FAQ

1) What evidence do you have to demonstrate the performance of the system in these conditions:

a) freeze/thaw conditions
b) wet/dry extremes in surrounding soils and in the material itself
c) settling of waste beneath cap (material tensile properties come into play)
d) arctic conditions
e) arid conditions
f) atop poor subgrade
g) atop soil with high organic content

Although comparisons with conventional asphalt paving may be natural, MatCon^® is designed to overcome several perceived shortcomings of asphalt pavements. The near-absence of air voids (<3% vs. 6-8% for asphalt paving) effectively inhibits the entrance of water, normally the worst enemy of asphalt mixtures. The first project, an incinerator ash dump owned by RECOMP, is now nearly twenty years old; testing of recently obtained core samples has shown the density, impermeability, and general condition to be unchanged since construction. So in response, we can say that MatCon is:

• Very resistant to extremes of temperature. The strength and flexibility characteristics are far less sensitive to temperature because of modifiers used - it is stiffer (stronger) in summer and softer (less brittle) in winter conditions where exposed to the elements.
• Freeze-thaw or wet-dry damage is not a factor in the absence of water in the system. Temperature induced volume change is resisted by the stronger matrix of modified asphalt.
• Very dry conditions (for example, an uncovered cap) could create some long-term aging, but low air voids essentially eliminate this potential problem. At the very surface (top 1 to 2 mm), drying or aging is readily overcome by periodic (10 years) maintenance such as fog sealing. If the surface is not exposed to air, then this potential is essentially nonexistent.
• Tensile and shear strength of MatCon layers is relatively high because of the nature of the crushed rock and modified binder matrix. Laboratory test values such as resilient modulus values are typically 2 - 3 times that for pavement mixtures. Resistance to long-term settlement caused by poor subgrade or consolidation of waste materials is improved by engineered modifiers. While no test data for MatCon yet exists, observation shows the layers tend to bend and conform to changing shape without cracking.
• The binders and aggregate materials are nearly inert to most materials with the exception of concentrated nitric acid and petroleum solvents such as gasoline or diesel fuels over time. And the impermeable nature of MatCon, coupled with modifiers, minimizes potential damage to the asphalt binder. No long-term data exists, but it is expected that soils with high organic content would not affect MatCon. (see no. 2 below)

2) How does one measure the chemical resistance properties of the material?

Early in our evaluation of MatCon performance, we consulted Dr. Henry Haxo, then President of Matrecon, Inc. about testing for chemical resistance. Using his previous experience in developing performance standards for geomembranes, we devised a testing program to evaluate the resistance of MatCon to attack by leachate from municipal solid waste. Matrecon then tested different MatCon modified binder formulations representing possible mixture designs, using procedures customarily associated with geomembranes. These included long-term exposure to model leachate solutions.

The research was conducted in two phases: on the binder only (without mineral aggregates), and on modified asphalt concrete mixtures meeting the MatCon design criteria. In Phase 1, the binders were exposed, in a well-defined procedure, to dilute aqueous solutions of MEK, TCE, toluene, and o-xylene. Evaluation was based on both absorption of the organics and the viscosity of treated binders following exposure. The absorption results showed that the distribution coefficients for these organics in aqueous solutions contacting the MatCon binder appear to be similar to those of geomembranes. It was also found that, as expected, the MatCon binder affected the viscosity of the modified asphalt binder, but it was recognized that the concentrations used would be far higher than those normally encountered in MSW (Municipal Solid Waste).

In Phase 2, specimens of the full MatCon mixture (including mineral aggregates) using a range of modified binders were exposed to tap water and a dilute aqueous solution of toluene (selected from Phase 1 as having a strong effect on the binder). Every 30 days over a 240-day exposure period, the toluene in solution was measured and the specimens were removed and tested for resilient modulus (MR - a dynamic loading test that simulates heavy truck loading) to evaluate the structural integrity. Results of these studies showed the MatCon modified mixtures absorb far less leachate (<0.39%) than those made with conventional asphalt binders. Low void content (<3%) and thus low water absorption, is important to overall performance. MR appeared to correlate with leachate absorption, but the MatCon modified mixtures retained modulus much better than unmodified. It was concluded by Dr. Haxo that even though these mixtures performed well using the model leachates, it was recognized that absorption by in-service MatCon liners "should be less than the absorption by the specimens immersed in the dilute aqueous toluene solution because of the presence of dissolved salts in MSW or MSW incinerator ash landfills."

3) Which chemicals destroy the hydraulic conductivity of the material or cause it to radically alter physical properties?

The most destructive chemicals would be petroleum hydrocarbons. Since it is unlikely that these would be present in MSW, the aqueous solutions of these were used in the laboratory tests described in question no. 2.

4) How are liners consisting of this material constructed on steep slopes? On normal landfill slopes; 3H:1V?

To place MatCon on slopes 3 to 1 or steeper it is necessary to winch the paver and rollers up and down the slopes. We recommend that slopes be 4 to 1 or flatter to eliminate the need to winch equipment.

5) What construction quality assurance measures are required to verify these properties:

a) liner thickness
b) hydraulic conductivity of impermeable and permeable layers
c) material continuity

Quality control during construction is a key factor in the successful performance of MatCon. Thickness of each layer is monitored by the laydown equipment and can be carefully controlled by the operators. For example, a 4-inch lift thickness can be laid uniformly at, say, 4±0.2 inches, depending on the smoothness of the underlying subgrade or base layer. This tolerance is well within the safety margins required for performance in both permeability and structural integrity.

The expected hydraulic conductivity is measured indirectly during construction by measuring the density and void content, a normal quality control test for asphalt paving mixtures. The values obtained are compared with permeability-density-voids test results (a control curve) that is developed at the time the laboratory mix design is made, using the project materials and the final mixture design. Final independent confirmation can be made using core samples tested in the laboratory, if needed.

Material uniformity is typically quite good throughout the mixture both laterally and vertically. Joints between passes of the equipment are maintained hot by limiting the distance paved before backing up and placing the adjacent pass, or lane. Further assurance can be provided by joint heaters, specially designed equipment to maintain heat in the area of joints until final compaction can be achieved. A special overlapping geometry (cross section) can be used to help assure a tight, impermeable joint, as well. Experience has shown that careful preparation and attention can result in joints readily meeting the required density and impermeability expected throughout the liner.

6) What advantages become evident when comparing this material to geosynthetic composite barriers?

1. Liner strength is such that it does not need a protective cover to protect from puncture and can withstand heavy equipment loading. The advantage of the ability to operate heavy equipment on this surface is that if a cell needs to be cleaned out for any reason, it can be accomplished without risk to the integrity of the liner.

2. Significantly larger number of potential manufactures and installers. (300 to 400) vs. (10 to 20)

3. If leak detection system is installed, it can be constructed to isolate leakage area.

4. Can be installed in more varied weather conditions. (Not affected by fog, light rain or wind conditions.)

5. On site storage not necessary.

6. Because it is not susceptible to UV damage, it can be left uncovered. This permits a greater variety of site uses and facilitates observation and easy repair of any damage that may occur.

7. MatCon can be designed to cope with arid or arctic climatic conditions.(See response to question 1)

7) What are the conditions of the patent on this material?

A U. S. patent was issued in 1992 for the system. Currently (1998), additional patent protection for other features of the MatCon system is in progress.

8) What is the life expectancy of the material under;

a) buried conditions
b) exposed conditions

Asphalt materials, whether the residue from refined crude oil or from natural deposits such as Trinidad Lake, are very durable. Surviving examples of use in construction or preservation in antiquity are still found today. Natural deposits of asphalt have existed for thousands of years with virtually no change in properties.

Well-designed layers used in the MatCon system would be expected to survive for a very long time. Buried MatCon membranes, not exposed to air would remain essentially unchanged. Exposed layers, such as caps used for parking or storage, can also survive for long periods with routine maintenance. Fog sealing to preserve the surface could be done routinely, say every 10 years as needed. If load-associated cracks or other damage occurred, the exposed surface could be easily repaired using conventional paving techniques.

9) To what degree will you warrant the hydraulic and physical performance of this material?

A typical HDPE liner material guarantee warrants that the material will be free from defects and be able to withstand normal weathering from 15 to 20 years of normal use in approved applications. They exclude from their warranty any damage due to natural events such as earthquakes, floods, or piercing hail. They further define normal use as excluding any exposure to harmful chemicals, abuse of the liner by equipment or people, and excessive pressures or stress from any source.

Their typical workmanship warranties range from 1 to 3 years. They exclude any damage due to subsurface or overburdened soil conditions or from total or differential soil settlements.

Their typical workmanship warranties range from 1 to 3 years. They exclude any damage due to subsurface or overburdened soil conditions or from total or differential soil settlements.

10) Is it possible to use this material in conjunction with a geomembrane, if necessary?

It is possible to lay a geomembrane directly on a MatCon surface, however if a MatCon layer is to be placed over most geomembranes, an intervening layer of fine aggregate would probably be needed for protection of the geomembrane.

11) Describe the construction sequence;

a) where is the material prepared?
b) how is it moved to the site
c) what provisions are necessary to eliminate cold joints?
d) what subgrade preparation is necessary?

a. MatCon is manufactured in a conventional drum mix or batch asphalt plant that has the controls necessary to produce high quality state or federal specification hot mix asphalt pavement material. The MatCon binder is delivered in standard asphalt tanker trucks to the hot liquid storage tanks at the asphalt plant. In most cases the aggregates used in the production of MatCon will be similar to those used in high quality asphalt pavements.

b. After the aggregates have been heated and mixed with the MatCon binder, this mixture will be deposited into conventional end dump trucks and hauled to the job site where it will be dumped into a standard asphalt paver which lays MatCon on the prepared surface. MatCon will then be rolled with vibratory or rubber tired rollers until the desired density is achieved.

c. Cold joints can be eliminated by paving as wide as possible, up to 22 feet, paving shorter distances so that the joint does not cool. A special cold joint geometry is used see figure 1 here.

d. MatCon can be placed upon any subgrade that is firm and uniform.

12) What testing has been done to measure internal shear and interface friction properties (between the material and the subgrade soil);

a) in the short term
b) in the long term
c) under earthquake accelerations

This question was also addressed in No. 1. In recent years, new testing procedures have been developed as part of the Strategic Highway Research Program (SHRP). These include dynamic shear, short-term fatigue cracking, tensile strength (particularly cold weather), water sensitivity, and aging. Although only some of these methods have been applied during the evaluation of MatCon, they can be used to evaluate important or crucial projects. To date, these test methods are providing better insight into the engineering behavior as compared to older empirical tests. The results can be applied to special applications such as side slope stability (creep, for example) as well as pavement loading (storage, traffic, etc.).

The design of MatCon, with very low voids and high binder content provide good resistance to short term cracking (fatigue) and long term bending without cracking. Similarly, it would be able to resist modest earthquake loading, but we have no direct experience with larger earthquakes except in pavement such as airfields. Sliding resistance between subgrade and pavement layers is normally quite high because of the total contact to the inherent rough surface at the interface.

13) Do you perceive any regulatory barriers to the implementation of this product?

No. The basic regulations for hazardous waste landfill design are contained in RCRA Parts 264 and 265, Subpart N - Landfills. These state the requirement that landfills "must install two or more liners and a leachate collection and removal system above and between such liners…"

The definition of a liner is given in RCRA Part 260, Subpart B-Definitions:

"Liner means a continuous layer of natural or man-made materials, beneath or on the sides of a surface impoundment, landfill, or landfill cell, which restricts the downward or lateral escape of hazardous waste, hazardous waste constituents, or leachate."

A MatCon-modified asphalt concrete liner engineered and installed to prevent contaminant migration does meet the RCRA definition of a liner. Use of geomembrane liners and recompacted clay have become the "standard" over time as a result of material availability and engineering practice. Specifications, testing and analysis have centered upon the measurable properties of the membrane systems. However, to our knowledge, there are no codified regulatory barriers to alternative liner systems including a MatCon modified asphalt.

14) It is perceived that roadway pavement is highly permeable. How is this product fundamentally different from roadway pavement?

The fundamental advantage of MatCon over conventional asphalt pavements is low permeability. The aggregate gradation, coupled with modified asphalt makes it easier to construct a low void, highly durable layer.

Typical permeability values may be about as follows:

For asphalt pavement (6 - 8% air voids): k @ 1x10-5 cm/sec

For MatCon (<3% air voids) k < 1x10-8 cm/sec

Hydraulic asphalt containing large amounts of fine material has often been promoted in the past for water or waste containment, but usually results in a design that is difficult to build and is inconsistent in its ability to reduce permeability. In order to obtain reduced permeability, the design requires large amounts of fine aggregate (dust). This, in turn, requires very high quantities of hard based (high viscosity) asphalt binder to coat the fines. This attempt to reduce permeability results in a mixture that is difficult to compact (tender), thus inconsistent in uniformity of voids and permeability. It is very difficult to compact on slopes because of its instability. In fact, one project in California failed when the side slope collapsed in "accordion fashion" when warmed by the sun. A further shortcoming is the inability to use hydraulic asphalt caps for parking or storage because it is inherently unstable (soft) under standing loads; equipment will often punch through in summer weather.

15) Can layer thicknesses be altered?

The layer thickness of MatCon can be varied even as it is being placed from 2 to 12 inches.

16) What possibilities threaten the performance of this material when used in waste containment?

The hazards to performance for any waste containment system would be similar, especially for catastrophic situations such as earthquakes or large landslides. As indicated earlier, hydrocarbon such as oil or gasoline could be detrimental, but it would be unlikely that these would be placed in a landfill. However, the design of MatCon with low voids and a modified asphalt binder provide superior resistance. This was pointed out in the evaluation by Dr. Henry Haxo, when he concluded that MatCon and geomembranes would behave similarly when in contact with aqueous organic solutions.

17) What properties and/or test methods are employed that are not common to the evaluation of paving materials?

As discussed in No. 13, new tests are available for evaluating fundamental engineering properties. Permeability is a property that is very important to MatCon, but not normally used for pavements. Dynamic or resilient modulus is used in the design and also as a means to measure and monitor structural integrity with time. Water sensitivity (stripping and/or softening) is important to MatCon but is not always used for pavement evaluation. Chemical resistance may become a standard test for liners/caps, but would not be used for pavements.

18) Is it possible for a cover system to double as a parking area?

One of the distinct advantages of using MatCon as a cover, is that the owner ends up with a very strong and durable surface that can have a number uses, such as parking, storage, and sport courts.

19) Is it possible to place cover soil and vegetation above the liner to restore the land for other use?

There should be no difference between MatCon and conventional liners for placement of cover soil except that most liners require a minimum depth of cover in order to protect if from damage, where MatCon would not.

20) How does one prevent clogging in the drainage layer?

Typically the drainage layers on top of the liners which transport the leachate to collection points are sand. The MatCon system is not susceptible to puncture so a much larger free draining aggregate, which would be less prone to clogging, could be used. The leak detection system can be enhanced to inject dyed water into the system to determine if there is a clog in any given sect.