual illustration of two-lift concrete pavement. pavements are an innovative approach to optimizing the characteristics of each layer and, hence, the overall pavement structure. For example, the upper lift may consist of abrasionresistant and more durable materials optimized for surface characteristics such as noise and texture, while the lower lift may employ recycled or marginal aggregate materials and possibly higher quantities of supplementary cementitious materials (SCMs), such as fly ash or slag cement. The result is a pavement structure with equal (or better) performance as a single-lift pavement but with lower environmental impacts and reduced material costs. This paper provides an overview of two-lift concrete pavement technology in the United States, including its background, benefits, and design and construction considerations, and also describes several recent two-lift paving projects. Two-Lift Pavement Evolution in the U.S. Most of the first concrete pavements constructed in the U.S. in the early 1900s were of the two-lift variety. These commonly featured a 3- to 4-inch (75- to 100-mm) bottom lift (consisting of larger aggregate and reduced cement contents) that was capped with a thin 1- to 2-inch (25- to 50-mm) top lift incorporating a smaller, more durable aggregate. What stimulated the use of this approach during those early days of concrete pavement was overall constructability (thinner lifts were easier to place and control than thicker lifts) and the limited availability (and cost) of hard and durable aggregates that were needed to withstand the traffic loading of the day, which included horses clad with steel horseshoes. ASCP 4 th Concrete Pavements Conference 2

3 The first use of concrete in a street application in the U.S. is widely acknowledged as occurring in Bellefontaine, Ohio in 1891 in front of the courthouse. That initial project was so successful that the City went on to pave the other three streets surrounding the courthouse in 1893 with concrete. That 1893 construction, one of which is still in service today, featured a 4-inch (100- mm) bottom lift with a 1.5-inch (38-mm) maximum top size aggregate, and a 1-inch (25-mm) top lift with a 0.5-inch (13- mm) maximum top size (Snell and Snell 2002). The slabs were formed in 5-ft (1.5-m) squares with tar paper placed between adjacent slabs, and imparted with a grid pattern on the surface to provide traction for horse traffic. Other early concrete pavements also featured two-lift construction, a core Figure 2. Core of 1906 twolift concrete pavement (courtesy Tom Van Dam). of which from a project built in 1906 in Calumet, Michigan is shown in figure 2. This photo clearly shows the two different paving materials that were used in the construction of the pavement. As concrete pavement technology evolved, most agencies began moving away from two-lift pavements. However, as the value of mesh reinforcement in controlling cracks was documented, agencies began to adopt jointed reinforced concrete pavement (JRCP) designs featuring longer slab lengths, mid-panel mesh reinforcement, and doweled joints. This was a common pavement design used by many U.S. highway agencies and represented a significant portion of the original interstate system that was built in the U.S. in the 1950s, 1960s, and 1970s. The movement to these types of designs served to resurrect two-lift paving, not in terms of employing different materials but rather to accommodate the placement of the mesh reinforcement between separate lifts of concrete. However, by the 1990s, JRCP designs had fallen out of favor in the U.S. as most agencies gravitated to jointed plain concrete pavement (JPCP) designs, and two-lift paving once again declined. ASCP 4 th Concrete Pavements Conference 3

4 Although a few agencies continued to evaluate the use of two-lift concrete pavements, it was after a series of tours of European concrete pavements by U.S. researchers and practitioners that two-lift paving technology re-emerged (Darter 1993; Hall et al. 2007; Tompkins, Khazanovich, and Darter 2010). Those tours demonstrated the positive experience of several European countries (most notably Austria but also Germany and the Netherlands) with two-lift pavements and documented the material savings and environmental benefits associated with their use. This gave rise to an early wave of two-lift pavement projects in the 1990s (Michigan, Kansas), followed by a larger flurry of projects in the 2000s and early 2010s (Kansas, Missouri, Illinois, Minnesota). A major study was conducted in the U.S. from 2007 to 2011 that led to the development of improved guidance for the design and construction of two-lift pavement structures (Rao et al. 2013). Advantages and Benefits As alluded to in the preceding discussion, two-lift paving offer a number of potential advantages and benefits, as summarized below: Reduced costs. Use of local/recycled aggregates. Use of reduced cement contents. Use of increased supplementary cementitious materials. Optimize surface characteristics (for noise, friction, ride). Several of these also have an environmental and sustainability component, which has been a recent driving force behind the growth of two-lift pavement construction. For example, many parts of the U.S. are seeing a scarcity in high-quality aggregate suitable for use in concrete pavement structures. These agencies are then faced with having to import those aggregates from elsewhere, which not only has a cost implication but also an environmental impact in terms of energy consumption and greenhouse gas emissions associated with the material hauling and transport. If those agencies could use more widely available local aggregates of perhaps more marginal quality, they would realize both financial savings and ASCP 4 th Concrete Pavements Conference 4

5 reduced environmental impacts. Furthermore, with more and more agencies recycling and reusing materials from existing pavements, there are opportunities to reuse both recycled concrete aggregate (RCA) and reclaimed asphalt pavement (RAP) as an aggregate source in the lower lift, helping to reduce environmental impacts by incorporating recycled products, drawing down on existing stockpiles, and by lessening aggregate mining and extraction activities. One concern about two-lift concrete pavements is the potential for an increase in costs due to the use of multiple batch plants and the mobilization of two paving crews (Cable and Frentress 2004). However, these costs can be offset by the use of higher quantities of locally available and/or recycled materials in the bottom concrete layer (which are cheaper and negate the need for hauling/transport of higher quality aggregates) and a reduction in the cement contents (and addition of SCMs) in the bottom layer. Although many early projects in the U.S. showed a significant increase in construction costs, these were generally smaller, pilot programs and did not reflect large-scale projects where the potential for large costs savings would be possible (Hu et al. 2014). Indeed, when the Illinois Tollway constructed a number of large, two-lift paving projects on I-90, the agency secured a significant reduction in bid prices when compared to single-lift construction (Gillen et al. 2013). Design Considerations The mix designs used in the individual lifts are determined by the highway agency based on local material availability and overall agency goals. The bottom lift commonly features the use of marginal aggregate that may not be suitable for use as a surface layer or the use of recycled materials (both RCA and RAP), whereas the top lift uses an agency s specified surface aggregate or perhaps a harder aggregate for surface durability. Reduced cement contents and use of SCMs are used in the bottom lift, with agency standard mix designs commonly employed in the top lift. Table 1 summarizes some of the mix designs that have been used in selected two-lift concrete pavements in the U.S. ASCP 4 th Concrete Pavements Conference 5

6 Table 1. Example mix designs used in two-lift pavements (compiled from Fick 2009; Gillen et al. 2013; Rao et al. 2013; and Pierce et al. 2016). Top Lift of Concrete Bottom Lift of Concrete Kansas DOT Illinois Tollway* Minnesota DOT** Tennessee DOT I-70, Salina I-90 (various segments west of Chicago) Mn/ROAD Research Facility, Albertville I-65, Nashville Thickness, inches (mm) 1.2 (30mm) 3 (75mm) 3 (75.mm) 3 (75mm) Coarse Aggregate Max Size and Type Coarse Aggregate Content, lb/yd 3 (kg/m 3 ) Fine Aggregate Content, lb/yd 3 (kg/m 3 ) Cement Content, lb/yd 3 (kg/m 3 ) SCM Content, lb/yd 3 (kg/m 3 ) Req d Compressive Strength, lbf/in 2 (MPa) 0.62 inch (16mm) rhyolite 1576 (935) 1518 (901) 438 (260) 110 (65) (Class F flyash) 0.75 inch (19mm) crushed dolomite 1857 (1102) 1245 (739) 405 (240) 175 (104) (Class C flyash) inch (10mm) crushed granite 1976 (1172) 843 (500) 616 (365) 109 (65) (Class F flyash) 0.75 inch (19mm) surface limestone 1800 (1068) 1244 (738) 289 (171) 113 (67) ( Class F flyash) 169 (100) (Slag cement) 3000 (20.7) 3000 (20.7) w/c Thickness, inches (mm) 11.8 (300) 9 (229) 6 (150) 10 (250) Coarse Aggregate Max Size and Type 1.5 inch (38mm) limestone Coarse Aggregate Content, lb/yd 3 (kg/m 3 ) 1743 (1034) Fine Aggregate Content, lb/yd 3 (kg/m 3 ) Cement Content, lb/yd 3 (kg/m 3 ) SCM Content, lb/yd 3 (kg/m 3 ) Req d Compressive Strength, lbf/in 2 (MPa) 0.75 inch (19mm) crushed dolomite 0.75 inch (19mm) FRAP 1575 (dolomite) (934) 273 (FRAP) (162) 1221 (724) 1196 (710) 548 (325) 375 (222) (27.6) 145 (86) (Class C flyash) 60 (36) (Slag cement) 3500 (24.1) (14 days) w/c Bottom Lift inch (38mm) Class A 0.75 inch (19 mm) Class A Bottom Lift 2 1 inch (25 mm) RCA 1.5 inch (38 mm) Class A Bottom Lift (467) (for 1.5- inch [38mm]) 1102 (654) (for 0.75 [19mm] inch) Bottom Lift (546) (RCA) 825 (489) (Class A) Bottom Lift (749) Bottom Lift (712) Bottom Lift (142) Bottom Lift (214) Bottom Lift (214) Bottom Lift (142) 1.5 inch (38mm) limestone 0.75 inch (19mm) surface limestone 765 (454) (limestone) 1150 (682) (surface limestone) 1290 (765) 289 (171) 105 (62) (Class F flyash) 132 (78) (Slag cement) 3000 (20.7) 3000 (20.7) Bottom Lift 1: 0.29 Bottom Lift 2: 0.39 * Slight variations in structural and mix designs used over project length, typical values given. ** Minnesota project featured two separate sections with different mix designs used in the bottom lift ASCP 4 th Concrete Pavements Conference 6

7 The structural design of two-lift pavements can be performed using available design procedures that incorporate a concrete-on-concrete overlay component. In essence, the process assumes the placement of a new concrete overlay on an existing concrete pavement, but since two-lift represents the construction of a new pavement structure, no prior cracking or damage should be input for the existing pavement (Rao et al. 2013). Layer-specific mix design properties and structural characteristics can be provided for the different paving lifts. In the U.S., the AASHTO Pavement ME software program can be used to design two-lift pavement designs, but the program is currently limited to top layer thicknesses of no less than 4 inches (100mm). There have been some concerns about the differences in the coefficient of thermal expansion (COTE) between the two lifts and the potential for possible debonding if those are significantly different. Extensive analytical evaluation of composite pavement behavior has shown that this is not a concern, and the analysis results are backed up based on the longterm experience of Austria and other European countries that have never experienced a debonding failure between paving lifts (Rao et al. 2013). Construction Considerations With multiple trucks, pavers, and belt placers, the construction site for a two-lift paving project can be a bustling work area (see figure 3). Although construction activities associated with two-lift concrete Figure 3. Overview of two-lift pavement construction site (courtesy Kurt Smith). pavements generally follow conventional paving practices, there are a few items that require special attention or consideration, as described below. Batching, Transport, and Delivery ASCP 4 th Concrete Pavements Conference 7

8 The batching, transport, and delivery of two separate mixes for use in the two-lift paving can create some production and logistical issues, and requires careful planning and consideration as follows: Batching. Either two plants or a single plant can be used, with some projects employing a local ready-mix facility. The use of two plants for two-lift paving is common in Europe, although many projects in the U.S. have used a single plant. The goal is to ensure the production of adequate quantities of both materials, with recognition that greater quantities of bottom-lift materials will need to be produced since it is thicker than the top lift. Transport. The transport of materials to the job site is performed using conventional means (end dump trucks, transient trucks, etc.), but the materials must be delivered in sufficient quantities to meet the demands of the paving equipment. The contractor can determine the quantity requirements of a particular job and then establish a delivery target of bottom-lift trucks to top-lift trucks; for example, a two-lift paving project on I-65 in Tennessee targeted the delivery of four bottom-lift trucks to the delivery of one top-lift truck (Pierce et al. 2016). Delivery. A critical concern regarding the delivery of the paving materials to the project site is that each truck deliver to the correct paver. While seemingly obvious, this is important to ensure that bottom-lift materials are delivered to the paver placing the bottom lift and that top-lift materials are delivered to the paver placing the top lift. A number of methods have been used to ensure the proper delivery, including the distribution of colored cards to truck drivers to match up with the paving machine and the use of different transport trucks to discriminate which mix is being delivered. Timing of Lift Placements It is imperative that the top lift be placed while the bottom lift is still plastic in order to promote the development of a strong bond between the two lifts. Specifications ASCP 4 th Concrete Pavements Conference 8

9 requirements can vary by agency, but generally stipulate time periods of no more than 45 to 90 minutes between lift placements. Dowel Bars Dowel bars for two-lift paving can be placed in baskets on grade in front of the paver or by insertion methods. Dowel insertion methods are commonly used in Europe but in the U.S. most two-lift pavement projects have placed dowels on baskets. In either case, it is important that the dowel bars be placed at mid-depth of the total concrete thickness (see figure 4). Agency requirements for dowel Figure 4. Dowel location in a two-lift pavement structure. alignment and location then apply. If a paver is used for the placement of the bottom lift, the vibrators must be raised above the dowel bars to avoid contact. Similarly, when the top lift is placed, the vibrators on the second paver should be raised above the interface between the two lifts to avoid intermingling of the materials. Vibration levels can vary, but values of 8000 rpm for the bottom lift and 4000 rpm for the top lift have been used (Pierce et al. 2016). Joint Sawing As with conventional concrete pavements, the creation of joints in two-lift concrete pavements is a critical construction item. This is generally achieved by sawing partial-depth in the young concrete to create a weakened plane that promotes the development of a crack at that location. The joint sawing should begin as soon as possible after the slab has achieved adequate strength, and under most normal conditions this may be between 4 and 12 hours after placement. For two-lift concrete pavements, the depth of the sawcut in the slab should be the deeper of (Rao et al. 2013): 1/3 of the total slab thickness (top lift + bottom lift), or Top lift thickness + ½ inch (13mm). ASCP 4 th Concrete Pavements Conference 9

10 This reason for this is to ensure that the sawcut penetrates into the bottom lift so that the potential for horizontal delamination is minimized. Selected Two-Lift Paving Projects in the U.S. Two-lift concrete pavements have seen a growing history of recent usage in the U.S. and are emerging as a viable alternative on a number of paving projects. This section provides a brief overview of four two-lift paving concrete projects in the U.S. that illustrate a range of designs, materials, and applicability. Table 2 summarizes some of the characteristics of these projects (mix design characteristics were presented previously in table 1), with additional details provided in the following sections. Table 2. Characteristics of selected two-lift concrete pavement projects in U.S. (compiled from Fick 2009; Gillen et al. 2013; Rao et al. 2013; and Pierce et al. 2016). Location Kansas DOT Illinois Tollway* Minnesota DOT Tennessee DOT I-70, Salina (eastbound) I-90 (various segments west of Chicago) Mn/ROAD Research Facility, Albertville I-65, Nashville (northbound) Year Project Length Top Lift Thickness (aggregate type) Bottom Lift Thickness (aggregate type) Base Type and Thickness Transverse Joint Spacing Dowel Bars Material Production Time Specification Between Lifts Curing Surface Texture 5 miles (8 km) (7 sections) 1.2 inches (30mm) (rhyolite) 11.8 inches (300mm) ((limestone) 6 inches (152mm) cement-treated granular base 20+ miles (32+ km) 3 inches (75mm) (crushed dolomite) 9 inches (229mm) (crushed dolomite + FRAP) 3 inches (75mm) warm-mix asphalt base 1000 ft (305 m) (2 sections) 3 inches (75mm) (crushed granite) Bottom Lift 1 6 inches (150mm) (Class A aggregate) Bottom Lift 2 (Class A + RCA) 8-inches (200mm) granular base 5000 ft (1524 m) 3 inches (75mm) (surface limestone) 10 inches (250mm) (limestone) 4 inches (100mm) asphalt-treated permeable base 16.4 ft (5 m) 15 ft (4.6 m) 15 ft (4.6 m) 15 ft (4.6 m) 1.5-inch (38mm) epoxy coated bars on 12-inch (300mm) centers 1.5-inch (38mm) epoxy coated bars on 12-inch (300mm) centers 1.25-inch (32mm) epoxy coated bars on 12-inch (300mm) centers 1.5-inch (38mm) epoxy coated bars on 12-inch (300mm) centers Central batch plant Central batch plant Ready-mix plant Ready-mix plant 60 minutes minutes minutes 45 minutes White pigmented (plastic sheets for exposed aggregate) Various (longitudinal tining, longitudinal grooving, astroturf, grinding, NGCS, EAS) White pigmented Longitudinal tining Curing/retarder compound EAS and diamond grinding * Slight variations in structural and mix designs used over project length, typical values given. White pigmented Transverse tining ASCP 4 th Concrete Pavements Conference 10

11 Kansas Department of Transportation, Interstate 70 The Kansas Department of Transportation constructed a demonstration project of two-lift concrete paving in In addition to making use of a local limestone aggregate for the bottom lift, the project featured a number of surface textures, including transverse and longitudinal tining, grinding, next generation concrete surface (NGCS), and an exposed aggregate surface (EAS) (Fick 2009). A limited life-cycle assessment (LCA) which considers the environmental impacts associated with different pavement designs was performed on the project and indicated that the two-lift design offered a 15 percent reduction in global warming potential and a 20 percent reduction in energy usage when compared to conventional concrete pavements (Hu et al. 2014). Illinois Tollway, Interstate 90 In 2011, the Illinois Tollway embarked on 15-year, multi-billion dollar program to rehabilitate, reconstruct, and expand its freeway system (Gillen et al. 2013). For a portion of that work on the I-90 Jane Addams Expressway west of Chicago, the Tollway used two-lift construction during the construction seasons due to the availability of recycled materials and because the rural setting of the project site offered sufficient room for haul roads and equipment maneuverability. The two-lift concrete pavement featured fractionated reclaimed asphalt pavement (FRAP) in the bottom lift of the pavement, enhancing the sustainability of the overall construction process and helping to reduce costs. The FRAP was recovered from the existing asphalt pavement and graded, with only the coarse components (greater than No. 4 sieve) used in the bottom lift at typical contents of 15 to 25 percent (Gillen et al. 2013). Minnesota Department of Transportation, Mn/ROAD Facility Under the SHRP2 R21 project, the Minnesota Department of Transportation constructed a two-lift concrete pavement in 2010 at the MN/ROAD facility near Albertville, MN (northwest of Minneapolis). The purpose of the project was to demonstrate the feasibility of two-lift concrete pavement systems, and featured two different sections with unique mix designs in the lower lift: one a low cost concrete with Class A aggregate and low cement content and ASCP 4 th Concrete Pavements Conference 11

12 one a recycled concrete featuring a 50/50 blend of Class A aggregate and RCA (and low cement contents). Both sections included an exposed aggregate surface, but a portion of that surface was diamond ground in the low cost section (Rao et al. 2013). Tennessee Department of Transportation, Interstate 65 This project was placed in 2014 as part of the reconstruction of I-65 through Nashville and featured the use of a polish-susceptible limestone aggregate in the lower lift. The twolift component was constructed on the outside shoulder of the project but at the same thickness as the mainline pavement. Based on the results of this project, TDOT noted that the placement of the two-lift composite pavement was a success and determined that it was a viable alternative to the placement of full-depth concrete pavements (Pierce et al. 2106). Summary Two-lift concrete pavements the placement of two separate lifts of concrete on one another in a wet-on-wet paving process have experienced a re-awakening in the U.S. as highway agencies recognize the potential for cost savings and sustainability benefits. The use of two-lift concrete pavement allows agencies to optimize the use of materials within a pavement structure, effectively utilizing higher quality aggregate in the surface course to meet durability requirements while employing local or marginal quality aggregates (and reduced cement contents) in the lower surface to meet structural demands. This saves costs by reducing the demands for high-quality, more expensive aggregate and allows agencies to make use of locally available material or to incorporate greater quantities of recycled materials (both asphalt and concrete), which also translates into positive sustainability impacts. Many of the design and construction aspects of two-lift concrete pavements are similar to what is done for conventional concrete, but there are some unique concerns and considerations to recognize. Several U.S. highway agencies have experience with two-lift pavements and its use is expected to grow as agencies become more familiar with the process and its performance and sustainability benefits are documented. ASCP 4 th Concrete Pavements Conference 12

13 References Cable, J. K. and D. P. Frentress Two-Lift Portland Cement Concrete Pavements to Meet Public Needs. Report No. DTFH61-01-X (project 8). Center for Transportation Research, Iowa State University, Ames, IA. Darter, M. I Report on the 1992 U.S. Tour of European Concrete Highways. FHWA- SA Federal Highway Administration, Washington, DC. Fick, G National Open House, Two-Lift Concrete Paving, October 15-16, 2008, Salina/Abilene, KS Interstate 70. Concrete Pavement Technology Center, Ames, IA. Gillen, S. L., A. S. Brand, J. R. Roesler, and W. R. Vavrik Sustainable Long-Life Composite Concrete Pavement for Illinois Tollway. Proceedings, Illinois Tollway Open House: Sustainable Concrete Paving Practices, August 20-21, 2013, Rosemont, IL. Hall, K., D. Dawood, S. Vanikar, R. Tally Jr., T. Cackler, A. Correa, P. Deem, J. Duit, G. Geary, A. Gisi, A. Hanna, S. Kosmatka, R. Rasmussen, S. Tayabji, and G. Voigt Long-Life Concrete Pavements in Europe and Canada. Publication No. FHWA-PL Federal Highway Administration, Washington, DC. Hu, J., D. Fowler, M. S. Siddiqui, and D. Whitney Feasibility Study of Two-Lift Concrete Paving: Technical Report. Report Texas Department of Transportation, Austin, TX. Pierce, L. M., M. B. Snyder, K. D. Smith, and J. L. Waller SHRP2 R21Project Implementation: Two-Lift Concrete Paving on I-65, Nashville. Proceedings, 11 th International Conference on Concrete Pavements, San Antonio, TX. Rao, S., M. Darter, D. Tompkins, M. Vancura, L. Khazanovich, J. Signore, E. Coleri, R. Wu, J. Harvey, and J. Vandenbossche Composite Pavement Systems, Volume 2: PCC/PCC Composite Pavements. SHRP2 Report S2-R21-RR-3. Transportation Research Board, Washington, DC. Snell, L. M. and B. G. Snell Oldest Concrete Street in the United States. Concrete International, Volume 24, No. 3. American Concrete Institute, Farmington Hills, MI. ASCP 4 th Concrete Pavements Conference 13

14 Tompkins, D., L. Khazanovich, and M. I. Darter Survey of European Composite Pavements. SHRP2 Report S2-R21-RW-1. Transportation Research Board, Washington, DC. ASCP 4 th Concrete Pavements Conference 14