EARLY AGE MONITORING OF CEMENTITIOUS MATERIAL …demo.webdefy.com/rilem-new/wp-content/uploads/2016/10/pro061-143… · EARLY AGE MONITORING OF CEMENTITIOUS MATERIAL ... reflection

1st International Conference on Microstructure Related Durability of Cementitious Composites 13-15 October 2008, Nanjing, China

EARLY AGE MONITORING OF CEMENTITIOUS MATERIAL PROPERTIES

Surendra P. Shah (1), Zhihui Sun (2) and Giri Venkiteela (1, 2)

(1) ACBM Center, Northwestern University, USA

(2) Department of Civil and Environmental Engineering, University of Louisville, KY, USA

AbstractEarly age monitoring of concrete mechanical properties are essential to assure the quality

and the performance of concrete structures. In this paper, a non-destructive one-sided ultrasonic technique, ultrasonic wave reflection method (UWR), was proposed and its capabilities in monitoring the early age concrete properties at micro and macro structural levels were briefly reviewed. In addition to the applications of UWR testing, a comprehensive modeling procedure for estimation of concrete mechanical properties based on the cement pastes properties (measured by UWR method) was also presented herein. Having the many advantages over the other in-situ non-destructive testing (NDT) methods, UWR method can be reliably applied in field to measure the various early age properties besides the compressive strength of concrete.

1 INTRODUCTION

Concrete, the most widely used construction material in the world, requires strength, durability, versatility and economy in the construction of concrete projects. In concrete, hydration happens immediately after the mixing of cement and water. During the hydration process cement paste stiffens and further hardens to make solid concrete mass. In comparison with the life cycle of concrete, early age is a short period (4 days), during this period, due to the lack of proper quality control and under/over estimation of concrete properties, in many cases, construction failures and wastage of material occur [1]. In order to guarantee the quality and life time performance of concrete, it is extremely important to have a reliable field testing method to estimate the changes in properties during early age.

Over the past few decades nondestructive testing (NDT) methods are commonly used to monitor the property changes of cementitious materials [2]. A continuous monitoring NDT method for concrete behavior throughout its setting and hardening procedure is highly desired in field. Also, it is important that the technique can monitor other properties of a concrete besides the compressive strength. The most generally used in-situ early age NDT method is the maturity method [3]. In this method the maturity of a concrete specimen is calculated as a

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function of the age and curing condition of the specimen. This maturity is then related to the strength of concrete. Using this method the in-situ strength can only be reliably estimated when the concrete used in the field has exactly the same mix proportions and properties as the calibration concrete used in the lab. However, measuring the physical parameter (temperature) instead of mechanical property of the material becomes a drawback of this technique. Moreover, the accuracy of strength prediction depends very much on the chosen maturity function. Therefore, in-place techniques are necessary for continuous monitoring the strength gain of concrete in field.

Among the various in-situ NDT methods, due to their ease of application and the capability in observing the mechanical properties, the ultrasonic techniques are extensively used in continuous observation of the concrete properties [4]. With the prior knowledge on ultrasonic wave propagations in materials, it is possible to estimate the mechanical properties of materials. One kind of such technique, which requires only one side access to the structural member to perform testing, a new NDT method called ultrasonic wave reflection (UWR) method was developed at the Center for Advanced Cement-Based Materials (ACBM) [5]. This method measures the shear wave reflection loss (RL) at an interface between the hydrating cement paste and a buffer material. Further, the wave reflection loss can be correlated numerically and theoretically with the mechanical properties of fresh concrete.

The objective of this paper is to present a brief review of capabilities and applications of UWR method in early monitoring of concrete with the aid of research work conducted in ACBM center. Based on the measured micro and macro structural level properties of cement pastes, a comprehensive modeling approach was proposed for the estimation of concrete mechanical properties. From the experimental study, it has been concluded that the UWR method is a promising technique in the observations of concrete property changes during early age.

2 METHODOLOGY OF ULTRASONIC WAVE REFLECTION TECHNIQUE

2.1 Wave reflection at boundary From the principles of wave reflection theory, when a sound wave hits the interface formed

by two materials, it is partially reflected back and partially transmitted in to the medium on the other side of the interface [6]. The reflection coefficient r represents the amount of wave energy, which is reflected at the interface, which can be calculated as

1122

1122

vvvv

AAr

i

r (1)

Here, 1 and 2 are the mass densities of material 1 and 2, and v1 and v2 are the wave velocities respectively. Ai and Ar are the amplitudes of incident and reflected waves respectively. By knowing the density and the shear wave velocity (vT), shear modulus (G) of material can be calculated as

G= .vT2 (2)

From the above equations, it is observed that the wave reflection coefficient is governed by the shear modulus of the tested material, which is a key mechanical property of the material.

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2.2 Experimental apparatus and procedure The apparatus setup for UWR test is as shown in Fig. 1. Here, two shear wave transducers

with central frequency of 2.25MHz are coupled to the steel plate, which works as a buffer material with a thickness of 12mm. The transducers are connected to a pulsers/receiver system, which in turn is connected to a computer. Data collection is performed by a comprehensive computer program [4]. During the test, time-signals of shear waves are collected. In order to calculate the reflection coefficient r, this time domain signal needs to be transformed into frequency domain. For this purpose, the Fast Fourier Transform (FFT) algorithm is employed as shown in Fig.2. The reflection coefficient r is then calculated as the ratio between the amplitudes of the second and the first echos as in Fig.2. To eliminate the amplitude losses due to transducer coupling and attenuation in steel plate, a calibration process is required; more details about this procedure can be found in [4].

Figure 1: Apparatus Setup for UWR Method

Figure 2: Reflected Echoes in Time Domain and Transformed Frequency Domain

Since amplitude ratios in ultrasonics are usually measured in decibels, the reflection coefficient r is represented as reflection loss, RL as shown in equation (3). A typical wave reflection loss curve for cement paste is shown in Fig.3. It can be seen that the rate of reflection loss is increasing as the hydration is progressing (between Point A and point B). After reaching a certain value, the increasing rate of reflection loss slows down (after B).

RL= -20. Log(r) (3)

Figure 3: Development of Reflection in Cement paste

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3 MONITORING CONCRETE PROPERTIES WITH THE UWR METHOD

3.1 Setting behavior Concrete setting is defined as on set of rigidity in fresh concrete. During concrete setting,

cement pastes stiffen and bond the aggregate together. To know the potential of reflection loss in monitoring of the setting time, first, the turning point A in Fig.3 was correlated with the ending point of dormant period in the heat evolution curve (Calorimetric method). For this purpose, three different mortars turning points in reflection loss curves (Fig.4 (a)) were related to their corresponding ending point of dormant periods. The linear relationship in Fig.4 (b) describes the capability of wave reflection loss in monitoring the setting behavior of cementitious materials. Second, the reflection loss was directly correlated to setting times measured with Vicat needle tests. From the Fig.5, it can be observed that the initial setting times (ti) for three cement pastes appear at the same level of reflection loss values (RL), where as the final setting time (tf) reflection loss values (RL) are showing some variations. Even though the numerical difference between ti and tf are similar (2~3 hr) for three cement pastes, the differences in the increasing rate of reflection losses explains the difference in RL values for the final sets [4].

Figure 4: (a) RL Curves for Three Mortars with Different Chemical Admixture (b) Correlation of the Turning Point A and the End of the Dormant Period

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Figure 5: Correlation between Reflection Loss and Setting Time

3.2 Viscoelastic properties Viscoelastic properties of cement paste change during the hydration of the cement

particles. This development of viscoelastic properties can provide fundamental information about the physical state of a particle suspension system transforming from a flowable viscous semisolid to a solid system. The UWR technique was used to monitor the viscoelastic properties of cement pastes at very early age by observing the evolution of the storage shear modulus and the viscosity [7]. For different cement pastes, the calculated storage shear modulus (G’), which represents the elastic properties of the materials, and viscosity were compared to the results measured directly from the Oscillatory Rheometric (OR) and Step Rheometric (SR) methods respectively as in Fig.6. The dynamic shear modulus is also theoretically correlated to the reflection loss by using the equation (4). In this equation, Zs isthe acoustic impedance of the buffer steel plate, is the density of the test material and r isthe reflection coefficient.

2

22

11

rrZ

G sWR (4)

Figure 6: Comparison of Viscoeleastic Properties

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3.3 Compressive strength The compressive strength (fc) of concrete is the most desired property in the construction

of concrete projects. For this purpose, various cement paste, mortar and concrete mixtures’ compressive strengths were tested and correlated to the corresponding reflection loss developments [4]. From the Fig.7 it can be observed that the wave reflection loss trend is in consonance with the strength development of three different cement pastes. From the experiments it was also observed that the reflection loss measurements on mortars and concrete were governed by the cement pastes properties near to the buffer material (steel) and were independent of w/c ratio and curing temperatures.

Figure 7: Correlation between Reflection loss and Compressive Strength

3.4 Comparison with other in-situ methods To improve and expand the applicability of UWR method, it is worth to compare the UWR

method to other in-situ methods like, the ultrasonic wave transmission technique and the maturity method. An attempt was made by [8] to compare the wave transmission to that of wave reflection method with the help of experiments on the penetration resistant, the in-situ temperature raise, the adiabatic heat release and the chemical shrinkage. This research showed that the wave reflection loss and pulse velocity measurements were following the same trends for various mortar and concrete mixtures with the developments in tested parameters. On the other side, comparison of maturity method and UWR method in evaluating the early age compressive strength of mortars was also studied [9].During this research it was found that the relationship between compressive strength and wave reflection loss is independent of curing conditions and w/c ratio, on the other hand the relationship between equivalent age and compressive strength is significantly influenced by these parameters. The comparison also showed that the UWR method can be improved in monitoring the early age properties by applying the maturity concept to the shear wave propagation.

4 MODELING CONCRETE MICROSTRUCTURES

4.1 Modeling the microstructure of cement pastes In concrete, the setting behavior is governed by its cement paste’s microstructure

evolution, which is a complex issue associated with cement particles agglomeration and flocculation, percolation and wide variety of other reactions. To study the application of UWR in understanding the cement pastes microstructure evolution, a computer simulation model

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called HYMOSTRUC3D was used [10]. In this model, degree of hydration, the volume fraction of total solids, and the capillary porosity are chosen as three basic parameters that describe the volume change of solid phase in microstructure of the cement paste during hydration. For a given condition, in order to assure the quality of simulation, calibration was carried out by conducting experiments include particle size distribution analysis and thermogravimetric analysis (TGA). Beside these parameters, in order to account for the connectivity of different phases, percolation threshold of the solid phase was also simulated (Fig.8). The percolation threshold is considered as an indicator of initiation of setting process.

(a) Initial status: solid particles suspended in water (b) solid phase is percolated

Figure 8: Percolation of Solid Phase during Hydration

In Fig.9, at the same degree of hydration, the cement paste with water:cement mass ratio (w/c) 0.35 has much more solid phase than the paste with higher w/c ratio. Since the solid phase is the dominant factor for wave propagation, a larger amount of solid phase in microstructure corresponds to higher reflection loss. The difference in the slopes of the regression lines for correlation between reflection loss and degree of hydration indicates that the reflection loss is not only a function of the progress in hydration but also a measurement of microstructural properties that are influenced by the w/c ratio.

Figure 9: Correlation between Reflection Loss and Degree of Hydration

Figure 10: Correlation between Reflection loss and Capillary Porosity

The correlation of reflection loss and capillary porosity (Pc) (by HYMOSTRUC3D) is shown in Fig.10. This type of correlation can also helpful to monitor the permeability of the cement paste using the UWR method. Based on the microstructural simulation of each

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mixture, the fraction of total solid phase (Ts) and the fraction of connected solid phase (Cs)were calculated [11]. Reflection loss and solid phase developments for w/c= 0.35 are shown in Fig.11. The reflection losses at the percolation threshold (tp) of the solid phase and the time when the solid particles are almost connected with each other (te) are plotted as solid dots in Fig.11. It was observed from the experiments that, the threshold (tp), turning point A in Fig.3 and the initial setting time (ti) in Fig.5 were occurring at, more or less, the same time for each mixture. This observation provides useful information about the reflection loss capability in defining the initial setting time of cement pastes and in providing another option with solid phase percolation to describe the setting behavior of Portland cement pastes.

Figure 11: Correlation between Reflection loss and Solid Volume Fraction

Figure 12: Correlation between Reflection loss and Specific Contact Area

The capability of reflection loss in explaining the inter particle bonding in microstructure was also studied. HYMOSTRUC3D model provide the information about specific contact area of solid phase. The correlation is as shown in Fig.12. Based on these observations, it can be concluded that the UWR method is not only sensitive to the volume of the connected solid particles, but also to the intensity of interparticle bonding of the solid phase in the microstructure of cementitious materials. Further, more details about HYMOSTRUC3D model simulation and experimental procedures can be found in [4].

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4. 2 Modeling concrete properties In concrete, the elastic moduli govern the deformation when it is subjected to external and

internal loads. Thus, elastic modulus is considered as one of the important mechanical properties by both engineers and researchers. Using the advantage of UWR method in accurate determination of the shear modulus of cement pastes, in order to estimate the mechanical properties of concrete and mortar in field based on the properties of cement paste, modeling is required on the elastic properties of concrete composites. For this purpose a comprehensive three level modeling approach was proposed as expressed by the flow chart shown in Fig. 13.

Figure 13: Modeling the Elastic Properties of Concrete on Various Levels

In level 1 and Level 2, concrete composite was modeled as a two phase theoretical model and a three phase computer simulated 2D microstructural model [4] respectively. During these levels of modeling, concrete elastic properties are modeled with hydration age, curing temperature, w/c ratio, aggregate content and aggregate size and dispersions. But it was observed that these models were failed to study the influence of the aggregates on elastic behavior of concretes. To study this, In Level 3, various concrete and mortars were modeled with the aid of the Virtual Cement and Concrete Testing Laboratory (VCCTL) cement hydration module and a differential effective medium theory (D-EMT) [12].

Figure 14: Geometry Used in D-EMT Simulation

E, G of paste matrix

E, G of aggregates

Upperbound

Lowerbound

Hirsch’smodel

Level 1

ITZ

Aggregatedistribution

GMCLevel 2

Microstructure

AggregatePSD

D-EMTLevel 3

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0

2

4

6

8

10

12

14

16

18

0 100 200 300 400 500 600 700Hydration Age (hours)

Dyn

amic

Shea

r M

odulu

s (G

Pa)

concrete w:c:s:g=0.5:1:2:2aggregate volume fraction=64.84%

Level 1: EITZ=0.8Epp; Ebulk=Epp

experimental data

D-EMT Level 1

Figure 15: D-EMT Simulation for Concrete

The VCCTL cement hydration module, an extension of the microstructural development model CEMHYD3D, has the capability in simulating the ITZ regions around a single aggregate particle [4]. It places cement particles randomly in a box, dissolves, diffuse, and reacts the particles to form a cement paste microstructure. Fig. 14 shows the geometry that is typically used, with periodic boundary conditions. In the study by [12], the simulated ITZ was mapped onto an effective particle with uniform elastic moduli. The resulting simpler composite, with a bulk paste matrix, was then treated by the usual D-EMT. During this study, the changes in elastic properties of ITZ (EITZ) and Bulk paste (Ebulk) were considered with reference to the elastic properties of plain paste (Epp). By assuming the ratio of the elastic properties between the ITZ and the plain paste to be a constant (a) in equation (5), the simulation curve in Fig.15 of the elastic modulus has very similar trend as compared to the experimental data obtained directly with the resonance frequency method measurements. This hints that the D-EMT has the potential to be used for simulating the elastic properties of concrete composite, even if the volume fractions of the aggregates are beyond the dilute.

EITZ=aEpp (5)

Although Fig.15 shows relatively good simulation results for the elastic properties of concrete composites, there is neither theory nor experimental data that can support these assumed values of the parameter a or assuming that the bulk paste elastic moduli values are the same as the plain paste values (Fig.15). In additional, the ITZ w/c ratio in concrete is higher than the overall average value specified in the mix design [12]. Due to these reasons, in the modeling, the contrast of elastic moduli (R1& R2) between the ITZ and the bulk paste matrix were considered as a function of hydration age as shown in Fig.16. Further, R1 and R2were modified by considering the gradient in w/c ratio and hydration products in the ITZ.

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Figure 16: Modeling the Elastic Properties of (a) ITZ (b) Bulk Paste Matrix

Finally, based on the experimental work and the modeling, possible relationships between compressive strength and shear modulus were developed. It was observed that these relationships are temperature independent [13] and it was also observed that compared to the compressive strength, the dynamic shear modulus is more sensitive to the aggregate content.

5 CONCLUSIONS AND FUTURE RESEARCH WORK

Due to the versatility and the ease of application in early age in-situ monitoring of the concrete properties, the UWR method can be employed as a promising NDT technique in field. Using UWR method, reflection loss can be correlated to various concrete properties at macro and micro structural level include, viscosity, setting behavior, dynamic shear modulus, compressive strength, degree of hydration, capillary porosity, percolation threshold and specific contact area of solids. The proposed comprehensive modeling approach is very useful to predict the concrete composite properties in field based on the mechanical properties of cement paste measured by UWR method. To improve the UWR method measurements, the focus of the future research work include, the real observations on cement paste’s microstructure developments, the quantification of ITZ influence on the reflection loss developments and finally modeling of these micro and macro structural level mechanical properties by using a single modeling approach.

REFERENCES

[1] Feld, J. and Carper, K.L. ‘Construction Failure’, 2nd edition, New York: John Wiley & Sons,Inc., 1997.

[2] Malhotra, W.M., and Carino, N.J. ‘Handbook on Nondestructive Testing of Concrete’, 2nd Edition, USA: CRC Press, Inc.2004.

[3] Carino N.J. ‘The maturity method: theory and application’. Cement. Concr. Aggr 6(2) (1984)61-73

[4] Sun. Z, Monitoring the Early-Age Properties of Cementitious Materials with Ultrasonic Wave Reflection Method at Macro- and Micro-Structural Levels. PhD. Thesis, Evanston: Northwestern University, 2005.

[5] Öztürk, T., Rapoport, J., Popvics, J.S. and Shah, S.P. ‘Monitoring the setting and hardening of cement-based materials with ultrasound’. Concr. Sci. Eng. 1(2) (1999), 83-91.

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[6] Akkaya, Y., Voigt, T., Subramaniam, K.V. and Shah, S.P. ‘Nondestructive measurement of concrete strength gain by an ultrasonic wave reflection method’. Mater. Struct, 36(262) (2003), 507-514.

[7] Sun, Z., Voigt.T., and Shah.S.P., ‘Rheometric and ultrasonic investigations of viscoelastic properties of fresh Portland cement pastes’. Cem. Concr. Res 36(4) (2006), 278-287.

[8] Voigt, T., Grosse, C.U., Sun, Z., Shah, S.P. and Reinhardt, H.W. ‘Comparison of ultrasonic wave transmission and reflection measurements with P-and S-wave on early age mortar and concrete’. Mater. Struct. 35(5) (2005), 858-866.

[9] Voigt, T., Sun, Z. and Shah, S.P. ‘Comparison of ultrasonic wave reflection method and maturity method in evaluating early-age compressive strength of mortar’. Cem. Concr. Compos. 28(4) (2006), 307-316.

[10] Ye, G. Experimental Study & Numerical Simulation of the Development of the Microstructure and Permeability of Cementitious materials, PhD Thesis, Delft: Delft University, 2003.

[11] Sun,Z., Ye, G. and Shah, S.P., ‘Microstructure and early age properties of Portland cement pastes-effects of the connectivity of solid phases’. ACI Mater.J, 102 (2005).122-129.

[12] Sun, Z., Garboczi, E.I., and Shah,S.P. ‘Modeling the elastic properties of concrete composites: experiment, diffractive effective medium theory, and numerical simulation’, Cem. Concr. Res. 29(1) (2007), 22-38.

[13] Sun,Z., Voigt, T. and Shah, S.P., ‘Temperature effects on the strength evaluation of cement based materials with ultrasonic wave reflection techniques’. ACI Mater.J, 102(4) (2005) 272-278.

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