Master of Science in Engineering (MSE)
Civil and Environmental Engineering and Construction
First Committee Member
Second Committee Member
Jin O. Choi
Third Committee Member
Fourth Committee Member
Number of Pages
The Center for Sustainable Transportation Infrastructure (CSTI), in its 2012 study, estimated that out of all the concrete leaving the plant, between 2% and 7% of concrete returns to the plant unused as a returned fresh concrete (RFC). Disposal of both the truck wash water and RFC is a growing concern for the industry. Most industry personnel contacted during the investigation, agree that reusing is superior to recycling of this substantial RFC economically and environmentally. This study will determine if the reuse of RFC in subsequent batches compromises the quality of newly blended concrete.
The effect of RFC on fresh and hardened characteristics of subsequent batches was studied. This research will be performed in a laboratory where setting time and compressive strength will be tested for both the control and blends of varying proportion and age of plain or retarded RFC with subsequent fresh batches.
This study will discover the C1798/C1798M-16 (Standard Procedure for reusing returned fresh ready-mixed concrete) recommendation. In this procedure, it is stated RFC up to 8-hours old at 100F, treated with hydration stabilizing admixture, can be blended in up to 50% proportion with a new batch of RMC without adversely affecting the fresh and hardened characteristics of the blend. The reuse of RFC has been neither explicitly banned nor allowed by end users due to the uncertainties of the effects of the RFC on the properties and characteristics of the blend. This standard procedure has not encouraged the end users to reuse of RFC, despite its liberal allowance of reuse of RFC. Not only that, the prohibitive practices of the states of California and Iowa experience in the reuse of RFC and the limitations set by ASTM C94 reinforces the negative perception surrounding the reuse of RFC in concrete blends.
A commonly used, Clark County qualified mix design No. 101, was batched both indoors and outdoors. Each batch was tested shortly after batching as a control sample. The concrete was then held for 1hr, 2hrs, 3hrs or 4hrs to simulate RFC. The simulated RFC was then mixed with newly batched concrete in various proportions. Both the control and blends were tested for slump, air entrapped, unit weight, setting time, and compressive strength as per ASTM standard and specifications. Thirty samples blended with indoor batched RFC and other 40 samples blended with outdoor batched RFC were tested in this investigation. The test results of this investigation showed that, for the mix design 101, retarded RFC up to three-hour-old and plain RFC up to two-hour-old can be used in 30% and 20% proportions respectively without affecting the fresh and hardened characteristics of subsequently blended concrete.
Returned Fresh Concrete; Reuse; Sustainablity
Civil Engineering | Sustainability
Gebremichael, Negasi Niguse, "Investigation of Setting Time and Compressive Strength of Ready-Mixed Concrete Blended with Returned Fresh Concrete" (2017). UNLV Theses, Dissertations, Professional Papers, and Capstones. 3130.