Superabsorbent polymer (SAP) is a fairly novel admixture in concrete and other cement-based materials with various potential applications, e.g. internal curing, improving freeze-thaw resistance, aiding self-sealing and self-healing, control of rheological properties, and others [1–3]. SAP rapidly absorb large amounts of mixing water from the freshly prepared concrete, typically in excess of 10 to 40 times their own dry mass. While absorbing part of the mixing water, they swell and create macro-sized water reservoirs. The absorbed liquid is later released into concrete in response to osmosis or capillary pressure, the latter driven by the process of self-desiccation or drying [2, 4].
While in most applications SAP are used to improve the performance of concrete during hardening and in the hardened state [5–7], they can also affect the behavior of freshly mixed concrete in various ways. While absorbing part of the water, they in general reduce the flowability [8, 9]. On the other hand, they may stabilize the concrete mix due to increased viscosity. The behavior of concrete with SAP strongly depends not only on the absorption capacity of mixing water (or, more precisely, pore fluid), but also on the absorption and desorption kinetics of the SAP [4, 10–12]. For most applications, it is desired that SAP absorb the mixing water rapidly and retain it for a desired period, e.g. until setting. This kinetics will also strongly interact with the specific mixing and casting procedures.
With the recent introduction of SAP into the industrial practice of cement-based materials, appropriate testing and characterization methods are sought that could aid practitioners in verifying the presence and performance of this admixture. Some procedures have been developed for quantifying the amount and even the size distribution of SAP-originated pores, similar to those used for assessing the air void system with air-entraining agents, e.g. X-ray tomography or microscopy [2, 13–15]. However, such methods can only be applied to hardened concrete and in most cases require elaborate specimen preparation (cutting, polishing) and evaluations. The presence of SAP and their overall effect can also be assessed in tests that aim at a specific performance of the concrete. A prominent example are measurements of autogenous shrinkage, in which the reduction of shrinkage with specific types of SAP becomes evident [16].
At the same time, a simpler test method is sought to detect the presence of SAP at the construction site in as-delivered, fresh concrete. The desired test method would be similar in its simplicity and accessibility to the fresh-state tests used for example to assess the action of water-reducing admixtures (spread, slump, etc.) or air-entrainers (air content in fresh concrete). Such tests deliver immediate answers (in a matter of minutes) and rely on rather simple, repeatable procedures and affordable devices. In the case of the proposed test, the verification of the presence of SAP (and possibly the quantification of their amount in the concrete mix) would serve as a step in the verification of the mixture composition. By confirming the presence of SAP, the expected performance of concrete may be anticipated, e.g. with regard to internal curing or other applications. This can be considered analogous to the aforementioned air-entrainment test. In this test, the content of air is determined in the bulk of concrete using a pressure porosimeter. Although the mere fact of a high air content does not automatically translate to better frost resistance of concrete (because the air void size distribution is not measured on site), it allows to verify that indeed a proper amount of air was entrained. This means that the concrete with entrained air most likely also has a sufficient amount of small air bubbles spaced at sufficiently short distances to exhibit good frost resistance. To come back to the intended case, concrete in which SAP have been detected (and possibly their amount measured) can be expected to benefit from internal curing and to exhibit lower autogenous shrinkage in an analogous way in which concrete with extra air content is expected to exhibit better frost resistance.
A major technical challenge in assessing the amount of SAP is that usually very small quantities of SAP are used in concrete, typically 0.1–0.3% by mass of cement [17]. After absorbing part of the mixing water they constitute only a couple percent of the concrete volume. At such small quantity, and owing to the usually small size of the swollen SAP particles (usually fraction of a millimeter) and their translucent appearance, the SAP are very hard to be spotted visually in the bulk of fresh concrete, let alone be quantified. We therefore developed a method in which the SAP particles are first separated from the concrete mix by washing and sieving so that the SAP particles can be visually detected. This constitutes the first, qualitative part of the test procedure proposed in this paper (Test-1). In the next step, the amount of the collected SAP is quantified with relatively simple thermogravimetric test procedures (Test-2). This second type of test requires longer times (owing mainly to the time necessary for the drying of the collected material) and more resources (e.g. ovens, scales) and hence falls outside the domain of typical field tests. However, it constitutes a rather simple procedure compared to more elaborate tests carried out on hardened concrete. It may thus be used in certain cases where a relatively fast outcome is necessary or limited resources are available at a testing laboratory. The aim of this study is to aid practitioners in applying the test methods in practice and assessing their reliability.
To test the repeatability of the methods and identify potential issues, an interlaboratory study was carried out as part of the activities of RILEM TC 260-RSC. Fourteen laboratories from twelve countries participated in the tests. All tests were carried out using two different SAP products mixed in concrete with prescribed volume proportions. Additionally, blind tests were carried out at one laboratory where the operators tested concrete mixes with and without SAP without a prior knowledge on the concrete mix compositions.