FINAL TECHNICAL REPORT

June 15, 1998 through January 1, 2000

 

Project Title:     LARGE-SCALE FIELD UTILIZATION OF ILLINOIS PCC BOTTOM ASH FOR ROADWAYS AND PARKING LOTS OF A YOUTH, SPORT, SAFETY COMPLEX

 DCCA Project Number:  96-205103 (CRG-270)

Principal Investigator:       Nader Ghafoori, Southern Illinois University

Project Manager:             Wayne Bahr

 ABSTRACT 

In the past five years, Southern Illinois University at Carbondale (SIUC), under the sponsorship of Materials Technology Center and Illinois Clean Coal Institute (ICCI), has conducted a series of laboratory studies and  a small-scale field demonstration project to obtain engineering characteristics and durability of vibratory-placed and roller compacted structural-grade concretes containing bituminous pulverized coal combustion (PCC) dry bottom ash.  These efforts have provided promising results and identified a number of potentially viable commercial paving applications for Illinois PCC dry bottom ash residues.

In view of prior promising laboratory investigations and a small-scale demonstration project, the investigation presented herein was intended to embark on a final endeavor in order to bring the commercialization aspect of the initial research program a step closer to reality by conducting a comprehensive large-scale field study.  Nearly 140,000 ft2 (surface area) of a youth, sport, safety complex were used.  An entrance/access road, parking lots, and a children safety village belonging to this facility were paved using a pre-identified optimum PCC dry bottom ash structural-grade concrete by two contractors who have expressed an interest in the construction of the project.  The youth, sport, safety complex was located on the campus of the University of Illinois at Springfield, Illinois.  The PCC dry bottom ash concrete pavements were extensive engineering studies and monitored for approximately two years for both short-, medium-, and long-term performance.  In-depth and comprehensive destructive and non-destructive evaluations were conducted in order to demonstrate the viability of Illinois PCC dry bottom ash as a suitable filler/binder fine aggregate for the concrete/paving construction industry.  The field results were also compared, through destructive and  non-destructive testings, with an equivalent field control concrete (made with conventional natural fine aggregate) test pavement and a companion laboratory-made mixture.  Field cores, taken from paving slabs were tested for leaching of solid waste to obtain aqueous solutions which were used to determine the materials that could leach into groundwater under the specific testing conditions.  The results were then compared with the requirements of Class I and Class II of IEPA Groundwater Quality Standards. 

EXECUTIVE SUMMARY

Most modern pulverized-coal boilers have dry bottom furnaces; that is, the ash is intended to be removed as a dry solid before complete melting occurs.  The PCC units operating within the state of Illinois are all dry boilers.  These furnaces generally have open grates at their bases.  Below the open grates there is generally a water-filled ash pit designed to receive the ash from the furnace.  Although a small amount of molten slag will form on the internal walls of the boiler and find its way into the ash pit, a large portion of the dry bottom ash is collected in a dry state. By collecting the ash in a dry state, the physical properties of dry bottom ash are quite different from wet bottom slag.  The dry bottom ash has the appearance of natural sand and, when examined under magnification, the spherical nature of these particles appear to be internally porous rather than externally porous.  In addition, the predominant material is light in color and has a sand paper-like surface texture.  The dry bottom ash is also lighter in weight than the slag bottom ash and, thus, generates a lighter product.

 The objective of this research program was to examine the performance of Illinois PCC bottom ash structural-grade concrete pavements by conducting a large-scale field project at a designated site on the campus of the University of Illinois at Springfield, Springfield, Illinois.  The field project consisted of paving 140,000 ft2 of a youth, sport, safety complex which included an entrance/access road, parking lots, a children safety village, and a slab-on-grade for a storage building.  The major tasks assigned to this field study fully examined the engineering characteristics, long-term performance, and economical and environmental issues involved in large-scale utilization of Illinois PCC bottom ash structural-grade concrete pavements.

This final report is intended to review the milestones of the project and present a discussion of the results presented in the nineteen previously submitted monthly reports.

The matrix constituents, which were used in the preparation of pavements, include PCC bottom ash, natural fine and coarse aggregates, ASTM Type I Portland cement, water, and chemical admixtures.

During the second reporting month and after a site visit was made to Dullman’s ash pond, it was determined that the required amount of the bottom ash would not be available on time.  The existing piles, although not ideal, had to be used as a source of bottom ash for use in concrete mixtures.  During the third reporting month, sieving operations produced sufficient bottom ash to complete the paving areas.  Site preparation, including grading, leveling and compacting, was also completed.  Due to some paperwork delays by the purchasing office of SIUC, Carbondale, the actual construction of roadways and parking lots began on September 29, 1998.  Nearly 3500 yd3 concrete of different matrix constituents was used to pave 140,000 ft2  of a youth, sport, safety complex which included an entrance/access road, parking lots, a children safety village, and a slab-on-grade for a storage building.  The step taken to complete the construction phase of this project were documented in the fifth monthly report.  The construction of the entire project was completed during the first ten days of December 1998.

There were three distinct mixtures, namely “A”, “B”, and “C.”   Concretes “A” had 100% PCC dry bottom ash for their fine aggregates.  Mixtures “B” had equal volume of PCC dry bottom ash and natural sand as a fine aggregate component of the matrix.  Groups “C” were reference concretes.  The availability of greater quantity of fines in the PCC bottom ash made it impossible to achieve the desired level of air content within freshly-mixed concretes of groups “A” and “B”.  This factor, in turn, adversely impacted the resistance to freezing and under accelerated laboratory testings for the samples of the said mixtures.  Fresh characteristic of the trial mixtures (including consistency, bleeding, initial and final setting times, air content, and adiabatic temperature) were examined. For a practically identical water-to-cement ratio and workability, the amount of bleeding water for the PCC bottom ash mixtures were significantly below that of the control mix.  This is explained through finer bottom ash particles, which reduced channels available in concrete for bleeding.  The experimental results also reveal that the inclusion of PCC bottom ash reduced initial and final setting times.  When the fine aggregate component contained 100% bottom ash, the initial and final setting times of the field pavements decreased, over those of the equivalent mixes with 50 and 0% PCC bottom ash, by nearly 8 and 13%, and 10 and 16%, respectively.  The peak adiabatic temperature rise and the corresponding elapsed time for all three mixture proportions were nearly identical. The availability of greater quantity of fines in the PCC bottom ash made it impossible to achieve the desired level of air content within freshly-mixed concretes.

During the 9th and 10th monthly reporting periods, over 300 cylinders and 100 prisms were cored.  They were transported to Carbondale, sized in the laboratory, and subjected to different testings to obtain their bulk and long-term characteristics.  The cored locations of the roadways and parking lots were filled with the concrete of the mixture “C”.  The experimental program included: compression (ASTM C 39), splitting tension (ASTM 496), static flexure (ASTM C 78), flexural fatigue, modulus of elasticity and Poisson’s ratio (ASTM C 469), drying shrinkage (ASTM C 157), abrasion (ASTM C 779, Procedure C, ball bearings), chloride permeability (AASHTO T 127), external sulfate resistance (ASTM C 1012), and rapid freezing and thawing (ASTM C 666, Procedure A).

The unit weight of the PCC bottom ash concretes was slightly below that of the reference paving slabs.  However, they remained in the range typically expected for normal weight concrete.

Field specimens were also tested at different ages (from 7 to 180 days) under two distinct testing conditions (wet and air-dry) for compression, (4 x 8 inch cylinders), splitting-tension (4 x 8 inch cylinders), and flexure (8 x 8 x 26 inch prisms).  Under both testing conditions, the PCC bottom ash concretes (mixtures "A” and “B”) exhibited higher strength properties than that of the control mixtures.  Highest strength characteristics were obtained for the mixtures containing both PCC bottom ash and natural fine aggregate as a fine component of the matrix (mixture “B”).  The splitting tensile-compressive strength ratios and flexure-compression ratios for mixtures “A” and “B” reproduced the results obtained for the control concretes.

 During the course of this investigation, the cylindrical cores for roadways were subjected for rapid chloride permeability using AASHTO T-277.  The test samples had nominal dimensions of 4 x 2 inches. All three mixtures, A, B, and C, were rated as moderately resistant to rapid chloride permeation.

The beam-shaped cores obtained from various paving slabs were also tested for resistance to freezing and thawing using ASTM C 666, Procedure A. The results of the accelerating laboratory tests were quite discouraging, even though field slabs have not shown any signs of deterioration stemming from freezing and thawing cycles of the winter climate.  The inferior performance of group “A” may be attributed to the excess fines existed in the PCC dry bottom ash which, in turn, made the air-entraining process extremely difficult. However, the most disturbing results were of group “C” of the road paving slabs.  Upon visual examination of the deteriorated specimens, it was clearly evident that the reference concrete had some bottom ash in it.  This could have only occurred during blending at the ready-mix plant.  The results for group “C” of the parking lot further corroborate this claim.

 During the investigation, the beam-shaped cores for various paving slabs were also evaluated for resistance to abrasion using ASTM C799, Procedure C, ball bearings. The depth of wear increased with time in all testing samples.  The bottom ash concrete (group A) displayed the least resistance to abrasion when compared with the control mix (group C) and concrete containing 50% bottom ash by volume (group B).  The majority of coefficient of variations, a measure of reproducibility, were well within the allowable limit of 17%.  The coefficient of variation also decreased with time which is in line with the expected performance when the ball-bearing abrasion test is used.

The Portland cement, PCC dry bottom ash, and sample cores of paving slabs from roadways and parking lots were tested for leachate and pH studies.  In general, all paving slabs under consideration complied with requirements of Class I and Class II of IEPA Groundwater.  The pH of all core specimens remained in the range not conducive for corrosion of steel reinforcements.

 The resistance to external sulfate attach of roadway specimens corresponding to groups A, B, and C were studied.  At the end of six months, both mixtures “A” and “B” exhibited higher resistance to sulfate attack when compared to that obtained for the reference concrete.  In addition to the results reported above, field cores were evaluated for possible mass loss once immersed in a sulfate-rich environment. No mass loss was detected during the six months test period.