Testing of chloride penetration and carbonation of concrete with superabsorbent polymers
Zelfhelend beton: oplossing voor Brusselse tunnelproblematiek?
Beton is vandaag de dag het meest gebruikte bouwmateriaal ter wereld. Huizen, kantoorgebouwen, fabrieksgebouwen maar ook tunnels, bruggen en wegen worden gemaakt uit dit sterke, maar tevens brosse materiaal. De treksterkte van beton is immers tien keer kleiner dan de druksterkte, wat beton zeer gevoelig maakt voor scheurvorming. Via deze scheuren kunnen water, schadelijke stoffen (zoals chlorides aanwezig in zeewater of strooizout) en gassen (zoals CO2 aanwezig in de lucht) het beton binnendringen en zo tot een versnelde corrosie van de wapening leiden. De gevolgen hiervan zijn niet te onderschatten: naast een verminderde duurzaamheid en structurele integriteit van het bouwwerk, kunnen onderhouds- en reparatiekosten zeer hoog oplopen. Ook de indirecte kosten die hieruit volgen - denk aan het sluiten van de tunnels in Brussel en de hiermee gepaard gaande files of de erbarmelijke staat van sommige wegen die leidt tot schade aan voertuigen en ongevallen - swingen de laatste jaren danig de pan uit.
Oplossingen uit het verleden zoals extra wapening plaatsen of scheuren manueel dichten zijn vaak duur en arbeidsintensief. Idealiter herstelt het beton zichtzelf wanneer een scheur gevormd wordt, vergelijkbaar met het genezen van een snee in het menselijk lichaam. Beton bezit de intrinsieke eigenschap om scheuren te herstellen, de zogenaamde autogene heling, mits het om zeer fijne scheuren (< 30 µm) in de aanwezigheid van water gaat. Dit water is van groot belang aangezien het zal reageren met cementpartikels (de zogenaamde hydratatie-reactie) en alzo beton zal vormen. In realiteit komen echter frequent grotere scheuren voor en is water niet altijd voor handen. Daarom wordt er de laatste jaren veel onderzoek verricht naar mechanismen die in staat zijn grote scheuren in beton te dichten, zelfs in afwezigheid van water. Een veelbelovende oplossing is het gebruik van superabsorberende polymeren (SAPs) in beton.
Superabsorberende polymeren kunnen tot maar liefst 500 keer hun eigen gewicht in vloeistof absorberen, met een gezwollen hydrogel als resultaat. Hedendaagse toepassingen van SAPs zijn talrijk: de hygiëne-industrie (luiers), de medische industrie (intelligente pillen), de agrarische industrie (irrigatie-alternatief)… Ook in de bouwnijverheid hebben SAPs recent hun intrede gemaakt als hulpstof in zelfhelend beton. Voor deze toepassing worden SAPs in poedervorm bij de andere bestanddelen van beton (zijnde cement, zand, granulaten en water) gemengd. Wanneer SAPs vervolgens in contact komen met water dat via een scheur het beton binnendringt, zwellen ze en vullen alzo de scheur. In een later stadium geven de SAPs hun geabsorbeerd water af aan het omringende beton. Dit leidt tot een verdergaande hydratatie van ongehydrateerde cementpartikels en stimuleert de vorming van calciumcarbonaatkristallen. Deze laatste worden gevormd door de reactie van calcium aanwezig in het beton met CO2 opgelost in water. Het gecombineerde effect van de verdergaande hydratatie en de calciumcarbonaatkristallen kunnen scheuren tot 100 µm volledig herstellen. Figuur 1 toont een scheur van 112 µm breed die na 28 dagen volledig hersteld is door toevoeging van SAPs. De vorming van witte calciumcarbonaatkristallen zorgt ervoor dat de scheur sluit en de dichtheid van het beton herwonnen wordt. Hierdoor wordt de indringing van schadelijke stoffen in het beton via de scheur afgeremd of zelfs helemaal gestopt, met een aanzienlijke daling van wapeningscorrosie als resultaat.
Figuur 1: Volledige heling van een 112 µm scheur na 28 dagen door de toevoeging van superabsorberende polymeren (SAPs) aan beton. De getallen in de rechterbenedenhoek geven de ouderdom van het beton in dagen.
De volgende fase van dit onderzoek focust op de invloed van de toevoeging van SAPs op de weerstand van beton tegen chloride-indring en carbonatatie. Wanneer chlorides het beton binnendringen en de wapening bereiken, tasten zij de beschermingslaag van de wapening aan met chloride-geïnduceerde corrosie tot gevolg. De belangrijkste bronnen van chlorides in beton zijn zeewater en dooizouten. Carbonatatie-geïnduceerde corrosie doet zich voor wanneer door de reactie van koolstofdioxide (CO2) uit de lucht de wapening begint te roesten. Beide types corrosie doen de duurzaamheid van het beton drastisch dalen en kunnen op termijn zelfs tot falen van de constructie leiden. Corrosie is immers een expansieve reactie die het beton (verder) doet scheuren en afschilferen en tot een gereduceerde wapeningsdoorsnede en verminderde hechting tussen wapening en beton leidt.
Wanneer SAPs toegevoegd worden aan vers beton, zullen ze een deel van het mengwater absorberen en zwellen. In verhard beton geven de SAPs het geabsorbeerde water af aan het beton en kunnen scheuren geheeld worden. Bij deze waterafgave zullen de SAPs echter krimpen en lege macro-poriën achterlaten in het beton. De toevoeging van SAPs verhoogt dus de porositeit van het beton. Deze verhoogde porositeit heeft een negatief effect op zowel de chloride-indringsweerstand als de carbonatatieweerstand, aangezien chlorides en CO2 makkelijker en dieper kunnen penetreren in poreus beton. Wapeningscorrosie zal bijgevolg sneller optreden wanneer SAPs worden toegevoegd aan beton.
Dit negatieve effect van SAPs op chloride-indringing en carbonatatie kan echter tegengegaan worden door het gebruik van pH-responsieve SAPs. Dit type SAP zwelt weinig bij hoge pH, zoals tijdens het mixen en verharden van het beton (pH 12,8) zodat een minder poreus beton wordt gevormd. Wanneer er echter een scheur optreedt, zal de pH dalen naar 9 à 10, bijvoorbeeld door de indringing van CO2 of chloriden. PH-responsieve SAPs zullen bij deze lage pH wel veel water absorberen en zwellen, wat ideaal is voor het opvullen en herstellen van scheuren.
PH-responsieve superabsorberende polymeren stimuleren de zelfhelende eigenschappen van beton zonder in te boeten op chloride- en carbonatatieweerstand, met een hogere duurzaamheid, minder onderhoud en verlaagde kost van betonnen constructies tot gevolg.
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