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11-10: BASE AND SUBBASE OPTIONS FOR CONCRETE PAVERS
| Keywords: |
asphalt treated base, base, base course, base design, cement treated base, concrete pavers, flexible base, pavement design, pavers, rigid base, structural coefficients, subbase, subgrade |
INTRODUCTION
Concrete pavers form the surface course of pavements walkways and patios. They provide a smooth, safe, nonslip surface and are aesthetically pleasing. They are capable of withstanding aggressive traffic regimes and environmental conditions. Similar to other flexible and rigid pavements, the effects of traffic and environment on the subgrade are reduced with the use of other structural layers/materials between the surface course (pavers) and the subgrade. These layers are known as the base and subbase courses. Figure 1 identifies each layer and its position in the pavement structure. A base course is used in all pavements and walkways, but the subbase course may not be required in all situations. The selection of the base and subgrade materials thickness depends on the inteded use of the paver area.
BASE COURSE
The base course is the main structural component in the pavement section. The materials in the base layer need to be capable of resisting the imposed effects of vehicular and pedestrian traffic.
The materials used in the construction of the base course should be of high quality so that they do not deform or deteriorate as a result of the imposed stresses. The most common materials consist of unbound (plain) or bound (cemented) granular materials. For unbound granular materials, the quality of the aggregate particles is especially important, as it is the interaction of the particles that provides the strength and durability of the layer. With bound granular materials, the bonding material, be it portland cement or asphalt, also contributes to the strength and durability of the layer.
SUBBASE COURSE
In some applications, a correctly designed base course is capable of spreading the loads sufficiently to the underlying subgrade thereby alleviating the need for a subbase. This is particulary true for patios and walkways subjected to pedestrian traffic only. However, for heavier wheeled traffic using a subbase layer can often lead to economic benefits in circumstances where the subgrade is of low strength or when it is frost susceptible.
Under load, the subbase is subjected to lower stresses and strains than the base course. As such, the materials used for the subbase do not need to be of such high quality as for the base. With stabilized subbases, where materials are added for strengthening such as cement, lime, asphalt, crushed stone, or shell, it is often possible to construct a subbase layer using the on site subgrade materials.
AGGREGATES
Aggregates form an important part of pavement construction and are present in most pavement layers. There are several sources for the various grades of aggregates, with rock quarries and sand and gravel pits being the major suppliers. There are three classes of rock that may be quarried. They include 1) igneous and metamorphic rocks that are usually very hard and durable, 2) sedimentary rocks that are softer but generally acceptable for most purposes and 3) industrial and mining bi-products and recycled materials. The last category can include slag, mine tailings, and concrete and asphalt recovered from pavement and building demolition waste.
Sand and gravel pits produce aggregates that are smooth and partially rounded in shape, while crushed aggregates are rough-faced, angular particles. Both are normally processed and screened for uniform gradation. Fines (particles passing the No. 200 sieve) are usually considered as undesirable and are removed as they are frequently moisture susceptible.
Other bi-product and recycled aggregates may be processed by passing them through a crusher and screens. However, recycled aggregates can be more variable as the source of the material frequently changes.
FLEXIBLE BASE LAYERS
The base courses in flexible pavements distribute the imposed loading to the subbase or subgrade by load spreading as depicted in Figure 2. The load spreading is primarily a function of the thickness of the base layer. The type and the quality of the materials also have a significant effect.
The base course materials must be strong enough to resist the effects of the wheel loads without excessive deformations (lateral spread or rutting). They should be unaffected by water penetrating through the surface course or by freeze and thaw conditions.
Aggregate Base Course
The major specification requirement for a dense-graded, crushed aggregate base course is the particle size distribution. Typical gradation envelopes are set out in AASHTO M-147 and ASTM D-2940 (ref.3), along with other properties such as durability and plasticity. Durability is frequently determined using the LA abrasion test ball-mill equipment and by sodium or magnesium sulphate soundness testing. The strength properties of the compacted material are only occasionally required, and California Bearing Ratio test results exceeding 80 or 100 may be required. The material should be tested to determine the maximum dry density and optimum moisture content using the modified Proctor method (ref. 2, 6).
Thickness requirements for aggregate base courses vary greatly according to usage. Minimal thicknesses of 3 to 4 in. (76 to 102 mm) are common for patios and walkways and can be 12 in. (305 mm) or more for parking lots and roadways.
Aggregate base course is spread in layers of up to 6 in. (152 mm) in compacted thickness, dependent upon the proposed compaction process. It can be spread by grader or spreaders, and care is required to avoid segregation. The material should be moisture conditioned to within 2% of the optimum to enable satisfactory compaction. Compaction should be a minimum of 98% of modified Proctor (ref. 2, 6) utilizing a static or vibratory roller that can achieve the target density through the full depth of the layer. Additional finishing may be required to achieve the required surface profile.
One special type of aggregate base is an open graded drainage layer that may be specified at the bottom of the base course. This consists of a specially graded crushed aggregate that contains few fines and limited fine aggregate so that water can drain through it. This material is typically compacted with pneumatic tire rollers or vibratory rollers, and it should be covered with the next pavement course as soon as possible.
Geosynthetics
Geosynthetics are man-made products used to improve the performance of soils and granular pavement layers. They are primarily manufactured from plastics, such as polypropylene and polyethylene and from fabrics/yarns such as polyester. The materials chosen are stable and resistant to many chemicals encountered in the ground, and when properly protected (coatings or carbon black additives) to the deteriorating effects of sunlight. There are three main types that are used in pavement construction: geotextiles or fabrics; geogrids; and geocells or webs.
Geotextiles are formed from filaments or yarns that are formed into fabrics and supplied in rolls. When geotextiles are placed directly on top of the subgrade, and under the base or subbase, this separates the two layers and prevents the intrusion of the finer soil particles into the granular layer and punching of larger aggregates into the subgrade. This enables the base to retain its strength longer. Geotextiles can also perform as filters to prevent clogging of under-drains and drainage blankets.
Geogrids are formed from filaments, yarns or plastics that are structured to form a grid or net with openings typically around 1 in. (25 mm) square. These materials are also supplied in rolls and act as a reinforcing element in the pavement. The aggregate particles from the overlying layer penetrate the openings in the grid so that they are prevented from moving laterally under loading. This has the effect of increasing the stiffness and shear strength of the layer and increasing its load spreading properties. Furthermore, geogrids minimize the potential for an aggregate base or subbase to punch into the subgrade.
Geocells are formed by securing strips of plastic together so that when opened like a concertina, they form a series of pockets that can be filled with aggregate or other materials. They are supplied in sections that expand to cover around 200 ft2 (18.6 m2). Various thicknesses are available from 3 to 8 in. (76 to 203 mm). The material compacted into the pockets is contained by the cell walls. This has the effect of increasing the stiffness and shear strength of the layer and increasing its bearing capacity and load spreading properties.
Cement Treated Base Course
Cement treated base is a carefully controlled mixture of portland cement, aggregates and water that is compacted and cured to form a stiff, durable layer. It acts much like a rigid base in that it distributes the load over a much larger area than normal flexible bases; however, they are almost always designed as flexible systems. The aggregates are generally imported from off site and may be crushed or gravel, and are graded from 2 in. (51 mm) to dust. A mix design is undertaken in the laboratory to determine the required proportions of the materials in order to achieve the design strength. This may be from 250 to 1,000 psi (1.72 to 6.89 MPa). Cement treated bases in the lower strength category are often termed soil-cement bases and utilize select soil type materials for the aggregate. Cement content varies between 5 and 12% by weight of the total mixture. The water content must be sufficient to achieve the required compaction and full hydration of the cement. Tests are also undertaken to establish durability in regard to wetting and drying cycles and freezing and thawing cycles. Cement treated base course may be mixed in a central mixing plant or mixed in place.
Central plant mixing enables materials to be accurately proportioned, and so it is generally adopted when higher quality and strength is required. The materials are mixed and transported to the site for spreading by a spreader or grader. They are then compacted using rollers.
In-place mixing entails first spreading and grading the untreated aggregate, then spreading the cement, and then mixing in place with specialized machinery. Water is then added and the materials are remixed prior to compaction. A special form of in-place mixing is recycling where the existing pavement is initially pulverized and utilized instead of spreading aggregates. The mixing with cement and water follows in the manner previously described.
Once the water has been added to the cement and aggregates, the materials should be graded and compacted within two hours and before initial set of the cement. Laboratory testing of the materials determines the optimum moisture content and maximum dry density. A relative density of at least 96% of the laboratory value should be achieved in the pavement. The compacted and finished cement treated layer is cured by water misting or the application of an asphalt emulsion cure coat. It should not be subjected to traffic loads for at least 7 days.
As the cement treated base materials cure, they shrink and cracks begin to appear at regular spacing. The aggregate particles on the face of the cracks continue to be interlocked with the opposite face so that load is transferred to the other side preventing differential settlement.
A permeable cement treated base course may be used where vertical and lateral drainage are required in the pavement section. Permeable cement treated base is a combination of high quality coarse aggregate with limited fine material, cement, and water, so that any water that penetrates the overlying layers can drain through it. The material is spread and compacted by a static steel wheel roller. Curing is by water misting. Care is required to ensure that the drainage characteristics are maintained until this layer is covered. See reference 5 for more information on cement treated base courses.
Asphalt Treated Base Course
Asphalt concrete base materials consist of a mixture of aggregates and asphalt cement that are mixed at a central hot-mix plant. Asphalt concrete aggregates are usually blended to achieve a gradation from 3/4 or 1/2 in. (19 or 13 mm) to dust. The materials are proportioned to comply with a laboratory prepared mix design.
A permeable asphalt base course may be used where vertical and lateral drainage are required in the pavement section. Permeable asphalt base is a combination of high quality coarse aggregate and asphalt cement, with limited fine material, so that water can drain through it. It is hot mixed and spread as conventional asphalt concrete. Compaction is by a static, steel-wheeled roller. Care is required to ensure that the void structure remains free draining until it is covered.
RIGID BASE LAYERS
Rigid bases (portland cement concrete) distribute the imposed loading to the subbase or subgrade by flexure as depicted in Figure 3. The load spreading is primarily a function of the thickness of the base layer. Some local authorities require the base meet their standards for portland cement concrete pavements, essentially ignoring the structural properties of the concrete paver layer. However, if the local jurisdiction will consider thickness determination by a design analysis, the pavement structure can be designed using the structural coefficients listed in Table 1 for each pavement layer.
Construction details for rigid bases are virtually the same as for portland cement concrete pavement with the exception that a high tolerance and skid resistant surface is not required. They can be either of the three main rigid pavement types: 1) Jointed Concrete Pavements with or without dowels, 2) Jointed Reinforced Concrete Pavements or 3) Continuously Reinforced Concrete Pavements. More information on design and construction of each of these pavement types can be obtained from the American Concrete Pavement Association, Skokie Illinois.
Joints
Joints in a concrete base are virtually the same as for a concrete pavement. They are used to control cracking of the slabs. They must provide for adequate load transfer across the joint and prevent the ingress of moisture and infiltration of foreign matter. There are three basic joint types: 1) contraction joints used to accomodate shrinkage during curing, and to accommodate subsequent expansion and contraction of the concrete due to changing environmental conditions, 2) construction joints used at the juncture of sections of base placed more than 2 hours apart, and 3) isolation joints used to isolate the base from objects that may have a different relative movement such as foundations.
Joint spacings for jointed plain (unreinforced) concrete bases are typically between 10 and 20 ft (3.0 and 6.1 m) with joint widths up to 1/8 in. (3.2 mm). Jointed reinforced concrete bases have wider joint spacings, typically 20 to 40 ft (6.1 to 12.2 m) with joint widths up to 1/4 in. (6.4 mm) and require dowels for load transfer.
Since thermal changes in the base will result in slab movement and opening and closing of the joint, the design needs to consider preventing intrusion of the bedding sand into the base joints. Joint sealing and/or prudent use of a geotextile over joints will reduce this potential. Where rigid bases are used under concrete pavers, close centered joints should be adopted. Widely spaced joints in the base lead to opening of overlying paver joints and settlement of the bedding sand. The opening of overlying paver joints will result in a loss of structural integrity of the surface course. Isolation joints and other joints that permit significant movement should be avoided under the pavers, or carried through to the surface with a suitable edge restraint provided on either side.
Continually reinforced concrete bases are designed without transverse joints and the acceptance that transverse cracking will occur. The cracks, however, are held tightly together by the steel reinforcement. Cracks may develop at closely spaced centers, 2 ft (0.6 m) is common, but they do not open significantly and therefore do not need to be sealed or protected against intrusion by the bedding sand.
SUBBASE LAYERS
It is frequently an economical practice to construct a layer of material that has better strength than the subgrade, but is not as strong as the base. This layer can provide a good working platform on which to construct the base, particularly if high compaction standards and elevational tolerances are required. It may also provide additional protection to the subgrade where freezing and thawing are design consideratons.
Granular Subbase
Granular subbase layers consist of semi-processed or unprocessed, unbound aggregates. Many of the same properties that apply to aggregate base course materials apply to granular subbase; however, the standards are less restrictive. Typical gradation envelopes are set out in AASHTO M-147 and ASTM D-2940 (ref. 3), along with other properties including durability and plasticity. The California Bearing Ratio of a granular subbase is often required to be a minimum of 20 or 30.
Granular subbase is spread and compacted in layers, generally by a wheeled grader or bulldozer. Maximum layer thickness is 6 to 8 in. (152 to 302 mm), and the materials are compacted to a minimum relative density of 95% of modified Proctor (ref. 2, 6).
Stabilized Subbase
It is frequently economical to improve existing soils or low quality materials to form a subbase in a pavement structure. Stabilization involves mixing these materials with agents that will result in strengthened compacted material. Several binding agents that can be used for this purpose include cement, lime, crushed stone, and asphalt among several.
Portland cement can be mixed with water and most inorganic soil types to achieve a material suitable for subbase construction and most often is referred to as soil-cement. It is most effective when used with granular materials including sands. In general, the finer the material being stabilized, the greater the percentage of portland cement that is required. This may range from approximately 3 to 12% by weight of the total mixture. The exact proportion of cement will be determined by a mix-design process that uses compressive strength, wetting and drying, and freezing and thawing durability tests. Compressive strength is often the controlling property and values can range between 100 and 500 psi (0.69 and 3.45 MPa), depending on the governing authority. The cement is generally spread over the surface of the material in place, and is then mixed in by specialized machinery. It is then watered, remixed and compacted to a controlled density. A curing regime of misting or an asphalt emulsion coating is required.
When using lime for stabilization, cohesive soils are more suitable than granular soils. There are several methods of mix design adopted by various national and state authorities. These are based upon reduction in plasticity and strength tests such as compression and California Bearing Ratio. Typical lime content varies between 3 and 10%. The construction process is similar to cement stabilization. However, the binding process is different. The lime changes the chemical nature of the clay particles and leads to a change in the soil texture. This process is known as flocculation and agglomeration, and results in a material that is more readily compacted and less vulnerable to moisture changes. There is also a long term pozzolanic cementing action. Hydrated lime is used more frequently than quick lime, but the latter is most beneficial in wet site conditions. Lime can also be used with some granular materials when a supplementary pozzolan such as fly-ash is added. This can increase the range of suitable materials.
Asphalt materials can be used for stabilizing granular soils, including sands and silts. For subbase construction the usual material is asphalt emulsion and construction is undertaken in place. Construction entails spraying asphalt emulsion onto the material surface and then mixing it in with specialized machinery. It is then allowed to aerate prior to compaction. There are two benefits of using asphalt materials for stabilization of soils. The first is a waterproofing ability most effective on fine grained materials. The second is a lubrication and adhesive property, most effective on coarse grained materials.
Other modifiers include calcium chloride, tree resin products, liquid polymer products, sulfuric acid and enzymes. These products are beyond the scope of this publication.
DESIGN CONSIDERATIONS
Structural Properties
The structural design of concrete paver pavements is typically undertaken using AASHTO design principles. The NCMA Pavior PlusTM computer program (ref. 1) and TEK 11-4 Structural Design of Interlocking Concrete Pavements for Roads and Parking Lots (ref. 4) were developed on this basis. Guidance is given in the program's help files and the TEK in determining appropriate structural coefficients for various pavement materials. Table 1 expands on that list.
Drainage
The presence of high moisture levels in a pavement section increases deterioration of the materials and reduces the structural strength of the pavement. Saturation of pavement materials can lead to a rapid breakdown of the integrity of the materials, as the water acts as a lubricant and creates high hydrostatic pressures under the effects of traffic. Moisture changes in the underlying subgrade can result in shrinkage and swelling that result in reduced ride quality. In wet conditions, frost action is more likely to cause problems.
Rapid removal of water from the pavement section is an important consideration during the pavement design process. Frequently, it is beneficial to incorporate a positive drainage system to facilitate this. Permeable base layers discussed previously assist in directing water to the edges of the pavement where it can be collected in a subsurface drainage system. In large pavement areas, it may be necessary to incorporate a system of under-drains. More information on drainage and permeable bases can be found in reference 5.
Frost
Concrete paver pavement sections perform very well in a frost environment. However, freezing conditions should be considered in cold weather regions as moisture freezing in the larger pores of the soil influences water to rise above the water table through capillary action in the smaller pores. This moisture migrates to the ice in the larger pores causing them to increase in size, developing into ice lenses. This freezing and expansion leads to heaving of the pavement surface.
When thawing, the upper ice lenses melt before the lower ones, leaving saturated and loose materials. This creates weakness in the pavement and can result in rapid premature failure. To avoid such problems it is important to use non-frost susceptible materials above the level that frost will penetrate. Where subgrade soils are known to be frost susceptible, they can be removed and replaced, or a thicker pavement may be required. Improved subsurface drainage may also be beneficial.
REFERENCES
1. Pavior PlusTM Concrete Paver Software, CMS-11311. National Concrete Masonry Association, 1999.
2. Standard Method of Test for Moisture-Density Relations of Soils Using a 10 lb (4.54 kg) Rammer and an 18 in. (457 mm) Drop, AASHTO T-180-90. American Association of State Highway and Transportation Officials, 1990.
3. Standard Specification for Graded Aggregate Material for Bases or Subbases for Highways or Airports, ASTM D2940-98. American Society for Testing and Materials, 1998.
4. Structural Design of Interlocking Concrete Pavements for Roads and Parking Lots, TEK 11-4. National Concrete Masonry Association, 1993.
5. Subbases and Subgrades for Concrete Pavements, Technical Bulletin No. 11, American Concrete Pavement Association, 1991.
6. Test Method for Laboratory Compaction Characteristics of Soil Using Modified Effort (56,000 ft-lbf/ft3)(2,700 kN-m/m3), ASTM D 1557-00. American Society for Testing and Materials, 2000.
Provided by: CCI Industries Ltd.
