Visualization of water penetration in cementitious materials with superabsorbent polymers by means of neutron radiography D. Snoeck a , S. Steuperaert b , K. Van Tittelboom a , P. Dubruel b , N. De Belie a, ⁎ a Magnel Laboratory for Concrete Research, Department of Structural Engineering, Faculty of Engineering, Ghent University, Technologiepark Zwijnaarde 904, B-9052 Ghent, Belgium b Polymer Chemistry and Biomaterials Group, Department of Organic Chemistry, Faculty of Sciences, Ghent University, Krijgslaan 281, B-9000 Ghent, Belgium abstract article info Article history: Received 14 September 2011 Accepted 4 May 2012 Keywords: Cracking (B) Permeability (C) Transport properties (C) Polymer (D) (Hydrogel) Self sealing Concrete cracks due to its low tensile strength. As both harmful gases and fluids may enter the concrete by migrating into cracks, the durability is endangered. The service life decreases, repair costs rise and buildings could structurally decline. In the current research, crack sealing is enhanced by the use of superabsorbent polymers (SAP). When cracking occurs, SAP particles are exposed to the humid environment and swell, sealing the crack. By means of neutron radiography, the moisture distribution is studied during capillary absorption and water permeability tests. Capillary absorption in a crack and water permeability through a crack are reduced in specimens containing SAP particles. SAP particles are able to seal the crack, thus allowing a recovery of the water-tightness of the structure. The total uptake of potentially harmful substances hereby lowers, leading to an enhanced long-term durability and lower maintenance costs. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Concrete is prone to cracking and the cracks interconnect flow paths for water, possibly containing harmful substances [1]. Water can move in different states and therefore water is usually referred to as moisture. The moisture movement is a combination of water vapour in the pores, liquid water in larger pores, bound water at the pore walls and bound water in the gel [2]. The ingress of water could induce steel corrosion, frost attack, chemical attack and internal expansion, endangering the durability of a structure. One of the main factors which endangers the durability in cracked concrete is the water movement through cracks. A smart cementitious material which has the property to stop or to partially prevent water movement in a crack would therefore have a clear added value. Hydrogels, or superabsorbent polymers (SAP), have the ability to absorb a significant amount of liquid from the surrounding environ- ment (up to 500 times their own weight) and to retain the liquid within their structure without dissolving. SAP particles swell due to the osmotic pressure difference between the hydrogel and the external solution. This property has promoted the use of SAP in the hygiene industry, but SAP particles are also useful in building applications. SAP particles have been used in concrete to decrease the autogenous shrinkage by providing internal curing [3,4]. Lee et al. [5] and Snoeck et al. [6] investigated the incorporation of SAP in concrete in order to obtain self-sealing properties. When liquids enter a crack, SAP particles along the crack faces will swell and block the crack. This is reflected in a decrease of water permeability through a crack. Also, Snoeck et al. [6] investigated the use of SAP to obtain self- healing properties as water absorbed by SAP can be transferred to the cementitious matrix for further hydration and the precipitation of calcium carbonate. The above mentioned qualities of SAP in cementitious materials are a result of the swelling capacity. The water absorption capacity of SAP can be measured in several ways, all based on mass or volume changes or the measurement of an indirect quantity [7]. An example of the latter is a measurement of the concentration change of an exposure liquid component, which is not absorbed by the SAP particles [8]. By this indirect measurement, the amount of liquid absorption can be determined. The absorption of single SAP particles can be measured by gravimetric methods (difference in mass), as well as optical microscopy. The tea bag method is frequently used for multiple particles. Dry SAP particles are hereby placed into a permeable bag, which is subsequently submerged into the liquid. The difference in mass before and after submersion equals the absorption capacity. The water held by capillary forces between SAP particles, however, cannot be totally removed, even by centrifugation. This leads to a miscalculation of the absorption capacity and this is a drawback in the tea bag method. A better method is the one described by Jensen [7] which stipulates the volume increase of loosely packed SAP particles by means of graduated cylinders. The swollen SAP particles are heavier than the fluid, so the packed amount can be recorded. This amount is linked to the absorption capacity, excluding Cement and Concrete Research 42 (2012) 1113–1121 ⁎ Corresponding author. Tel.: + 32 92645522; fax: + 32 92645845. E-mail address: nele.debelie@UGent.be (N. De Belie). 0008-8846/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2012.05.005 Contents lists available at SciVerse ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp