FINAL TECHNICAL REPORT

November 1, 1999, through October 31, 2000

 

Project Title:             EXTRUDED FIBER-REINFORCED CEMENT COMPOSITES CONTAINING FLY ASH

ICCI Project Number:            99-1/2.1B-2

Principal Investigator:               Dr. Surendra P. Shah, Northwestern University

Other Investigators:                  Dr. Alva Peled, Michele F. Cyr, Northwestern University

Project Manager:                     Dr. Ronald Carty, ICCI

 

ABSTRACT

 The major goals of the present research are to: 1) develop extrudable compositions of fiber-reinforced cement composites that contain substantial quantities of Illinois fly ash; 2) identify the best compositions for targeted products and 3) promote the most promising products for commercialization. The existing technologies and materials for producing precast elements are not suitable for this market. With further development, the extrusion technology can overcome these limitations.  This high value-added product is targeted for the industrial and residential construction industries, including applications such as roofing materials, exterior siding, floor tiles, and wall, ceiling, and floor panels.  This year involves development and testing of different compositions and specimen cross-sections.  Optimum material compositions and combinations of hybrid fibers are studied to enable tailoring of the composite performance.  The use of hybrid fibers enables a reduction in the cost and an improvement of the performance to take full advantage of the beneficial aspects of fly ash.  The effect of fly ash on the durability of composites was evaluated, as was the effect of fiber dispersion on composite performance.

 

In previous years, it was shown experimentally that fiber-reinforced cement composites containing large amounts of Illinois fly ash could be extruded.  As much as 80% by volume of the cement could be replaced by the Illinois fly ash to produce extruded fiber-reinforced composites with high strength and toughness. High-performance-fiber-reinforced composites were developed that can be used for lightweight elements for the industrial and residential construction industries. The flexural performance of extruded composites can be controlled and optimized with the use of hybrid fibers. The addition of polyvinyl alcohol (PVA) fibers to a glass and polypropylene (PP) hybrid composite significantly increases both the strength and the toughness of the composite and produces a strain-hardening response. Moreover, the long-term durability of the glass:PVA:PP hybrid composites is better than that of the glass fiber composites and the PVA fiber composites.  The addition of fly ash further improve the durability of most composites.  The composite cross-section was shown to influence its mechanical properties. Cellular cross sections with circular openings performed better than the those with square openings when tested dry; however, when tested saturated, the reverse trend was observed. 

 

EXECUTIVE SUMMARY

 

The overall goals of the present research are to develop and promote the use of extruded fiber-reinforced cement composite components for residential and industrial construction.  These products include exterior panels for industrial buildings, roofing materials (tiles, shingles, corrugated sheets), exterior residential siding, indoor-outdoor floor tiles, indoor wallboard, and decorative indoor wall, ceiling and floor panels.  This is accomplished by: 1) finding the optimal composition with the maximum allowable content of the Illinois fly ash; 2) evaluating hybrid fiber reinforcement, i.e., combinations of different types of fibers such as polypropylene (PP), glass, and polyvinyl alcohol (PVA) fibers, to reduce the cost and to tailor the composite performance for a specific application; 3) evaluating the durability of the composites to improve performance, especially of glass fiber composites, with the addition of fly ash and hybrid fibers; 4) optimizing the cellular cross-sections, which offer reduced weight and increased rigidity, to minimize element weight and improve performance; 5) identifying the best compositions, cross-section shapes, and long-term durability for targeted products in industrial and residential construction; 6) working closely with ACBM’s Industrial Affiliates to transfer this technology to the market.

 

Fibers are incorporated in the brittle cement matrix to control cracking by bridging the cracks, to provide high ductility, and to improve impact resistance and tensile and flexural strength.  The relationship between the elastic modulus of the fibers and the elastic modulus of the cement matrix (about 15-30 GPa) influences the mechanical performance of the composite.  Hybrid composites containing two or more different types of fibers can be considered to control the cost and optimize desired properties of the composite by taking advantage of the different properties of different types of fibers.

 

Extrusion is a processing technique that has been shown to impart high performance characteristics to fiber-reinforced, cementitious materials.⁽[1]⁾  Extrusion is a forming process in which a highly viscous, plastic-like mixture is forced through a die, a rigid opening of desired cross-section.  There is growing interest in the use of the extrusion process in the fiber-reinforced cement composites industry.  Extrusion yields a dense matrix with a strong fiber-matrix bond and good fiber alignment. It also allows greater flexibility in element shape than is possible with standard board manufacturing processes.  It is a potential candidate for low cost commercial applications, such as roofing tiles, flooring tiles, building panels, and pressure pipes. 

The extrusion technology has been shown to be superior to conventional production methods for cementitious composites in that the extruded composite is stronger, more ductile, making it better able to withstand hazards such as hail and wind load, and largely impervious to water penetration, offsetting freeze/thaw concerns.  The resulting composite has been evaluated by a major producer of industrial siding and found to be cost-competitive.  The existing technologies and materials for producing precast elements are not suitable for the residential construction market, since the products are too heavy and not cost effective.  This has prevented market penetration of these products.  With its lightweight, high value-added elements, extrusion technology can provide a more competitive product.

 

The proper viscosity (rheology) is essential for successful extrusion.  If the rheology is not ideal, defects can form during extrusion leading to reduced performance of the composite.  The round-particle morphology of fly ash improves the rheology of the extrudate, in addition to the ecological and economical benefits of its use in cement-based systems.  Partial replacement of cement with fly ash not only makes cement-based elements less expensive; it also reduces the total life cycle energy consumption by reducing the amount of cement needed.

 

A laboratory ram extrusion rheometer was used to extrude small-scale sheets and cellular cross-sections.  Specimens containing Illinois Class F fly ash as a replacement for cement were prepared with different fibers, PVA, glass, polypropylene, and hybrid fiber combinations.  In most cases, 70% by volume of cement was replaced by fly ash.  For comparison, specimens without fly ash were also prepared.  The fiber volume fraction (V_f) was 2% for PVA fibers and 5% for the glass, polypropylene and the hybrid fibers.  All these composites were extruded as 4 mm sheets.  PVA fiber composites with a fiber volume fraction of 3% and 60% by volume of the cement replaced by fly ash were prepared to study the different specimen cross-sections, including 4 mm and 8 mm thick sheets, a cellular cross-section with square openings, a cellular cross-section with circular openings and a cellular cross-section with square openings whose corners are rounded. 

 

After extrusion, the specimens were covered with a plastic sheet for 1-3 days.  Then they were steam cured at 90°C and 100% relative humidity (RH) for 2 days or moist-cured for 28 days in 100% RH at room temperature.  After removal from the curing environment, the specimens were stored at 22°C, 50% RH for 24 hours, dried at 105°C for 24 hours, and stored at 22°C, 50% RH for another 24 hours before testing in flexure. Moist-cured specimens were also tested under a saturated, surface-dry condition to study the effect of moisture content on the mechanical behavior.  These specimens were kept under moist curing conditions until immediately prior to testing.  To study the effect of fly ash on the long-term durability of the composite, glass, PP, PVA, and hybrid composites both with and without fly ash were stored in an 80°C water bath for 6 weeks, after being moist cured 28 days, to accelerate aging.  The mechanical performance was evaluated by determining the flexural strength and toughness of the composites using three-point bend tests.

 

It was found that mixtures containing substantial quantities of Illinois Class F fly ash are extrudable and provide enhanced flexural properties.

 

One of the main advantages of the extrusion technique is that a greater variety of cross-sections can be produced than is possible with the standard manufacturing processes.  One of the goals of the present work is to produce lightweight elements for precast applications.  For these applications, lightweight elements, such as cellular cross-sections are important to reduce the cost of transportation and erection. A variety of cross-sections were extruded using a number of different mixtures to optimize mixture design and cross-section performance.  It was found that the composite cross-section influenced its mechanical properties with sheets generally performing better than cellular cross-sections. Cracking at the corners of the cellular specimens tested dry was observed with a scanning electron microscope (SEM). Cellular cross-sections with circular openings performed better than those with square openings when tested dry.  However, when the specimens were tested wet, the square opening performed better than the circular opening.  The drying process may increase cracking at the corners of the cross-section leading to a reduction in mechanical performance. The circular opening may improve the mechanical performance of the composite, but it also increases the unit weight of the element, a critical consideration for precast panels. In an effort to reduce the element weight while improving performance, cellular cross-sections with square openings whose corners are rounded were extruded.  The unit weight of this cross-section was similar to that of the plain square openings, but after a 2-day steam cure, the performance was similar as well.  Therefore, further modification of the die design is needed to minimize cracking and reduce the element density.

 

The flexural performance of extruded composites can be controlled and optimized with the use of hybrid fibers.  A hybrid composite performs better than composites reinforced with each of its constituent fibers, indicating that hybrid fiber composites do allow for tailoring of composite performance and improved performance over individual fiber composites.  Hybrid composites containing glass and polypropylene fibers showed a significant drop in load-carrying capacity at a relatively small deflection after the glass fibers failed. Even with a minimal amount of glass fibers, the drop in strength exhibited by the composite was not desirable.  The addition of PVA fibers to these glass/PP hybrid composites significantly increases both the strength and the toughness of the composite and produces a strain-hardening response.  The primary drawback of PVA fibers is that they are significantly more expensive than polypropylene fibers. A 40:40:20 glass:PVA:PP ratio composite with a total fiber volume fraction of 5% was found to be the optimal hybrid combination because it gave high performance (similar to a 40:60 glass:PVA hybrid composite) with relatively low cost.  SEM analysis was used to examine the fracture surfaces of the plain and hybrid composites to determine if there were differences in microstructure that might explain the improved performance of the hybrid composite.  However, no significant differences were observed among the composites.

 

All hybrid composites were tested with 70% of the cement in the matrix replaced with fly ash.  The most successful hybrid composites, 20:80 glass:PP and 40:40:20 glass:PVA:PP, were also tested without fly ash.  The presence of fly ash improved the flexural performance of the glass and polypropylene composite, however it did not have a significant effect on the performance of the glass, PVA and polypropylene composite. Similar trends were obtained for steam-cured specimens and moist-cured specimens.

 

In general, composites without fly ash are more adversely affected by aging.  The glass and glass:PVA:PP hybrid composites with fly ash show little change in strength but a significant reduction in toughness with aging.  Without fly ash, these composites show a reduction in strength and a much greater reduction in toughness.  The polypropylene fiber composite with fly ash shows significant increases in both strength and toughness with aging.  These increases are much less when fly ash is not present. The increase in strength and the reduction in toughness may be explained by an improvement in fiber-matrix bonds with aging, as the fibers fracture instead of pulling out at failure. The increase in bond strength may be a result of a decrease in porosity with aging.  This decrease was observed in mercury intrusion porosimetery and SEM analysis of the glass fiber composite. Because the polypropylene fiber composite has a relatively poor fiber-matrix bond, the improvement in bond strength with aging is not great enough to cause fibers to fracture, which may explain the increase in strength and toughness with aging. The durability of a composite can be controlled by hybrid fiber combinations and, in most cases, improved with the addition of fly ash.

 

Glass fiber composites were prepared with bundled and dispersible glass fibers to examine the effect of fiber dispersion on composite performance.  The dispersible fiber composite performed much better than the bundled composite at a lower fiber volume fraction.  Good fiber dispersion may improve composite performance because the fiber-free, or unreinforced, areas of the matrix are smaller.  The addition of fly ash further enhanced the improved performance of the dispersible glass composite, suggesting that fiber dispersion may be aided by fly ash.

 

This project has attracted commercial interest.  A Colorado based start-up company, which is engaged in developing the concept of modular concrete housing, is interested in using extruded composites as its construction modules.  ACBM has been working closely with this company to develop an extrudable mixture using a new rapid-setting cement.  A significant amount of Illinois Class F fly ash is added to achieve an extrudable mixture.

 

Summary and Conclusions The flexural performance of extruded composites can be controlled and optimized with the use of hybrid fibers.  Increasing the glass fiber content while reducing the polypropylene fiber content increases the hybrid composite strength over the polypropylene fiber composite.  The addition of PVA fibers to a glass/PP hybrid composite significantly increases both the strength and the toughness of the composite and produces a strain-hardening response.  The 40:40:20 glass:PVA:PP ratio composite was found to be the best combination because it gave high performance (similar to a 40:60 glass:PVA hybrid composite) with relatively low cost.  A composite with cellular cross-sections was successfully extruded to produce lightweight elements for precast applications. It was found that the composite cross-section influenced its mechanical properties.  The shape of the opening of a cellular cross-section as well as the moisture content of the element affects the mechanical performance.  A circular opening performs better than a square opening when the composite is tested dry, but a square opening performs better than circular when tested wet.  Hybrid fiber combinations can control the durability of a composite, and the addition of fly ash can make them more durable. Fiber dispersion can have a significant effect on performance.  It can be concluded that the replacement of cement with fly ash is beneficial for extruded fiber-reinforced cement composites.