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

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 mix propor- (2) Two-step mixing, or pre-slurry mixing—In this tion. Since water absorbs microwave energy approxi- mixing procedure, cement and water were first mixed mately as 500 times (depending on frequency) as a dry into a slurry, and then fine and coarse aggregates were concrete material, the moisture sensor readings primar- slowly added into the slurry for further mixing. (In addi- ily reflect the moisture content of the concrete tested. tion to SSD coarse aggregate, OD coarse aggregate was In the present study, a pan mixer was used to mix also used in this mixing procedure); and 0.0566 m3 (2 ft3) of concrete. The moisture sensor was (3) Multiple-step mixing—In this mixing procedure, placed at a location of 60 mm from the edge of the all coarse aggregate and a half amount of mixing water, mixer (Fig.1a). It is positioned with a net space of 25 together with AEA, were loaded in a mixer and mixed mm from bottom of the pan mixer and with an angle of for 30 seconds, and sequentially sand, cement, and the 55o from central line of the mixer (Fig.1b and Fig.1c). rest of the water were slowly added into the mixer dur- Recommended by manufacturer, the angle was set to ing mixing; provide consistent compaction of the mixing materials The mixing time varied in the tests, depending upon against the ceramic faceplate. The space between the the time required for the concrete mixtures to be mixed sensor and the bottom of the pan mixer was adjusted so uniformly. that most aggregate particles can pass through, and the concrete mixture was able to cover the whole area of the 2.4 Definitions of mixing time sensor’s ceramic faceplate. During mixing, the sensor When two or more steps of mixing are involved in a recorded the moisture content of the concrete mixture at mixing process, the mixing time needs to be defined a speed of four readings per second. clearly. Since hydration occurs as soon as cement is in The sensor was calibrated with saturated surface dry contact with water, mixing time can be defined as the (SSD) sand before use. The calibration curve provides time from cement in contact with water to the time to users with a linear relationship between the moisture the complete stop of the whole mixing process. Thus, content of the material tested and the microwave energy the mixing time includes the time for loading of all con- detected by the moisture sensor. During tests, the mois- crete materials. However, during loading of concrete ture content of concrete mixtures was calculated based materials, the mixing energy applied to the mixture var- on the calibration curve. ies with time because the volume of the concrete mixture is changing. Some loading processes may take 2.2 Concrete materials longer time than the actual mixing time. As a result, if Type I cement with 20% slag replacement, river sand loading time is counted as a part of mixing time, evalua- (fineness modulus of 2.92), and limestone (nominal tion of the effect of the mixing time on properties of the maximum size of 25 mm) were used. Air entraining mixtures made with different mixing methods becomes agent (AEA) was employed for improving freeze-thaw less rational. Based on ACI 304R (ACI 2000), for sta- durability of the concrete. The concrete mix proportion tionary mixing, the mixing time should be measured is presented in Table 1, and it has a w/b of 0.43. In or- from the time all cement and aggregates are in the mixer der to have a consistent concrete mixture, all coarse drum. Based on ASTM C94, for central mixed concrete, aggregate (CA) used was sieved and recombined to a the mixing time shall be counted from the time all the given gradation. The coarse aggregate was also soaked solid materials are in the drum. To be consistent with in water and dried to a SSD condition before use, except that defined in these standard documents, in the present for the two-step mixing procedure, where the effects of paper, the mixing time is defined as the time from all both SSD and oven dry (OD) coarse aggregates on the materials loaded into the mixer. Detailed definitions for mixture properties were investigated. the specific time involved in a mixing process are given below: 1. Required mixing time is defined as the time re- 374 K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005 quired for a mixture from finishing loading all materials tube and an internal vibrator. The vibrator used has a 25 to achieving stable moisture content; mm-diameter steel head, with a frequency of 11,000 to 2. Extended mixing time is the time from achieving 12,000 rpm and amplitude of 1.02 mm from centerline the stable moisture content to finally stopping mixing; to side. With the internal vibrator that consolidates con- 3. Actual mixing time is the time from finishing load- crete mixture during a test, the Vibra-John test simulates ing all materials to finally stopping mixing. It is equal to the field concrete under a dynamic placing condition. the sum of the required mixing time and extended mix- Previous research has indicated that such a vibration test ing time; is beneficial to assess the workability of stiff concrete. 4. Time for cement in contact with water is the time The concrete subjected to vibration behaves as a shear- from the beginning of any cement in contact with water thinning fluid (Tattersal 1991). The small changes in to the time the whole mixing process is finally com- concrete slump may lead to much larger changes in the pleted; and workability results when the concrete is under vibration 5. Total mixing time is the time from beginning load- (Koehler et al. 2004). ing materials to finally stopping mixing. It includes both To conduct the test, concrete was filled into the loading time and actual mixing time. 0.028-m3 (1.0 ft3) vertical cube in two layers. Each layer was pre-consolidated with a temping rod for 50 strokes 2.5 Concrete tests and methods while the side door was kept closed. The surface of the After mixing, concrete workability was evaluated by concrete was then leveled with a trowel. To begin a flow various test methods, including the slump tests (ASTM test, the side door on the vertical cube was opened and 2000), Vibra-John apparatus tests (described below), the vibrator was turned on. When the vibrator was on, and vibrating slope apparatus (VSA) tests (Wang et. al. the concrete mixture started to flow into the horizontal 2005). Other tests, such as air, water, and coarse aggre- tube. The times for the front of the concrete mixture to gate content measurements of the mixtures, were also travel from the starting point to the lines 0.15 m and performed. The results from the ASTM slump and Vi- 0.30 m away from the vertical tube (T0.15m and T0.30m, bra-John tests are presented in this paper; the rest will respectively) were recorded. The average flow rate of be published separately. the concrete, described by 0.15m/(T0.30m-T0.15m), was The Vibra-John apparatus was developed by the Iowa calculated. (The authors found that the concrete flow Department of Transportation (IADOT) in the 1970s rate measured from Vibra-John tests had a good linear and used for low slump (less than 50 mm) pavement relationship with that measured from VSA tests.) The concrete. Unfortunately, there is no document available higher the flow rate, the better flow-ability the concrete for the development and application of this device. As has. shown in Fig. 2, this apparatus consists of an “L” shape After workability tests, concrete was cast in cylinders,

0.15m 0.15m

0.30m Side Door

0.30m

0.10m

0.46m 0.075m

0.30m 0.025m diameter vibrator head

Fig. 2 Vibra-John Apparatus. K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005 375 and the 3-day compressive strength of the concrete was sor readings became stable. At about 35 seconds after tested based on ASTM C39. A scanning electron micro- the start of mixing, the moisture sensor readings dis- scope (SEM) was also used to study the microstructure played a stable moisture content of 15%, very close to of the concrete, including the pore distribution and the the one calculated from the concrete mix design. In the interface between cement paste and aggregate. present study, the mixtures were considered as uniformly mixed when a stable moisture reading was de- 3. Results and discussions tected by the moisture sensor. The trial test results indicated that three repeated tests reached the same stable 3.1 Reliability of the moisture sensor moisture content (15%) at the approximately same time After calibration of the moisture sensor, three trial tests (32–35 seconds after mixing). It evidenced that the were performed to verify the reliability of the sensor. moisture sensor provided reliable readings in the trail Figure 3 shows the results from the three repeated tests tests. for the same concrete mix subjected to the same mixing Based on the definitions given in Section 2.4, a mix- procedure (one-step mixing: all concrete components ture was mixed thoroughly during the required mixing were loaded into the concrete mixer before mixing). As time. The additional time spent after the moisture sensor observed in the figure, in the first 32–35 seconds of reached stable readings to the time when mixing is fi- mixing, the moisture content of the concrete mixtures nally stopped is called the extended mixing time. As recorded by the moisture sensor fluctuated significantly, shown in Fig. 3, three different extended mixing times or was unstable. This is because at the beginning of (7, 33, and 55 seconds) were selected and their effects mixing some concrete materials in the mixtures were on concrete properties (slump and compressive strength) wet while others were dry, and moisture was not uni- were studied. Table 2 presents the test results. It is ob- formly distributed. The shapes of the moisture content– served that the extended mixing time appears to increase mixing time curve in the early mixing period also ap- concrete slump slightly but has little effect on 3-day peared different in the three repeated tests. This is pos- compressive strength of the concrete. sibly because before mixing the distributions of con- It is noted that the 32-35 second required mixing time crete materials (especially water) loaded in the mixer is significantly shorter than what commonly used in the were quite different from each other. US concrete practice. However, in France, ordinary As the mixing proceeded, the water, together with concrete is generally mixed for 35 seconds (Larrard and other concrete components, became more uniformly Cazactiu 2004). The investigators believe that for the distributed in the mixture. Therefore, the moisture sen- given mixing condition (equipment and materials), con-

Loading all materials and Stable moisture mixing begin

7s 20

Batch 1-C 10

33s 20

Batch 1-B 10

Moisture content, % content, Moisture 0 55s 20

Batch 1-A 10

0 0 102030405060708090 Time (s)

Fig. 3 Moisture sensor test results from three repeated one-step mixing procedure. 376 K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005

Table 2 Repetition of one step mixing procedure. that in the concrete with SSD coarse aggregate. The slurry moisture content was stable at 10%, although it Batch number (see Fig. 3) 1-C 1-B 1-A took a longer time for the slurry to reach stable moisture Required mixing time(s) 33 32 35 content. At about 10 seconds after all concrete components were introduced into the mixer, the Extended mixing time (s) 7 33 55 moisture content of the concrete mixture was stable at Slump (mm) 57 57 64 15%. In contrast to the concrete with SSD coarse 3 day strength (MPa) 21.4 20.9 21.7 aggregate, the moisture content of the mixture with OD coarse aggregate varied more significantly when the aggregate was introduced into the mixer. Figure 7 presents the results from the moisture sensor crete components might have been uniformly distributed test of the concrete subjected to a multiple-step mixing in the mixture after approximate 35 seconds of mixing. procedure. As observed in the figure, the moisture sen- Further mixing time, or extended mixing time, might sor reading scattered when each concrete component significantly influence cement hydration rather than the was introduced into the mixer. After all concrete com- uniformity of the mixture. ponents were added into the concrete mixer, it took approximately 23 seconds for the concrete mixture to 3.2 Effect of mixing procedures on moisture reach the stable moisture content of 15%. distribution in a concrete mixture Figures 4–7 demonstrated that the moisture sensor After the above trial tests, a new set of moisture sensor readings well captured the subtle changes, such as load- tests was designed to investigate the effect of mixing ing each concrete component, in concrete mixing. Table procedures on the moisture distribution of the concrete 3 summarizes the time required for all the concrete mix- mixtures. Figures 4–7 provide the test results from a tures at different mixing stages. As observed, different given concrete mixture subjected to three different mix- mixing procedures required different time for the con- ing procedures: (1) one-step mixing, (2) two-step mix- crete mixture to reach the final stable moisture reading. ing, or pre-slurry mixing, and (3) multiple-step mixing. Interestingly, the one-step mixing procedure required Similar to the results from the trial test, Fig. 4 once approximately 32 seconds for the given concrete mix- again illustrates that it took approximate 32 seconds for ture to reach the stable moisture content after all con- the given concrete mixture subjected to one-step mixing crete components were loaded in the mixer, while the to reach stable moisture content, or to be mixed uni- two-step mixing (pre-slurry mixing) procedure required formly. The stable moisture content was approximately only about 10 seconds, and the multiple-step mixing 15%. In Figs. 4-7, dots indicate the readings measured required about 23 seconds. These results implied that by the moisture sensor; and lines represent the moving the pre-slurry mixing procedure provided the highest average of every four readings, or the average reading mixing efficiency of the mixing procedures used. measured by the moisture sensor in every second. Figures 5 and 6 demonstrate the results from mois- 3.3 Effect of mixing procedures on concrete ture sensor tests of concrete subjected to the two-step workability mixing procedure, or pre-slurry mixing procedure. As After the moisture sensor displayed stable moisture con- observed in Fig. 5, where SSD coarse aggregate was tent, the concrete mixture was mixed further for en- used, it took less than 10 seconds for the moisture con- hanced uniformity. Then, the workability of the concrete tent of the slurry became stable at 10%. The moisture was evaluated. Figure 8 illustrates the effect of mixing readings during the slurry mixing period appeared scat- procedures, especially the mixing time, on concrete tered significantly, mainly due to the small amount of workability (slump from ASTM tests and flow rate from slurry in the mixer (the surface area of the moisture sen- Vibra-John tests). Again, the actual mixing time was sor sometimes could not be fully covered by the slurry). defined as the time from finishing loading of all con- At about 30 seconds after the beginning of testing, SSD crete materials to finally stopping the concrete mixing. coarse aggregate and fine aggregate were slowly added It was observed from Fig. 8 that when actual mixing into the concrete mixer. During the addition of the ag- time increased from 57 to 75 seconds, concrete slump gregate (taking about 35 seconds), the moisture sensor increased linearly (approximate 25%, which is not sig- readings became unstable again, indicating non-uniform nificant due to the test variation); while the concrete distribution of the concrete components in the mixture. flow rate, measured from the Vibra-John tests, increased About 10 seconds after all concrete components were exponentially (approximate 66%). The enhanced flow- introduced into the mixer, the moisture sensor readings ability of the concrete with increased actual mixing time became stable again, implying that the concrete mixture may result from improved uniformity of the mixture and was homogeneous. The stable moisture content was also a better cement hydration provided by the good distribu- approximately 15%. tion of cement particles in water and the longer hydra- As shown in Fig. 6, the trend of moisture content in tion time. The improved cement hydration could reduce the concrete with OD coarse aggregate was similar to the friction between aggregate particles in concrete, thus K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005 377

Loading all materials Stabilized at 32s after and mixing begin loading all materials

25 Actual mixing time = 65s

33s

10 Moisture content, %

0 0 20 40 60 80 100 120 140 160 Time (s)

Fig. 4 Moisture sensor test result of concrete mixed by one-step mixing procedure.

Loading cement and Loading Stabilized at 10s after water and mixing begin Aggregate loading all materials

Actual mixing time = 57s

20 10s 47s

10 Moisture content, %

0 0 20406080100120140160 Time (s)

Fig. 5 Moisture sensor test result of concrete mixed by two-step procedure. 378 K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005

Loading cement and Loading Stabilized at 12s after water and mixing begin Aggregate loading all materials 25

Actual mixing time = 47s 20 12s 35s

10 Moisture content, %

0 0 20406080100120140160 Time (s)

Fig. 6 Moisture sensor test result of concrete mixed by two-step procedure (OD coarse aggregate).

Loading Loading 1/2 water, Loading Loading Loading the Stabilized at 23s after aggregate mixing begin sand cement rest of water loading all materials

Actual mixing time = 75s 20 23s 52s

10 Moisture content, %

0 0 20406080100120140160 Time (s)

Fig. 7 Moisture sensor test result of concrete mixed by multiple-step mixing procedure. K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005 379

Table 3 Time required by different mixing procedures. Mixing time* One-step mixing Two-step mixing Multiple-step mixing ( CA moisture condition) (SSD) (SSD) (OD) (SSD) Required mixing time (s) 32 10 12 23 Extended mixing time (s) 33 47 35 52 Actual mixing time (s) 65 57 47 75 Time for cement in contact with water (s) 65 120 95 100 Total mixing time (s) 65 120 95 165 * See Section 2.4 for definitions of mixing time.

same sequence. Further research is necessary to separate 100 the effects of the other material loading sequences from the mixing time. 80 Multi Two steps One steps (OD coarse Two step 3.4 Effect of mixing procedures on concrete 60 aggregate) steps strength SSD Figure 9 presents the 3-day compressive strength of 40 Aggregate

Slump (mm) concrete manufactured with different mixing procedures.

20 The figure illustrates that concrete strength varied with mixing procedure, especially with the actual mixing

0 time. Concrete manufactured with the multiple-step 40 50 60 70 80 mixing procedure for the longest actual mixing time of Actual mixing time (s) 75 seconds had the highest strength, while the concrete manufactured with the two-step mixing procedure for the shortest mixing time of 57 seconds had the lowest (a) Slump strength. As mentioned before, the extended mixing time might allow more time for cement to contact water 0.05 after the cement particles are well distributed in water, thus facilitating cement hydration and improving con- 0.04 Multi crete strength. Previous research has indicated that for a steps Two steps given sufficient mixing time, the mixing sequence may (OD coarse have less influence on the uniformity and strength of 0.03 aggregate) One Two step steps concrete (Chang and Peng 2001). More research is 0.02 needed to verify the findings. SSD

Flow rate (m/s) rate Flow Aggregate In the two-step mixing (pre-slurry mixing) tests, con- 0.01 crete made with dry coarse aggregate demonstrated much higher strength than that made with SSD coarse 0 aggregate. Since the water content (including the mixing 40 50 60 70 80 water added and the water absorbed by aggregate) in Actual mixing time (s) both mixtures was the same, the improved strength probably resulted from the improved interface between (b) Flow rate aggregate and cement paste. This finding is also consistent with the previous research (Chang and Peng 2001). Fig. 8 Effect of mixing time and sequence on concrete workability 3.5 Effect of mixing procedures on concrete microstructure Figure 10 displays the backscattered electron images of improving concrete flow. concrete manufactured with different mixing procedures. Note that the actual mixing time in Figs 8 and 9 re- Attention was paid to the agglomeration of cement sulted from the concrete mixtures mixed with different and/or slag as well as distribution of air voids. Based on mixing sequences. The mixing sequence also has sig- the image study, no noticeable agglomeration was ob- nificant influence on concrete behavior. Interestingly, served in both the concrete mixed with the one-step the trend showing the effect of actual mixing time on mixing procedure for actual mixing time of 65 seconds concrete slump in Fig. 8 was similar to that presented in and the concrete mixed with the multiple-step mixing Table 2, where concrete mixtures were mixed with the procedure for a mixing time of 75 seconds. This indi- 380 K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005

23 Multi Two steps One ' (MPa) step step c,3 (OD coarse Two 21 aggregate) steps SSD Aggregate 19

17 3 daystrength, f

15 40 50 60 70 80 Actual mixing time (s) (a) One-step mixing Fig. 9 Effect of actual mixing time and mixing sequence on concrete strength. cates that both concrete mixtures might be “truly” mixed uniformly since they all reached the stable moisture content. However, the void content in the two mixtures was quite different. The concrete made with multiple-step mixing (Fig. 10b) displayed more air voids, or pores, than the concrete made with one-step mixing (Fig. 10a). Further study on pore distribution of concrete (as discussed below) confirmed this observation. Ten backscattered electron images were taken from one hardened concrete sample. Concrete samples made with all three different mixing procedures were exam- (b) Multiple-step mixing ined. The concrete images had a magnification of 40x in an area of 2300µm x 3000µm. The distributions of con- Fig. 10 Microstructure of concrete. crete pores within a range of 10–1000µm were analyzed Note: The concrete made with multiple-step mixing (Fig. from the images. The results were summarized in Fig. 10b) displayed more air voids, or pores, than the con- 11 and Table 4. crete made with one-step mixing (Fig. 10a). Figure 11 and Table 4 demonstrated that the accumu- lated percentage of pores for all sizes, especially the pores less than 200 µm, in the concrete mixed with the multiple-mixing procedure was approximately twice entraining air voids (d400 µm) in the two-step mixing that of those in the concrete mixed with the one-step concrete with OD coarse aggregate was even lower than mixing procedure. The total volume of all pores meas- the one-step mixing concrete, which would significantly ured (d1000µm) was 6.17% in the concrete mixed with influence concrete freeze/thaw durability. As a result, the multiple-mixing procedure but only 3.58% in the additional AEA may be necessary when dry aggregate is concrete mixed with the one-step mixing procedure. used for concrete. This indicated that mixing aggregate with water, to- To further verify concrete uniformity, standard devia- gether with AEA, before introducing cement into con- tions of the pore contents from 10 images of a given crete might be an effective way to provide proper air concrete sample were calculated. The results indicated entrainment. that the standard deviations calculated from concrete Figure 11 also shows that the pore size distribution made with different mixing procedures were very close, curve of the two-step mixing process with SSD coarse which implied that no severe non-uniform pore distribu- aggregate lay between the multiple-step and one-step tion occurred in the concretes tested. mixing concrete results. When OD coarse aggregate was The above pore-structure analyses suggested that the used, the pore content of all sizes of pores, especially concrete mixing procedure (material loading sequence the small pores, was significantly lower than that of and mixing time) and aggregate moisture condition may concrete with SSD coarse aggregate. The dry aggregate significantly influence the size and amount of air/pore possibly absorbed some water, together with AEA, into content in concrete, but they do not extensively affect or on the surface of the aggregate, thus reducing the uniformity of pore distribution as long as the mixture is effectiveness of AEA. In particular, the volume of small uniformly mixed as detected by the moisture sensor. K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005 381

7 One-step 6 Two-steps Multi-steps 5 Two-steps (OD CA)

Average of cumulate percentage of percentage of cumulate Average 1 pores smaller than a given size in 10 images in size a given than smaller pores 0 0 200 400 600 800 1000 Pore diameter (micro-meter)

Fig. 11 Effect of mixing procedures on concrete pore distribution.

Table 4 Pore content in concrete, %. 4. Conclusions Mixing Multiple- Method One-step Two-step step The following conclusions can be drawn based on the (CA condi- preliminary investigation: tion) (SSD) (SSD) (OD) (SSD) (1) The moisture sensor used provided reliable test d 50 Pm 0.24 0.43 0.15 0.61 results describing moisture distribution in concrete mix- d 200 Pm 1.71 2.13 1.06 3.22 tures. The sensor readings well captured the subtle d 1000 Pm 3.58 5.47 3.89 6.17 changes, such as the loading sequence of concrete materials, in concrete mixing process. This technique appeared to be a useful tool for controlling concrete uniformity and studying optimization of concrete mixing; Figure 12 shows the images of the interface between (2) Different mixing procedures required different aggregate and cement paste of concrete manufactured time for a concrete mixture to reach stable moisture with different mixing procedures. It was observed that content, or to be mixed uniformly. One-step mixing re- the one-step mixing procedure generally provided con- quired the longest mixing time, while two-step (pre- crete with relatively large and weak interfacial transition slurry) mixing required the shortest mixing time for the zone (ITZ), shown by the wide and dark-gray strip be- mixtures to be mixed uniformity after loading all mate- tween aggregate and paste in Fig. 12a. The concrete rials into a mixer. Regardless of mixing procedures, the mixed with the multiple-step mixing procedure usually stable moisture content detected by the moisture sensor exhibited a median size of interface (Fig. 12d). Con- was almost the same as the one calculated from the crete mixed with the two-step mixing procedure often original concrete mix design; showed much less noticeable interface than that of con- (3) Concrete mixing time, together with material crete mixed with any other mixing procedures used. loading sequence, significantly influenced concrete Although concrete made with OD coarse aggregate had rheological behavior. When mixing time increased from much higher strength than the concrete made with SDD 57 to 75 seconds, concrete slump increased slightly and coarse aggregate, no clear differences in the ITZs of the linearly, while the flow rate measured from the Vibra- two concrete samples were observed from the present John tests increased exponentially (approximately study. It is possible that the reduced air content in the 66%); concrete made with OD coarse aggregate had significant (4) The concrete mixing procedure (such as material effect on the concrete strength. Further research is loading sequence and mixing time) and the aggregate needed to quantify the characteristics (such as width and moisture condition considerably influenced concrete porosity) of the interfaces of the concretes manufactured strength. For a given concrete mix proportion, when the with different mixing procedures. two-step mixing (pre-slurry mixing) procedure was used, the concrete mixture made with dry coarse aggregate 382 K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005

(a) One-step mixing (b) Two-step mixing

Fig. 12 Interfaces of concrete made with different mixing procedures. Notes: (a) The one-step mixing procedure generally provided concrete with relatively large and weak interfacial transition zone (ITZ), shown by the wide and dark-gray area between aggregate and paste; (b) and (c) The concrete mixed with the two-step mixing procedure usually showed much less noticeable ITZ; and (d) The concrete mixed with the multiple- step mixing procedure exhibited median size of interface.

demonstrated much higher strength than that made with Portland Cement Concrete Pavement Technology (PCC SSD coarse aggregate. Images from SEM evidenced Center) at Iowa State University and the Federal High- that concrete having higher strength generally had im- way Administration (FHWA), USA, for their sponsor- proved interface between cement and aggregate; and ship of this project. Technical advice from Dr. Scott M. (5) Besides mixing time, mixing sequence and aggre- Schlorholtz at the Materials Analysis and Research gate moisture condition demonstrated more important Laboratory (MARL) and testing assistance from student influence on the concrete air void system. When the Shihai Zhang, Iowa State University, are also sincerely multiple-step mixing procedure was used, the concrete appreciated. had approximately doubled the amount of air as the concrete mixed with one-step mixing procedure. For the References same mixing procedure (two-step mixing), concrete American Concrete Institute (2000). “Guide for made with dry coarse aggregate had much fewer air Measuring, Mixing, Transporting and Placing voids (especially small air voids) than the corresponding Concrete.” ACI 304R-00, Reported by ACI concrete made with SSD coarse aggregate. However, Committee 304, Farmington Hills, Michigan 48333, the above factors (mixing sequence, time, and aggregate USA. moisture) had less influence on the uniformity of con- Assenheim, J. G. (1993). “Moisture measurement in the crete as long as the mixture was uniformly mixed as concrete industry.” Concrete Plant and Production, detected by the moisture sensor. 14 (5), 129-131. ASTM C94/C94M (2000). “Standard specification for Acknowledgements ready-mixed concrete, A1: Concrete uniformity The authors would like to acknowledge the Center for requirements.” West Conshohocken, PA: ASTM. K. Wang and J. Hu / Journal of Advanced Concrete Technology Vol. 3, No. 3, 371-384, 2005 383

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