Two mix modifications were applied to both groups of concretes studied. - modification 1
The major alteration was introduction of internal curing or, as intended for comparison reasons, one of its elements i.e. either pure SAP or extra water. Unlike the widespread manner of formulating the composition of internally cured concrete, where some part of aggregates of one or more kind is removed in order to maintain original paste content constant (e.g. see [Mec 06] for latter), rival and more practical approach was followed. In particular, both SAP and extra (curing) water or, alternatively, one of the two of variables was added as new components to the concrete mix. In other words, they were put ‘on top’ of original mix composition, hence the name given: ‘on top’ approach. This meant that volumetric fractions of ingredients in 1 m³ underwent some reduction, however, mutual proportions including relation to cement mass remained unchanged, see Appendix C and Section 3.2.5, respectively.
Even though above-mentioned approach was not perfect, for fair interpretation of IC effects, it appeared to be more appropriate solution to use. The number of arguments gathered in the short review given in Table 3.3 does in fact show this very clearly.
Table 3.3: Short analysis of different aspects related with ‘on top’ approach.
Action Impact1 Brief description of the gist and problem solution (if necessary)
Addition of new components -
Volume of the concrete’s shrinking part decreases with every increase of IC variables. On the other hand, because ‘on top’ approach is used, the degree of internal restraint is largely fixed and thus allows easy elimination of effect caused by original mixture modification with IC. To make comparison fair, the corresponding result for the mixture without IC would only need to be rescaled for the amount of cement, and, consequently, other ingredients equivalent to mixture with IC (i.e. assuming neglected part undergoes no shrinkage).
Maintaining basic ingredient of the mix in their original
mutual proportions +
Degree of internal restraint (attributed to aggregates and perhaps unhydrated cement cores as well as some hydrates) is preserved when ‘on top’ approach is applied.
Table 3.3: Short analysis of different aspects related with ‘on top’ approach (continued).
Action Impact1 Brief description of the gist and problem solution (if necessary)
Maintaining basic ingredients of the mix in their original mutual proportions
+
Because paste/aggregate interfacial zone can occupy significant part of total cement paste volume in typical concrete (other than UHPC though!), there would be an ease of water transfer between the cement paste/aggregate interfacial zone and the bulk phase, e.g. [Hal 95][Sch 07b]. A small variation in aggregate content such as 5 % for aggregate fraction 50-60 % could change percolation of such zones and thus impact the permeability. With ‘on top’ approach, however, initial permeability very likely remains unchanged.
+
Assuming all extra water has been absorbed by SAP, in ‘on top’ approach, each particle remains covered with selfsame paste and water film thickness. This does not hold true for the alternative approach, see Section 4.2.2 for more details.
+
Additionally to providing internal restraint, aggregates of any specific surface area or absorption can be involved in various processes related to shrinkage as well as basic cause of its origin in autogenous conditions (hydration). In theory, this involves aggregate shrinkage [Hyo 13] and stimulation of hydration [Haa 75][Tas 98][Mou 11][Rah 12]. By applying ‘on top’ approach this issue is not a concern.
+
Because no aggregates are removed in ‘on top’ approach, there is no interference into secondary phenomena which may affect concrete’s workability, ball-bearing phenomenon for instance. This would be of paramount importance given absorption of IC agent can be estimated from consistency measurement (cf. Section 4.2.2).
1 refers to effect on interpretation precision; could be positive (+) or negative (-)
Important decision made in the framework of modification regarded execution of the IC- incorporating mix design in practice. Of the two possible manners of polymeric admixture introduction, dry application and homogenization of SAP with other dry ingredients was preferred. This meant that the extra water had to be introduced separately, the moment for which was shifted towards wet mixing and involved blending with other wet components to be added beforehand (note that the same solution was also applied when no SAP addition was planned). The basis for such decision was the success of uniform distribution of SAP porosity in rival composition of UHPC studied by the author [Dud 06] and only one study [Lam 05] showing otherwise i.e. poor dispersion of quality-equivalent IC agent. It has been assumed that it is the usage of very low w/c and high superplasticizer content as well as fines, and, consequently, high viscosity that secures preferable distribution of the SAP particles in UHPC. As experience shows, pre-saturation of SAP with tap water prior to mixing clearly fails in this respect, see [Wan 09]. If so, neither alternative approach becomes option for UHPC, for which best possible packing of ingredients is at origin of improvement of mechanical properties, nor could the IC effect be fairly quantified.
Important enough, it must be borne in mind that only on addition in ‘dry configuration’, there was guaranteed control over IC agent’s sorption behaviour. All other reasons aside, this would not hold true in case of the alternative approach due to change of pH. As in such case the pH changes abruptly from lower value (that of distilled or tap water absorbed by SAP) to higher one (immersion of swelled polymer in pore solution), there would be considerable partial release of the fluid carried, in analogy to result showed in Figure 3.2 or the similar study result on pure SAP by Pourjavadi et al. [Pou 13]. Meanwhile, only some of the water deposed of could be absorbed back under the new circumstances. This argument finally prevailed on implementation method chosen.
Figure 3.2: Swelling response of an exemplary superabsorbent polymer to cycles of immersion in distilled water
and, subsequently, in saline solution in comparison to behaviour after single swelling in the latter, after [Shu 11].
- modification 2
The original M2Q and B5Q were proposed as fibre-incorporating versions of UHPC. In own experiments, however, the fibre reinforcement was removed for majority of finely grained
UHPCs tested and some of mixtures containing coarse aggregates. This modification was
motivated by number of problems related to usage of fibre, largely discussed in [Epp 10].
Usage of steel fibres is challenging for at least two more reasons. Certainly and despite opinion of some researchers [Bon 97], fibres are critical component of UHPC as they play fundamental role mitigation of autogenous shrinkage, see review in Appendix B. The success, however, very much depends on the fibre orientation. Without measures to control reinforcement alignment, being even more difficult for UHPC with coarse aggregate [Cwi
Time [h] 700 600 500 400 300 200 100 0 Sw e lli n g c a p a c it y [g o f w a te r / g o f SAP] Swelling Cycle-1 Deswelling Cycle-1 Swelling Cycle-2 Deswelling Cycle-2 Swelling Cycle-3 Deswelling Cycle-3 Swelling in 0.003 M NaCl Model fit 0 6 12 18 24 30 36 42 48 54 60 66
08], it seemed reasonable to exclude fibres from the mix. Only doing so, separation of potential effect of fibres from the one ascribed to IC was possible. Be that as it may, for sake of argument, few mixtures containing steel fibres were tested as well, this including mixtures varied by the presence of coarse aggregates and IC, see UHPCs having additional “f” in their labels in Table 3.4 and 3.5 (to follow).
Whenever the second modification was executed, change in concrete composition analogous to that exerted by inclusion of extra water was attained. Speaking about details, upon elimination of fibres the rest of components increased in content, however, with no change to mutual proportions.