Concrete Pavement Construction Basics Tech Note Table of contents General principles • Everyone at the construction site, particularly Site preparation ......................................................2 foremen and supervisors, is responsible for Preparing the grade, or road bed .......................2 recognizing and troubleshooting potential problems Establishing the stringline ..................................3 as they arise. Placing dowel baskets .......................................4 • Batches of concrete should be consistent and uniformly mixed. Slip-form paving operations ..................................5 • A major cause of pavement failure is unstable Wetting the grade ...............................................5 subgrade. The subgrade should consist of uniform Delivering the mix ..............................................5 material, and the subgrade system must drain well. Placing the concrete ..........................................6 • Dowel bars are important for load transfer at Spreading and consolidating the concrete .........6 transverse joints on pavements with high truck volumes. Dowels must be carefully aligned, Setting header joints ..........................................7 horizontally and vertically, to prevent pavement Placing tiebars during construction ....................7 damage at the joints. Finishing .............................................................7 • Stringlines control the slipform paver’s horizontal and Texturing ............................................................7 vertical movement and ensure a smooth pavement Curing ................................................................7 profile. Once stringlines are set, they should be Insulating ............................................................7 checked often and not disturbed. • Overfinishing the new pavement and/or adding Jointing ....................................................................8 water to the surface can lead to pavement surface Why joints are important ....................................8 problems. If the concrete isn’t sufficiently workable, Rules of thumb ...................................................8 crews should contact the project manager. Changes to the mixture or to paver equipment may reduce the Sawing joints ......................................................8 problem. Cleaning and sealing joints ................................8 • Proper curing is critical to preventing pavement damage from rapid moisture loss at the pavement Weather considerations .........................................9 surface. Hot, dry weather paving .....................................9 • A well spaced and constructed system of joints is Cold weather paving ..........................................9 critical to prevent random cracking. Rain ....................................................................9 • Joints are simply controlled cracks. They must be sawed during the brief time after the pavement Troubleshooting ...................................................10 has gained enough strength to prevent raveling Common problems ...........................................10 but before it begins to crack randomly (the “sawing Hot and cold weather .......................................10 window”). Sawing problems .............................................11 • Seasonal and daily weather variations affect Sealing problems .............................................11 setting time and other variables in new concrete. Construction operations should be adjusted appropriately. Stabilization Site preparation Stable soils are generally defined as those that can support Before construction begins, the construction site must loads. Loam or clay loam of glacial till origin or clean sand be carefully prepared. This includes preparing the grade, is typically desirable. establishing control mechanisms (the stringline), and placing Just as important as the soil’s quality, however, is its dowel baskets. uniformity. During excavation, localized pockets of various kinds of soil may be encountered where water absorption Preparing the grade, or roadbed rates, densities, and expansion and contraction rates may The majority of concrete pavement failures are not caused vary. These pockets lead to different levels of support. by failure of the concrete slab but by problems with the materials beneath the slab. These problems can include poor To stabilize the subgrade so that it will provide uniform drainage, unstable or nonuniform materials, or poor compac- support to the rest of the pavement structure, localized pock- tion. ets of undesirable material should be removed and replaced with the dominant soil type or with select backfill, or the soil Adequate preparation of the roadbed—the subgrade, sub- should be treated with additives. base, and base—is essential for a strong, durable concrete pavement system. Typical stabilizing materials are select soils or back- fill—loam or clay loam, clean sand, or other stable soil • Subgrade: earth that has been graded to the desired material—excavated from road cuts or borrow sites. elevation. (In county and municipal paving projects with low traffic volumes, concrete is often placed Typical stabilizing additives include the following: directly on the prepared earth subgrade.) • Lime: either quicklime or hydrated lime, either high • Subbase: a course of material that is placed on the calcium or dolomitic. Through chemical reactions with subgrade to provide drainage and stability. Three kinds soil, lime reduces soil plasticity and increases com- of subbases may be used, based on the need to balance pressive strength. It is sometimes used to stabilize wet drainability and stability: soils. Granular subbase is the most drainable subbase. It is a • Portland cement: hydrates with moisture in the soil mixture of granular material that is uniformly shaped and hardens. Portland cement performs best with well and minimally compacted. It does not provide sig- graded, sandy, and gravelly materials with 10 to 35 nificant structural support; no construction traffic is percent fines. More cement is usually needed for soils allowed on a granular subbase. with little or no fines and with clay soils. Modified subbase is moderately drainable. It contains • Fly ash: a byproduct of coal furnaces. When mixed a greater percentage of crushed particles and a denser with soil and water, it acts as a binder. Self-cementing gradation than granular subbase, providing more sta- Class C fly ash can be used for treating subgrades. bility. A layer of polymer geosynthetic, called geogrid, may Special backfill provides more stability and support be used as a foundation under special backfill to provide but is the least drainable. It is generally a uniform underlying tension reinforcement. The grid interlocks with mixture of crushed concrete or crushed limestone, or aggregates in special backfill and spreads wheel loads over a a mixture of gravel, sand, and soil, with or without large area. crushed stone. Special backfill or modified subbase is often used under pavement in urban areas to support construction traffic. • Base: a course of fairly rigid material, sometimes Fly ash can cement- or asphalt-treated, that is placed on the sub- be used as a soil-stabilizing base to provide a stable platform for the concrete additive. pavement slab. Grading First the site is graded to cut high points and fill low areas to the desired roadway profile elevations. Generally, cut material can be used as embankment fill. However, peat, organic silt, or soil with high organic content should not be used; borrow material should be used instead. 2 Concrete Pavement Construction Basics Compaction and trimming (Side profile view) Before compacting, subgrade material may have to be brought to the optimum moisture content. The subgrade must be thoroughly compacted, generally with a sheep’s foot roller, to provide a strong platform for construction activi- ties. After compaction, a proof-roller (a heavy, loaded, pneu- matic-tired vehicle) is driven over the subgrade to locate soft The stringline controls the pavement profile. areas or other problems that may require additional stabiliza- tion. (Side profile view) Finally, an electronically controlled trimmer machine is used to trim the subgrade surface to the profile grade. The trimmer runs off the stringline for both elevation and alignment control. Trimming should occur within a week of proof-rolling. The subgrade cannot be used for a haul road. Trucks are only allowed to drive on the grade for dumping. Trimming a granular subbase A damaged stringline can cause a dip in the profile, making the slab thinner at the dip. Once the stringline is set, be careful not to bump it out of alignment or knock the wands loose. Do not operate trucks or other equipment close to the pad line; this could pump the ground around the stakes and disturb the stringline align- ment. Any of these situations could cause the paver to deflect from the correct elevation, creating a dip or a bump in the concrete slab. Establishing the stringline During paving operations, the stringline must be A stringline, supported by stakes alongside the paving eyeballed often for correct alignment. If you notice a distur- lane, guides the paving equipment horizontally and bance in the stringline, notify your supervisor. vertically. The pad line, or track line, is the space between the stringline and new pavement; it must provide adequate room for paver tracks and have a stable, smooth surface.