Farmacodinamia

- theoretical (stages in evolution of P-wave velocity)

The most common curve of P-wave velocity (UPV) in function of time as obtained from ultrasonic testing is presented in Figure 4.50. It could be observed that the curve possessed S- shaped pattern, a typical for this kind of measurement and recorded before e.g. by [Rei 96][Lee 04][Zha 12b]. In agreement with [Smi 02], that pattern is picturing well microstructural evolution of low w/c material at the curing temperature of 20 °C. On this condition, the evolution could be characterized by at least 4 main stages in analogy to [Lee 04][Zha 12b].

0 500 1000 1500 2000 2500 3000 3500 4000 4500 0 4 8 12 16 20 24 Time t [h] U lt ra s o n ic p u ls e v e lo c it y U P V [ m /s ] original curve fit

Microstructural development of hydrating paste and related to it behaviour of pulses in concrete as well as meaning of stages could be interpreted as follows:

During stage 1, typically no signal was received, resulting in UPV equal to nil57. For mixtures that contain high amount of entrapped voids as being currently the case of finely grained UPC with only few exceptions (see Section 4.2.3), this effect could be attributed to high attenuation capability these internal discontinuities possess [Kea 89]. In low frequency range of measurement58, they likely dominated the scattering mechanism in wave propagation over the fine quartz aggregates, even if latter may be playing similar role as well [Agg 05][Zha 12b]. This impact lasts from minutes to hours and finishes at random time which, as a rule of thumb, is always before setting.

When stage 2 commenced, the first values recorded were relatively small irrespectively of mix combination, in fact falling below UPV in water (around 1500 m/s) or even air (340 m/s). It implies that the role played by the scatterers was most likely continued though this period as well but new reasoning appeared as plausible59. Be that as it may, given that cement grains were suspended in still fluid-like material state and no connected passage (of solids) existed,

57 _{In some cases, a considerable scatter of values with no defined trend has been observed during the stage 1.}

This confirms results of Pessiki and Carino [Pes 88] and yields generally larger signal attenuation ability compared to more advanced stage of hydration.

58 _{High frequencies can be assumed to be completely damped until set.}

59 _{It could be assumed, for instance, that low velocities were alternatively generated in result of solution process}

of cement in water and consequent change of internal viscous forces [Haa 75], they alternatively occurred as sign of insufficient coupling of cement particles with water [Fey 01] and/or, more generally, were result of the fact that in fresh UHPC the wave transfer media were all three: liquid, solid and gas, although restricted to fluid phase before initial set [Zhu 11a].

Figure 4.50: Stages in evolution of UPV-time curve and the corresponding characteristic events.

A

B

IP1

IP2 IP3

oscillation and motion of emulsion phase rather than propagation took place [Rei 96], leading to elongation of the wave-path length and low UPV.

As stage 2 progressed, the UPV increased only insignificantly to a higher value but of unchangeably low level. This indicated general high sensitivity of UPV to (massive) hydrate formation [Smi 02], likely ettringite [Moe 10], as well as to relative density variation [Cho 01]. None of the reasons behind it60, however, could yet pay a direct contribution to connectivity/integrity of solids and resultant construction of load-bearing structure at this hydration moment.

The so-called solid percolation threshold61 (point ‘A’ in the graph) put an end to stage 2 and gave rise to stage 3. From this important event (found mathematically as the intersection point of two straight lines tangent to data in stage 2 and 3 acc. to [Smi 02][Lee 04]) onwards, the UPV increased rapidly. The appearance of first event informed that new propagation path became available, in particular the wave propagation was granted through the solid phase62. The latter, on the other hand, yielded its network and linkage of which was gradually increasing due to silicate hydrate formation and especially C-S-H phase [Moe 10]. Soon, the advancement of connectivity brought about stiffening, including phenomenon ascribed to first inflection point in UPV curve and time-zero (or its vicinity) in this thesis. Additional events were also recorded but their meaning remained vague, see Section 4.5.3 for more details.

Eventually, stage 4 took place with a mathematically specified start point (point ‘B’ in the graph) defined as intersection point of two straight lines tangent to curve in stage 3 and stage 4. During this stage, UPV curve did not show significant increment any more and rather levelled off, to reach plateau within few days time. Such dramatic change of course indicated acquirement of fully connected solid frame, approaching the final stiffness by slid skeleton

60 _{Examples of former and the latter are ettringite formation [Voi 05] and declining rate of gravitational setting}

[Voi 05], respectively. Another school of thought presented by Popovics [Pop 94], however excludes increase in velocity due to formation of large quantities of solid hydration products if having no interconnections, or when the interconnected solid frame is tenuous (i.e. still non-rigid, non-elastic. Therefore other explanations appear plausible such as increase in viscosity of liquid phase [Pop 94], air bubble migration and workability loss [Rob 11], chemical shrinkage [Cho 01], and/or in general- increasing number of physical contacts between particles [Fey 01].

61 _{It is moment describing finalization of processing of isolated events between cement grains, their clustering}

and mutual building up bridges. The event is often ascribed to phenomena such as creation of first interconnected solid phase and creation of first interparticle bonds.

62 _{The solid phase is preferable path of ultrasonic wave propagation. It can be assumed to be attained at critical}

and filling up of last remaining pores by hydration. Only part of this so-called hardening phase could be followed due to reasons discussed in Section 4.5.4.

- experimental

Summarizing the foregoing section, the transitions points A and B as well as important changes in the UPV rate reflected as inflection points IP1, IP2, IP3, all taking place during stage 3, could be considered as important events in microstructural development in early age. Since falling in the period when the connectivity is generally only increasing, they seem good points of reference to be taken into account when analysing moment of strength build-up, setting and especially effect of IC.

The same statement cannot be applied to case of some UHPC with highest amount of extra water but containing no SAP. Examples are shown in Figure 4.51.

0 500 1000 1500 2000 2500 3000 3500 4000 4500 0 4 8 12 16 20 24 Time t [h] U P V [ m /s ] F-R.04 F-R.08 inflection points F-R.04 inflection point F-R.08

Compared to other mixtures, the curve picture for mixtures F-R.04 and F-R.08 was different and only sometimes the S-shaped pattern was recorded. This was very much likely triggered by pronounced viscosity reduction due to increase of basic w/c by extra water bringing about increasingly lower content of entrapped air, see trend in Section 4.2.3. Air is well known as major modifier of UPV curve [Zhu 11] similarly to curing temperature [Smi 02] and superplasticizer type [Trt 13b], which were maintained constant in own tests. In some cases, it brings about picture like recorded for mix F-R.08, being picture characteristic for the air void- Figure 4.51: Evolution of UPV in time for mixtures with extra amount of water, but no addition of SAP.

free mixtures of low w/c [Cho 01]63. In other cases, for certain low values of air entrapped, this modifier appeared mainly to perturb the curve evolution built in stages 1, 2 and partly 3, even when the selfsame composition was concerned and the only difference being different production dates. In consequence of apparent modification, important events to report appeared to be alteration of UPV which changes from constant or quasi-constant value higher than longitudinal velocity in water towards much lower one or just the opposite, i.e. UPV increase starts. However, to maintain regularity of data evaluation, only the well definable inflection points were concerned in the following.

ii. Approach based on temperature

To understand changes occurred in early ages even better while also to examine meaning of infection points, changes in the in-situ temperature evolution were examined in addition to ultrasonic test. In principle, referring to a measure of heat of hydration and comparing it to a tool of monitoring mechanical processes is fully justified given possibility of translation of both results into degree of hydration or reaction, see examples in [Moe 09] and [Rob 11], respectively.

- theoretical (stages in evolution of temperature curve)

For assessing exothermic output of hydration reactions typically diverse calorimetric methods are used where the heat produced is expressed after taking binder quantity into account64. More recently, however, it has been showed that this step might not be necessary. According to Trtnik and Turk [Trt 13b], a platform for correlation between results of in-situ temperature (in °C) and P-wave velocity should also exist when referring to important changes in their evolution, example being their rate maxima65. Other researchers [Özt 06][Gab 11] go even

63 _{UPV higher than in water for concrete that has not set yet is rare but has been sometimes recorded for low w/c}

materials that contained theoretically no air entrapped. It has been ascribed to low quantity of water [Cho 01], although other underlying reasons might be imagined as plausible, e.g. higher concentration of solid phase at low w/c or presence of agglomerates.

64 _{Calorimetric measurements (or in other words heat of hydration investigations) can be generally performed}

under three different conditions, i.e. isothermal (in the so-called conduction calorimetry), where it is the ambient temperature that is maintained constant, semiadiabatic/adiabatic, where heat exchange is prohibited due to insulation of different level of efficiency or according to heat of solution method where heat of hydration is assessed by measuring the temperature rise when the cement is decomposed in an acidic solution. Of these three, the method used at present (i.e. in-situ temperature record of a sealed sample) most resembles isothermal/conduction calorimetric method and possesses similar advantages including allowance of early measurement start. On contrary, relatively large specimen made of low conductivity material could be tested at present, which should be borne in mind as a step towards overcoming the major drawback of the isothermal conduction calorimetry, i.e. lack of realistic and repeatable results owing to small sample size.

65 _{As a rule of thumb, measurements of temperature on the same batch of concrete but cured under different}

conditions (with respect to heat insulation measures and similarly to curing temperature) are likely to give different time association of important evolution events, an issue also verified in this work, see Section 4.3.2 and

further in simplifying the comparison but without loosing the scientific correctness. In particular, instead of performing the measurements in semi-adiabatic conditions or similar (e.g. those following which temperature rise in test samples is more than extra 20 °C over room temperature) they suggest less severe test conditions, these being isothermal or ones in which temperature of sample increases only by few degrees relative to ambient. Experience showed that following them, sufficient reflection of hydration process resembling that in calorimetric conditions is found [Özt 06] while changes in both variables, P-wave velocity and in-situ temperature, are very likely to be connected when they follow similar trend [Voi 05]. This should apply especially to period between end of induction/dormant period or beginning of resistance test and maximum of the temperature, i.e. of main interest currently.

- experimental

A typical temperature vs. time curve as obtained from tests on selfsame batch of exemplary concrete as in P-wave velocity measurement is shown in Figure 4.52. It can be seen that, as expected, evolution of temperature followed the classic heat of hydration curve and the characteristic stages of its progress. This involves pre-induction period (also known as initial exothermic reaction period) when heat only reduces, induction/dormant period when temperature stabilizes, acceleration period (also called as nucleation and growth period) when heat production regains on strength again, and, eventually, deceleration period when the temperature reduces for the second time after attaining the maximum (2nd peak).

4.3.3. The effect will be similar if not identical when the sample size changes alone or simultaneously too and no variation in the binder content has been acknowledged. Therefore, only under specific set of conditions the two may equalize the effect exerted on other properties, one of examples being presented in [Med 11b]. In this work, by using the samples of identical size and their exposure to the selfsame curing conditions during UPV and temperature measurements as well as with temperature sensor being embedded at exactly the selfsame position, the potential sources of errors were eliminated.

20 21 22 23 24 25 26 27 28 0 4 8 12 16 20 24 Time t [h] T e m p e ra tu re T [ °C ] original curve fit

Among the stages, one important event is beginning of the temperature rise [Smi 02] or, in other words, start of accelerated exothermic reactions of cement [Özt 06] which could be defined as minimum on temperature curve [Özt 99]. The singularity is assumed to coincide with the end of induction period and the beginning of stiffening process that is also bringing about coalescence of stable skeleton of cement matrix [Özt 99]66. It is different to intersection point (ITP) of two straight lines tangent at point of minimum temperature and first inflection point of temperature curve; here another relevant event is addressed, in particular the moment when temperature after initial slow increase started to rise at incomparably higher rate, as already seen in Figure 4.3867. Yet, since preliminary data analysis showed that inflection points in UPV-t curves appeared at later ages, other characteristic points in temperature evolution were taken into account as well, this including maximum temperature rate, i.e. first inflection point on T-t curve (IP1) and second peak temperature (2nd PEAK). Evaluation finally involved parameter important from engineering viewpoint and referring to measure of heat developed ( T).

66 _{Differences in monitoring systems used as well as in mix compositions could be named as underlying reasons}

for significant change in slope of UPV-t curve (point A in Figure 4.50) to occur earlier than one in case of in-situ temperature (ITP in Figure 4.52) as opposite to [Cho 01][Smi 02] but in agreement with [Voi 05][Gab 11][Trt 13b]. Still, the meaning of the event referred to as beginning of temperature rise could be somewhat similar, e.g. start of formation of minimum hydrates content or otherwise modification of nature of the bridges being the prerequisite towards the abrupt rise in UPV.

67 _{Note that both points mentioned could be highly important for UHPC, since low w/c materials set soon after}

the end of dormant period [Özt 06][San 09], i.e. when the temperature of sample due to increasing hydration heat starts to increase, having confirmation for UHPC provided e.g. in [Sch 02b][Yoo 13].

Figure 4.52: Characteristic events in temperature-time curve and their determination.

ITP IP1

2nd PEAK

∆_T