From the GAP 2019 transportation conference proceedings: In “Design and Construction of Bases and Subbases for Concrete Pavement Performance,” Shreenath Rao, Hesham Abdualla, and Thomas Yu, P.E. use a mix of data and case studies to show the impact that the base/subbase has on increasing or decreasing the overall concrete pavement performance. The case studies included in this paper document the substantial effect of drainage on the structural and functional performance of concrete pavements. Well-designed and well-constructed drainage systems are crucial for long-term pavement performance, especially in areas where the potential for moisture damage is high.
Base course is a layer of the pavement structure immediately beneath the surface course. It typically consists of high quality aggregate such as crushed gravel, crushed stone, or sand that provides a uniform foundation support and an adequate working platform for construction equipment. Base may consist of unbound materials, such as gravel or crushed stone, or stabilized materials, such as asphalt-, cement- or lime-treated materials. The subbase course is typically a granular borrow that is placed between the base and subgrade. It can be constructed as either a treated or untreated layer. Untreated or unbound aggregate subbase layers are characterized in a manner similar to the subgrade in pavement design. The material quality requirements of strength, plasticity, and gradation for subbase are not as strict as for a base. The subbase course must be better quality than the soil subgrade, the subbase is often omitted if soil subgrades are of high quality. Depending on site conditions, subgrade improvements may also be performed. However, the role of different base and subbase layers and rationale for using different base types and layering are not well documented as many agencies specify standard or typical base and subbase layers based on historical performance and their own experiences. For example, it is unclear where and why a treated base should be used, or why one type of treated base is preferred over another.
Pavement engineers generally agree that foundation layers perform important functions, including providing uniform support, controlling pumping and erosion, protecting against frost heaving, and reducing moisture related damage to paving materials. Based solely on structural analysis, the benefits of unbound aggregate base cannot be demonstrated as the structural models used in mechanistic-empirical (ME) pavement design do not show significant impact of foundation layers on pavement performance. From structural perspective, the most efficient means of providing adequate structure is by providing sufficient surface thickness, concrete or asphalt. However, experience shows that concrete pavements placed directly on subgrade do not perform well in most areas, because of pumping and migration of fines resulting in variability in foundation support. So, it should be clear that the foundation layers perform a different function than the surface layer, which is the main structural component to resist the applied loads. However, when evaluating the benefits of foundation layers, attempts are often made to quantify benefits only in terms of structural effect.
Ultimately, the benefits provided by the foundation layers can be related to the structural and functional performance; however, the benefits are more in the way of preventing bad things from happening that can lead to localized or progressive failures and increased roughness. The relatively simple, idealized structural models used in ME designs are not meant to consider complex mechanisms involved in failures resulting from subgrade and foundation problems. This is not to say that the current ME design models are deficient; it would not be practical nor necessary for design procedures to model complex failure mechanisms associated with foundation problems. For the purposes of pavement design, qualitative understanding of the failure mechanism and protecting against those failures is sufficient to devise effective pavement layer designs that avoid foundation problems. A pavement foundation that does not degrade over time does not need to be replaced. A permanent foundation has obvious advantages in environmental impact, and it could also have significant economic advantages. In congested areas, not having to replace the foundation could be highly advantageous in expediting pavement rehabilitations and reconstructions.
2. FUNCTION OF PAVEMENT FOUNDATION IN RIGID PAVEMENTS
Pavement foundation in rigid pavements has less appreciable impact on the structural capacity and the primary function of the foundation layers is providing a uniform support for the concrete slabs. A uniform and sound quality support layer enhances the rigid pavement performance more than a stronger and non-uniform support (ACPA 2007, Hein et al. 2017). The functions of the rigid pavement foundation are as follows:
- Provide a uniform support to the PCC layer with adequate stiffness.
- Offer a stable construction platform.
- Prevent loss of slab support due to erosion and pumping.
- Provide resistance against frost-heave and soil expansion.
- Separate the subgrade from the main structural component.
- Improve drainage and prevent moisture-related damage.
- Provide a gradual vertical transition in layer moduli (stiffness) from the slab to the subgrade.
If the primary functions of the rigid pavement foundation are not adequately considered during the design process or it is not properly constructed, the pavement system may not achieve the desired performance. Moreover, misuse of a foundation layer beneath a rigid pavement may lead to premature deficiencies. For instance, the base and subbase type and thickness should be selected based on specific site conditions. More often than not, the base and subbase type is selected based on a variety of factors such as the agency policy, cost and availability of materials, and past experience. Given these conditions, the base type and thickness should still be selected to meet the needs (e.g., drainage, protect against frost heaving, protect against swelling and unstable soils) of the project site.
3. QUALITATIVE DESCRIPTION OF PAVEMENT FOUNDATION
3.1 How Rigid Pavements Work
PCC slabs have elastic modulus that is an order of magnitude higher than asphalt concrete. The typical flexural strength is about 700 psi and the modulus of elasticity is about 5 million psi. Therefore, unlike flexible pavement structures, which transfers the wheel load gradually to the layers underneath (see Figure 1), the traffic load applied to rigid pavement structures is primarily distributed by the concrete slabs over a wider area before it is transmitted to the layers beneath the slabs (Hein et al. 2017) . As such, the pavement responses induced in the layer below the concrete slabs, including the stresses (i.e., pressure) and strains as well as the deflections, are relatively smaller.
Previous studies showed that the load-induced compressive stress on top of subgrade in rigid pavements is substantially lower than its bearing strength. As an example, 12,000 lb tire load with 100 psi contact stress applied on a typical rigid pavement structure induces compressive stress of about 7 psi for the corner loading. In this case, the induced stress on top of subgrade drops to as low as 3 psi for interior loading. Such observations confirm that concrete pavements obtain the desired structural capacity from concrete slabs and therefore, the uniformity and stability of support layers in rigid pavements is a more important than their stiffness and strength (ACPA 2007).
3.2 Failure Mechanisms
The purpose of a uniform support for concrete pavement is to ensure that pavement will attain its service life and uniformly distribute loads over the foundation throughout the entire service life. A uniform support can be achieved by reducing the effect of three key factors: frost heave, pumping of fine-grained soils, and volume changes of the soil. Other factors responsible for non-uniform support include variability in compaction, in cut/fill and transitions, and ineffective drainage system. Table 1 summarizes the causes and effects of non-uniform support on the performance of concrete pavement and the recommended practicable solutions to eliminate such problem (Hein et al. 2017, ACPA 2007, ACPA 1995, Christopher et al. 2006, Snethen et al. 1977).
4. DESIGN OF BASES AND SUBBASES FOR CONCRETE PAVEMENT
The selection of base and subbase type for a given a project should be based on (1) the function of base/subbase layer with the pavement structure, (2) improve the short and long-term performance, (3) cost-effective approach, and (4) local experience (Hall et al. 2005). Modulus of subgrade reaction (known as k-value) is typically used to quantify the stiffness (strength) of rigid pavement support. Composite k-value is a representative of pavement foundation stiffness consisting base and subbase. The k-value is determined by plate load test in accordance with AASHTO T122 and ASTM D1196. The stiffness of pavement support may increase by placing subbase and base layer on top of subgrade. However, increasing the support strength (or stiffness) to reduce the PCC thickness, to expedite the construction process, or as a surrogate for improving the durability of the base is not recommended. Increasing k-value within the typical range does not substantially affect the required thickness of concrete slab (ACPA 2007).
Aggregate base and subbase with 15% or more fines (i.e., passing the sieve No. 200) are highly prone to pumping. The use of non-erodible or treated base and subbase materials can control and prevent pumping. The requirements in AASHTO M155 entitled “Standard Specification for Granular Material to Control Pumping under Concrete Pavement” should be followed when the unbound granular materials is to be used (AASHTO 2004). In general, the higher the application of heavy truck traffic, materials with lower fine content and lower plasticity should be selected.
Stiffer bases are not necessarily better support under rigid pavements as they fail to conform to the shape of the curled PCC slabs and may lead to loss of support, higher curling stresses, and subsequent cracking. It should be noted that providing thicker concrete slab, higher concrete strength, the use of dowel bars and widened slabs are more economical to substantially reduce the cracking potential in concrete slabs and pumping of materials. A stiff support has potential to cause cracking because of the higher environmentally-induced stresses in the slabs. This can be detrimental for relatively young concrete slabs leading to development of random cracks. It is recommended that the compressive strength of cement treated bases and lean concrete basses should range from 300 to 800 psi and 750 to 1,200 psi, respectively (Hein et al. 2017).
Stabilized bases including cement treated bases and lean concrete bases have potential to expand and contract due to moisture and temperature variations. These movements can sometimes induce stresses greater than the strength of freshly placed surface PCC (when the strength in the freshly placed PCC is low as it is hydrating and gaining strength), thus increasing the potential for early-age cracking in the PCC layer. In addition, rough slab-base interfaces increases frictional forces at the interface due to the excessive axial restraint to volumetric shrinkage and to thermal expansion and contraction (Hall et al. 2005). To mitigate this potential risk, it is common practice to have a debonding separator layer (like a plastic sheet) between the cementitious stabilized base and the PCC layer. However, an unbonded base contributes less to the long-term fatigue performance of concrete pavement as compared to a fully bonded base, and this may need to be considered in the pavement design process, for example, by increasing the thickness of the PCC layer. This is less of an issue with dense asphalt-treated bases which are sufficiently flexible and do not expand and contract due to thermal effects to the same extent as cementitious stabilized bases.
To provide drainable base layers, permeable granular or stabilized bases with drainage system or free draining delighted bases can be used. Permeable granular layers should only be used where there is potential for moisture damage to pavement on roadways with medium to heavy truck traffic, and should be properly designed and constructed. However, the owner agency should have a commitment to regular inspection and routine maintenance of the edge-drains or the exposed (daylighted) area of the aggregate drainage layer. An open-graded base needs a suitable separator layer beneath it to prevent subgrade fines from migrating up into and clogging the base. This may be an appropriately graded untreated aggregate subbase, an appropriate geotextile fabric, or a layer of subgrade soil treated with sufficient lime or cement to achieve good long-term stability and resist erosion. Stabilized open-graded drainage layers have very little aggregate passing the No. 200 sieve. Asphalt cement contents typically range between 1.6 and 1.8 percent by mass of aggregates. Cement treated open graded drainage layers are typically produced with a water to cement ratio of 0.37 and a cement content of 185 to 220 lbs/yd3 (Hein et al. 2017). Permeable bases must be constructed strong enough to resist construction traffic and paving machine without deformation (Hall et al. 2005). The recommend permeability values are ranging from 500 to 800 ft/day with taking in consideration the stability of the bases (Hein et al. 2017).
The use of “daylighted” base course that is exposed to the open along the edge of the pavement is recommended to drain water infiltrating from the surface into the base layers, particularly in situations where moisture conditions are not extremely severe. Daylighting allows water to slowly drain out of the pavement structure without the use of edge drains. Daylighted bases are well suited for roadways with flat grades (1 percent or less) and shallow ditches, where it is difficult to outlet drainage pipes at an adequate height above the ditch. However, it requires careful construction and periodic maintenance to keep the exposed edge clear of soil, vegetation, and debris, and prevent clogging. Typical maintenance activities include weeding and manual removal of debris. The bottom of the exposed edge of the daylighted base should be at least 6 in. above the 10-yearstorm flow line of the ditch to prevent water from backing up into the daylighted base during or after a heavy rainfall. Daylighting the base layers is more “forgiving” than using edge drains. With edge drains, there is the potential for trapping water within the pavement layers causing a “bathtub” effect and resulting in significantly greater damage, if they get clogged from not being regularly maintained or from improper installation. However, when properly maintained, edge drains are effective and drain water efficiently out of the pavement system, particularly in areas with high water tables and cut sections.
5. CASE STUDIES FOR EFFECTS OF BASES AND SUBBASES ON PAVEMENT PERFORMANCE
5.1 U.S. 460 Bypass, Appomattox County, VA
The project is located on the U.S. 460 bypass in the northern part of Appomattox County, Virginia. The doweled jointed plain concrete pavement (JPCP) section, an approximate 2.8-mile long section, of the U.S 460 bypass exhibited premature failure at several location approximately 5 years (i.e., 1998) after paving. The Virginia DOT engineers and researchers conducted field and laboratory investigations to identify the causes of premature failures and evaluate the condition of the pavement section (Hossain and Elfino 2005, Elfino and Hossain 2007). The project is located in a wet-freeze climate and the average daily traffic (ADT) in 2003 was 13,000 with 10% truck traffic.
5.1.1 Design and Construction
The U.S. 460 bypass was designed to carry an equivalent of 8 million single axle loads (ESAL) with an expected design life of 30 years. The following design was used for this section:
- 9.0 in. doweled JPCP slab with 15 foot spacing.
- 4.0 in. cement-stabilized open-graded drainage layer (OGDL).
- 6.0 in. cement-treated soil, using 10% hydraulic cement by volume.
- 9.0 to 6.0 in. variable depth jointed concrete undoweled tied shoulder.
- 4.0 in. aggregate base materials for shoulder (VDOT Type 1, Size 21A).
- Pavement edge drain UD-4 in accordance with the standard pavement edge drainage and outlet pipes.
The subgrade soil was classified as A-7-5 red clay and silt with a CBR of 9.
A visual survey was conducted to evaluate the cause of premature pavement failure. The survey results showed that about 24% of the eastbound slabs were distressed, compared with 12% of the westbound slabs. The pavement exhibited mid-slab cracks, broken joint seals, lane-shoulder drop-off and pumping, and joint faulting. Field and laboratory investigations were performed to assess the causes of pavement distresses. The overall observation of the laboratory and field investigation can be summarized as follows:
- A majority of the drainage layer was clogged and filled with red soil (see Figure 3a).
- Cracks propagate through the drainage layer in the mid-slab crack core sample.
- Water trapped underneath the slab was observed under damaged slabs during coring.
- The OGDL was not extended over the edge drainage in some areas (see Figure 3b).
5.1.3 Lesson Learned/Summary
- A poor drainage system and increased truck traffic can significantly affect pavement performance.
- If the OGDL is not continued to the edge drain, trapped water in the drainage layer will seep vertically and cause an increase in base/subbase and subgrade moisture.
- Water abrades the soil cement base/subbase under repeated heavy loads leading to localized loss of support, disintegration, resulting in pavement distresses, including structural and durability Figure 4.
5.2 U.S. 63, Callaway County, Missouri
The Missouri River flooding significantly damaged pavements, culverts, bridges, etc. in Jefferson City, Missouri, resulting in the closure of roadways, delay in traffic, and economic losses to the city. The roadway was completely washed out due to the flooding in 1993. The project is located on southbound US-63 in Callaway County, MO, just across the Missouri River from Jefferson City, Missouri. The original pavement design of southbound US 63 sections consisted of 9 inches of joint reinforced concrete pavement (JRCP) with 61 foot joint spacing on 4 inches of dense graded crushed rock base. The Missouri Department of Transportation (MoDOT) researcher and engineers conducted a comprehensive study to further enhance the pavement design of US 63 sections to resist such environment. The major finding of the study led to developing a new standard specification provision of a thick daylighted rock base, which has capabilities to drain water from the pavement structure and to improve the load-bearing capacity of pavement structure.
5.2.1 Design and Construction
The new design of US 63 section consisted of 12 inches of doweled joint plain concrete pavement (JPCP) with 15 foot joint spacing over 24 inches daylighted rock base and was constructed in 1994. This was the first implementation of daylighted rock base in Missouri. The 24 inches base was selected to increase the structural capacity as well as improve drainage during heavy rain or flood periods. The grade was raised about 6 feet because of the flooding damage. The daylighted rock base was placed on the top of the subgrade. A cross slope gradient from the median to the outside fill slope was provided on the top surface of the subgrade to remove water effectively from the pavement structure, prior to the placement of the 24 inch rock fill base. The subgrade soils in this area consisted of A-6 and A-7-6 soils.
A visual survey of US-63 sections conducted in 2016 and revealed that all sections performed in excellent condition and there were no signs of cracking or faulting (see Figure 5a). In early 2018, a second survey was conducted indicated excellent conditions, which could be attributed to the effectiveness of the daylighted rock base (see Figure 5b). The pavement section was constructed in October of 1994 and has performed extremely well with minimal cracking, faulting, and roughness. Minimal maintenance has been done on the section since construction and all joints look exceptional. The section experienced another flood in 1995 and maintained in good condition. The success of this pavement was attributed to the daylighted 2-foot rock base and its superior drainage capabilities. After 24 years of relatively heavy traffic, US 63 section is still in perfect structural condition and no repairs were performed. IRI measurements were taken from 2007 to 2017 for the project and the data shows the consistency of the roughness of the pavement over 10 years of initial life.
5.2.3 Lesson learns
- The stability and drainability of base material is essential to enhance the performance of pavement during heavy rain or flooding incidents.
- The original base, 4 inches dense graded crushed rock, was filled with sand and relatively undrainable. Concrete pavement constructed over a dense-graded rock base has a high risk of being damaged during flooding incidents due to ineffective base drainability.
- The use of a thick daylighted rock base significantly improved the long term pavement performance. The 24 inch daylighted rock base was effective for removing water from the pavement structure, which enhanced the JPCP performance and eliminated the moisture-related damages.
5.3 U.S. 23, Monroe County, Michigan
In 1992, Michigan DOT constructed an Aggregate Test Road on southbound US-23 with the main purpose of studying the effect of frost susceptible coarse aggregate on concrete durability. The project starts just north of the US-23 and US-223 interchange and ends at the border line between Michigan and Ohio. The test road was constructed with concrete mixtures including five different coarse aggregates (Groups A through E) of varying degrees of freeze-thaw properties. The coarse aggregate type for group A was 6AA crushed limestone, group B was 6AA blast furnace slag, group C was 6A naturel gravel, group D was crushed limestone from different quarry, and group E was natural gravel. All other factors of the concrete mix design were kept the same. The average annual daily traffic (AADT) was about 20,000 with 18% of commercial (Hansen et al. 2007, Quiroga 1992).
5.3.1 Design and Construction
The original pavement was removed to the existing sand subbase. The new pavement structure consisted of a 10.5 inches jointed reinforced concrete pavement (JRCP) with 27 ft. joint spacing, on a 4 inches asphalt treated permeable base (ATPB) on a 3 inches gravel separator layer. Half of each of the five test sections was built on the original poorly-draining subbase while the other half constructed on a well-draining permeable sand subbase to evaluate the effect of subbase layer on concrete performance. The existing subbase material exhibited a much finer mix compared to the new subbase which greatly affects the drainage properties of the materials. The existing subbase was considered impermeable, and the new subbase is very drainable. The other half was constructed on a well-draining, special select subbase which showed extremely high drainability values ranging from 198 ft/day to 288 ft/day, well exceeding the specification requirement of 7.7 ft/day. The subgrade soil below the pavement structure consist of a wet clay. During reconstruction, subgrade undercuts were performed at locations with unstable grade, followed by the installation of a 4.0 inch underdrain and backfill.
The main purpose of the test road was to study the effect of freeze-thaw on pavement performance. All JRCP sections did not exhibit any distresses related to freeze-thaw problems such as joint deterioration or D-cracking. The ATPB was a major factor in preventing D-cracking along with a good air-void system of the concrete. Mid-panel deflections were measured and there were smaller deflections underneath the well-draining subbase compared to the existing poor subbase. Dowel bar looseness was also observed which contributed to higher deflections and poor load transfer. After 23 years, all section performed well with the exception of section B. Section B (i.e., aggregate type was blast furnace slag) exhibited significant full lane width mid-panel cracks in about 75 percent of the truck-lane panels which was followed by crack spalling. Full depth repairs were made after 19 years of service. It was observed from coring that some minor deterioration of the ATPB at the crack edge occurred in isolated case which caused some erosion issues in section B. All joints did not exhibit pumping and joint faulting was less than 0.04 inches. Overall, the project performed very well regarding freeze-thaw resistance, durability, drainage, and distresses. The freeze-thaw performance was attributed to the well-draining ATPB layer which prevents water from accumulating at the bottom of the PCC layer.
5.3.3 Lessons learned
- A well-draining base/subbase structure improved pavement performance as well as freeze-thaw resistance.
- Higher mid-panel deflections were observed for the “existing” poor-draining subbase compared to the well-draining subbases.
- The ATPB has the potential to drain water from pavement, prevent pavement becomes saturated, which eliminating the effect of freeze-thaw damage and moisture related distresses.
5.4 Ontario, Canada
The Ontario Ministry of Transportation (MTO) is responsible for the management of 10,300 miles of paved roads and rigid pavements are about 6% of the total. The Ontario MTO conducted several forensic investigations and gathered the information from other highway agencies in North American to develop specification for three types of open graded drainage layers (OGDL). They categorized the OGDL into three types; (1) untreated, (2) asphalt cement treated, and (3) Portland cement treated. Beginning in the early 1980s, the MTO constructed a series of test sections to monitor the performance of the drainage system and pavement. The key design considerations for the OGDL layers include:
- Permeability of the OGDL (ensure water movement away from the travel lanes).
- Stability/strength (to allow proper placement and compaction as well as support for the pavement surface).
- Collector system (ensure water entering the pavement will be moved away from the travel lanes and ensure long-term performance of the system – not clog).
- OGDL protection (ensure the OGDL and drainage system are not clogged by fine aggregate and soil particles reducing the system permeability).
Based on the key findings of the OGDL research investigation, the MTO developed new specifications requiring that a 4 inch layer of OGDL be placed beneath the concrete slab in all new rigid pavement designs (Marks et al. 1992, Hajek et al. 1992, Bradbury and Kazmierowski 1993, and Kazmierowski et al. 1999). The gradation of the OGDL consists of coarse aggregates retained on the No. 4 sieve size. As untreated aggregates were not considered to be stable enough to support construction traffic without distortion, the OGDL is treated with 1.8 percent asphalt cement. In addition, the longitudinal drainage system was modified to be integral with the OGDL to ensure that water entering the system exits the pavement as soon as possible. The OGDL should be extended 3 feet past the edge of the concrete pavement or paved shoulder, if present.
5.4.1 Design and Construction
This section demonstrates the design and construction of highway using three types of OGDL basses. The highway 115 is located near the city of Pererborough. A section of Highway 115 Pererborough with a total length of 10.20 mile was built in 1991 with three different OGDL to evaluate the performance of each type. Section one (0.6 mile long) consisted of 8 inches JPCP, on 4 inches of untreated OGDL with increase in percent of passing No. 4 to increase layer stability, on 4 inches of aggregate base, over 12 inches of aggregate subbase. Section two was similar to section one, but with 4 inches of cement treated base (200 lb/yd3) instead of untreated OGDL. Section three was similar to section one, but with asphalt cement treated base (1.8 percent) instead of untreated OGDL. The purpose of 4.0 aggregate base was used to act as the filter layer between OGDL and the subgrade. The longitudinal subdrain was placed under the shoulder, 2 feet away from the lane edge. A 4 inches diameter outlets was placed at 330 feet intervals to roadway ditches. The cement treated OGDL was placed with the concrete slipform that was used to place the concrete and there was no issues during the placement reported. Minor damage of the surface of the cement treated OGDL was observed during the placement of concrete pavement. The cement treated OGDL was cured by water “sprinkling” every 2 hours for 8 hours. The asphalt treated OGDL was placed using hot mix asphalt paver with no placement issues. The untreated OGDL was placed using trucks and a grader to achieve the design profile.
Laboratory testing was conducted to evaluate the permeability of the three OGDL. The results indicated that all three types of OGDL met the initial permeability and stability requirements. The untreated OGDL was able to carry construction traffic without any significant damage. The FWD testing was conducted in 1992-1993 indicted that the deflection of cement treated OGDL was 17 percent less than asphalt treated OGDL and about 28 percent less than untreated OGDL. In general, the performance of the Highway 115 pavement has been excellent. There were several issues related to late season construction that were documented on this contract including; cold weather concrete delivery, subgrade instability in cut to fill transition, and premature cracking due to late saw cutting of transverse joints.
In 2005, a pavement evaluation was undertaking to identify and prioritize concrete pavement restoration requirements for the pavements. The evaluation included a detailed pavement surface condition survey, Falling Weight Deflectometer (FWD) testing, subgrade and pavement layer materials testing, MIT Scan to check dowel bar alignment, and Ground Penetrating Radar (GPR) testing and test pits at the side of the roadway to verify the operation of the drainage system. The results of the pavement investigation completed in 2005 revealed 0.5 percent cracked slabs (2 slabs) in the eastbound direction of the highway and 2.4 percent (50 slabs) in the westbound direction at an age of 13 years. By this time, the pavement had carried approximately 4.67 million equivalent single axle loads (ESALs). These slabs were replaced in a 2006 construction contact. The majority of the slab replacements were at cut to fill transition areas. In addition, the concrete pavement was diamond ground for smoothness/friction in 2011 and then grooved in 2014. As of 2017, the pavement had carried approximately 13.3 million ESALs.
5.4.3 Lessons learned
- Open graded drainage layers and their drainage system should be protected from the intrusion of fines. The movement of fine soil particles such as silty clay by pumping action of repetitive axle loading can lead to early pavement failures.
- OGDL layers should be separated from the subgrade through the use of a granular layer. The use of granular layers was found to be more effective than using a geotextile.
- The continuity of the OGDL and subdrain system to remove water from the pavement is critical to prevent water from being trapped within the pavement structure.
- The OGDL should not be left uncovered for long periods or over the winter.
- Asphalt treated OGDL can more easily be completed when the layer has been allowed to cool below a temperature of 150o
The goal of this paper was to identify and document the useful information related to the effect of the pavement foundation on the performance of concrete pavements. The function of pavement foundation include prevention of pumping, protection against frost action, drainage, prevention of volume change of the subgrade, increased structural capacity, and a stable construction platform. The primary function of base is to prevent pumping, and therefore it must be free draining or highly resistant to erosion. Various case studies of the effect of pavement foundation on concrete performance were summarized based on data/field investigations. These case studies show the effect of base/subbase in terms of increasing or decreasing the overall pavement performance. The case studies included in this paper documents the substantial effect of drainage on the structural and functional performance of concrete pavements. Poorly designed or constructed drainage systems have a detrimental effect of pavement performance, while removal of water through well-designed and well-constructed drainage system is crucial for long-term pavement performance in areas where the potential for moisture damage is high.
ABOUT THE AUTHORS
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