Use of coal combustion by-products (CCBs) and waste foundry sands in flowable fill

Subrahmanya T Bhat, Purdue University

Abstract

The purpose of the present study is to investigate the use of waste foundry sand in flowable fill. However, the concepts developed are applicable to flowable fills containing any type of sand. Even though the flowable fill is presently popular as a trenchfilling material, this study addresses a much broader perspective in order to be able to develop uses of this material in geotechnical applications. Since sand is the major component of flowable fill, replacing sand with a waste material may be beneficial from an economical as well as an environmental point of view. One such waste material produced in large quantities is waste foundry sand, which is a by-product of metal casting industries where sand is used for making molds and cores. Green sands from ferrous foundries in which clay and water are used as binder materials, are mostly non-hazardous. In this research, only green sands were taken. The fly ash used was class F type. Flowability, hardening characteristics and 28-day unconfined compressive strength are the important aspects to be considered in the design of any flowable fill. Flow curves representing fly ash content vs water-solid ratio were developed. The flow curves help explain the mechanics of flow, and also help in choosing the proportions for an ideal mix, which produces a denser, less porous material with minimal amounts of cement. The 28-day unconfined compressive strength was correlated with the water-cement ratio. A simple mix design procedure was developed based on flowability and strength criteria. The long-term strength beyond 28 days was also studied. The hardening behavior was evaluated by a mortar penetrometer. The penetration resistance was correlated with the unconfined compressive strength estimated by a soil pocket penetrometer. The penetration resistance necessary to ensure walkability on flowable fill was estimated to be 450 kPa(65 psi). The time required to achieve this penetration resistance is defined as "walkable time", and this can be used to compare the hardening behavior of different mixes. The pore structure of hardened flowable fill was studied using mercury intrusion porosimetry technique. The permeability characteristics were studied using constant head permeability testing. One of the objectives of the research was to compare the behavior of this material with that of soil. Consolidated drained and undrained triaxial tests were conducted which led to a better understanding of the stress-strain-strength behavior. The environmental aspect was studied by a bioassay toxicity test. The pH of pore solution offer 28 days was also studied. An accelerated strength test was developed using a hot water bath technique to predict the 28-day unconfined compressive strength.

Degree

Ph.D.

Advisors

Scholer, Purdue University.

Subject Area

Civil engineering|Environmental science|Environmental engineering

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