SAP is relatively new material for purposes of IC but it is hardly new admixture in the concrete technology. Potentially the first application could be dated back to year 1989 when a water-absorbent polymer was used in production of high-strength concrete with aim of improving strength and durability [Has 89]. Many other uses have been successively accomplished since that moment [Tak 91][Ito 94] until the admixture was finally suggested for curing [Tsu 99]. In-between 2001-2002 the most efficient curing methodology using SAP, so-called water entrainment, was introduced and theoretically background by Jensen and Hansen [Jen 01b][Jen 02]. Due to their input, the hypothesis and experimental evidences of curing method effectiveness forwarded since 1950s and ascribed to same process, i.e. internal curing [Mat 70], could be scientifically validated for yet another IC agent.
- the advantages of using IC in general and SAP in particular
Proper curing, while being a mitigation strategy against shrinkage, is one of the most important requirements for optimum performance of a cement-based material in any environment or application [Mee 99]. Best mix designs cannot overcome all imperfect properties related to associated material, with cracking sensitivity due to autogenous shrinkage being the Achilles’ heel for UHPC, see Section 2.2.3.
The review of advantages resulting from applying internal curing using SAP in various cement-based materials is presented in Table 2.6. It can be summarized that internal curing seems truly acceptable if not optimum curing methodology for UHPC. It indeed provides the solutions for attaining all fundamental goals of curing in least invasive way, therefore in spirit of UHPC design. When SAP is used as IC agent, some disadvantages related to usage of rival materials can be furthermore overcome too. Last but not least, as signalized in Section 2.5.1, the polymeric materials do show best perspectives with regard to IC optimization which can be attained by engineering properties of the polymers. All these elements add up to great potential of SAP in the IC perspective.
Table 2.6: Analysis of curing requirements in terms of fulfilment by IC using SAP. Curing requirement aimed at
or specific benefit resultant Manner of fulfilment
Adequate water content [Mee 99] By supplying curing fluid water from the tiny internal reservoirs, the problem of capillary discontinuity that would limit curing water migration distances is prevented. The experimental evidence of maintaining satisfactory moisture content is the higher RH compared to systems without IC under sealed conditions [Jen 02][Ben 02][Wan 09][Pié 06][Est 09], and in theory reflecting free access to water. When material is additionally exposed to drying to ambient, RH distribution seems yet improved too [Zha 06].
Maintenance of adequate temperature and preservation of reasonably uniform temperature throughout the concrete body [Mee 99]
Since the IC agent can be distributed uniformly throughout concrete member (e.g. [Lur 08a][Dud 08b]), no humidity and thus heat of hydration-related temperature gradients are expected to occur. Further benefit especially from using SAP refers to advantageous control over αT [Wyr 14][Wyrzykowski and Lura 2013 Ibid.Wyr 14].
Adequate time for sufficient hydration [Mee 99]
Because the water of curing fluid needed and to be carried by the IC agent can be precisely assessed (Powers’ model based estimations [Jen 93][Jen 01b], other [TC 196-ICC]), and it will be released only when needed (i.e. in controlled manner), hydration can continue until maximum attainable degree of hydration is obtained.
Adequate protection from damaging disturbances during the early period of curing [Mee 99]
Although any curing type must protect concrete from mechanical disturbances especially when microstructure is being developed, IC does it internally. Stresses due to restrained autogenous shrinkage and cracking likelihood are indeed reduced when IC is applied. Nonetheless, fast loss of water from IC agent particles and their pronounced volume decrease, without corresponding changes in the surrounding matrix, may sometimes lead to local defects in direct vicinity of the IC agent particles, e.g. [Lam 05].
Improvement of concrete properties [TC 196-ICC]
Hydration is prolonged with IC which provides trigger for improvement in all properties related to degree of hydration. Final effect depends on SAP particularities used e.g. smaller IC agent particles may appear to be more preferable choice in enhancing strength owing to secondary effects exerted, see Section 2.5.3 for more details.
Minimizing shrinkage [TC 196-ICC] Autogenous shrinkage is reduced or even becomes converted into expansion with IC, e.g. [TC 196-ICC][TC 225-SAP][RILEM pro052][RILEM pro074]. Observations of positive impact of IC are also made for plastic shrinkage [TC 225-SAP][Dud 10b] and total shrinkage [Mec 06], including that of UHPC, see [Geo 10b] and [Sol 11][Sol 12], respectively. However, little changed [Pié 06] or even higher [Mec 09] total shrinkage of concrete with IC has been also noted, showing dependence of final effect on the time-zero choice or other factors (mix composition etc.).
Other benefits related to operation with moisture (flow barrier function, moisture buffer performance)
Beside poor curing practices, concrete can be exposed to various environmental conditions. Important in this respect, SAP as contrary to many absorptive materials characterizes with very favourable moisture buffering capacity and velocity of adjustment to RH changes [Cer 09]. In terms of durability, the potential function of cutting off capillary network ascribed to SAP [Pai 09] might be very useful too.
Other benefits SAP unlike light-weight aggregates can be added dry to the mix, known since 1989 [Has 89]. This assures homogenous distribution of polymers (e.g. [Lur 08a][Dud 08b]) and prevents them from formation of gel-blocks, being a serious problem related with usage of SAP in the pre-saturated form [Lam 05][Wan 09]. Dry application makes furthermore handling of the IC measure easier. It might be though required to make addition moment adjusted to size and content of SAP [Moe 09].
- mechanisms of IC by means of SAP
The effectiveness of IC by means of SAP is closely linked to ability of distributing the curing fluid from the particle of polymeric material to furthest region undergoing self-desiccation in its vicinity. The associated travel distance referred to as ‘paste – IC agent particle proximity’ [TC 196-ICC] and constituting so-called sphere of influence [Zhu 11] or protected paste volume [Jen 01b] will be decided by the properties of the matrix developed, on one hand, and properties of SAP, on the other hand (Section 2.5.1). It can be very complex and specific property and for each material must be investigated individually. The general belief is though that water discharge proceeds in stages (Figure 2.21) and, as recent modelling findings reveal, is governed by demand-supply mechanism rather then being triggered by capillary suction [Wyr 11]. However, should this be disturbed and/or microcracking [Hua 10] as well as growth of hydrates in pores occupied by SAP occur [Jen 02][Kle 13][Jus 15], different and more complex scenario might be expected.
- other remaining questions and problems related to usage of IC
Since no regulations regarding use of superabsorbent polymers in concrete technology have been proposed to date, although being among goals of running RILEM Technical Committee 260-RSC, there is understandable lack of consensus regarding method of implementing SAP and manner of determining the amount of water to be absorbed by it. With intention of IC purposes, the former is currently conducted with extra water (‘water entrainment’), with partial workability adjustment using superplasticizer or with SAP being applied alone (‘water sealing’). Although using SAP in option ‘water entrainment’ seems most meaningful among them when taking into consideration the origin of autogenous shrinkage, two other Figure 2.21: Generalized scheme of internal curing obtained using SAP from dispersion stage (left), through
finalized solution absorption of IC agent (middle) until cessation of IC activity (right) [TC 225-SAP], adopted from [Moe 09]. Measurements prove in general this view in cement paste, see e.g. [Trt 10].
combinations mustn’t be ruled out. In fact, since phenomena triggering autogenous shrinkage are taking effect in pores, modification of the pore system structure is reported to appear as important factor in reducing AS as the amount of curing water [Jen 01b][Lur 06]. Meanwhile, very little is known about the outcomes of e.g. pure SAP addition (i.e. without extra water) and potential benefits that could be imagined for UHPC.
Implementation also regards the method both IC variables, i.e. SAP and extra water, are introduced in the mix. Common solution is addition of SAP and extra water during dry and wet mixing, respectively, and, on stage of mix design, some replacement of fine aggregates having size similar to that expected from swollen SAP. In case of cement-based materials without aggregate of required diameter, the optional ‘on top’ method certainly appears important alternative to consider.
It is eventually noted that the most controversy continuously regards estimation of water absorbed by SAP. Many methods has been advanced and used for purpose of estimating absorbed IC fluid so far. The most goal-fit methods are certainly protocols in which fluid absorption of SAP is assessed both under load and excluding factor of changing absorption in dependence on concrete composition, therefore, where the SAP behaviour is examined in the particular cement-based material under modification. These may include, although needn’t limit to, tests on fresh concrete [Mec 06][Dud 08b][Moe 09][Ass 14a], refined pore analysis of concrete cross-sections [Jen 02][Ass 13] or, less handy and more costly, X-ray microtomography [Lur 08a], Neutron Tomography [Trt 10] or Nuclear Magnetic Resonance [Nes 09]. However, only little attention has been devoted to the tea-bag test [Zoh 08][WSP 11], which, although not perfect, is easy to perform and can be applied for any liquid, therefore also pore solution, while giving surprisingly good results in comparison to other methods [Ass 14a]. It could be an important alternative in light of changing absorption of SAP in time and being specific for SAP type applied.