Use of a Moisture Sensor for Monitoring the Effect of Mixing Procedure on Uniformity of Concrete Mixtures Kejin Wang1 and Jiong Hu2

Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, October 2005 / Copyright © 2005 Japan Concrete Institute 371 Scientific paper Use of a Moisture Sensor for Monitoring the Effect of Mixing Procedure on Uniformity of Concrete Mixtures Kejin Wang1 and Jiong Hu2 Received 9 January 2005, accepted 18 June 2005 Abstract The present research is to explore a new approach to monitoring uniformity of concrete mixtures. A given concrete mix was subjected to three different mixing procedures. A moisture sensor was installed inside a pan mixer to monitor moisture content of the concrete mixtures during mixing. The concrete mixtures were considered as uniformly mixed when stable moisture content was detected by the moisture sensor. The concrete workability and strength were then evaluated, and the concrete’s microstructure (pore distributions and aggregate-paste interface) was examined. The preliminary results indicated that the moisture sensor provided reliable test results describing moisture distribution in concrete mixtures. The sensor readings well captured the subtle changes, such as the loading sequence of concrete materials, in the concrete mixing process. The material loading sequence, mixing time, and aggregate moisture condition had significant influences on the concrete workability, air void system, and strength. These research results provide researchers and engineers with insight into the control of concrete mixing quality and the optimization of mixing procedures in the lab and field. 1. Introduction Thorough mixing is essential to produce a homogeneous mixture. A well-mixed mixture will permit all Concrete production consists of multiple and closely concrete components to distribute in its system uni- interrelated steps, including batching, mixing, consoli- formly and allow cementitious materials to hydrate uni- dation, finishing, and curing. Each step of the process formly, thus providing the hardened concrete with ho- makes a unique contribution to the quality of the final mogeneous microstructure and better performance. concrete product. Insufficient attention to any process- Concrete mixing is affected by the type of mixer, mixing step may result in poor concrete from what other- ing time, material loading sequence, and mixing energy wise is a well-designed mix. Today, concrete needs (Ferraris 2001). As various fine cementitious materials, more attention in its production process than ever before low w/b, and high binder content are increasingly used particularly due to the following reasons (National Re- in modern concrete, optimization of mixing procedures search Council 1997): becomes essential. The agglomeration of fine cementi- (1) Economic pressures on high-speed construction tious particles often occurs in inappropriately mixed and growing demands for an extended service life of concrete, which not only impairs hydration of cementi- concrete structures, which require optimal processing tious materials but also reduces workability of the con- procedures; crete (Williams et al. 1999). Sufficient mixing time and (2) A variety of supplementary cementitious materials energy are required to break down the particle clusters (SCMs) and chemical admixtures are increasingly used and to obtain a homogeneous and workable mixture. On in modern concrete. Various combinations of the SCMs the one hand, excessive mixing does not necessarily and/or admixtures might require different production improve concrete quality; but it instead may simply ex- procedures to achieve a homogeneous mixture and du- tend construction time and increase energy consumption. rable concrete; and Sometimes, excessive mixing may cause losses of (3) Low water-to-binder ratio (w/b) and high binder slump, air, and abrasion resistance of concrete, thus re- content are used in growing applications of high- ducing concrete workability and/or durability. As a re- strength and high-performance concrete. Such usage sult, it is important to monitor the uniformity of con- makes it more difficult for the concrete to obtain uni- crete mixtures and to develop an optimal mixing method form mixing, adequate consolidation, and efficient cur- that renders a homogeneous concrete mixture using ing with conventional processing methods. minimal time and energy. In current concrete practices, uniformity of concrete mixtures is measured by variations in either concrete components (ASTM 2000) or concrete macroscopic 1Assistant Professor, Civil, Construction and Environ- properties (Ferraris 2001). The commonly used methods mental Engineering, Iowa State University, USA. include flow measurement (Zain 1999), water content E-mail: [email protected] measurement (Nagi and Whiting 1994), water-to-cement 2Ph.D. Candidate, Civil, Construction and Environ- ratio (w/c) determination (Naik and Ramme 1989), and mental Engineering, Iowa State University, USA. 372 K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005 aggregate settlement (Petrou et al. 2000). These existing 381mm test methods can neither describe mixing efficiency nor monitor concrete uniformity. The present research is to explore a new approach to monitoring concrete mixture Moisture uniformity during mixing using an inserted moisture sensor head sensor. Center A great deal of work has been done using sensor 55o of mixer technique to study properties of cement-based materials. Direction Khalaf and Wilson (1999) studied the movement and of material special distribution of water in fresh concrete. Bois et. al. flow (1998) investigated the hydration of cement paste and concrete using a near-field microwave sensing technique. Mubarak et. al. (2001) evaluated w/c of in situ fresh concrete by inserting a monopole antenna probe 60mm into concrete after mixing. Various water meter sensors are often installed in concrete trucks or truck mixers. (a) Angle setting and position of the sensor Using truck meter sensors, Boscolo et. al. (1993), Board (1997), and Assenheim (1993) examined moisture content of concrete mixtures. However, these sensors are generally used for controlling the water content of a concrete mixture, or for ensuring a correct concrete mixture proportion, rather than for evaluating uniformity of the concrete mixture. Limit work has been done Concrete level using a moisture sensor for monitoring uniformity of in- Ceramic situ concrete mixture. The present investigation has Faceplate suggested that appropriate use of a moisture sensor can (H=76mm, not only verify the water content of concrete mixtures W=61mm) but also evaluate the mixture uniformity, and the re- 25mmMixer floor search results can be used for the control of concrete mixing quality and the optimization of mixing procedures. (b) Height setting of the sensor 2. Experimental work In the preliminary investigation, a moisture sensor was installed inside a pan mixer, and it monitored moisture distribution of the concrete mixture during mixing. Three different mixing procedures (with different material loading sequences and mixing time) were employed for a given concrete mix. Two moisture conditions (saturated surface dry and oven dry) of coarse aggregate were considered. The effectiveness of the mixing procedures and their effects on concrete workability, strength, and microstructure were examined. 2.1 Moisture sensor The moisture sensor, shown in Fig. 1, was manufactured by Hydronix. It has a ceramic faceplate with a dimen- (c) Moisture sensor in a pan concrete mixer sion of 76 mm (height) x 61 mm (width). After installed in a mixer, the faceplate will contact the concrete mix- Fig. 1 Moisture sensor and installation. ture in the mixer, and the sensor will record the moisture content of the mixture. The moisture sensor works prin- cipally based on the microwave reflection concept. The fields of the electromagnetic waves. A simple material moisture sensor generates a low-power field of micro- in an electromagnetic field can be characterized by the wave energy into concrete, and it detects the energy material permittivity H* and permeability P*, described absorbed. Each concrete component (cement, aggregate, as follows: water, or air) has a unique set of electromagnetic proper- * ties and provides a unique way of interacting with the D H E (1) K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005 373 B P * H (2) Table 1 Concrete mix design (SSD). Mix proportions (kg/m3) 2 w/b where D is electric flux density (Coulombs/m ), E is Cement Slag Water Sand Limestone electric field strength (V/m), B is magnetic flux density 2 286 72 154 794 999 0.43 (webers/m ), and H is magnetic field strength (A/m). The permittivity indicates the polarizability of a material, and it characterizes the ability of the material for storing and absorbing energy. The permeability de- 2.3 Mixing procedures scribes the ease with which a magnetic field can be set Three different mixing procedures (with different mate- up in a material. Both permittivity and permeability of rial loading sequences and mixing times) were applied concrete are related to the electromagnetic properties of to the above concrete mix. They are as follows: all components (cement, aggregate, water, and air) as (1) One-step mixing—In this mixing procedure, all well as their volume contents. As a result, the micro- aggregate, cement, and water were loaded into a con- wave energy reading from a moisture sensor inserted in crete mixer at once, and they were then mixed; concrete can be correlated to the concrete

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