4 Results and discussions 1 Introduction
4.4 Determination of time-zero 1 Introduction
4.4.4 Final comments on time-zero
The fact that time-zero likely matches final set might be important for numerous reasons, discussion on which reader may find in Appendix G. Nonetheless, one final remark should be made on coincidence of time-zero as read from deformation curve and final set. In comparison to study of Assmann [Ass 13], who matched the end of setting with maximum deformation rate, earlier occurrence of the final set and its particular coincide with first minimum absolute value of the strain rate is revealed in own study. Since the comparable measuring system and curing conditions have been used, whereas impact of setting behaviour testing method could be excluded (based on study of [Zha 12b] and discussion in the previous section), this difference in opinion could be reasonably linked to configuration of material under investigation (UHPC vs. cement paste of w/c = 0.3 and CEM I 42.5 R as binder).
Two different aspects of the regarded impact are likely necessary to be considered here. One could be directly referred to incorporation of extra ingredients to the paste of UHPC as well as change in the effective w/c and the cement class/composition. Evidently, when the content of hydrating material is reduced, and so does effective w/c (also due to unintended adsorption mechanism of aggregates), other conditions for hydration process must result. This especially involves less paste which binder will be packed more dense and which, owing to higher fineness, can interact/dissolute faster (relative to cement in paste) in lower particle spaces after dormant/induction period ends. Under such circumstances, the particle-to-particle bridges and the shear-rigid bonds develop earlier, leading to faster rate of/more abrupt shear modulus development in UHPC than in the paste. It will be supplemented by effect owed to presence of aggregates which by provision of global mechanical effect, abrasive effect and the effect of site [Mou 11] will contribute to more favourable development of Young’s modulus
too. The manifestation of these changes is observed shift in deformation curve56. This effect finds clear validation in literature, where different composition changes are shown to affect time-ascription of time-zero in reference to setting and/or shrinkage rate, this including cement type [Epp 09], w/c or incorporation of pozzolans [Cha 07] (although for some reasons and in respect to w/c not agreeing with opinion of [Zhu 08]). It is furthermore analogous to similar factors affecting correlation between knee-point or TG parameter and setting, this typically revealed to be both cement type and w/c [Jus 00] as well as w/c and aggregate incorporation (from comparison of [Gam 13] and [Trt 13a]), respectively. In comparison, the change of mix composition could plausibly trigger other phenomena. Strictly speaking, in paste that possesses higher basic w/c, contains no aggregates or binder other than cement, meanwhile has to be vibrated during casting (which is assumption based on own experience with pastes), the likelihood of bleeding is typically higher compared to UHPC. Thus, given the bleeding affects the shape of deformation curve [Tia 08], other coincidence of two discussed points should result, as observed. Eventually, different viscosity of mixes is not concerned, although it is admitted that for penetration tests this could be important issue to consider as well, e.g. [Med 11b].
It could be concluded that deformation curve cannot be fully trusted in respect to time-zero determination whenever change of mix configuration and/or bleeding takes place.
4.4.5 Summary and concluding remarks
New criterion for determination of time-zero directly from strain-time curve was proposed in subsequence to analysis of alternative assessment approaches. According to the proposal, time-zero is experimentally defined as the last extremum before the maximum deformation rate is attained. It is far before a flattening of the autogenous shrinkage curve occurs and being often considered as mark of transition between chemical shrinkage and self-desiccation shrinkage [Justnes et al. 1996 Ibid. Che 10 or Jus 00]. The big advantage of new approach is support in other important changes taking place in concrete around the time when the fluid-to-
56 Comment 1: Why end of setting and not some other point is manifested as characteristic point in the shrinkage
curve, this could only be speculated. However, taking into account that development of Young’s modulus is close to this time-point, in deformation curve it should be manifested as first, like observed. It follows that rate of deformation reduces after end of settlement and on advancing the stage of shear-rigid bonds development (increasing stiffness of material); meanwhile, shrinkage can be regained only to certain degree afterwards provided that self-desiccation is pronounced enough. The reason for reduction of shrinkage rate in subsequence of the second maximum (which has been referred to maximum rate of deformation) is different, and could be likely attributed to resistance to the contraction of volume provided by the negative pressure in the internal voids [Yoo 14], changes in pore-size distribution [Hab 06a][Med 11b], crystallization processes and topochemical reactions of inner C-S-H [Hab 06a]. These could be as important, if not more, as self-restraining of chemical shrinkage by hardening, paying certain contribution as well [Hab 06a][Yoo 14].
solid transition is noted in low w/c systems. Another purely practical pro is obviously the handiness. On the other hand, some disadvantage found was the fact of remaining somewhat conservative as to stress generation onset or initiation of the compressive stresses reduction when using new time-zero. Decision of putting forward new description of time-zero was finally dictated by other observations made, in particular the difficulty of tracing stress generation in case of mixes with IC, producing very little free (Section 4.6.3) and simultaneously restrained autogenous shrinkage (Appendix I) as well as presence of SAP volume changes without accompanying changes in pore structure before, e.g., maximum deformation rate in shrinkage curve was recorded. Subsequently, the meaning of time-zero was analysed with support of destructive and non-destructive methods, having done additional review regarding detection possibilities and particular definitions of setting points or other for the latter (Appendix G). Both specific value of ultrasonic pulse velocity (UPV = 1165 m/s) as well as the first inflection point in UPV-t curve showed as coinciding with time-zero, and, simultaneously, as corresponding to final setting time although strictly according to American standard ASTM C403. Since results depended on norm and not the meaning behind occurrence of particular setting point as such (in theory: rapid strength development), it was concluded that matching time-zero with final set done to date on regular basis is incorrect and should be investigated in more depth in future.