Guidelines for Detection, Analysis, and Treatment of Materials-Related Distress in Concrete Pavements

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Chapter 5. Summary and Conclusions

This Volume 1: Final Report for the project entitled, "Detection, Analysis, and Treatment of MRD in Concrete Pavements," presented a synthesis of relevant information and a description of how this information was used to develop a set of three guidelines.   It then summarizes the results of a total of six case studies in which the guidelines were applied.  It was found that the first two guidelines were effective in making reasonable diagnoses of MRD, providing a valuable structure for both the engineer and petrographer to operate within.

The synthesis discusses the various deterioration mechanisms that result in MRD in concrete pavement. These distresses can generally be classified as either physical or chemical processes.  Freezing of concrete in a saturated or near saturated state can result in freeze-thaw damage to the paste, coarse aggregate, or both.  The application of chemical deicers can magnify the physical distress mechanism and, in some cases, deicers may react chemically to the detriment of the concrete structure.  Common chemical attack mechanisms can be associated with the paste or an interaction between the paste and aggregate.  Alkali–aggregate reactions, external sulfate attack, internal sulfate attack, and corrosion of embedded steel are chemical distress mechanisms.  For each MRD, the distress mechanism, manifestation, diagnosis, treatment, and prevention are discussed.

The synthesis reveals that although some MRD types are well understood, there are many unanswered questions regarding the specifics of the deterioration mechanisms.  Typically, multiple theories exists, none of which seems to completely explain laboratory observations.  In addition, more than one distress mechanism is typically at work at the same time.  The primary mechanism begins the deterioration process by producing microcracking in the paste, which in turn opens up the structure to the ingress of other deleterious substances.  For example, it is common to find ASR and DEF acting together, but it is difficult to determine which mechanism initiated the distress.  Similarly, it has been speculated that SEF in some instances fills pore void space to the point where the air-void system is compromised, resulting in paste freeze-thaw deterioration.  This complicity of more than one mechanism makes it very difficult to determine the exact cause of deterioration, which in turn complicates the selection of appropriate treatment and prevention strategies.

The visual manifestations on the pavement surface of many of these deterioration mechanisms appear similar, especially early in their development.  Cracking and staining in the vicinity of joints is an indicator of MRD, but this visual analysis alone does not provide positive identification of what mechanism is at work.  For example, it is believed that in the past some cases of external sulfate attack, possibly resulting from deicer impurities, may have been misdiagnosed as aggregate freeze-thaw deterioration (D-cracking).  It has also been speculated that DEF has commonly been misdiagnosed as ASR, even when conventional laboratory procedures have been used (Wolter 1997).

To address the difficulties of accurate diagnosis, standardize diagnostic methods executed by well-trained personnel are required.  This includes training of both the field crews collecting visual information and concrete samples, and the laboratory staff carrying out chemical and petrographic analyses.  In some cases, advanced analytical methods based on the use of SEM and XRD may be required to establish what mechanism(s) is at work.  And it must be fully understood that the complexity of the problem might be so great that the best result of a diagnostic investigation is a prioritized list of probable causes.

The major component of this study was development, evaluation, and refinement of the first and second guidelines.  To accomplish the evaluation, six test sites were chosen with at least one being in each LTTP climate zone.  The guidelines, provided in Volume 2 of this Final Report, were applied at the six sites with the cooperation and support of the SHAs.  Both Guideline I – Field Distress Survey, Sampling, and Sample Handling Procedures for Distressed Concrete Pavements and Guideline II – Laboratory Testing, Data Analysis, and Interpretation Procedures for Distressed Concrete Pavements were evaluated. The third guideline, Guideline III – Treatment, Rehabilitation, and Prevention of Materials-Related Distress in Concrete Pavements, was only superficially applied since detailed information regarding agency maintenance, rehabilitation, and mixture design policy were unknown.  In general, the guidelines seemed to direct the necessary work well and provide a systematic method of gathering and recording data.  A detailed description of the case studies is provided in Volume 3 of this Final Report.

The first guideline presents a systematic approach for performing a field distress survey, sampling the distressed pavement, and sample handling procedures.  The procedures were applied and few modifications were required from the draft guidelines.  The only significant modifications were changes to the forms used for recording construction records data, which was done to better reflect the data that were available on the sites examined.  One key point learned from applying the guidelines was that construction records for the selected sites were incomplete or limited.  In part this may be due to the age of the pavements.  However, it may be indicative of a systemic lack of methods and procedures for accurately recording construction data.  Even data as fundamental as the job mix formula for the mix design was unavailable in many cases.  Information such as climatic conditions during placement is non-existent.  Improving data collection, most probably by an automated data collection system during concrete placement, could greatly add to the information available to help diagnose the causes of pavement distress, including MRD.  It is suggested in Guideline III that SHAs adopt a more rigorous data collection and storage methodology in line with what is presented in ACI 126.1R, Guide to a Recommended Format for the Identification of Concrete in a Materials Property Database.

Applying Guideline I provides an engineer with a detailed assessment of the current condition of the pavement.  This not only provides the current information needed for analysis, but also provides a baseline for monitoring the rate of pavement deterioration when compared to data gathered in the future.  This greatly improves the ability of the engineer to maintain the pavement and extend its life, while providing a means to judge the effectiveness of various treatments.  This also illustrates one positive aspect of the developed guidelines.  They are intended to be applied together but they can easily be applied in different time frames.  As an approach, a SHA may use Guideline I as a means of screening pavements and prioritizing maintenance and reconstruction after the specific MRD(s) present has been identified

Of the three guidelines developed, the second guideline is the heart of the research effort, as it proposes an approach to diagnosing MRD.  The recommended laboratory procedures provide a systematic method for analyzing distressed concrete.  The diagnostic flowcharts and tables provide a step-by-step approach to use when trying to determine the exact cause of MRD.  Clearly, there will be cases where the guidelines fail to isolate the cause to any one MRD mechanism and, in many cases, multiple MRD mechanisms will be identified as possible contributors to the observed distress.  However, it is believed that in most cases the data collected using the methods discussed in Guideline II will provide a more complete understanding of the distress.  Based upon the results of this evaluation, the majority of cases can be resolved.  In four of the six case studies used to evaluate the guidelines, definitive and most probable causes of MRD were established.  Of the other two, one was identified through execution of the guidelines as not likely being affected by an MRD.  It is noted that the laboratory investigation conducted on this site bore out this conclusion, even though early stages of MRD were observed, but were not yet (and may never be) associated with pavement deterioration.  In the last site presented, a diagnosis could not be reached using the guidelines, as it became evident early in the evaluation that a different approach would need to be taken to investigate the problem.

The researchers were satisfied with Guideline II in terms of its efficacy and broad applicability.  However, in a couple instances, techniques not originally proposed were employed.  Specific examples are the use of epifluoresence microscopy as a means of estimating the effective w/c for the concrete and the use of a flat bed scanner as a low cost imaging tool.  Neither of these techniques is precluded by the guidelines.  In fact, in the case of epifluoresence microscopy, it is presented in the synthesis.  The methods proposed in the guidelines were never intended to be the sole methods of analysis or interpretation.  They are simply designed to provide guidance as to the common methods.  In the case of the flat bed scanner, this technology was used to show SHA personnel a low cost alternative to conventional microscopy.  The flatbed scanner will not supplant the petrographic or stereo optical microscope, but it may provide an economical way for a SHA to begin the process of laboratory analysis by providing a means of documenting specimens so specific areas can be identified for analysis when sending specimens to an outside lab.  When combined with staining techniques, the flat bed scanner is a useful addition to any concrete analysis laboratory.

For engineers working on this project, Guideline II proved to be very useful for helping them understand the process of laboratory analysis.  For many engineers, this process is a mystery and misunderstandings can result if the procedures used are not understood by the person interpreting or otherwise using the data—the engineer.  When following the guideline, the choice of tests was understood and the engineers knew that the laboratory personnel progressed through the diagnosis without stopping at the first distress identified. It is important when diagnosing MRD that the analyst keeps investigating all possibilities to discover all mechanisms that may be active.

Unfortunately, for laboratory personnel familiar with the various analytical techniques, the guidelines were reported to be too confining.  Laboratory personnel examining concrete are, in general, slow to rush to judgment.  The inherent variability in concrete, and the limited sampling possible from most pavements, makes it difficult for an analyst to make yes/no decisions about observations, as is required in the diagnostic flowcharts presented.  Laboratory personnel are more comfortable with decisions that are not absolute or are somehow weighted for their significance.  As the recipient of the data, the engineer has to understand that absolute decisions are rare and that in the end, the petrographer or analyst can only provide their best judgment.   However, the laboratory personnel need to understand the engineers’ needs.  Namely, they need to make yes/no decisions about replacement or rehabilitation and, therefore, require the clearest possible diagnosis from the laboratory in order to proceed.  Performing the laboratory analysis in accordance with Guideline II removes ambiguity and provides a comprehensive look at all possible distresses.

A strong point of Guideline II is that it does not force the diagnosis to resolve at one specific cause.  Numerous MRD mechanisms can be active and each should be clearly identified, without bias.  The guidelines serve as an interface between engineers and laboratory personnel.  Although some MRDs will not be unambiguously diagnosed by using the guidelines, the more common distresses will be identified.  Even when absolute diagnosis is not possible, the guidelines help the engineer understand the likely possibilities and the tests available to diagnose the problem further by contracting with outside laboratories.

With regards to Guideline III, the review of the literature suggests that the various strategies used to treat pavements affected by MRD are not very effective.  Most treatments are short-term fixes, such as the application of surface sealers in an attempt to slow the ingress of moisture and deleterious compounds.  Some suggested treatments, such as the use of lithium salts in treating ASR, show promise.  But in general, long-term treatment of a pavement seriously affected by MRD almost always requires major rehabilitation, either through rubblization and overlaying or complete reconstruction. 

Thus, the best method to treat MRD is to prevent it.  In new construction, it is recommended that a holistic approach be adopted, in which the overall quality of the concrete is emphasized.  Long-term strength is only one measure of quality and it is important that permeability also be considered.  It is believed that 28-day strength is not the best measure of concrete quality and, in fact, the use of such short-term strength testing may be complicit in the increased observation of MRD.  Instead, the emphasis should be shifted to producing dense, impermeable concrete having relatively defect-free insoluble paste microstructural characteristics. This requires the use of durable, non-reactive aggregates arranged to minimize the paste fraction.  The paste should have low permeability and solubility.  The use of high quality fly ash or GGBFS may offer advantages in achieving the desired concrete properties.  And care must be exercised during all phases of construction to ensure that the concrete reaches it full potential.

To construct truly durable concrete pavements, it is believed that SHA incentives will need to be modified by changing construction specifications and practices to focus on long-term durability, de-emphasizing rapid construction and short-term strength gain unless project constraints absolutely demand "fast track" construction.  A number of studies have suggested that the movement toward increased construction speed has sacrificed concrete quality.  This has included the use of ever shorter mixing times, increased speed of paving, high frequency vibration, and poor curing practices, all of which may have a negative effect on concrete durability.  The general consensus is that the knowledge exists to build high quality, durable concrete pavements, but that the industry incentive is biased toward rapid construction and early opening.  The best way to increase long-term durability of concrete pavements is to revise the incentive away from rewarding rapid construction (which uses short-term 28-day strength as a primary criteria) toward rewarding long-term concrete properties such as 90-day strength and permeability.  This must be accompanied by an increased emphasis on selecting high quality, durable materials.  It is realized that this will lead to an increase in initial costs, putting concrete pavements at a competitive disadvantage if life cycle costing is not considered.  Therefore, the revision of highway agency policies must not be restricted to the engineering level, but also must include a commitment to accept higher initial costs to achieve high performance, durable concrete pavements that will provide many years of maintenance-free service.  Without this commitment, it is unlikely that proposed changes can be implemented.

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