Recycling Concrete with LBP

CERL researcher looks at whether it is safe to recycle concrete with lead-based paint

By Stephen D. Cosper

Editor’s Note: The author, an engineer with the Construction Engineering Research Laboratory of the U.S. Army, worked with the CMRA some years ago on the first project ever to examine any dangers of concrete painted with LBP. The full report on this second project, which was the basis of his presentation at the recent C&D World event in Las Vegas, can be downloaded at http://www.cecer.army.mil/techreports/ERDC-CERL_TR-10-1/ERDC-CERL_TR-10-1.pdf.

Within the Army, hundreds of Korean War-era barracks (at approximately 90,000 sq ft each) and associated concrete-masonry buildings, which contain lead-based paint (LBP), are being replaced with modern barracks complexes. Although these older barracks represent some of the most common current demolition projects, the problem of LBP in concrete masonry buildings is not unique to the Army. Throughout the U.S. Department of Defense (DOD), many such constructed before the 1970s contain LBP.

Some Army installations report that construction and demolition debris (C&D) constitutes 80% of their solid waste stream. Of this amount, about 63% is estimated to be concrete materials (Cosper 2004). The past few years have seen a growing trend to reduce C&D waste by reusing or recycling the materials made available by the demolition of excess buildings. The presence of LBP in this waste, however, can present a real barrier to the reuse for some of these building materials. If the standard leaching test (i.e., the toxicity characteristic leaching procedure [TCLP]) results on the material are 5 mg/kg or more, the material is classified as a Resource Conservation and Recovery Act “hazardous waste upon disposal.” Dealing with such hazardous waste is problematic. Tipping fees for hazardous waste such as LBP-containing C&D debris are usually many times that of non-hazardous C&D wastes. Also, the large volume represented by C&D concrete waste can significantly reduce the life of the landfill, a further cause for concern. Furthermore, if landfilled, an otherwise recoverable and useful resource may be needlessly buried.

The Strategic Environmental Research and Development Program (SERDP) solicited proposals for the development of environmentally friendly methods and techniques used in the deconstruction (demolition) of masonry buildings and other permanent (concrete) structures on military installations that are contaminated with lead-based paints. The SERDP Exploratory Development (SEED) Statement of Need (SON) indicated that LBP-containing concrete waste is commonly landfilled as a hazardous waste.

Actually, this is not typical practice since the chemistry of the concrete will usually buffer the TCLP test so it would not be classified as a hazardous waste. Acidification of soil does change the chemistry and mobility of heavy metals. However, the degree of mobility varies by specie, e.g., cobalt, zinc, cadmium, nickel are more mobile in an acid environment. Lead (Pb) is particularly immobile and tends to accumulate in organic materials (Kennedy 1992).

Moreover, the mass of the concrete in the demolition waste is much larger than the mass of the LBP since the Pb concentration in the bulk demolition material is usually very low. Nevertheless, there are concerns about the disposal of LBP-containing concrete aggregate in a landfill or reuse such as in a road base. A guidance document put out by the Wisconsin Department of Natural Resources, for example, advises the use of caution when reusing concrete containing LBP (WDNR 2004). Still, recent preliminary tests indicate that Pb will not leach from concrete road gravel even when directly exposed to the elements. There is a need to determine all of the elements of the Pb-concrete chemical system and the interactions that may affect the migration of the Pb into the environment.

Objectives
The objective of this work was to determine the mobility of residual Pb content in crushed demolition concrete in the environment when exposed to rainfall. This study was accomplished in four steps:

•    The Pb concentration in recycled concrete aggregate (RCA) was estimated based on paint sampling and construction quantity take-offs, pre-demolition.
•    These figures were compared to RCA samples taken, post-demolition and crushing.
•    A laboratory experiment was designed to model Pb leaching from RCA as commonly applied to the landscape.
•    The ultimate acid rain buffering capacity of RCA was determined.

Project Model
The Army has been making efforts to reuse and recycle waste from Army demolition and construction projects for many years. In 2006 and 2007, the Army Construction Engineering Laboratory worked with the Army Environmental Center (now the Army Environmental Command) and Concurrent Technologies Corp. to document some C&D waste diversion measures at Fort Jackson in Columbia, S.C. In one case, the Fort Jackson Directorate of Logistics and Engineering planned to demolish a large concrete cold storage building (a heavy, reinforced building of 19,750 sq ft), and to then crush the resultant debris for reuse on the installation, mainly as basic paving material. The native, clayey soil is prone to erosion and is difficult for vehicles to traverse when wet; there is a recurring need for paving. The main product produced was “3 inch minus” for paving. Experience shows this to be a common, economical, realistic way to reuse concrete, especially on a government installation. This process eliminates the costs to haul and dispose of the waste materials off-site, and precludes the purchase of virgin materials.

Most old concrete buildings will have at least some LBP present that becomes incorporated into the RCA matrix upon recycling. In industry, the presence of Pb is usually ignored, for it is generally understood that the Pb concentration is very low when diluted by the huge mass of concrete. In some cases, where the RCA will be employed in a sensitive area, or when the overall project has a high profile, Pb contamination is more closely scrutinized. (The concern is that Pb from the paint will dissolve in acidic rainfall and migrate into the environment.) This study was undertaken to test whether this actually happens, through a variety methods. Because the disposition of the CSB is representative, this SEED project used the following experience as a case study:

•    The total Pb concentration of the RCA was predicted through paint sampling data.
•    The total Pb concentration of the RCA was measure directly.
•    In the laboratory, representative columns of RCA were subjected to synthetic rainwater to quantify leaching.
•    The buffering capacity of the RCA was measured.

Conclusions and Recommendations
The total Pb values for the RCA samples used in this study (Table 1) were found to be similar in Pb content to urban soils (less than 300 mg/kg). Also, the SPLP values in the synthetic precipitation leaching procedure data for RCA with LBP (Table 2) were well below the 0.015 mg/L Pb EPA maximum contaminant leval (MCL) for drinking water,  and the extract from all of the concrete blank columns from the leaching experiment was also at or below the MCL. Based on these findings, this study concludes the Pb concentration in runoff from RCA with reasonable, real world amount of LBP, is extremely low.

Given the low concentrations of Pb in the RCA samples tested relative to regulatory limits and the buffering capacity of concrete, this study concludes that the environmental risk from Pb-containing crushed concrete applied to land is negligible. Moreover, the uses for crushed concrete as road base or fill conveys little risk of environmental or human exposure; in such cases, the RCA would be placed underneath thick layers of soil or paving materials, thus limiting environmental and human exposure.

On any given demolition project, it is recommended that some sampling and calculations be performed to ensure unusually high LBP-to-concrete ratios are appropriately addressed. It is also recommended that future work to expand and improve on this study include:

•    Identification of the variation in buffering capacity of concrete of different ages and from different regions, to determine whether this variation is great enough to significantly affect rainwater Pb leaching described in this report.
•    Correlation of SPLP and TCLP results.
•    Development of a computer tool to ease prediction of Pb concentration in RCA, pre-demolition.
•    Modeling the leaching properties of other common uses of RCA.
•    Determination of state-by-state testing requirements for RCA use, and any perceived limitations.

Cosper can be reached at 217-398-5569 or by e-mail at This e-mail address is being protected from spambots. You need JavaScript enabled to view it . 

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