| Summary: | An empirical response surface method (RSM) was used to evaluate the effect of key parameters on the properties of controlled low-strength material (CLSM) mixes produced with unprocessed-fly ash (u-FA) and recycled fine aggregate (RFA) was conducted. Design-Expert software was used to develop the design matrix and analyze experimental results. Three key design parameters were selected to derive mathematical models for assessing plastic and hardened properties. For the design and testing of twenty CLSM mixes, water-to-total cementitious (w/cm) ratio (2.53–2.73), u-FA replacement contents (50%–100%), and total binder contents (160–200 kg/m3) were used. The parameters and models' influences were tested to assess and improve their effects, including flowability, bleeding, segregation, initial stiffening time, water absorption, fresh density, air-dried density, oven-dried density, and 3, 7, and 28-day unconfined compressive strengths. Three optimum fly ash CLSM (u-FA-CLSM) mixes with high statistical desirability were designed and simulated using the developed models. As a result, robust u-FA-CLSM mixes that satisfy the Department of Transport (DOT) CLSM backfill criteria were made successful. In addition, the projected mix design models were revealed as valuable methods for recognizing the relationships between the mixing parameters that significantly affect u-FA-CLSM properties. This concept can shorten the process of the mix design and the testing involved because the model knows the relative importance of each parameter and offers valuable data to enhance the design of the mix, thus reducing the effort necessary to optimize CLSM mixes and promising a balance between the parameters that affect the plastic and hardened properties. For instance, 3-D response surfaces adoption is provided to illustrate the single and coupled impacts of mixture parameters on measurable properties, thereby highlighting the value of models. The u-FA-CLSMs minimize construction pollution, enhance design solutions and innovations, prolong structure service life, facilitate construction industry sustainability, and allow for lower cement content, resulting in cost savings. © 2022 The Authors
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