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CONSTRUCTION MATERIALS
OPTIMIZATION OF PORTLAND-POZZOLAN CONCRETE, AIRPORT RUNWAYS Lynn A. Bergman, PE & LS
Author
Lynn Bergman earned his Bachelor of Science in Civil Engineering at the University of North Dakota (UND) School of Engineering and Mines in Grand Forks, North Dakota. Mr. Bergman became interested in transportation and engineering early in life and studied Fundamentals of Transportation Engineering under "Mr. Fly Ash", Oscar Manz of UND, a pioneer in the utilization of coal combustion byproducts. Mr. Bergman's work with The Falkirk Mining Company included efforts to demonstrate the benefits of coal combustion byproducts, particularly "fly ash" and "bottom ash", in road and bridge construction.
He believes that a major challenge of this new century is to utilize the so-called "waste products" of the world's industrialized and increasingly environmentally conscious society. He believes the "optimization of percent replacement of Portland cement with fly ash" to be a logical first step toward the parallel challenge of constructing, early in this century, concrete that exhibits far greater economy, strength and durability than that of the last 50 years.
Abstract
This report briefly reviews the history of rigid concrete pavements. The basic constituents of concrete are defined, along with their physical and chemical properties. Pozzolans, and their importance to the durability of concrete, are described, from ancient use of volcanic ash in concrete constructed by the Romans, to modern day use of coal combustion byproducts along with air-entrainment and water-reducing admixtures in Portland-pozzolan concrete. Existing empirical equations for Portland-pozzolan concrete mix design are defined, tested, and refined. A Portland-pozzolan concrete optimization theorem (for determination of optimum percent replacement of Portland cement with fly ash) is presented. The results of demonstration projects are shared. Pitfalls in design, material specification, construction, and finished concrete protection are identified. Social and political considerations are described, along with suggestions to address them. History of Rigid Concrete Pavements Hydraulic Cement Natural cements were first produced from naturally occurring clayey limestone having approximately the same proportions as modern day Portland cement. The clayey limestone was made powdery by the action of heat (called calcining). These cements, in the presence of water (hence the term, hydraulic cement), produced a cementitious material. Pozzolan Fine volcanic ash, deposited by winds from the eruption of volcanoes, or pulverized volcanic pumice, are pozzolans. Pozzolans are siliceous or siliceous and aluminous materials that, in the presence of water, will combine with a calcium hydroxide activator (lime, cement or kiln dust) to produce a cementitious material.
Concrete pavements have been engineered and constructed by mankind since ancient times. The Romans combined calcined cements with naturally occurring pozzolans, aggregates and water to make the first hydraulic cement-pozzolan concrete. Portland Cement An English mason, Joseph Aspdin, patented Portland cement in 1824. It produced concrete that was the color of the natural limestone quarried on the Isle of Portland, a peninsula in the English Channel west of the Isle of Wight. The first Portland cement made in the United States was produced by a plant at Coplay, Pensylvania in 1872. Fly Ash Fly ash is the finely divided residue that results from the combustion of pulverized coal and is transported from the combustion chamber by exhaust gases. The fly ash is removed from exhaust gases by electrostatic precipitation. Fly ash produced by the combustion of lignite coal is found in ample supply in North Dakota. The fly ash from Great River Energy's Coal Creek Station near Washburn, North Dakota contains approximately one-sixth calcium hydroxide by weight and so is reactive with water to form a cementitious material. It meets the requirements for both Class C and Class F as defined by ASTM. Aggregates Aggregates comprise about 75%, by volume, of a typical concrete mix. Aggregates include natural sands and gravel, as well as crushed rock. To be most effective, aggregates should be well graded, strong, angular and clean. To be adequate in strength, aggregates should be able to develop the full strength of the cementing matrix. Aggregates should be hard and resistant to deformation due to heat or changes in moisture content. Flat or elongated particles have a detrimental effect on workability of the concrete mix. Course aggregates The course aggregates (those retained above the No 4 sieve) most likely to be as strong as the cementitious matrix are those produced by crushing the Canadian Shield igneous and metamorphic rock found around Minot, North Dakota. The rock found around Washburn, North Dakota contains significant amounts of flint and softer sedimentary rock derived from western North Dakota drainages and is slightly lower in quality. The course aggregates found to be readily available in most areas are those passing the one-inch sieve (3/4 inch minus). Larger aggregates may be specified, usually at greater additional cost than that warranted by additional strength achieved. Fine aggregates Fine aggregates (those passing the No 4 sieve) should be relatively free (i.e. 0-10%) of excess fines (those passing the 100 sieve). Bottom Ash Bottom ash is another byproduct of coal combustion. It can be used as a partial replacement for fine aggregate in concrete after pulverization combined with removal of excess fines. It's dark color is advantageous for concrete produced in northern climates due to the finished concrete's ability to absorb heat from the sun in winter, tending to sublimate and/or evaporate surface moisture (i.e. snow, ice). Portland-pozzolan Concrete
During the hydration process, some of the lime in the cement matrix becomes free (free lime) and available for further reactions. Tricalcium silicates and dicalcium silicates in Portland cement react with water to form tobermorite gel and calcium hydroxide. The resulting calcium hydroxide in water solution is the source of the "free lime". So the amount of "free lime" from tricalcium and dicalcium silicates can be determined from the resulting tricalcium and dicalcium hydroxides. When Fly ash is used as a partial replacement for Portland cement, the fly ash particles react with the "free lime" to produce additional cementitious material. Also, fly ash particles are spherical in shape, lowering the viscosity (resistance to flowing) of the plastic concrete for a given water-cement ratio. Air-entrainment (for freeze-thaw resistance) also tends to lower the viscosity. For these reasons, the water/cement ratio can be effectively reduced while maintaining workability. Prior to the last 15 years or so, fly ash was employed as a partial replacement for Portland cement primarily in mass concrete (i.e. dams, thick walls and slabs) to reduce the heat of hydration and resultant thermal cracking. Cold mixing water and ice chips were also used to keep temperatures down. The resulting concrete maintained an even temperature throughout curing, which took longer than Portland cement concrete. It was this slow curing that prevented Portland-pozzolan concrete from being used on projects where the concrete was required to resist early (i.e. 3 to 7 days after placement) traffic loading. Portland-pozzolan concrete pavement design has evolved to include the use of high-range water-reducing admixtures (called superplasticizers) to lower water/cement ratio, increasing early strength while maintaining acceptable workability. Optimum replacement, on a 1:1 basis by weight, can be determined by laboratory testing and has been generally been found to range from 30% to 50%. The following theorem has been developed for preliminary determination of the optimum replacement rate. This theorem should not be used in the absence of testing to validate the optimum replacement rate. Testing at the theoretical optimum and at replacement rates 5 percent above and 5 and 10 percent below theoretical optimum is recommended as a minimum. Portland-pozzolan Concrete Optimization Theorem Copyright ã 2000 Lynn A. Bergman
Where "P" represents the optimum percent replacement of Portland cement with fly ash in concrete for durability and strength.
Portland-pozzolan concrete has been proven to be more durable than conventional Portland cement concrete. Specifically, it: · Reduces permeability; fly ash reacts with "free lime" in Portland cement to generate additional cementitious compounds that "block" bleed channels · Reduces "bleeding" of water to the surface of the plastic concrete · Decreases chloride ion penetration (from road salts) · Improves resistance to sulfate attack · Resists the detrimental reaction of silica in aggregate with alkali in the cement (alkali-silica reaction, ASR) · Reduces drying shrinkage · Improves elasticity (i.e. flexibility) · Reduces "creep" (weight-induced deformation over time) · Improves freeze-thaw durability (when air-entrained) · Reduces heat of hydration "spike", particularly helpful in reducing plastic shrinkage of mass concrete Case History Riverdale Haulroad Grade Separation - Lignite Ash Demonstration Project Introduction The Falkirk Mining Company has been the supplier of lignite coal for Great River Energy's Coal Creek Station since the late 1970s when the power plant first came on line to meet the energy needs of a large portion of Minnesota. The Falkirk Mining Company found it necessary, in 1998, to construct a grade separation structure to allow McLean county road traffic to pass over Falkirk's Riverdale haulroad. A grant was received from the North Dakota Industrial Commission's Lignite Research Council, resulting in extensive testing for several durability parameters. Lynn Bergman, Senior Mining Engineer, responsible for all aspects of civil construction projects at Falkirk Mine, was designated in the grant proposal as the Owner's Representative to manage and coordinate the project. Other key personnel included: · Richard L. Gunderson, PE Houston Engineering, Inc Civil Engineering Consultant · Gary L. Arman, PE Arman Engineering Testing, Ltd. Geotechnical Consultant · Oscar F. Manz, Professor Emeritus Manz Associates CCBs Expert · Dr. James L. Jorgenson, PE Retired Professor, NDSU Structural Consultant · David Kaufman The Falkirk Mining Company Owner's Representative* * Assigned as Owners Representative after retirement of incumbent Owner's Representative Testing performed for the project included: ASTM C39/39M-99 Compression Strength of Cylindrical Concrete Specimens ASTM C457-98 Microscopical Determination of Parameters of the Air Void System in Hardened concrete ASTM C642-97 Density, Absorption, and Voids in Hardened Concrete ASTM C512-87 (1994) Creep of Concrete in Compression ASTM C1012-95a Length Change of Hydraulic-Cement Mortars Exposed to a Sulfate Solution ASTM C157/C157M-99 Length Change of Hardened Hydraulic-cement, Mortar, and Concrete ASTM C469-94 Static Modulus of Elasticity and Poisson's Ratio of Concrete in Compression ASTM C1202-97 Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration Structural elements for which fly ash replacement was employed included: · Pre-cast Concrete Pilings · Cast-in-place Concrete Abutments · Pre-cast Concrete Bridge Beams · Concrete Bridge Deck · Pre-cast Concrete "T-Wall" Elements (Vertical Retaining Wall) · Pre-cast Reinforced Concrete Pipe Test results contained in the final report indicated the following: · Optimum replacement of Type I-II Portland cement with Class C Fly Ash in reinforced concrete pipe was 40% by weight to minimize water absorption, enhancing long term durability · Results for creep, rapid chloride permeability and shrinkage were excellent for 30% to 40% replacement of Type I-II Portland cement with Class C Fly Ash Case History Runway 13-31 Reconstruction -Stage I - Minot International Airport Introduction The City of Minot has entrusted Kadrmas, Lee & Jackson (KL&J), since the late 1980s, with engineering services related to improvements of airport facilities. An important element of the recently completed Master Plan Update (submitted to the Federal Aviation Administration on November 22, 2000) for Minot International Airport is the Reconstruction of Runway 13-31. Runway Strength & Life-cycle Cost Comparison Current and future strength requirements were considered. Pavement sections were designed for hot bituminous and concrete pavement. These sections were determined based on initial loading of 121,000 pounds MTOW for the DC9-50 (largest MTOW of the DC9 family). Life cycle costs were determined for hot bituminous pavement ($65.24/SqYd/20yrs) and concrete pavement ($49.68/SqYd/30yrs). The concrete pavement design section, with 10 inches of recycled aggregate sub-base, 6 inches of crushed aggregate base, and 13 inches of concrete pavement, required initial flexural strength 700 psi. The 166,500 pounds MTOW for the Airbus A319, however, required a future flexural strength of about 800 psi with the same pavement section. Because of the need for strength increase from 700 psi currently to 800 psi as soon as year 2005, Portland-pozzolan concrete pavement was employed. Portland-pozzolan Concrete Mix Design The Federal Aviation Administration (FAA) requires a minimum design flexural strength of 600 psi. The Portland Cement Association's Design of Concrete Airport Pavement states "Experience indicates that concrete with a modulus of rupture from 600 to 700 psi at 28 days will usually result in a pavement at the least cost". Accordingly, a flexural strength of 650 psi was used as a starting point for the trial mix designs. The prediction of corresponding compressive strength is approximated by the square of flexural strength divided by a constant, 100 (constant "100" derived from empirical data for Portland cement concrete). While it is known that Portland-pozzolan concrete can exhibit higher flexural to compressive strength ratios than normal Portland cement at later ages, a design compressive strength of 4,225 psi (650x650/100) was deemed appropriate to start from. Although there are no current guidelines for the determination of required field compressive strength for runways employing Portland-pozzolan concrete, the American Concrete Institute (ACI) has developed such guidelines for building construction in the form of two empirical equations, as follows: f 'cr = f 'c + 1.34s and f 'cr = 0.9 f 'c + 2.33s where: f'c is the design compressive strength of Portland-pozzolan concrete, s is the standard field deviation (13 percent or about 550 psi for a 6 ½ sack mix of Portland-pozzolan concrete), and f'cr is the required field compressive strength of Portland-pozzolan concrete. Substituting: f 'cr = f 'c + 1.34s = 4,225 + 1.34(550) = 4,962 psi f 'cr = 0.9 f 'c + 2.33s = 3,802.5 + 1281.5 = 5,084 psi Accordingly, a 6 ½ sack mix (611 pounds of cementitious material per cubic yard of concrete) with compressive strength of 5,000 psi at 28 days was used for the Portland-pozzolan Concrete Trial Mix Designs. Corresponding field flexural strength was specified at 700 psi (square root of 500,000 = 707). In accordance with the previous discussion of Portland-pozzolan concrete, a high-range water-reducer and air-entrainment were also employed at the manufacturer's suggested rates. Course aggregate was specified to be 100 percent crushed. Results of the trial mix designs are illustrated on the following pages as "Flexural Strength vs Percent Fly Ash" and "Compressive Strength vs Percent Fly Ash". Bureaucratic Reality Despite 28-day flexural strength of about 960 psi for 40% replacement of Portland cement with fly ash, the Federal Aviation Administration's Airport District Office (FAA/ADO), in consultation with the FAA's Office of Aviation Research in Washington, DC, allowed only the current arbitrary limit of its specifications, 20% replacement. The 20% replacement mix exhibited 28-day flexural strength of about 860 psi. Increased Ultimate Strength The improved ultimate strength of Portland-pozzolan concrete has been known since the early 1900s. It is this improved increase in strength over time that allows design for initial 28-day flexural strength 700 psi and increase in flexural strength to 800 psi six months later. In fact, the 20% replacement mix design employed for Runway 13-31 Reconstruction exhibited 28-day flexural strengths of about 850 psi and 180 day flexural strengths of about 1,000 psi. These strengths correspond to an ability of Runway 13-31 to resist loading of dual wheel gear aircraft to 180,00 pounds after 28 days and 200,000 pounds after six months.
Lower Cost, Environmental Benefits, Non-destructive Testing The delivered cost of Class C fly ash in North Dakota is about $25 per ton. The delivered cost of Type I-II Portland cement is around $90 per ton. Every ton of fly ash used in Portland-pozzolan concrete reduces, by a ton, the requirement to dispose of fly ash as a waste. Each ton of fly ash used also reduces the need for Portland cement by a ton, along with the pollution associated with Portland cement production. A secondary benefit is the reduction in cost of electricity due to the reduced requirement (and associated costs) for disposal of fly ash. Destructive testing was not employed on this project. Surveying was performed, in excess of that normally required, to record actual pavement section thickness parameters. Destructive testing of base and sub-base was also avoided in favor of non-destructive methods of testing.
Pitfalls The use of a 100 percent crushed coarse aggregate was critical in achieving maximum ultimate strength. Since the strength of the cement bond eventually exceeds aggregate strength, the ultimate flexural strength limit of Portland-pozzolan concrete is a function of aggregate strength.
The fly ash must be supplied in dry form, just like Portland cement. Certification of the fly ash should include a statement guaranteeing it has not been prematurely wetted. Review of fly ash laboratory test results should also be performed to ensure that the fly ash maintains a uniform carbon content (LOI) to mitigate unacceptable fluctuations in entrained air. Testing with local aggregates, cement and fly ash should be performed prior to bidding to confirm concrete strength in accordance with the mix design. Finishing of Portland-pozzolan concrete should be kept to a minimum. A burlap drag finish with minimal edge finishing is recommended for both formed and slip-formed paving. Application of curing protection should begin immediately following placement and finishing while a "sheen" is still present on the concrete surface. As with Portland cement concrete, water must not be added to the surface to facilitate edge finishing. Water sprinkled on the surface can double or triple the water-cement ratio at the concrete surface, resulting later in unsightly spalling of the concrete surface. Because of its lower water-cement ratio, Portland-pozzolan concrete must be doubly protected from overworking and addition of water. The bid documents must apprise the contractor of the "tenderness" of Portland-pozzolan concrete, particularly during the first 24 hours. Premature saw cutting has a tendency to displace the aggregate (especially when rounded aggregates are employed), effectively destroying the concrete within a distance dependent upon the aggregate's maximum size. Additionally the saw blade will tend to eject a large amount of cement paste which will tend to adhere to the concrete surface nearby. Saw cutting may generally be delayed until from 13 to 20 hours after placement without risk of premature cracking because of the significant reduction in drying shrinkage afforded by Portland-pozzolan concrete. Early form removal can be destructive to the edges adjacent to the forms. Whenever possible, form removal should be performed from 18 to 24 hours after placement. Form removal should be followed immediately by the application of curing protection to the exposed concrete.
Information Gleaned from Stage I and Stage IIa Reconstruction of Runway 13-31 Results of the Trial Mix Designs indicated that the prediction of corresponding compressive strength of Portland-pozzolan concrete is approximated by the polynomial: C = 0.0061FxF + 1.172F + 19 where: C = Compressive strength, and F = Flexural strength. After substituting for F = 650 psi, C = 3,358 psi (not 4,225 psi). This value for compressive strength may then be substituted into the equations for determining required field strength, Substituting: f 'cr = f 'c + 1.34s = 3,358 + 1.34(437) = 3,943 psi f 'cr = 0.9 f 'c + 2.33s = 3,022 + 1,018 = 4,040 psi where: f'c is the design compressive strength of portland-pozzolan concrete, s is the standard field deviation (13 percent of 3,358 or about 437 psi), and f'cr is the required field compressive strength of Portland-pozzolan concrete. From the above, a 6 sack mix (564 pounds of cementitious material per cubic yard of concrete) with compressive strength of 4,075 psi at 28 days could have been used for the Portland-pozzolan Concrete Trial Mix Designs. Corresponding field flexural strength would likely have been specified at 725 psi (from above polynomial equation). The savings associated with reduction from a 6 ½ sack mix to a 6 sack mix amounts to the cost of 47 pounds of cementitious materials (9.4 pounds of fly ash and 37.6 pound of Portland cement) or about $1.81 ($0.12 + $1.69) per cubic yard of concrete. Reduced admixture costs would have added slightly to the savings. The paving contractor expressed concern with Portland-pozzolan concrete being placed during the month of October when temperatures can dip below freezing overnight. These concerns are likely a carryover from the days when Portland-pozzolan concrete (without high range water-reducers) was used only in mass concrete and other projects where slow strength development was an asset or where ample time for strength development allowed the economy of Portland-pozzolan concrete. The remaining stages of pavement reconstruction of Runway 13-31 were scheduled in early spring (mid-April through May) when similar cold weather concerns were just as likely to surface. Cold weather concerns are appropriate to both Portland cement concrete and Portland-pozzolan concrete with a high-range water reducer. Accordingly, it was believed to be prudent to use the same mix design (6 ½ sacks) for Stages IIa and IIb. Seven-day flexural strength for the Portland-pozzolan concrete was about 760 psi, well in excess of the 550 psi flexural strength required to reopen the runway. References: Fly Ash Facts for Highway Engineers, Report No. FHWA-SA-94-081), American coal Ash Association, Aug 1995 Standard Handbook for Civil Engineers, Frederick S. Merritt, McGraw-Hill Book Company, 1968 Design and Control of Concrete Mixtures, Twelfth Edition, Portland Cement Association, 1979 |
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