The density of concrete varies, but is around 2,400 kilograms per cubic metre 150 lbcu ft. Thus, for light traffic situations, the pavement may be placed directly on a properly prepared subgrade. Concrete pavement construction basics compaction and trimming before compacting, subgrade material may have to be brought to the optimum moisture content. Jointed plane concrete pavement 12 to 20 ft transverse joints with dowels longitudinal joint with tie bars plan view 12 to 20 ft. Performance of continuously reinforced concrete pavement. In a cut section the subgrade is the bottom of the cut figure 71. The subgrade and subbase factor concrete construction. Its role is to spread the load evenly over the subgrade. This pivotal work on rigid pavement design by the u. Note that a pavement built directly on the subgrade with no subbase would still. Concrete bases and pavement are constructed on a subbase which is placed on a prepared subgrade. In this publication, a subbase is defined as the layer of material that lies immediately below the concrete pavement. When do you need to use a subbase under concrete pavements. Overloading, loss of subgrade support, and insufficient andor improperly cut joints acting singly or in combination are also possible causes.
Bases and subbases for concrete pavements aashto classification a6 or a7 soil groups or the unified soil classification system ch, mh, and oh soils. This can involved such activities as replacing localized. Subgrade soils are subjected to lower stresses than the surface and subbase courses. Base and subgrade layers must provide adequate and moisture resistant strength that meets the design modulus, in addition to durability and stability. Where frequent heavy truck loads are anticipated, a subbase is needed to prevent pavement pumping. In other words, the design must consider the allowable opening of these joints over its service life with consideration of the riding quality of the. Subgrade beneath concrete slabs thaidavid40 structural 10 aug 15 23. This report describes test results and comparative analysis from 16 different portland cement concrete pcc pavement sites on local city and county. Subbase is often the main loadbearing layer of the pavement. The subgrade has a substantial impact on base and subsurface drainage requirements and on longterm pavement ride quality and overall performance. Bases and subbases for concrete pavement performance. Prior to constructing the subbase, the subgrade must be treated in accordance with section 207.
This condition may result from loss of fines, frost heave, loss of load transfer device key, dowel, etc. Eb204 subgrades and subbases for concrete pavements acpa. Subbase design manual for portland cement concrete pavements. In design and construction of bases and subbases for concrete pavement performance, shreenath rao, hesham abdualla, and thomas yu, p. To ensure a stable, longlasting, and maintenance free. Discuss the fundamentals of bases and subbases for concrete. It may be omitted when there will be only foot traffic on the pavement, but it is necessary for surfaces used by vehicles. Guide to subgrades and subbases, which support concrete slabs. This can involve such activities as subgrade stabilization, overexcavation of poor subgrade, applying a prime coat or compacting the subgrade preparing an existing pavement surface for overlay. In fact, once a crack initiates in a concrete pavement topdown at a.
Fulldepth repairs that are properly designed and constructed will provide good longterm performance for as long as the surrounding concrete pavement. Most native soils are not too uniform and thus require some improvement or additional layers to compensate. The subgrade supports the subbase andor the pavement section. Surface preparation generally takes one of two forms. Depending on site conditions, subgrade improvements may also be performed. Concrete pavement distresses a cracking cracks in concrete pavements often result from stresses caused by expansion and contraction or warping of the pavement. The subgrade is to be constructed so that the material has a uniform density throughout.
Basessubbases for concrete pavements transportation research. The blowups usually occur at joints or cracks in the pavement and the concrete at the blowup appear to be weak or deteriorated at that point. Subgrade compacted soil layer which forms the foundation for the pavement system. Pavement performance can be largely attributed to the care given in designing and preparing its foundation, which is comprised of the subgrade and base layers. A subbase is a thin layer of material placed on top of the prepared subgrade. However, a distinction in terminology needs to be made. The subgrade must be thoroughly compacted, generally with a sheeps foot roller, to provide a strong platform for construction activities. Early cracking of concrete pavement causes and repairs. Revised april 2007 chapter 7 pavement maintenance frost protection layer provides a barrier against frost action and frost penetration into the lower frostsusceptible layers. Settlement or faulting is a difference in elevation at a joint or crack caused by upheaval or nonuniform consolidation of the subgrade or subbase material.
Subbase is composed of the layers of material between the subgrade and the concrete pavement. Pavement type, pavement thickness, subbase type, and subgrade were all found to affect deflection. Subbase and subgrade performance investigation and design. Pavement loading loads applied to a pcc pavement spread over a large area of the basesubbase and subgrade this permits the use of thinner bases for rigid pavements than for flexible pavements. Some engineers call this a base course since that term is used to designate the first layer beneath an asphalt surface. Support in concrete pavement sustainability txdot project 6037 alternatives to asphalt concrete. The field performance of edgedrains installed without a permeable base has been mixed. When placed below the base aggregate, the geocomposite shortens the. In concrete pavement terminology, the subgrade the native soil on which the pavement is built must be protected and compacted to ensure it stays in its place and isnt damaged. Specifications for subgrade and subbase construction are limited to its effects on concrete pavement construction.
Army corps of engineers led to a much better understanding of the importance of the use of bases and subbases, their uniformity, and degree of. Settlement cracks settlement of the subgrade and subbase can cause the cracking of the concrete pavement. Concrete has a very low coefficient of thermal expansion, and as it matures concrete shrinks. Theoretical and practical studies of concrete pavement have determined that the optimal spacing between joints depends on the slab thickness, subbase stiffness, and concrete strength. Slab movements concrete pavement movement refers to the movements at the joints for jointed, reinforced, and posttensioned slabs.
Cracks propagate through the drainage layer in the midslab crack core sample. The reason the asphalt cracks usually do not cause a crack in a concrete. Stabilized subbase friction study for concrete pavements. Thinner pavements may not crack through at gutter joint, causing longitudinal cracks at midpanel. The concrete and subbase costs from tables 41 and 42 are then entered in columns 6 and 8. Concrete which is subjected to longduration forces is prone to creep. Other factors responsible for non uniform foundation include variability due to a number of reasons including, natural causes, excavation and. Preparing the subgrade and granular base course for new pavement. In concrete paving terminology, the subgrade is the soil the pavement is built on. Figure 36 analysis of modulus of subgrade reaction effects. Types of distresses in concrete pavements and their causes. All concrete structures will crack to some extent, due to shrinkage and tension. Achieving uniform subgrade support during construction is the key to adequate pavement performance without the expense of a subbase.
Most state agencies specify transverse contraction joints in plain pavement at an interval between 4. Continuously reinforced different pavement types subbase subgrade subbase subgrade base asphalt layer concrete section asphalt section asphalt layer how pavements carry loads 3000 kg. A reasonably uniform subgrade or subbase, with no abrupt changes in support, is ideal for any concrete pavement. Concrete pavement bases and subbases specify concrete. Subgrade beneath concrete slabs structural engineering.
If the pavement is susceptible to blowups, they begin to occur after 3 to 5 years of age. Subgrade and subbase concrete construction magazine. Several reconstructed portland cement concrete pcc pavement projects in iowa were recently studied to evaluate the influence of subgradesubbase nonuniformity on pavement performance. Design and construction of joints for concrete streets. Usually it is treated by compacting it sufficiently to prevent the pavement from settling.
Subbasesubgrade geocomposite drain sand backfill 25 mm 100 mm. In the early days of rigid pavement construction, concrete slabs were placed directly on top of the subgrade without any basesubbase layers. In highway engineering, subbase is the layer of aggregate material laid on the subgrade, on which the base course layer is located. This is done by laying down a sub base, a layer of aggregate material that is placed above the subgrade to prevent soil displacement. From the gap 2019 transportation conference proceedings. The overall condition of the pavement and the extent of deterioration should be carefully examined to ensure that the placement of fulldepth. Among the many factors involved in placement of a quality concrete pavement, the subgrade is an important component often overlooked. In a fill section, the subgrade is the top of the embankment or the fill. The subgrade and subbase factor concrete construction magazine. Job meeting should be held with the owner, engineer, concrete producer, testing company and. This includes thickness design, subgrade and subbase preparation, and jointing.
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