FINAL TECHNICAL REPORT

September 1, 1996, through April 30, 1998

Project Title: FIELD STUDIES OF ILLINOIS PCC BOTTOM ASH FOR STRUCTURAL GRADE CONCRETE PAVEMENTS

ICCI Project Number: 96-1/3.1A-26

Principal Investigator: Nader Ghafoori, Southern Illinois University at Carbondale

Project Manager: Daniel Banerjee, ICCI

ABSTRACT

In the past four years, Southern Illinois University at Carbondale (SIUC), under the sponsorship of Materials Technology Center, has performed a series of laboratory research into engineering properties and long-term durability of vibratory-placed and roller compacted structural grade concretes containing bituminous pulverized coal combustion (PCC) bottom ash. These laboratory efforts have resulted in identification of a number of potentially viable commercial applications for PCC bottom ash by-product residues.

One potential and promising application of the Illinois PCC solid waste residues, which also accounts for the large utilization of coal-based by-product materials, is in pavement construction. The proposal presented herein is intended to embark on a new endeavor in order to bring the commercialization aspect of the initial laboratory project a step closer to reality by conducting a field demonstration of the optimized mixtures identified during the four-year laboratory investigation. Eight different pavement slabs (each 24 ft wide by 24 ft long) were constructed at an identified site located in the Illinois Coal Development Park, Carterville, by two construction contractors who were part of the industrial participants of the initial projects. All sections were subjected to an extensive engineering evaluation and monitored for approximately two years for short, medium, and long-term performance. The field results were compared to that of the equivalent laboratory-prepared mixes in order to ascertain the suitability of the proposed PCC bottom ash concretes for field paving applications. Field cores taken from paving slabs were also tested for leaching of solid waste to obtain an aqueous solution which are 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 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 program is to examine the performance of Illinois PCC bottom ash structural grade concrete pavements by conducting a demonstration project at a designated site in Carterville, Illinois. The major tasks assigned to this field study fully examine the engineering characteristics, long-term performance, and environmental issues involved in large-scale utilization of Illinois PCC bottom ash surface course highway pavements.

An experimental two-lane road, consisting of eight different vibratory-placed concrete slab sections, was constructed. Each 24 x 24 ft slab was 8 inches deep. In addition to longitudinal joints, shrinkage/construction joints were provided at 24 ft intervals. Dowels were used to transfer loads between slabs. A portion of the experimental road consisted of six solid slab sections. Three of them were made with 100% PCC bottom ash as a fine aggregate component of concrete. The remaining three solid sections were the control concretes made with 100% natural siliceous fine aggregate. There were two sandwich sections made with 4 in. deep of a richer concrete encased between 2 in. leaner mixtures. Cement factors of 550, 650, and 750 lb/yd³ were used. Concreted were prepared with a uniform consistency of 4 1/4 inches and an air-entrainment of 6 1%. All trial mixtures possessed an identical volume of solid particles and were prepared with a constant slump, water-cement ratio, and air content. However, to achieve these uniform characteristics, the amount of added water-reducing and air-entraining admixtures had to vary among mixes. The dry components and proportion of each field and laboratory mixture are shown in Tables 1 and 13, respectively, along with the actual water-cement ratio and the amount of admixtures utilized. Once consolidation and final finishing were completed, slab sections were sprayed with curing compound and covered with a plastic sheet for a period of 7 to 10 days prior to opening to vehicular traffic flow.

Short-Term Properties of Field Paving Slabs - Table 2 documents the fresh properties of paving slabs. For a practically similar water-to-cement ratio and workability, the amount of bleeding water for the PCC bottom ash concrete was nearly 90% lower than that of the control mix. This is explained through the finer PCC bottom ash particles which attached themselves to the cement particles, reducing the channels for bleeding and leaving very little free water available in fresh concrete for bleeding. Table 2 also reveals that inclusion of PCC bottom ash improved initial and final setting times when compared to that of the control concretes. When the fine aggregate portion contained 100% bottom ash, the initial and final setting times of the field mixtures decreased, over those of equivalent control mixes, by 17 and 20% respectively. Increases in cement factor reduced the initial and final setting times of both PCC bottom ash and natural fine aggregate concretes in an approximately linear fusion. The peak adiabatic temperature rise and the corresponding elapsed time for both PCC bottom ash and reference field mixes are shown in Table 2. Test results indicate that all mixtures under consideration produced peak temperature at a rate similar to each other. The number of hours elapsed to reach the maximum temperature was also similar for PCC bottom ash concretes and reference mixtures. And finally, Table 2 displays the measured air content of the field freshly-mixed concretes which remained within the intended range of 6 1%.

Medium-Term Properties of Field Paving Slabs - As shown in Table 3, the unit weight of the PCC bottom ash concrete were slightly below that of the reference paving slabs. However, they remained in the range typically seen for normal weight concrete. Table 3 also documents wet (soaked) and air-dry compressive strengths of both PCC bottom ash and natural fine aggregate (control) concretes. The PCC bottom concretes gained 75.6% of its 28-day compressive strength in the first seven days after casting, and the 28-day compressive strength was exceeded by an average of 19 to 28% for the pavement ages of 91 and 180 days, respectively. The strength development of the control slabs followed a similar pattern, averaging 80% of the 28-day compressive strength by the first week. At the end of 91 and 180 days, additional gains of 21 and 31% were recorded over that observed after 28 days of pavement age. Under air-dry conditions, the compressive strength of the PCC bottom ash and natural sand concretes was nearly 11% higher than that obtained under soaked conditions. When compared against the control field slab specimens, the PCC bottom ash concrete exhibited a 9.2%, 10.41%, and 16.93% strength gain for the mixtures containing cement content of 550, 650, and 750 lb/yd³, respectively.

The splitting-tensile strengths of the PCC bottom ash and natural sand paving slabs are illustrated in Table 4 for both wet and air-dry conditions. The splitting-tensile strength improvements, over that of the reference concrete, were approximately 2.5%, 15.7%, 14.8%, 9.5% at pavement ages of 7, 28, 91, and 180 days, respectively. The splitting-tensile resistance under air-dry conditions was 9.5% and 12.6% higher than those tested in wet conditions for the PCC bottom ash and natural sand concretes, respectively. The splitting tensile-compressive strength ratios were in the range of 0.101 - 0.121 for the PCC bottom ash slabs, reproducing the results obtained for the control concretes (0.993 - 0.128).

The progression of flexural strength with respect to cement content and curing age is shown in Table 5. The flexural strength ratio of the PCC bottom ash to natural sand concretes was 1.08, 1.09, and 1.22 for the mixtures containing 550, 650, and 750 lb/yd³ portland cement, respectively. The 91-day flexural strength exceeded the 28-day result by nearly 11 and 11.2% for the PCC bottom ash and reference mixes.


Table 6 documents the shrinkage strain of the test mixtures containing 550, 650, 750 lb/yd³ portland cement. The drying shrinkage of the paving slabs increased with time and stabilized after roughly 180 days from the time of initial casting. PCC bottom ash slabs displayed higher ultimate drying shrinkage strains (30.8, 17.6, and 17.7% at cement factors of 550, 650, 750 lb/yd³, respectively) than the equivalent paving sections made with control concretes.

Long-Term Properties of Field Paving Slabs - The results of accelerated chloride permeability tests are shown in Table 7. The control mixtures allowed, on average, 970% higher current flow than the PCC bottom ash concretes. All reference slabs are considered moderately permeable, whereas the PCC bottom ash paving sections can be classified as having very low permeability to chloride.

The resistance to abrasion of the PCC bottom ash and natural sand concretes (for the saw-cut surface of the cross section) is shown in Table 8 for the mixtures containing 550 through 750 lb/yd³ portland cement. The abrasion resistance (via depth of wear) of the control mixtures improved by 9.7 and 23.8% when cement content increased from 550 to 650 to 750 lb/yd³, respectively. The PCC bottom ash concretes also exhibited a similar trend, and resistance to abrasion improved by 12.2 and 23.4% when the same cement contents were utilized. The average depth of wear for the bottom ash mixes was nearly 18% higher than that of the natural sand (control) concretes.

The absorption, an indirect measure of moisture conductivity, of both PCC bottom ash and natural sand slabs is shown in Table 9. The higher amount of fines in PCC bottom ash, as compared to that of the natural sand, provided lower absorption for the bottom ash concrete slabs. The 7-day absorptions of the PCC bottom ash concrete was lower by nearly 30% than that of the equivalent control mixes.

The resistance to freezing and thawing with deicing salts expressed in mass loss and surface rating of the field paving slabs is shown in Table 10. Although the impermeability of the bottom ash concretes surpassed that of the control mixes, the porous nature of the less-dense PCC bottom ash aggregate and the higher water-cement ratio resulted in a greater resistance to freezing and thawing with deicing salts for the natural sand concrete slab sections.

As documented in Table 11, all field specimens completed 300 freezing and thawing cycles with the lowest durability factor recorded at 95.4%. The PCC bottom ash concretes exhibited a similar resistance to rapid freezing and thawing when compared to that of the control field samples. Although the failure criterion (relative dynamic modulus of elasticity of 60%) was never reached, a moderate amount of surface scaling was found (after 300 freezing and thawing cycles) in the bottom ash specimen containing a low cement content of 550 lb/yd³. The mass loss and surface rating of all test specimens taken after 50 freezing and thawing cycles, up to 300 cycles, are shown in Table 11.

Leachate Studies - The results of ASTM shake tests for portland cement, PCC bottom ash, and field paving slabs are reported in Table 12. In general, all paving slabs under consideration, complied with the requirements of Class I and II of IEPA Groundwater. However, elements T1 and Sb recorded noise levels and they may be closely examined under (1) recalibration of ASTM shake test or (2) graphite furnace atomic absorption test.

Fresh and Hardened Properties of Laboratory-Made Specimens - the matrix components and proportions, and the resulting water-to-cement ratios are shown in Table 13. The results of the tests conducted for fresh and hardened properties are documented in Tables 14 and 15, respectively, for the PCC bottom ash and natural sand concretes.

*Control Pavements                  S = Sandwich sections                 T, B, & M = top, bottom and middle

*Surface Rating ( ):  0- No scaling, 1- Very Light Scaling (1/8 in. Depth, Max, no coarse aggregate visible), 2- Slight to Moderate Scaling, 3- Moderate Scaling (some coarse aggregate visible), 4- Moderate to Severe Scaling, and 5- Severe Scaling (coarse aggregate visible over entire surface)

**(Rating of Middle Section, Rating of Outer Section)

*Surface Rating ( ):  0- No scaling, 1- Very Light Scaling (1/8 in. Depth, Max, no coarse aggregate visible), 2- Slight to Moderate Scaling, 3- Moderate Scaling (some coarse aggregate visible), 4- Moderate to Severe Scaling, and 5- Severe Scaling (coarse aggregate visible over entire surface)

- None, ND = Not Detectable, *Noise Level, and NA = Not Available

*Control Mixtures