A Deeper Look at Recycled Concrete
It may be time to take a closer look at using more broken-up concrete from previous buildings, runways, roads and other structures as the base material for new runways on military installations.
By Toy Poole, Ph.D., M.SAME
Recycling of demolished concrete for use as base material for military airfield pavements has never reached its full potential. This limitation is attributable to uncertainties in maintenance costs and long-term performance of the pavement because of potential latent properties of the recycled concrete that might have some adverse impact on the new pavement.
Yet there are powerful pressures pushing towards less use of virgin aggregate, and to find ways to use recycled materials more—creating both economic and environmental benefits.
concerns about performance
The main concern about the performance of concrete used as base material involves cases in which the demolished pavement was actively deteriorating as a result of alkali-aggregate reaction (AAR). AAR is a phenomenon that occurs in concrete where an expansive chemical reaction develops involving the Portland cement and reactive components present in some aggregates. This expansion first develops on a microscopic scale, but eventually progresses to the point that cracking and breakdown in the concrete pavement occurs. The expansion also results in gross expansion of the entire structure, which can have damaging effects on adjacent structures. There is no practical way to stop the progression of AAR as long as the concrete is exposed to water.
AAR requires the presence of water and so usually occurs in structures in outdoor exposures. There is almost always presence of sufficient water around airfield pavements to support an active AAR condition if the properties of the cement and aggregates are suitable for the reaction to develop. This is even true in desert environments. The soil under pavements normally contains some water vapor from deeper water in the underlying geology. This vapor condenses on the underside of a pavement in sufficient quantity to support AAR.
Most AAR reaction develops and causes damage rather slowly over a period of several years or possibly a decade or more. The reaction eventually runs its course, but this can take many decades and may persist well beyond the service life of the original structure. The concern is that this residual expansion in the recycled concrete could cause distortions and damage to overlying pavement. Because of this concern the U.S. Air Force has implemented a restriction on using recycled AAR-affected concrete as base for some airfield pavements.
The origin of this concern stems from an incident at Holloman AFB, N.M., reported in 2006. Recycled concrete pavement was used as base for a hangar and surrounding pavement structures. Damage was noted approximately one year after completion of the construction and was caused by significant expansion in the recycled concrete base.
The expansion was sufficient to cause significant faulting in the overlying pavement. And the faulting was enough to cause damage to aircraft using the pavement. Damage to the hangar structure built over the recycled concrete also was evident in distortion and cracking of walls and door openings.
Laboratory investigation identified an expansive chemical reaction between the cement in the concrete fragments of the recycled material and sulfates in the local geology. This reaction is a well-known phenomenon in concrete technology. The reaction is based on a different chemical reaction than occurs with AAR, but it does present the same expansion and deterioration phenomena as AAR.
This sulfate-concrete reaction is considered to be controllable by selection of sulfate resistant materials for use in the concrete. The principal control is by use of Type V Portland cement, which is specially formulated to resist reaction with sulfates.
The demolished pavement at Holloman AFB was originally constructed with Type V cement. The normally operative protection from Type V cement did not work in this case. This effect had not been previously encountered.
Air Force engineers concluded that, since an expansive sulfate reaction that was not expected to occur, did occur, that perhaps a similar latent problem with other expansive phenomena, such as in AAR-affected concrete, could also occur. As a result of this uncertainty, restrictions were placed on use of recycled concrete with a history of AAR.
The Air Force developed guidance to address this issue in Engineer Technical Letter 07-6, “Risk Assessment Procedure for Recycling Portland Cement Concrete Suffering From Alkali-Silica Reaction in Airfield Pavement Structures.” This guidance remains in place. The document describes procedures for assessing the risk level associated with use of AAR-affected recycled concrete as base in various types of pavement structures. It allows for use of the recycled concrete when the severity of the AAR is determined to be low and when the criticality of potential interruptions to the use of the structure is also low. The threshold for use in critical structures is rather rigorous. Runways are a particular example of cases for which it would be difficult to justify use of the recycled AAR-affected material. This is expected given the operational criticality of the runways.
Since the occurrence at Holloman AFB, recycling of AAR-affected concrete as base has been used in other non-military pavement applications (such as highway pavements) without apparent concern for residual AAR.
Cost and Environmental Pressures
Many in the community of concrete engineers believe that heaving similar to what happened at Holloman AFB is not theoretically plausible for AAR-affected concrete used as base. This skepticism is based on differences in the microscopic features in the physical development of the expansion. To date, there have been no reports of such problems; but there also have been no systematic studies on the matter. Additionally, given the slowly developing nature of the damaging AAR phenomenon, the absence of apparent damage in field service records may not be conclusive.
In locations where new construction plausibly could use recycled concrete as base, but does not because of concerns of AAR in the recycled material, avoiding the use of recycled concrete can result in significantly enhanced costs for new construction. Despite the potential problems involved in using recycled concrete in airfield construction, there are powerful forces pushing for the increased use of this material in construction projects.
The alternative to not using recycled concrete is to land fill the demolished material and to secure supplies of suitable virgin aggregate. These activities can involve significant hauling costs. Supplies of good aggregate are finite, and many of the most accessible deposits have already been used up. This pushes up the cost of the remaining resources.
Another reason for the increased cost involves the timelines and expense involved in obtaining environmental, social and economic approvals for expanding an existing pit, or starting a new one. Strong environmental interests have pressed for more rigorous environmental reviews, due to concerns about protection of farmland, natural habitat, and groundwater supplies. Local property owners and residents often oppose plans for new and expanded pits, meaning that when an already-permitted site is worked out, further aggregate must come from elsewhere.
Regulatory requirements for closure of a worked-out pit are increasing as well, both in terms of financial cost and the time required. The increased regulation-related costs for starting, expanding, operating and closing an aggregate pit must be reflected in the price of the product.
As with many construction materials, which tend to be heavy, bulky and low in value per ton, one of the biggest cost factors in aggregate is transportation. Large trucks carrying aggregate from pit to site can burn fuel at a high rate, pushing up the costs of construction. Crushed concrete, available onsite, avoids the need to pay for or transport virgin aggregate and subsequently reduces the need to transport and dispose of concrete rubble elsewhere.
Using recycled concrete
One of the keys to expanding the use of recycled concrete in new military airfield construction is the full implementation of the risk assessment procedures in Engineer Technical Letter 07-6. These involve an assessment of the estimated aggressiveness of the remaining AAR the criticality of the particular pavement or part of the pavement that might be affected. These criteria are rugged for critical structures. However, a significant amount of less-critical pavement exists on Air Force installations that probably would tolerate this recycled concrete as base without undue concern.
Many in the community of concrete engineers believe that heaving similar to what happened at Holloman AFB is not theoretically plausible for AAR-affected concrete used as base. This skepticism is based on differences in the microscopic features in the physical development of the expansion.
Another crucial point is to develop a research program that investigates the residual AAR problem in detail. One part of the research would be to identify cases in which AAR-affected concrete was used as base and to evaluate the current status of the structure. The most useful of these would be structures that have been in service for at least 10 years.
Also needed is development of a laboratory procedure competent to capture the effects of residual expansion in recycled materials. This need applies to AAR, sulfate reactions, and other materials, such as certain types of slag aggregate.
Use of field-placed test structures as test specimens has developed in recent years for investigation of anticipated field performance. This approach seeks to simulate realistic applications but in smaller structures that are easier to manage as test specimens. Although the results from such experimental field sites are rather slow to develop, such an approach may be the best for development of reliable information.
Understanding the Benefits
Changes to established procedure, particularly if it has to do with the ways vital infrastructure such as military installations are built, must be done only with great care.
However, the financial and environmental benefits of recycled concrete for use as base, combined with innovative ways to manage these projects, make this change worth a new look.