A Necessary Support System

March 1, 2017

Ever since the Romans used palm fronds and other plant fiber matting to reinforce the sub-bases of their famous roads, fabrics have been used to improve road longevity and performance. Modern road construction methods rely on a family of materials called geosynthetics to achieve the same results. But these more sophisticated materials can be used for multiple applications, including: sub-grade stabilization foundation reinforcement, separation of differing layers of materials, stress absorption, drainage, sealing and water proofing. They can strengthen marginally useful materials and allow for the construction of stable roadways in unstable conditions. Each type of roadway pavement—rigid concrete or flexible asphalt—utilizes specific geosynthetics to achieve differing design goals that derive from the physical differences between the two types of pavement.

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Rigid Concrete Pavement
Concrete is defined as a “rigid” pavement, because, unlike asphalt pavement, it doesn’t flex under applied vehicle loads. Instead, concrete pavement acts as a rigid structure which actually bridges the point of load applications under the vehicle and truck tires. As such, it does not transmit these loads directly down into the underlying soil of the pavement’s sub-base. This load transmittal mechanism is reflected in the design and construction of concrete pavement. The design consists of several layers.

First, there is the sub-grade of compacted or otherwise stabilized sub-base soils. This acts to provide a firm and level foundation for construction of the rest of the pavement.

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Second, a sub-base layer (often referred to as a base course) is installed over the sub-grade. This sub-base layer is not always necessary given the underlying soil strength nor does it have to meet the same physical characteristics as the aggregate layer supporting asphalt pavement since it since the concrete pavement does not impose the same physical loadings from vehicle traffic. The sub-base can be constructed of a number of materials including: highly permeable open graded aggregates, cement treated materials or even lean concrete.

Third, the concrete surface pavement itself is installed on the supporting sub-base. The main physical characteristic of concrete is that it is very strong in compression but relatively weak in tension. When a load is applied to a concrete pavement layer, it causes a localized flexural moment with forces the upper half of the pavement into compression and puts the lower half of the layer in tension. As such, the bottom half of the concrete slab can experience tension cracking that can result in the physical failure of the concrete layer. To avoid this, the lower portions of concrete pavements are reinforced by steel (which is very strong in tension) in the form of dowel bars, continues rebar, or wire mesh.

The concrete itself is primarily made of Portland cement which acts as a binder for the other materials to create a rigid matrix when dry. Cement is a powdery mixture of lime, silica, alumina, calcium sulphate, fly ash, and other binding materials. When exposed to water, these materials form a slurry that goes pozzalanic, a term referring to the chemical reactions that occur in the field at ambient temperatures to create a hardened cement. The remainder of the concrete mix consists of aggregates, sand, crushed rock, and even recycled chunks of broken up concrete from previous demolition efforts. These are strong filler material bound together by the cement matrix.

Once poured, the concrete needs to be cured for an extended duration (28 days is typical) to achieve necessary strength characteristics prior to the use of the roadway for vehicle traffic. An internal strength of 3,000 psi is commonly used. The combination of steel and concrete is more expensive than asphalt, but can be expected to last considerably longer (30 years or more, compared to up to only 10 years) without the need for extensive maintenance and repairs. So for many applications, the overall life cycle costs of concrete pavement can be lower than asphalt construction.

Flexible Asphalt Pavement
Asphalt pavement (often referred to as bituminous concrete) is considered to be flexible since it deforms slightly under applied loads from vehicle tires, and it is not reinforced by structural steel as is rigid concrete pavement. The deformation is propagated through the various layers that make up the asphalt roadway and then down into the underlying soils. The applied load is transmitted via contact between the aggregate components of the pavement and subsequent contact with the soils. The forces from the vehicle start as point loads at the surface where the tire comes into contact with the surface of the pavement. The faces then spread out as they are transmitted downward through the pavement in a kind of triangular patter with its apex at the surface and flattening out at the bottom. To ensure structural stability under these loading an asphalt pavement needs to be constructed with multiple layers of compacted soil, aggregate base layers, the asphalt pavement itself, and its exposed wearing surface.

The soil sub-grade can be prepared, compacted, and stabilized for asphalt pavement construction in much the same way as it is prepared for concrete pavement. A significant difference is the use of aggregate for the sub-base layers. While rigid concrete pavement can sometimes dispense with the aggregate sub-base, asphalt pavement often uses more than one layer of aggregate, each with different sized stone.

Credit: Tensar
Wire mesh helps avoid cracking.

The asphalt cement itself is made from an organic (fossil fuel derived) material that resembles a black, sticky tar. Made from polymer distillates (naphthene, hdyrocarbons, asphaltenes, etc.) it is mixed with large quantities of aggregates. The resultant material is typically a 20:1 mixture of sand and gravel with the asphalt cement. Like Portland cement acts a binding agent, but forms a heavy, viscous but flowable mixture that can be melted and made fluid again either intentionally during road recycling operations or by excessively high temperatures.

Like concrete cement, asphalt is allowed to cure and harden before it can accept vehicle traffic, but this happens in a relatively short period of time. Often, installed asphalt pavement is ready later that same work day. Compared to concrete pavement, asphalt is relatively cheap but can also wear out sooner, differentially settle or be subject to pot holes—requiring more care and maintenance during its operational lifetime.

Types of Geosynthetics and Their Uses
Geosynthetics are a diverse family of various types of polymer materials formed into extruded grids and nets, blown film sheets and membranes, as well as woven and nonwoven geotextile blankets. The earliest type of geosynthetics (going back to the Romans and other ancient civilizations) are the geotextiles, also known as geofabrics. Today, the amount of installed geotextiles utilized in construction applications (as measured by area placed) is greater than any other type of geosynthetic. While past engineers relied on plant matter to make these reinforcing mats, modern geotextiles are made from advanced polymers, typically polypropylene and polyester. Polypropylene (specific gravity of SG = 0.90) is used for nonwoven geotextiles. Polyester (SG = 1.38) is a heavier material used is stronger and more resilient under applied loads. Geotextiles are physically characterized by whether they are made from a woven or non-woven fabric. Non woven fabrics are randomly needle punched to form a felt like material. Its filtration and porosity properties make it desirable for drainage, soil stabilization and erosion control. Woven geotextiles are made from monofilament, multifilament or slit film fibers woven together into a regular pattern and uniform openings. Slit film woven geotextiles are have higher tensile strength while monofilament woven geotextiles higher permeability for critical erosion control and sub-grade drainage applications.

Geomembranes are the next largest segment of the geosynthetic market. These are impermeable sheets made from rubber, polyvinyl chloride (PVC), chlorosulfonated polyethylene (CSPE), chlorinated polyethylene (CPE), polypropylene (PP), high density polyethylene (HDPE), linear low density polyethylene (LLDPE), and are used to contain liquids within impoundments, ponds and landfills or prevent the intrusion of liquids as a component of a cap and cover system. Geomembranes are manufactured by either the calendar process or are extruded. Calendared geomembranes are formed by forcing molten plastic between counter rotating rollers which flatten it into sheets of varying thickness. This process is used to form geomembranes made from PVC, CSPE, CPE and PP. Extrusion involves the melting of resin chips (and pieces of reworked scrap plastic from precious production runs) and using a screw feeder to force the melted material trough a die. This die can either extrude the geomembrane as a flat sheet or as a tube of blown film which is then sliced open lengthwise by nip rollers to create the sheet. This process is used to manufacture geomembranes from HDPE, LLDPE, and PP. These sheets can be textured (either by dribbling exudates on its surface or roughening it with blasts of blown nitrogen) or coextruded using differing polymers to enhance strength and durability.

Geogrids and geonets are made from solid segments of extruded plastics formed into grid like patterns. These patterns have large openings (apertures) and thick, crisscrossing rods, ribs or ribbons. Geogrids are largely used to provide strength reinforcement to soil in much the same way that steel rebar reinforces concrete. There shapes can be modified to maximized fiction forces between themselves and adjacent soils, aggregates or other geosynthetics, locking them in place inside road foundations or embankments. Geonets are also made from extruded plastic ribs arranged in crisscrossing patterns either in two layers (bi-planar) and tree layers (tri-players). These configurations form a matrix that allows for high rates of in-plane fluid transmissivity.

Geocomposite combine the in-plane flow capacity of geonets with the filtering capabilities of geotextiles. These material form a factory bonded sandwich consisting of a lower geotextile cushion, a middle geonet drainage layer and an upper geotextile filter. The resultant materials is used for horizontal under drain systems and vertical drains for removal of soil moisture. The types of drains formed by geocomposites include blanket drains placed on the floors of impoundments and landfills to remove accumulated leachate, panel drains placed vertically against foundations and other structures, edge drains used to remove seepage from under roadways, and wick drains to remove water from saturated soils at depth.

Similar to geomembranes in their function as a hydraulic barrier, but radically different in design, are geosynthetic clay liners (GCL). Most types of liner systems are composite liners consisting of both low permeability clays and geomembranes. The GCL provides the equivalent of a compacted clay liner without the need for the extensive construction effort required to roll the clay and compact it in place. Like the geocomposite, the GCL’s structure is a sandwich consisting of a lower geotextile, a middle layer of low permeability bentonite clay and another upper layer of geotextile bound together and reinforced by needle punching.

The last significant area of geosynthetic application is to the task of erosion control. Though no directly used in the structure of a pavement, erosion control geosynthetics indirectly protect against sedimentation and erosion during construction and afterwards by providing reinforcement to drainage collection channels. These include erosion control nets, silt fences, meshes and biodegradable blankets. These last fabrics are made form natural fibers such as coconut hulls (coir), straw and hay, or jute. A more permanent reinforcement is provided by turf reinforcement mats (TRM) made from monofilaments arranged in a 3D random mesh blanket. These geosynthetics serve to prevent erosion in channels and runoff slopes until permanent vegetation can take hold.

Advantages of Geosynthetics Compared to Natural Materials
So why use geosynthetics to reinforce roadway construction? What are the benefits? Fist, geosynthetics provide multiple levels of saving. The provide both volume and thickness savings. Geosynthetics are thin sheets and blankets compared to the thicknesses of the layers of aggregate base and surface pavement in the roads themselves. Except in the most remote locations, they are also cheaper than an equivalent amount of soil or cement needed to perform the same function. For example, a roadway sub-grades can be strengthened by the introduction of cement mixed into the soil to a depth of a foot—or it can be reinforced by a layer of geogrid.

Manufactured to exacting quality standards, geosynthetics also provide much greater quality control and superior performance than natural materials. Even the best soil borrow source, sand pit, or aggregate quarry will provide a somewhat heterogeneous material with a gradation of soil types being present. Geosynthetics are manufactured so that each roll is identical to all other rolls delivered to the work site. Produced in regional factories and stockpiled on site or in local warehouses, geosynthetics are often more readily available in bulk quantities than natural materials. Some regions are lacking in aggregate while other sites may be deficient in clay borrow sources. Hauling bulk quantities of clay, sand and stone from a long distance can be prohibitively expensive. Sold an marketed nationally, geosynthetic prices can be consistent and predictable for each construction season and site location. Placement of geosynthetics is typically simpler and easier than dumping, spreading and compacting natural soils.

The above adds up to superior construction quality control and assurance. Starting with manufacturer’s quality control at the factory, laboratory and field tests ensure the quality of the material being used even prior to deployment. Not being dependent on thickness for overall performance, geotextile placement is easier to certify by field survey with only the limits of deployment needed to be delineated with no need to performed grid shots to ensure layer thickness.

Installing Geosynthetics
What kind of equipment is needed to install geosynthetics? Geosynthetic deployment methods vary with the material. Certain materials (such as silt fences) are placed manually. Geomembranes are typically deployed from rolls placed on spacer bars hoisted by excavators or front end loaders. Other geosynthetics such as geogrids, geotextiles and geocomposites are similarly deployed but require additional tasks to complete specific applications.

Geotextiles used for separating layers of aggregate from underlying sols or overlaying pavement and to provide reinforcement and stabilization are rolled out in the direction of traffic to minimize the number of cross seams. At these seams the geotextiles can simply be overlapped or sewn together depending on the strength of the underlying soil. For weak soils (having a California Bearing ratio—CBR—equal to 1 or less, geotextile seam have to sewn with an overlap of a least 9 inches. Moderately strong soils (with a CBR between 1 and 3) can have sewn seams varying from 3 inches to 8 inches or simile overlaps from 30 inches to 40 inches. Very strong soils (having a CBR value greater than 3) need only be overlapped at least 24 inches.

Once in place, the separating/reinforcing geotextile can have aggregate placed over it to the required layer thickness. The aggregate is dumped by a truck that backs onto areas that have already received aggregate (never driving directly on the geotextile) and spread to the desired thickness with a motor grader. Roads constructed on weaker soils may need a low ground pressure bulldozer to spread the aggregate. Multiple lifts of stone may be required with each lift of placed stone being at least 6 inches thick. Compaction of the stone the stone should be performed with a static drum roller if the aggregate lift is less than 12 inches, after this a vibratory roller can be used.

In addition to placement under the pavement, geosynthetics can be placed in asphalt pavements as overlays to provide reinforcement and absorb the stresses from applied vehicle loads. Prior to installing overlays, the surface needs to be prepared by cleaning and removing excess dirt, standing water, oil stains and debris piles. The surface receiving the overlay may also be fine graded to ensure surface water runoff. Once the surface is properly prepared, a tack coat is applied at a rate of about 0.25 gallons per square yard of road surface at a temperature up to 325°F. This is an emulsion identical to the cement used in the asphalt pavement surface course and serves as a fixing agent to “glue” the geosynthetic in place. Given the roughness of the receiving surface a leveling course may be needed as well. The geosynthetic can then be deployed manually or mechanically as described above, with their edges overlapping each other by 2 inches, to 6 inches. Reinforcing grids and geocomposites may be further fixed in place with nails or staples. With the geosynthetic in place the overlaying layer of asphalt can now be placed.

In addition to vertical wick drains and horizontal geocomposite under drains, geotextiles can be used to improve the performance of standard trench drains. These drains consist of perforated collection pipe set in a trenches, surrounded by and embedded in highly permeable filter stone. A geotextile is used as a wrap around the outside of the stone, lining the bottom sides and top of the trench, and acting as a filter medium and separation layer between the stone and the surrounding soil. The edges of the geotextile wrap overall each other at the top of the trench but are not typically sewn.

Geosynthetic Roadway Applications
As mentioned above, there are several reasons why geosynthetics are used in roadway construction; separation of pavement construction layers and underlying soils, absorbing stress from wheel loads, moisture management via surface sealing and subgrade drainage, stabilization, and base reinforcement. And, it is better in the long run to engineer these qualities into the initial construction of the pavement rather than wait to perform repairs and upgrades in the future. Though the damage to pave is directly from vehicle loads, the weakening of the pavement that makes this damage occur is due to the fact that asphalt pavements are permeable and allow up to half of precipitation to infiltrate down into lower layers of the pavement. This moisture build up softens and weakens underlying soils while displacing aggregate in sub-base layers (especially when subject to expansive freezing of the water that accumulates in the pore spaces of the stone). Vehicle loads can then cause cracking and differential settlement, which in turn accelerates the rate of water intrusion, weakening the sub-base even further can causing potentially dangerous potholes and other damage. The use of geosynthetics in the initial pavement construction can minimize water intrusion and strengthen the pavement foundation against its effects in a cost effective manner.

Geocomposites placed under a roadway foundation or off to the side as edge drains and wick drains. These materials have very high in-plane transmissivity rates while providing cross plane filtration to prevent clogging and migration of fines. This prevents piping of soil under the pavement which can create areas of foundational weakness.

Geosynthetics provide two mechanisms that ensure sub-grade stabilization and base reinforcement. These are “membrane action” and “lateral restraint”. Membrane action is the bridging capability that geosynthetics provide over areas of weak sub-soils. Applied normal loads from vehicles get translated into tensile forces acting within the geosynthetic, which in turn is held in place and anchored by the overburden of the pavement and aggregate place out to the edges of the geosynthetic. Lateral restraint acts to confine layers of cohessionless aggregate and prevent them from spreading out laterally. Again, the tensile strength of the geosynthetic responds to these horizontal loads by friction resistance along its interface with the aggregates. By these mechanisms, geosynthetics add horizontal stiffness and vertical strength to the roadway structure.

Layer separation is require to maintain discrete and stable types of material layers and prevent impingement of an upper layer into a lower layer (especially of aggregate into soft underlying soils). Overlays fixed with tack coats prevent similar impingement of an asphalt pavement course with underlying aggregate. The resultant configuration can also serve as a moisture barrier, preventing long term damage to the pavement from water infiltration.

Stress absorption protects the upper layers of the pavement surface in much the same way that geosynthetics stabilize a sub-grade. The application of overlay geosynthetics provides tensile strength to resist applied vertical loads. In doing so, they also prevent old cracks from underlying damaged pavement from propagating upward to the new surface (aka “reflective cracking”). Their tensile strength can reinforce even badly damaged pavement. Again, these overlays also act as a moisture barrier.

Major Industry Leaders
Tensar International provides TriAx geogrids for road building. These products provide for greater reductions in aggregate, asphalt or concrete thickness requirements in pavement structures, than Tensar’s older generation BX geogrid. This, in turn, simplifies construction (which reduces impacts on utilities and construction times along with related safety concerns). Their use both extends the life of a road way and reduces the amount of undercut needed to stabilize soft areas by over-excavation and backfill. An example of its use is the I-25 highway construction and lane balancing for the Colorado DOT. The project extended 16 lane miles and included several on/off ramps. It saved CDOT both nine months of construction time and approximately $3,275,000 in construction costs. A 6-inch-thick TriAx mechanically stabilized layer replaced 6 inches of aggregate base course and 24 inches of sub-base. Another roadway project, I-90 in Illinois, utilized a TriAx geogrid to improve performance of the aggregate base for an 8-mile stretch of reinforced concrete pavement. This resulted in a savings of 20 days on the construction schedule while cutting the thickness of the base in half to 6 inches.

Tensar’s Wells Draw project in Utah involved the construction of 14 miles of asphalt roadway. Two alternative pavement sections were bid out side-by-side. Both sections had identical total thicknesses, but had different thicknesses of asphalt and base. The first section was the conventional section. The alternative was a TriAx stabilized section that would allow for a slight reduction in the asphalt thickness while providing slightly greater design life. The stabilized section provided about a $1,000,000 savings over the conventional method, reducing the total costs for the project by roughly 9%. In addition to reducing pavement construction layer thicknesses, Tensar’s geogrids can stabilize week sub-base soils, as shown in their Rt. 95 project in Maryland. On this project, very soft soils (CBR value less than 1%) were encountered. Instead of a planned 6 to 8 feet over excavation, an alternative utilizing TriAx geogrid a allowed for minimal over excavation of only 2 feet with two layers of geogrid stabilized granular soil. Similar results were achieved in an urban setting, the rebuilding of Main Street in Clearfield, UT. Use of TriAx in this situation avoided the need for a thicker pavement which reduced construction costs while avoiding impacts to adjacent utilities.

In addition to empirical results from major roadway projects, full scale Accelerated Pavement Testing by the Corps of Engineers has also demonstrated that for rural roads, Tensar geogrids can be used to significantly extend pavement performance. These tests simulated a 20 year performance period for a rural collector road having a soil sub-base with a CBR value of 3%. The Heavy vehicle simulator applied loads between 10,000 and 20,000 pounds. The reports showed conclusively that non-stabilized sections rutted quicker than the TriAx TX140 stabilized section, and that the pavement life of the TX140 stabilized section delivered over 18 times the traffic as the unstabilized conventional section. Other testing has been performed on Tensar TriAx geogrid reinforced pavements through accelerated pavement testing laboratory at the US Army Corps of Engineers Waterways Experimental Station.

The same geogrid products can be used to stabilize rigid concrete and flexible asphalt pavement structures, but their stabilization mechanisms are different. For flexible pavements, Tensar TriAx geogrids resist lateral movement within the base course and can improve the modulus of the base. TriAx can also maintain the stiffness of the base course over time. For concrete pavements, TriAx can improve how a base aggregate performs, but it can also significantly improve the uniformity characteristics of a base rock—which affects how a concrete pavement will perform.

Layfield Geosynthetics has a range of geotextiles available for all applications including roadway construction. Layfield geotextiles can be cut and sewn to size for particular applications prior to delivery to the site, including double-wide and triple-wide panels of fabrics for road and embankment construction. Specialty geotextiles are available and Layfield can special order most geotextile materials including woven monofilament geotextiles (erosion and sediment control applications) and high strength wovens (embankment reinforcements, sub-grade reinforcement over soft soils), woven polypropylene slit-film geotextiles (sub-grade stabilization, road building and embankment construction as well as separation), non-woven geotextiles (separation and filtration), high strength wicking/hydrophilic geotextile (sub-grade reinforcement over saturated soils, frost heave prevention).

NAUE America manufactures Secutex PP nonwoven geotextiles which are specially designed products for use in separation layers in road construction. They have high puncture resistance, high tensile strength, and high elongation capacity. This last feature is critical for providing resistance to damage especially during roadway construction and allows them to accommodate irregular or soft sub-grades. Secutex nonwoven geotextile fibers will reorient themselves around aggregate stones and their jagged surfaces, preventing damage to the geotextile’s nonwoven structure. Their Secugrid geogrids have a very high modulus of elasticity and are able to manage large ensile loads with minimal elongation, thereby reducing deformation on the road surface. Combigrid is a geocomposite that combines a layer of Secugrid with a Secutex geotextiles, providing reinforcement and separation/filtration stability with adjacent soils.