DETERMINATION OF INITIAL DEGREE OF HYDRATION BY ULTRA-SONIC PULSE TECHNIQUE

Transcription

1 DETERMINATION OF INITIAL DEGREE OF HYDRATION BY ULTRA-SONIC PULSE TECHNIQUE MATIAS KRAUSS, KARIM HARIRI, FERDINAND S. ROSTÁSY Institute for Building Materials, Concrete Structures and Fire Research, Technical University Braunschweig, Germany Abstract During the first hours after mixing, the properties of concrete vary and change within a wide range of behaviour. Fresh concrete is more or less a Newtonian fluid which gradually changes into a Bingham type of material, finally attaining solid body properties with considerable compressive strength and stiffness. The development of mechanical properties can be described by the degree of hydration. For the prediction of mechanical properties of early age concrete as well as for the prediction of stresses caused by differences of temperature and autogenous shrinkage, it is essential to know the initial degree of hydration, from which on the development of strength and stiffness begins. This paper deals with the determination of the end of the dormant phase by using ultrasonic pulse velocity techniques. Using compression wave and shear wave transducers the hardening of concrete is observed under adiabatic curing conditions. From the development of the dynamic Young's modulus and Poisson's ratio, a model of the initial degree of hydration is derived to improve existing models of the development of tensile strength and modulus of elasticity for very early age concrete. A procedure to determine an upper and lower bound for the end of setting time is presented. Typical results are presented for different concrete compositions, especially for high strength concrete. 1. Introduction In order to predict mechanical properties of concrete at very early age, non-destructive test methods are advantageous. By combining such methods with destructive tests reliable information regarding the mechanical properties can be acquired. In this paper a test method is presented for the determination of the end of setting time, at which the initial Young's modulus can be measured non-destructively. The method was developed in order to improve existing models for the description of mechanical properties and to obtain a better understanding of hardening concrete. 199

2 . Problem and scope of interest All models quoted in literature regarding the assessment of the development of the mechanical properties vs. degree of hydration describes the test results quite well. However they contain uncertainties, particularly for a low degree of hydration. For further information viz. Rostásy & Krauß [6], v. Breugel [8], etc. Fig. 1 shows typical models of the normalized Young's modulus and axial tensile strength. The values f ct1 and E ct1 are hypothetical end values at α = 1. The degree of hydration α denotes the end of dormant period and the on-set of strength and stiffness. This value is determined by regression of measured tensile strength data, presupposing a linear fct - α relationship. This method exhibits two main uncertainties, which we have to take into account. Fig. 1: Normalized tensile strength and modulus of elasticity dependent on degree of hydration - models and real behavior The first one relates to the fact, that reasonable results for tensile strength can only then be measured if α >.4, because the scatter of test results at very early age is rather large. The second one relates to the difficulty to predict the degree of hydration at very early age precisely enough. To obtain improved values of α, tests under adiabatic conditions are necessary. But the evolution of mechanical properties starts earlier than at α (see fig. 1). To determine the corresponding smaller initial degree of hydration α i a non-destructive testing method like ultrasonic measurement is advantageous. 3. Experimental equipment and testing In Fig. the developed test apparatus is shown. It consists of the adiabatic calorimeter, its controlling unit, ultrasonic measurement device and controlling unit for an external water bath. All controlling units are integrated in a standard PC. A proprietary software steers the controlling unit. The water bath is used to store the sealed specimens under adiabatic curing conditions for the destructive testing. A detailed description of adiabatic calorimeter is presented in Gutsch [1] and Morabito [3]. 3.1 Ultrasonic measurement The ultrasonic transducers are integrated in the steel bucket (viz. Fig. ). The transducers are protected by a plastic tube. A rubber seal ring fixes the sensors securely. This con-

3 struction guarantees that no moisture is lost during testing. The transducers are coupled directly. For the receiver and transponder a commercial transient time measurement system is applied. Two types of commercial ultrasonic shear wave and compression wave transducers with frequencies of 5 khz and 1 khz are used. By measuring the ultrasonic pulse velocity of shear and compression waves the dynamic Young's modulus dyn E and the dynamic Poisson's ratio dyn µ can be calculated (Neisecke [4], Keating et al. []): ( vs / vc ) ( v / v ) 1 dyn µ = (1) dyn E s (1 + dynµ ) (1 dynµ ) (1 dynµ ) c = v c ρc (), with ρ c density of (fresh) concrete; ν c, velocity of compression wave and ν s, velocity of shear wave propagation. The velocities can be calculated from the measured transient times tc and ts with vc = dc / tc and vs = ds / ts with dc and ds, thickness of specimen in the corresponding direction. The dimensions dc and ds were evaluated for each single test. Values from 9 mm up to 11 mm are measured. Usually the transient time of compression wave is defined as the first significant change of measured signal. Such definition is not useful for young concrete because of its excellent damping property. The expected signals are flat and have small impulses. It is hence difficult to identify the transient time. Other difficulties are the disadvantageous signalnoise ratio and the age dependent transducer - specimen system. To take this difficulties into account, and to ensure stable measurements certain measures became nescessary. The first one is: if the maximum amplitude is detected, the incoming signal is amplified to 1 % of its intensity. The second one is: the expected transient time is defined as the point of time, at which the first amplitude reaches 5 % of the maximum one. To control this procedure, all measured a-scans are evaluated by hand and PC (see Fig. 3). Every 3-36 seconds ten evaluations are made and the average value is stored. The identification of the on-set time of shear pulses is more complicated, because the transient time can not be evaluated as the first significant change of signal from zeroline. The reason is that the shear wave transducer can not generate a natural shear wave. But if the ratio of shear wave signal to compression wave ratio is big enough, it is possible to identify ts (Rehm et al. [5]). It is pre-supposed the measured signal as a natural combination of a c-wave and a s- wave. Without any kind of restriction, we can suggest that the known c-wave can be described as a sine wave. From this point of view it is possible to define the starting point of transversal impulse as the first significant change of a sine wave. In general 1

4 there exist four different possibilities to analyse the incoming signal. The starting point can be depicted between a minimum and maximum, maximum and minimum or as a minimum or maximum of measured signal. In Fig. 4 the procedure is shown. Fig.. Test set-up for ultrasonic testing under adiabatic curing conditions For the calculation of transient time, the point of time is depicted, which exhibits the first significant change of the compensated ultra-sonic-signal. For the evaluation of the s- waves, the procedure for evaluation of c-waves is adopted. 3. Concrete Compositions In this paper the investigations of two HPC compositions are presented: CO1/R1-R and CO11/I1-I5. This latter HPC is denominated by Norwegian HPC, because all constituents stem from Norway. This type of concrete was used for the recent strait crossings, e.g. Øresund bridge. The composition CO1/R1-R is a comparative German HPC. All concrete compositions were comprehensively tested. Tab. 1 contains main test data of these concretes.

5 A [%] amplitude amplified to 1% T d) a) b) c) d) t ~ t [µs] T T T T -4-6 evaluated transient time t c [µs] Fig. 3: Identification of transient time of c-wave a) b) c 1 ) c ) d) T = starting point of the transversal impulse Fig. 4. Identification of transient time of s-wave Further investigations regarding the heat liberation and the development of mechanical properties were reported in Rostásy & Krauß [6] and in Morabito [3]. In this report only the test results on the HPC are dealt with in detail. Table 1: Concrete compositions Concrete No. Cement content [kg/m 3 ] w/c [-] T c [ C] cement type Remarks 1 CO1/R1-R CEM I 5.5R CO11/I1-I CEM I 5.5R-LA FA = fly ash SF = silica fume. [kg/m 3 ] SF German HPC 18.4 [kg/m 3 ] SF Norwegian HPC 4 Models and Test results 4.1 Material models For the description of mechanical properties, the models developed at the IBMB and in Scandinavia are presented. More to these can be found in Gutsch [1] and Westman [9] Models of IBMB The heat of hydration is expressed by Q(te) = Qpot α (te) (3) with α (t e ), degree of hydration modeled by 3

6 c1 t α = + e (te) exp ln 1 (4) t k There-in are: t e, effective age (Hansen-Pedersen approach); Q pot, maximum heat of hydration (Bogue calculus); Q (t e ), heat liberated until t e ; t k, concrete specific parameter [h]; c 1, concrete specific parameter [-]. The axial tensile strength f ct, compressive strength f c and the Young's modulus E ct can be expressed by n i 1 for E ct α α X i( α ) = Xi1 1, X i ( α ) { E ct,f ct, f c }, n i = 1 for fct. (5) α 3 for f c Here are: X i1, hypothetical end value of property at α = 1; n i, property specific exponent. Eq. (5) shows the values n i used by IBMB. The values X i1 and α are determined by regression of test results Scandinavian models The heat of hydration, degree of hydration, strength and stiffness are generally expressed by b i te Y i (te) = Yi1 exp. (6) a i The parameters Y i1, a i and b i must be determined on basis of tests (viz. Morabito [3]). 4. Material properties of HPC from destructive and non-destructive testing at early ages In this paper only a brief description of the development of mechanical properties is given. Fig. 5 shows the development of degree of hydration vs. equivalent age for both HPC. Fig. 6 shows the development of axial tensile strength vs. equivalent time for both mixes. The corresponding form parameters of Eq. (5) and Eq. (6) can be found in Rostásy & Krauß [6]. It can be noticed, that the heat release of the German HPC starts earlier. This can be explained by the higher content of C3A of the German CEM I 5.5R. It can be shown that the compressive strength of both mixes reaches values from 68 to 75 MPa. For the maximum values of axial tensile strength values between 3.1 MPa and 4.1 MPa are measured. The static Young's tensile modulus reaches values between 8 GPa and 34 GPa after 67 h. An interesting point of these investigations are the mechanical properties at very early ages. All mechanical properties of the Norwegian HPC start at nearly the same age. The 4

7 initial setting time is about t e 9 h. From Fig. 5, a degree of hydration of α i.1 can be derived. It can be noticed, that the end of the dormant phase for the Norwegian HPC is reached at t 8 h and at t 5 h for the German HPC..6.5 Norwegian HPC German HPC 5 4 Norwegian HPC German HPC.4 meas α [-].3. f ct [MPa] t e [h] Fig. 5: Development of degree of hydration vs. equivalent age t e [h] Fig. 6: Development of axial tensile strength vs. equivalent age The task was to improve this values by NDT. The improvement is depicted in Fig. 7 and 8. Fig. 7 and 8 show a comparison between the static and dynamic modules of elasticity. We see dyn E ct > stat E ct. Their ratio dyn E ct / stat E ct is in the range of 1.5 to 1.7 for t e > 5 h. Both moduli start at a distinct age: dyn E ct, t ei 6.5 h and stat E ct, t e 11 h. The value t ei can be exactly identified with Fig. 7. The value t e for stat E ct had to be estimated with tests at t > t e. These results agree with the observations presented in Fig Improvement of the models of IBMB On basis of these test results, the models described with Eq. (5) and (6) can be improved. So the early age growth of strength and stiffness is taken into account. Figure 9 shows the attempt. It is pre-supposed that the value α has been established with axial tensile strength tests and that the value α i been accordingly measured by ultrasonic tests under adiabatic conditions. The transition of value α i to the models, Eq. (5), is described by straight lines between α i and γ α, with γ 1. 5

8 5 5 5 Norwegian HPC E ct [GPa] 3 3 dyn E ct [GPa] E ct [GPa] 3 3 dyn E ct [GPa] 1 meas. dyn. mod. dyn. meas. stat. mod. stat t e [h] Fig. 7: Development of Young s modulus of elasticity vs. equivalent age This model can be written as 1 1 meas stat meas dyn mod stat mod dyn α [-] Fig. 8: Development of Young s modulus of elasticity vs. degree of hydration 1 α αi Xi ( γ α ) γ α αi for X ~ αi α γ α i ( α ) = n.(7) i α α Xi1 1 α for γ α < α 1 Herein the abbreviations of Eq. (5) are used. With the initial value α i, the associated effective age t ei is known. The Scandinavian models, Eq. (6) can be accordingly revised by substituting t e by (t e - t ei ). Tab. shows the relevant parameters of both modifications. Fig. 9. Improvement of the models acc. to Eq. (5) With respect to the test results and model modifications for the normal concrete compositions listed in Tab. 1 only few remarks must suffice. The type of cement, its inherent heat potential and amount strongly influence the values of α i and t ei. Rapid hardening cements, a high cement amount and an elevated fresh concrete temperature reduce the dormant phase. 6

9 Table : Parameters of improved model for the elastic modulus for the Norwegian and German HPC Parameters of IBMBmodels Eq. (7) Parameters of Scandinavianmodels Eq. (6) Norwegian HPC E ct1 GPa E ct1 GPa 6.6 N i -.5 A h α B α i -.5 t ei h 11 γ German HPC E ct1 GPa 34. E ct1 GPa Conclusions n i -.5 A h.47 α -.1 B α i -.3 t ei h 6 γ For the forecast of thermal stresses and cracking, the realistic modeling of the mechanical properties of early age concrete is essential. However many material models exhibit at very early age - characterized by the end of setting time of cement paste and beyond that certain weaknesses. The latter are connected with the difficulties to perform destructive tests at that age and to unambiguously relate the results to the degree of hydration. The gradual transition of concrete into solid matter can only be distinctly described by non-destructive tests. By measuring the ultrasonic pulse velocities of compression and shear waves it is possible to determine the end of the dormant phase for arbitrary concrete compositions. Satisfactory results can be attained if these measurements are performed under adiabatic curing conditions. The investigations presented in this paper show, that the end of the dormant phase depends on the same parameters as the degree of hydration. Parallel destructive tests were performed at early and very early age. Their results correlate well with those of the ultrasonic tests. On basis of these results, the material model can be improved in order to realistically forecast the early age behaviour of concrete. Acknowledgments The research reported here was supported by the German Research Foundation under Grant Ro 88/37-1/. The authors wish to express their appreciation for the financial support. 7

10 6. References 1. Gutsch, A.-W., Stoffeigenschaften jungen Betons - Versuche und Modelle, Doctoral Thesis, TU Braunschweig, Keating, J., Hannant, D.J. & A.P. Hibbert Comparison of shear modulus and pulse velocity techniques to measure the build-up of structure in fresh cement pastes used in oil well cementing, Cement and Concrete Research, 19 (1989), pp Morabito, P. ( Ed.), Round Robin Testing Program: Equipment, testing methods, test results, Research report Brite-Euram BRPR-CT , ENEL SPA,. 4. Neisecke, J., Ein dreiparametriges, komplexes Ultraschall-Prüfverfahren für die zerstörungsfreie Materialprüfung im Bauwesen, Doctoral Thesis., TU Braunschweig, Rehm, G., Waube, N.-V. & J. Neisecke, Ultraschall-Impulstechnik für Fertigteile, DAfStb-H. No. 43, W. Ernst & Sohn, Berlin, Rostásy, F.S. & M. Krauß, Modeling of degree of hydration on basis of adiabatic heat release, Research report Brite-Euram BRPR-CT , TU Braunschweig, Rostásy, F.S. & P. Onken, Konstitutives Stoffmodell für jungen Beton, Research report, IBMB, TU Braunschweig, v. Breugel, K., Numerical simulation of the effect of curing temperature on the maximum strength of cement based materials, published in RILEM, Thermal Cracking in Concrete at Early Ages. RILEM proceedings 5. R. Springenschmid ed. FN Spon, London, Westman, G., Concrete creep and thermal stresses, Doctoral Thesis, Luleå University of Technology,