A framework for engineering sustainable composites based on time-dependent material properties and environmental impact assessments: an application to bio-based composite design

Improved design measures for civil engineering materials are necessary to reduce the environmental impact of the built environment. Over the last century buildings have been one of the largest consumers of materials. Due to growing material demands in the construction industry associated with increased global population and economic demands, it is imperative that research on designing materials use sustainability metrics in conjunction with performance metrics. However, little research has been conducted on developing design methodologies to incorporate sustainability metrics in the field of sustainable civil engineering material design. Rather, most recent advances have been associated with comparative analyses of existing materials and typically lack consideration of use-phase properties in the environmental impacts. By examining the influence of constituent properties on composite materials, this dissertation focuses on linking environmental impact, material durability, and composite constituent selection through a unique design method.

The design procedure consists of three fundamental steps for improved material design: (1) consideration of a base domain of alternatives for composite constituents and characterization of these alternatives in terms of mechanical and time-dependent properties through experimental testing; (2) environmental impact assessment and consideration of material improvements through life cycle analysis; and (3) application of mechanical and time-dependent properties to environmental impact modeling to refine desired alternatives for assessment in step (1).

This thesis applies the design method through application to a class of bio-based
composites, composed of a biosynthesized polymer and varying natural fibers, which
offers a potentially lower environmental impact material option for the construction
industry. In this research, characterization of mechanical properties and environmental
impact properties of these composites as well as improvements in composite design
were considered through manipulations in composite reinforcement and production
techniques. By extending theories from mechanical design and life cycle analysis,
initial property comparisons for the influence of these manipulations and for the
influence of base units for comparison were made. To incorporate durability
performance metrics, this research examined creep deformation behavior, which is a
critical time-dependent material property for structural load bearing applications and
time-dependent material serviceability. Creep behavior was incorporated into life
cycle analysis and allowed for assessment of environmental impacts associated with
material quantities needed to maintain necessary material functionality.

The results of the design method proved effective: through an integration of the
analyses conducted, desirable constituents can be selected and processing methods can
be refined. While this research is applied to bio-based composites, the principles
developed are applicable to green engineering of any composite material. The iterative
design procedure presented can act as a springboard to new research in improved
analysis and design techniques for composites.