Investigation of setting time and compressive strength of ready-mixed concrete blended with returned fresh concrete

Abstract

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 iv 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 twohour-old can be used in 30% and 20% proportions respectively without affecting the fresh and hardened characteristics of subsequently blended concrete. v LIST OF ABBREVIATIONS ASTM-American Society for Testing and Materials AASHTO-American Association of State Highway and Transportation Officials SSR&BC -Standard Specification for Road and Bridge Construction USS (Clark County) -Uniform Standard Specifications Clark County Area RMC -Ready-Mixed Concrete RFC -Returned Fresh Concrete...

Fresh and hardened properties of cement mortars using marble sludge fines and cement sludge fines

“…This accounts for huge amounts of waste as the annual global consumption of concrete exceeds 10 billion tons [10]. The returned fresh concrete has several possible uses, including immediate recycling or use as a source for recycled concrete aggregates [11][12][13], however, the material adhered to the inside of the barrel is washed and stored in sedimentation tanks, as it has high alkalinity [14]. The alkaline water from the sedimentation tanks finds some use replacing fresh water in concrete production [15][16][17] and the remaining material, named cement waste slurry or cement sludge fines (CSF) finds little or no use.Both MSF and CSF consist of fine particles and are produced in wet condition, which makes handling and processing more difficult, since some processing is usually required prior to transportation or use [18].…”

Effect of Low Mixing Speed on the Properties of Prolonged Mixed Concrete

“…Bad mixing has bad effects on the mechanical properties of mortars. Ready mixed concrete has waste between 1% and 13% [25]. Part of this waste comes from job sites due to bad mixing.…”

Analysis of CO2 emissions and waste elimination capacity of different recycling strategies applied in ready-mixed concrete plants

“…For reusing waste concrete when it is still fresh, hydration reactions of the cement present in the waste that returns to the RMC plant are delayed for a few hours or even for a few days by adding special chemical products, called hydrationstabilizing admixtures (HSA). Gebremichael et al [10] demonstrated the possibility of adding the HSA to stop the hydration of the concrete and allow its reuse. They also indicated that in some cases using HSA-stabilized concrete as a raw material for new concrete may increase the compressive strength of the final product.…”