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| FHWA > Engineering > Pavements > Materials > Materials Notebook |
CHAPTER 5 - PORTLAND CEMENT CONCRETEMay 1997
The attached is Section 5-4-6 of the Materials Notebook which concerns the use of fly in portland cement concrete. The Materials Notebook will be issued in final form in July. Section 5-4-6 is being issued at this time due to the issuance of EPA guidelines on the subject. This section covers fly ash properties, substitution ratios, replacement percentages, blended cements, and acceptance procedures for fly ash and mix design procedures. It is suggested that this section be provided to the division offices and State highway departments for their use in developing the specification for allowing the substitution of fly ash for cement in portland cement concrete. The Geotechnical and Materials Branch is available for technical assistance in developing specifications. To request technical assistance or if there are any questions or comments concerning the attached information, please call Mr. Michael Rafalowski at FTS 426-0436. SIGNED David S. Gendell Attachment
1. BACKGROUND.The following is guidance for the substitution of fly ash for cement in portland cement concrete. This is in response to the EPA guidelines on the use of fly ash in concrete. Five topics are discussed: fly ash - general properties; fly ash -requirements; mix design procedures; blended cements; and exemptions. 2. FLY ASH - GENERAL PROPERTIES.Fly ash is a pozzolanic material. A pozzolan by itself has little or no cementing properties but in the presence of lime and moisture has cementing properties. Three types of pozzolans are listed in ASTM C-618 and AASHTO M-295. Class "N" is a naturally occurring material. Classes "C" and "F" are fly ashes that are produced from burning coal. These fly ash classes, "C" and "F," will be discussed in length. Fly ash is a finely divided residue that results from the combustion of ground or powdered coal. The properties of fly ash depend on the coal that was burned and the power plant operations. Class "C" fly ash is typically produced from lignite or subbituminous coal. This material has free lime which gives it cementing properties of its own. Class "F" fly ash is typically produced from anthracite or bituminous coal. This fly ash depends on the free lime in the cement for its cementing properties. Fly ash which is produced at base loaded electric generating plants is usually very uniform. The base loaded plants are those plants that operate continuously. The only exception to this uniformity is in the start-up and the shut-down of the plant. Contamination may occur from using other fuel to start the plant, and an inconsistency in carbon content occurs until the plant reaches full operating efficiency. The ash produced from the start-up and shut-down must be separated from that which is produced when the plant is running efficiently. In addition, when sources of coal are changed, it is necessary to separate the two fly ashes. Peak load plants are subjected to many start-up and shut-down cycles. Because of this, these plants may not produce much uniform fly ash. The two properties of fly ash that are of most concern are the carbon content and the fineness. Both of these properties will affect the air content and water demand of the concrete. The finer the material the higher the water demand due to the increase in surface area. The finer material requires more air-entraining agent to give the mix the desired air content. The important thing to remember is uniformity. If the fly ash is uniform in size, the mix design can be adjusted to give a good uniform mix. The carbon content, which is indicated by the loss on ignition, also affects the air entraining agents and reduces the entrained air for a given amount of air-entraining agent. An additional amount of air-entraining agent will need to be added to get the desired air content. The carbon content will also affect water demand since the carbon will absorb water. Again uniformity is important since the differences from non-fly ash concrete can be adjusted in the mix design. The use of fly ash in concrete will result in a more workable mix. This is due to the fineness of the material and its almost spherical shape. Fly ash will also reduce bleeding. This is again due to its fineness. The use of fly ash will also reduce the permeability of the concrete. The by-product of the pozzalanic activity reduces the permeability of the concrete. Some fly ash will also reduce the alkali-reactive aggregate reaction. 3. FLY ASH - REQUIREMENTS.A. Discussion.1. Specifications.As stated earlier, there are currently two existing specifications for pozzolanic material, AASHTO M-295 and ASTM C-618. The following is a comparison of major differences between the two specifications.
Currently most State highway agencies are specifying fly ash using ASTM C-618 with the exception of the requirement on loss on ignition (LOI). Most States are currently specifying a maximum LOI of 5 percent. 2. Acceptance Requirements.The standard method for sampling and testing fly ash is contained in ASTM C-311. The same procedure is listed in AASHTO M-295. The procedure calls for a sampling frequency of one sample for each 400 tons of fly ash. This would amount to one test for 7,000 cubic yards of concrete, if the fly ash replaces 20 percent of the cement in a six-bag mix on a ratio of 1 pound of fly ash to 1 pound of cement. Most States that are currently using fly ash have used approved sources and certification programs with check tests. The frequency of the check tests are either one every 100-500 tons or one per shipment. The State needs to insure that the fly ash is of uniform consistency. This will result in a concrete with uniform properties. B. Recommendations.
4. MIX DESIGN PROCEDURES.A. Discussion.1. Rate of Substitution.The substitution rate of fly ash for portland cement will vary depending upon the chemical composition of both the fly ash and the portland cement. The rate of substitution typically specified is a minimum of 1 to 1-1/2 pounds of fly ash to 1 pound of cement. It should be noted that the amount of fine aggregate will have to be reduced to accommodate the additional volume of fly ash. This is due to the fly ash being lighter than the cement. 2. Amount of Substitution.The amount of substitution is also dependent on the chemical composition of the fly ash and the portland cement. Currently, States allow a maximum substitution in the range of 15 to 25 percent. 3. Time Of Set.The use of fly ash will affect the time of set. Both classes of fly ash will extend the set time from 2 to 4 hours and will vary from fly ash source to fly ash source. The set time can be controlled by using accelerators. B. Recommendations.
5. BLENDED CEMENTS.The following will discuss only the Type "IP," Type "P," and Type "I (PM)" cements. The specifications for these cements are in AASHTO M-240 and ASTM C-595. Blended cements can be manufactured by either intimate blending of portland cement and pozzolan or intergrinding of the pozzolan with the cement clinker in the kiln. Type "I" (PM) (pozzolan modified cement) allows up to 15 percent replacement of cement with fly ash.- The Type "IP" and Type "P" are pozzolan-modified portland cements which allow 15-40 percent replacement with pozzolans. The differences in the two types of cements is in the ultimate strength and the rate of strength gain of the concretes. Most States specify limits on the pozzolanic content on Type "IP" cement. These limits are between 15 and 25 percent. 6. EXEMPTIONS.The EPA guideline on the substitution of fly ash requires the State highway agency to document the reasons for not allowing the substitution of fly ash for cement if it feels that it is technically inappropriate. The following two cases will not require documentation.
1. BACKGROUNDThe following describes the procedures that have been found to be effective in recycling concrete pavement. Three topics will be discussed. They are the concrete mix design, the use of D-cracked pavement, and the use of pavement with alkali reactive aggregate. In an effort to save money and conserve virgin aggregate, a number of States have decided to recycle their concrete pavements into aggregates for the new concrete pavements. This guideline will serve to address several problems and solutions to the problems that occur with the use of recycled aggregate. It should be noted that the use of D-cracking and alkali reactive concrete pavements, when used as aggregate will require testing to determine the type of treatment required to reduce the chances of deterioration of the new concrete. Some of this testing may take up to a year to perform which will require considerable lead time when recycling these problem pavements. 2. CONCRETE MIX DESIGNA. DiscussionCurrently many States do not use trial batches to approve concrete mix designs. The approval is based on cookbook proportions that are included in either the standard specifications or standard operating procedures. Due to the differences between the physical properties of the recycled concrete and virgin material, it is absolutely essential to perform a trial batch to verify the concrete mix design. This needs to be performed with the sizes and proportions of each aggregate both virgin and recycled that is to be used on the project. The material that is produced by the recycling (crushing) operations is very angular. Of particular concern is the particle shape and distribution of the aggregate passing the No. 4 sieve, i.e., the fine aggregate. These two properties of the fine aggregate affect the water demand, workability, and air content of the concrete mix. The air content can be modified by increasing the air entraining admixture. The other two items that will be discussed below are the water demand and the workability of the mix. B. RecommendationsThe following four methods can be used to alleviate the water demand/workability problem.
A combination of all or some of the above solutions may be used to develop an economical and workable mix design, but this can only be accomplished in conjunction with performing trial batches. 3. USE OF D-CRACKED PAVEMENT IN RECYCLINGA. DiscussionD-cracking is a durability problem relating to freezing and thawing in the presence of moisture. D-cracking is generally associated with the aggregates found in the midwestern States. The deterioration starts from the bottom of the pavement slab and works its way to the surface and typically becomes visible at joints and cracks first. Gravels are more prone to D-cracking, however, some crushed stone aggregates have also exhibited D-cracking. The pore characteristics of the aggregate actually determine whether the aggregate is susceptible to D-cracking. B. RecommendationsThere are two methods that have been used successfully to reduce D-cracking. These two methods have been used both on virgin aggregate and recycled aggregate mixtures.
It should be noted that neither of the above methods will eliminate D-cracking but only reduce the problem. If aggregates are known to be prone to D-cracking it will be necessary to perform durability tests on concrete specimens made from the recycled mix to assure that the treated material will perform satisfactorily over the design life of the pavement. The most reliable test procedures for determining durability provide for 300 to 350 freeze-thaw cycles on concrete specimens. AASHTO Test - Procedure T-161, "Resistance for Concrete To Rapid Freezing and Thawing," is equivalent to ASTM C-666. There are two methods in both procedures, Method "A" where the specimens are immersed in water at all times and method "B" where the specimens are immersed in water only during the thawing cycle. The Portland Cement Association has also established an expansion criteria. The following Table is a summary of practices by PCA, and the States of Iowa, Illinois. and Kansas
These test procedures require approximately 2-1/2 to 6 months to run. This time should be allowed for in the preliminary engineering stage to assure a durable concrete. 4. USE OF ALKALI REACTIVE AGGREGATESA. DiscussionThe alkali reaction is defined as expansion caused by the reaction of mineral constituents in the aggregate and the alkali in the cement. There are two types of the reaction, the alkali-silica reaction which occurs with some forms of silica aggregate (opal, chert, flint, and etc.), and alkali-carbonate reaction which occurs with some carbonate aggregates such as dolomites. It should be noted that only a relatively small proportion of carbonate aggregates will react with alkalis in an expansive manner. Both reactions require a warm moist environment. The resulting deterioration becomes evident as map cracking on the surface of the concrete B. RecommendationsThe following three methods have been used successfully with virgin mixes to reduce both types of alkali reactions.
It is recommended that several concrete mixes with varying amounts of fly ash and/or various cements with different alkali contents be tested with the recycled aggregate in accordance with ASTM C-227 which is an expansion test. The current test criteria in the procedure is a maximum expansion of 0.05 percent at 3 months and 0.10 percent at 6 months. The latest PCA research indicates that the crucial criteria is 0.10 percent expansion. Based on this, they recommend the following criteria: 0.05 percent expansion at 6 months, 0.10 percent at 1 year, and to continue testing after a year if expansion is occurring until the expansion stops or a value of 0.10 percent is reached. Since many aggregates react over a long period of time, it is recommended that this criteria be used instead of the criteria that is stated in ASTM C-227. 5. REFERENCESACI Committee 401, "Guide to Durable Concrete." ACI 207.2R 77, American Concrete Institute. D. C. Stark, "Characteristics and Utilization of Coarse Aggregate Associated with D-Cracking," Portland Cement Association, RD-047.01P (1976) D. C. Stark, "Alkali-Silica Reactivity Some Reconsideration," Portland Cement Association, RD 076.01T (1981) Design and Control of Concrete Mixtures, EB 001.12T, Portland Cement Association (1979) A. D. Haliverson, "Recycling Portland Cement Concrete Pavements," Minnesota Department of Transportation, Federal Highway Administration, Report FHWA-DP-47-3, February 1982
Attached is a copy of a final rule published in the May 1, 1995, Federal Register under which the Environmental Protection Agency (EPA) revised the Comprehensive Guideline for Procurement of Products Containing Recovered Materials by adding 19 new items. Also, attached is a copy of the Recovered Materials Advisory Notice, which contains EPA guidance on meeting the requirements of the Comprehensive Procurement Guideline (CPG). Of the 19 new items added to the CPG, only 1, cement and concrete containing ground granulated blast furnace slag (GGBFS), will affect the Federal-aid Highway Program. The Resource Conservation and Recovery Act (RCRA) requires that once an item is designated by the EPA, procuring agencies purchase it composed of the highest recovered materials content practicable, taking into account competition, price, availability, and performance. The RCRA also requires that procuring agencies develop an affirmative procurement program for designated items. The requirements of RCRA apply to Federal agencies and any State agency or agency of a political subdivision of a State which is using appropriated Federal funds for such procurement when the purchase price of the item exceeds $10,000 or when the total cost of such items purchased during the preceding fiscal year was $10,000 or more. However, a procuring agency is not required to purchase an item where it is not reasonably available, is available only at an unreasonable price, or does not meet the procuring agency's reasonable performance requirements. The procuring agency must, however, take the affirmative step of inquiring whether the item is or can be made available. Unreasonable price for recovered materials has been determined to be a price greater than that of virgin materials. In addition to the CPG, the EPA has published a Recovered Materials Advisory Notice (RMAN) containing recommendations on purchasing designated items in compliance with RCRA. The recommendations in the RMAN include guidance on recovered materials content and specification revision for procurement of the 24 items in the CPG. The following are the key recommendations relating to GGBFS:
The FHWA affirmative procurement program will consist of ensuring that the use of GGBFS is allowed where it is reasonably available and technically feasible. In those States where GGBFS is available at a reasonable price, specifications for projects authorized after May 1, 1996, must be revised to allow the use of GGBFS. States in which GGBFS is not available at a reasonable price will not be required to modify their specifications but must document the lack of availability. The Construction and Maintenance Division is developing technical guidance on the use of GGBFS in portland cement concrete. For more information contact, Mr. James Powell at (202) 366-8534. SIGNED GERALD L. ELLER 2 Attachments
This memorandum provides the follow up information referenced in our May 30, 1995, memorandum on the Comprehensive Guideline for Procurement of Products Containing Recovered Materials (CPG). Attached for reference are copies of ACI 226.1R-87 "Ground Granulated Blast-Furnace Slag as a Cementitious Constituent in Concrete" and "The Effects of Different Cementing Materials and Curing on Concrete Scaling". Additionally, our recommendations on the use of ground granulated blast-furnace slag (GGBFS) in concrete are detailed below. We recommend that the use of grade 80 GGBFS be avoided unless warranted in special circumstances. The GGBFS is graded by its hydraulic reactivity as either grade 80, 100, or 120. Concretes made with grade 80 GGBFS will exhibit lower strengths at all ages. However, concretes made with grades 100 or 120 GGBFS will exhibit lower early strengths but will meet or exceed control strengths at later ages and are therefore acceptable for use as cement replacement. In the absence of special circumstances or mix specific data, the substitution of GGBFS should be limited to 50 percent for areas not exposed to deicing salts and to 25 percent for concretes which will be exposed to deicing salts. While substitution of GGBFS for up to 70 percent of the Portland cement in a mix has been used, there appears to be an optimum substitution percentage which produces the greatest 28 day strength. This is typically 50 percent of the total cementitious material but depends on the grade of GGBFS used. Also, research has shown that the scaling resistance of concretes decreases with GGBFS substitution rates greater than 25 percent. Care should be taken to ensure that proper curing is maintained for concretes in which GGBFS has been substituted for a portion of the Portland cement. The reduced heat of hydration and reduced rate of strength gain at early ages exhibited by GGBFS modified concretes reinforces the need for proper curing of these mixes. With an increased time of set and reduced rate of strength gain, concretes containing GGBFS may be more susceptible to cracking caused by drying shrinkage. Additionally, the set retardation caused by GGBFS is temperature sensitive and becomes more pronounced at lower temperatures. During cold weather concreting, favorable curing temperatures should be maintained until the concrete has reached a sufficient strength to resist the effects of freezing temperatures and allow safe form removal. The use of matched curing or non-destructive testing can be used to determine in-place concrete strength for determination of removal of cold weather protection and safe form removal. For further information on this subject contact Mr. James Powell at (202)366-8534. SIGNED GERALD L. ELLER Attachments
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. CPG REQUIREMENTSOn 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. TECHNICAL RECOMMENDATIONSOur 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 days and 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 4.2.3.2 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. DOMESTIC SLAGIn 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 law and 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. SIGNED GERALD L. ELLER REFERENCES
LIST OF STANDARD PORTLAND CEMENT CONCRETESPECIFICATIONS OR TESTSI. Portland Cement
II. Aggregates
III. Water
IV. Admixtures
V. Field Control of PCC Mixtures
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