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|FHWA > Engineering > Pavements > Recycling > 2/20/96 Memo|
This memorandum is issued to clarify the requirements of the Comprehensive Procurement Guideline (CPG), to clarify the technical guidance provided in our October 6, 1995, memorandum on the use of ground granulated blast-furnace (GGBF) slag in Portland cement concrete, and to provide guidance on the use of domestic slag.
On May 1, 1995, the Environmental Protection Agency (EPA) published the Comprehensive Guideline for Procurement of Products Containing Recovered Materials, also known as the CPG. The CPG consolidated five existing item designations, and designated 19 new items that can be made with recovered materials. Of the items contained in the new CPG, one is of primary concern to the Federal-aid highway program: cement and concrete containing coal fly ash or GGBF slag.
In addition to the CPG, the EPA also published the Recovered Materials Advisory Notice (RMAN) which contains the EPA's recommendations to procuring agencies for meeting their Resource Conservation and Recovery Act (RCRA) obligations with respect to the existing and newly designated items. The key recommendations contained in the RMAN are:
Due to variations in coal fly ash, GGBF slag, cement, strength requirements, costs, and construction practices, the EPA is not recommending recovered materials content levels for cement or concrete containing coal fly ash or GGBF slag. Additionally, the EPA does not recommend that procuring agencies favor one material over the other. These recommendations are consistent with the FHWA's current policies regarding the use of coal fly ash in cement or concrete, and States currently in compliance with those requirements will not be required to change specifications for its use.
Our October 6, 1995, memorandum provided technical guidance on the use of GGBF slag in Portland cement concrete. This memorandum contained three principal recommendations on the use of GGBF slag:
Since the issuance of the referenced memorandum, we have learned that there may be some confusion over when GGBF slag should be used and the recommended GGBF slag substitution rates. Our technical guidance was not intended to change or modify practices in States currently achieving satisfactory results with GGBF slag, regardless of their substitution rate.
The grade of a GGBF slag is based on its activity index, which is the ratio of the compressive strength of a mortar cube made with a 50 percent GGBF slag-cement blend to that of a mortar cube made with a reference cement. For a given mix, the substitution of grade 120 GGBF slag for up to 50 percent of the cement will generally yield a compressive strength at 7 daysand beyond equivalent to or greater than that of the same concrete made without GGBF slag. Substitution of grade 100 GGBF slag will generally yield an equivalent or greater strength at 28 days. However, concrete made with grade 80 GGBF slag will have a lower compressive strength at all ages. To provide a product with equivalent or greater compressive strengths, only grades 100 and 120 GGBF slag should be used. However, in mass concrete, the heat of hydration may be an overriding factor, and the use of grade 80 slag may be appropriate.
The guidelines contained in our memorandum on GGBF slag substitution rates were intended to provide a starting point for States with little or no experience in the use of cement and concrete containing GGBF slag. States currently obtaining good performance at greater percentages should not reduce their substitution rates. Section 18.104.22.168 of ACI 318-89, "Building Code Requirements for Reinforced Concrete," indicates that substitution rates of up to 50 percent may be acceptable for concretes exposed to deicing chemicals. In addition, in mass concreting operations, the heat of hydration may be an overriding factor and substitution rates greater than 50 percent may be appropriate.
As detailed in ACI 226.1R-87, distributed with the October 6 memorandum, the use of GGBF slag in concrete can have many beneficial effects, including a possible reduction in alkali-silica reactivity, a reduction in concrete permeability, and an increased resistance to sulfate attack. However, as with any material, the full benefits of GGBF slag will not be realized without the use of proper construction practices. Usually, the use of GGBF slag will result in an increase in the time setting. In the absence of proper moist curing, the increase in the time of setting can lead to an increase in plastic shrinkage. Additionally, the set retarding effects of GGBF slag are more pronounced at lower temperatures. Therefore, as with any admixture that retards set, care should be taken to ensure the use of adequate temperature and moisture controls.
In preparing to meet the requirements of the CPG, a State raised the question of whether or not the use of GGBF slag may be limited to domestic GGBF slag. The Buy America provisions contained in 23 CFR 635.410 do not address GGBF slag. However, based on FHWA guidance in "Contract Administration Core Curriculum Participants Manual and Reference Guide, 1996," States are permitted by Section 165(d) of the STAA and 23 CFR 635.410(b)(2) to have Buy America provisions that are more restrictive than the Federal requirements. If the more restrictive State provisions are not required pursuant to an Act of the State legislature, the FHWA requires a State legal opinion that the requirements are authorized under State lawand do not conflict with the competitive bidding statutes of the State.
The EPA was contacted to determine if the CPG or the RCRA contained language precluding a State from requiring the use of domestic GGBF slag. The EPA responded verbally that neither the CPG nor RCRA would prevent a State from limiting the use of GGBF slag to material of domestic origin and that the use of material of foreign origin is a local issue. Therefore, based on FHWA policy and the EPA response, States may require that only domestic GGBF slag be used if the State highway agency is legally authorized under State law to impose more stringent Buy America requirements.
For further information on this subject, contact Mr. James Powell at (202) 366-8534.
Gerald L. Eller
This page last modified on 11/20/06