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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Any proposal to incorporate a nonconventional material, and particularly a waste or by-product material, into a pavement structure requires, in addition to an engineering evaluation, an evaluation of environmental, occupational safety and health, recyclability, economic and implementability issues. All of these issues need to be addressed prior to determining the acceptability of the material. Such an evaluation is complicated by the number of technical disciplines as well as institutional considerations that must be included in the process.
There is, in general, an absence of documented procedures, including evaluation tests, assessment procedures, and criteria to address many of the aforementioned issues. The purpose of this chapter is to outline a recommended procedural framework whereby one can decide if a waste or by-product material can be used as a substitute for a conventional material.
The framework presented is intended for use by a waste or by-product material generator who is interested in finding markets for his or her material, the transportation or environmental official who must evaluate the proposal and assess the suitability of the material and the application, or perhaps a legislative body that may be considering mandated use of a material in a specific pavement construction application.
THE EVALUATION FRAMEWORK
There are six major steps in a nonconventional material evaluation process that should be considered:
Figure 21.1 depicts a general schematic of the steps involved in this framework. Such a framework is not meant to be rigid in its order, since many of the individual steps could be combined or undertaken in different chronological sequence.
Figure 21-1. Non-conventional material pavement application evaluation framework.
STEP 1 - IDENTIFY RELEVANT ISSUES
Engineering, environmental, occupational health and safety, recyclability and economic issues can normally be identified by assembling a technical review team capable of identifying the specific requirements associated with using a material in a proposed application.
During this planning stage, emphasis should be placed on compiling and assessing existing data, which can include previous laboratory testing, field demonstrations, and the performance history from previous projects (including projects in other states) that have made use of the proposed material in the proposed application. Incorporating existing data into this process can be of great assistance in the task of defining all relevant technical issues and avoiding any unnecessary duplication of prior efforts.
Some recommended engineering questions that should be considered are as follows:
Some recommended environmental questions that should be considered are as follows:
Some recommended occupational safety and health questions that should be considered are as follows:
Some recommended recyclability questions that should be considered are as follows:
Some recommended economic questions that should be considered are as follows:
STEP 2 - ESTABLISH TESTS AND CRITERIA
Once the technical issues that require resolution have been defined in Step 1, then tests and criteria can be established to resolve these issues. There is, at the present time, however, an absence of generally accepted test methods and criteria to address all of the engineering, environment, occupational safety and health, and recyclability issues that in general will arise when assessing the use of nonconventional materials in pavement applications.
Despite this absence of standardized evaluation tests and procedures, nonconventional materials have been used and have performed in an acceptable manner. Materials that have been used with the greatest success have generally undergone significant laboratory and field demonstration testing to ensure that performance criteria defined for the material and application could be met. In some cases (e.g., blast furnace slag, recycled asphalt pavement and coal fly ash) special American Society of Testing Materials (ASTM) test methods and specifications have been developed and criteria for use established.
For many of the nonconventional materials being proposed for use (or those that might be proposed in the future), tests and criteria must be established for each technical issue.
The test methods and criteria that are typically used to verify the engineering suitability of conventional materials have been formalized in ASTM and AASHTO specifications. These specifications, which generally include test methods and criteria, address broad categories of material and products that are typically used in pavement construction, including aggregates and fillers, cementitious materials used in asphalt and Portland cement concrete, as well as materials used in embankment and fill and stabilized base (soil-cement) applications. ASTM and AASHTO test methods and criteria, however, have been established primarily for the evaluation of conventional materials, and are based, in great part, on empirical correlations between the measured material test properties of these conventional materials and recorded field performance that have evolved over several decades.
When a nonconventional material (and particularly a waste or by-product material) is proposed for use in a pavement application, unless the proposed material exhibits characteristics that are identical (or at least very similar) to that of conventional materials, the applicability of the ASTM or AASHTO test methods and criteria may be questionable, and test modifications or new test methods may be needed.
As part of the engineering evaluation process, it is recommended that the following engineering testing issues be considered:
Chapter 22 of this report presents a more detailed description of the environmental evaluation issues (including public health) and assessment methods that are available to evaluate waste and by-product material pavement application proposals. In general, the following environmental evaluation issues should be considered:
Occupational Safety and Health
The potential for exposure to dust, fumes, sharp edges, corrosive liquids, etc., are all issues that need to be considered when evaluating a waste or by-product material. To properly assess occupational safety and health issues, it is recommended that the following occupational safety and health testing issues be considered:
When new materials are introduced into pavements, there is a need to consider the potential recyclability of the material or product at the end of its functional service life. The original material or product, at that time, can become a new raw material or perhaps a component of a new product.
From an engineering point-of-view, assessing potential recyclability requires that the designer satisfy himself or herself that there are no significant engineering property considerations that might preclude future use or recycling of the material or product containing a waste or by-product material. Similarly, from an environmental or occupational safety and health viewpoint, it is necessary to ensure that recycling of the pavement will not result in any adverse impact to the environment.
It is recommended that the following recyclability testing issues be considered:
STEP 3 - PERFORM TESTS
The third step in the evaluation process is the implementation of testing and assessment procedures to determine whether the material will meet the criteria established.
STEP 4 - MODIFY MATERIAL OR PRODUCT
If a material is not capable of meeting established material or product criteria, it is useful to consider whether additional or alternative material processing or product modification could achieve the desired results.
For example, if a waste or by-product material contains a larger fraction of fine-grained particles than desirable for use as a substitute aggregate (i.e., the material is unable to meet the target specifications), screening or blending with coarse material could be used to modify the composite product gradation in order to comply with the required specifications. Alternatively, a modified product design (e.g., mix design) might be able to compensate for the fineness of the material.
STEP 5 - IMPLEMENTATION FACTORS
Even if it is possible to satisfy all of the engineering, environmental, occupational safety and health, recyclability, and economic issues associated with the use of a nonconventional material in a pavement application, there are still some intangible, nontechnical issues that could prevent widespread implementation. These nontechnical issues are lumped under the category of implementation factors. The implementation factors include institutional acceptability, political acceptability, and public acceptability.
Institutional acceptability is a factor that accounts for all the steps that will be required to gain acceptance of the material. This includes the initial testing and demonstration of the applications, the development of specifications for material use, and the degree to which engineers will be willing to accept and specify the material (if not mandated).
Depending on the particular material and its proposed use, the length of time required in such a process and the degree to which the material will be accepted should be considered in a decision to move forward with the proposed application. For example, a proposal to use a potentially reactive material as an aggregate in Portland cement concrete will, in most instances, require long-term field testing to establish the suitability of the applications and might ultimately result in a reluctance to use the material if the demonstration data are not conclusive. A material with marginal environmental properties could require a comprehensive health risk assessment process, which may require an investment of time and money that may not be commensurate with the benefits associated with recycling the material, and may still leave many engineers reluctant to use the material at the end of the process .
Political acceptability includes the expected level of support from public officials for the proposed use. This support may be directly related to the perceived benefits to the public if the proposed material use strategy is implemented. For example, one could be expected to receive greater political support for a proposal to use scrap tires (a highly visible waste material) in pavement construction applications than quarry by-products. Such political support can be expected to translate into institutional benefits, particularly as it relates to the time-line for demonstration and the development of specifications.
Public acceptability is a factor that must be considered in any process. Adverse public reaction to a proposed material-application strategy can be expected in many instances to ultimately erode political and institutional support. Sometimes it may be prudent to select applications that will be viewed more favorably by the public, even if technically it may be equally or more suitable in other applications. For example, the use of a waste or by-product material in a traffic lane wearing surface may be technically feasible, but it may be advisable to propose its use in a base course to provide a protective layer over the by-product modified pavement, or to use it in roadway shoulders rather than in traffic lanes to reduce any concerns that may be anticipated by the public.
STEP 6 - FIELD DEMONSTRATION
The sixth step in the recommended process is the field demonstration.
Demonstrations are normally undertaken to resolve technical issues (engineering, environmental, occupational safety and health, and recyclability) that cannot be fully addressed in a laboratory environment. They may also be undertaken to address implementation factors by providing field data to reassure the public that the proposed application will not result in adverse roadway safety or environmental problems, even if the laboratory data suggest that such is the case..
Proper planning is critical for the implementation of a successful demonstration to ensure that all monitoring equipment, construction, and quality control procedures are in place.
The ability to arrive at a final decision regarding the acceptability of a material for use will depend on the degree to which each of the aforementioned steps were planned and implemented. The establishment of a stepwise framework with specific objectives, procedure and criteria is critical to implementing an evaluation approach that will address all necessary issues.