Plastic shrinkage cracking and other evaporation-related impairments in 3D printed and cast concrete

Moelich, Gerrit Marius (2021-12)

Thesis (PhD)--Stellenbosch University, 2021.

Thesis

ENGLISH ABSTRACT: The Fourth Industrial Revolution (Industry 4.0) aims to make manufacturing more agile, flexible and responsive to the customer through intelligent and autonomous solutions. One Industry 4.0 technology that shows great potential for the con- struction industry is 3D concrete printing as it eliminates the need for formwork and is so-called formfree. Early estimations show the formfree attribute can re- duce construction costs and time by 28% and 50%, respectively. Furthermore, 3D printing concrete (3DPC) increases design freedom, produces less waste material and improves workplace safety. The advantages of 3DPC's formfree attribute are noteworthy. However, the absence of formwork leaves 3DPC vulnerable to rapid pore water evaporation im- mediately after extrusion, causing plastic shrinkage, plastic shrinkage cracking (PSC) and an impaired long-term durability and mechanical performance. On- site printing in dry and windy climates, common in Southern Africa, Australia, Central America and the Middle East, is of concern since the evaporation rates tend to be more severe. However, for 3DPC, the more benign evaporation rate of indoor climates cannot be precluded from evaporation-related damage. This research aims to improve the resilience of concrete printed or cast in ad- verse climatic conditions by quantifying, understanding, modelling and mitigating the consequences of rapid early age pore water evaporation. Ultimately, this study contributed several novel findings that amounted to seven journal publications. First, a method was proposed for using weather data to characterise the ex- pected on-site evaporation rate for a specific location. Active PSC mitigation measures were recommended based on the result. Thereafter, the effect of solar radiation exposure on the accuracy of the available evaporation estimation equa- tions and severity of PSC were evaluated. The results showed that high levels of solar radiation exposure significantly increase the severity of PSC, and the Jansen- Haise model was recommended to estimate the pore water evaporation rate in these conditions. A test method was proposed to induce and measure plastic shrinkage and early age cracking in 3DPC. 3DPC exhibited severe plastic shrinkage that started immediately after extrusion. Early age cracks appeared and increased in severity within the first one to two hours at a moderate evaporation rate. This behaviour was attributed to the low bleeding rate, absence of coarse aggregates, the high quantities of fines, and high surface area to volume ratio of 3DPC. It is estimated that the risk of PSC in 3DPC is three times as high as cast concrete. Decreasing the magnitude or delaying the evaporation rate decreased the plastic shrinkage and reduced the risk of cracking. Based on this result, an empirical model for the risk of PSC was proposed for 3DPC. To minimise the risk, the evaporation rate should be reduced to below 0.1 kg/m2/h until the final setting time. Evidently, 3DPC is also vulnerable to PSC at lower evaporation rates, typical in indoor climates. The eficacy of several mitigation measures for early age cracking in 3DPC was evaluated with the proposed test method. At a low dosage, short polypropylene microfibres prevented (100% reduction) the formation of PSC without adversely affecting the printability or buildability. Alternatively, increasing the structuration rate, with additives, can reduce early age cracking while improving buildability. Superabsorbent polymers also improved buildability by increasing the stiffening rate through continued pore water absorption. Discontinuities in the transfer of plastic shrinkage from one layer to the next were noticed and coined as interlayer slip. Interlayer slip and microcracking, caused by pore water evaporation from a restrained 3DPC specimen, did not reduce the mechanical and durability performance significantly. Therefore, visibly uncracked 3DPC shows satisfactory durability performance for a short pass time. Finally, internal curing, with superabsorbent polymers, increased the long- term flexural strength (19%) and interlayer adhesion (10%) of printed concrete by promoting the hydration of anhydrous cementitious particles near the interlayer. Evaporation of the interlayer moisture has the direct opposite effect. An analytical model for the interlayer adhesion was proposed based on the amount of interlayer surface moisture which was estimated from the initial surface moisture, the pass time, bleeding rate and evaporation rate. The model was validated for an interlayer bond strength reduction from 30% to 50%. However, applying the model to the experimental results of other researchers suggest that this range is extendable.

AFRIKAANSE OPSOMMING: Die Vierde Industriële Rewolusie beoog om vervaardiging meer buigsaam en re- aktief tot die behoeftes van die kliënt te maak deur middel van intelligente en outonome oplossings. 3D-drukwerk met beton (3DDB) toon groot potensiaal vir die konstruksiebedryf aangesien dit vormwerk vry is. Vroeë beramings toon dat 3DDB die konstruksiekoste en konstruksietyd kan verlaag met tenminste 54% en 75%. Verder verhoog 3DDB ontwerpvryheid, produseer minder afvalmateriaal en verbeter werkplek veiligheid. Die voordele van vormvrye beton is opmerklik, maar die afwesigheid van vorm- werk stel 3DDB bloot aan poriewaterverdamping onmiddellik na plasing wat kan lei tot aansienlike plastiese krimp, ernstige plastiese krimpkrake en 'n verminde- ring in die langtermyn duursaamheid en sterkte. Die verdamping van poriewater is veral 'n bron van kommer vir terrein drukwerk in droë en winderige klimaatstoe- stande; algemeen in Suiderlike Afrika, Australië, Sentraal-Amerika en die Midde-Ooste. 3DDB is egter ook kwesbaar vir plastiese krimpkrake in meer gematigde klimaatstoestande. Die doel van hierdie navorsing is om die weerstand van 3DDB geplaas in ongunstige klimaatstoestande te verbeter. Die nadelige gevolge a.g.v. vroeë ouderdom poriewater verdaping word bestudeer, gekwantifiseer, gemodeleer en versagmaatreëls word aanbeveel. n Metode word voorgestel vir die gebruik van liggingspesifieke weerdata om die verwagte verdampingstempo te karakteriseer. Aktiewe plastiese krimpingsver- sagmaatreëls word aanbeveel op grond van die resultate. Die effek van sonstraling blootstelling op die akkuraatheid van die beskikbare verdampingsberamingsverge- lykings word geëvalueer en die Jansen-Haise model word aanbeveel vir toestande met hoë sonstraling. Die erns van plastiese krimpkrake in gegote beton word ook geëvalueer en die resultate toon dat sonbestraling plastiese krake aansienlik verhoog. 3DDB toon ernstige plastiese krimping, selfs teen 'n lae tot matige ver- dampingstempo. 'n Toetsmetode word voorgestel om plastiese krimp en krake in vroeë ouderdom 3DDB te veroorsaak en te meet. 'n Groot hoeveelheid en hoë tempo plastiese krimping vind plaas onmiddellik na plasing met vroeë ouderdom krake wat verskyn binne die eerste een tot twee uur van verdamping. Hierdie gedrag word toegeskryf aan die lae bloeitempo, die afwesigheid van growwe aggre- gaat, groot hoeveelhede fynstof en die hoë oppervlak-tot-volume-verhouding van 3DDB. Daar word beraam dat die risiko van plastiese krimpkrake in 3DDB tot drie keer so hoog is soos gietbeton. As die verdampingstempo verlaag of vertraag word verminder die plastiese kraakrisiko. Op grond van hierdie resultate word 'n empiriese model voorgestel vir die risiko van plastiese krimpkrake in 3DDB. Die doeltre endheid van verskillende versagtingsmaatreëls vir plastiese krimp- krake in 3DDB word geëvalueer met die voorgestelde toetsmetode. 'n Lae dosis polypropyleen vesels voorkom plastiese krimpkrake sonder om die drukbaarheid of boubaarheid nadelig te beïnvloed. Die verhoging van die struktureringstempo, m.b.v. ymiddels, is voordelig in terme van kraakversagting sowel as boubaarheid. Diskontinuïteite in die oordrag van plastiese krimping van een laag na die vol- gende is opgemerk en die verskynsel is benoem tussenlaagglip. Inklemming van die 3DDB monster onder 'n verdampingslas veroorsaak beide tussenlaagglip en mikrokrake. Die verdampingsverwante skade het egter nie die meganiese en duur-saamheidsverrigting aansienlik verminder nie. Die gevolgtrekking is dat 3DDB met 'n kort laagtyd goeie duursaamheid toon nterne kuring, met superabsorberende polimere, het die langtermyn buig- sterkte en tussenlaaghegting van 3DDB verhoog deur die hidrasie van sement par- tikels naby die tussenlaag te bevorder. Verdamping van die tussenlaagvog het die teenoorgestelde e ek. 'n Analitiese model vir die tussenlaaghegting word voorge- stel gebaseer op die hoeveelheid tussenlaagvog teenwoordig wanneer die daarop- volgende lament geplaas word. Die oppervlakvog word bereken vanaf die aan- vanklike waarde, laagtyd, bloeitempo en verdampingstempo. Die model kan die tussenlaaghegting vermindering akkuraat voorspel tussen 30% en 50%. Hierdie perke sal waarskynlik in toekomstige navorsing uitgebrei word.

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