Understanding the role of rheology in the plastic settlement and shrinkage cracking of early age concrete

Understanding the plastic (settlement/shrinkage) cracking phenomena of early‐age concrete is important in‐order to establish a holistic approach to minimise its occurrence. One of the factors associated with early‐age concrete is the rheo‐related behaviour which occur simultaneously within the timeframe known for plastic cracking. It is therefore useful to establish their links to broaden the knowledge of plastic cracking. This study is a novel evaluation of the influence of rheo‐physical and rheo‐viscoelastic behaviour on the plastic cracking behaviour by systematically altering these behaviours of formulated concrete mixes and extensively characterising them. The theory and frameworks for linking the behaviours were presented and established via statistical and analytical approaches. Significant rheo‐related parameters found to influence plastic cracking phenomena include yield stress, structuration, creep and stress relaxation. The rheo‐mechanics modelling suggests that the plastic cracking initiation tends to be a ductile failure that is pressure insensitive and sufficiently represented by von Mises criteria. This study opens up a consciousness to start evaluating mitigation strategies directed towards the materials optimisation of concrete mixtures to minimise the occurrence of plastic cracking in early‐age concrete.The World Academy of Science cum South Africa’s National Research Foundation and South Africa’s Pretoria Portland Cement.http://letters.rilem.net/index.php/rilemam2023Civil Engineerin

Plastic shrinkage cracking in conventional and low volume fibre reinforced concrete

Thesis (MScEng)--Stellenbosch University, 2012.ENGLISH ABSTRACT: Plastic shrinkage cracking (PSC) is the cracking caused by the early age shrinkage of concrete within the first few hours after the concrete has been cast. It results in unsightly surface cracks that serve as pathways whereby corroding agents can penetrate the concrete which shortens the expected service life of a structure. PSC is primarily a problem at large exposed concrete surfaces for example bridge decks and slabs placed in environmental conditions with high evaporation rates. Most precautionary measures for PSC are externally applied and aimed to reduce the water loss through evaporation. The addition of a low dosage of polymeric fibres to conventional concrete is an internal preventative measure which has been shown to reduce PSC. The mechanisms involved with PSC in conventional and low volume fibre reinforced concrete (LV-FRC) are however not clearly understood. This lack of knowledge and guidance leads to neglect and ineffective use of preventative measures. The objective of this study is to provide the fundamental understanding of the phenomena of PSC. To achieve the objective, an in depth background study and experiments were conducted on fresh conventional concrete and LV-FRC. The three essential mechanisms required for PSC are: 1→ Capillary pressure build-up between the particles of the concrete is the source of shrinkage. 2→ Air entry into a concrete initiates cracking. 3→ Restraint of the concrete is required for crack forming. The experiments showed the following significant findings for conventional and LV-FRC: PSC is only possible once all the bleeding water at the surface has evaporated and once air entry has occurred. The critical period where the majority of the PSC occurs is between the initial and final set of concrete. Any preventative measure for PSC is most effective during this period. The bleeding characteristics of a mix have a significant influence on PSC. Adding a low volume of polymeric fibres to concrete reduces PSC due to the added resistance that fibres give to crack widening, which increases significantly from the start of the critical period. The fundamental knowledge gained from this study can be utilized to develop a practical model for the design and prevention of PSC in conventional concrete and LV-FRC.AFRIKAANSE OPSOMMING: Plastiese krimp krake (PSK) is die krake wat gevorm word a.g.v. die vroeë krimping van beton binne die eerste paar ure nadat die beton gegiet is. Dit veroorsaak onooglike oppervlak krake wat dien as kanale waardeur korrosie agente die beton kan binnedring om so die dienstydperk van die struktuur te verkort. Dit is hoofsaaklik ʼn probleem by groot blootgestelde beton oppervlaktes soos brug dekke en blaaie wat gegiet is in klimaat kondisies met hoë verdamping tempo’s. Meeste voorsorgmaatreëls vir PSK word ekstern aangewend en beperk die water verlies as gevolg van verdamping. Die byvoeging van ʼn lae volume polimeriese vesels is ʼn interne voorsorgmaatreël wat bekend is om PSK te verminder. Die meganismes betrokke ten opsigte van PSK in gewone beton en lae volume vesel versterkte beton (LV-VVB) is vaag. Die vaagheid en tekort aan riglyne lei tot nalatigheid en oneffektiewe aanwending van voorsorgmaatreëls. Die doel van die studie is om die fundamentele kennis oor die fenomeen van PSK te gee. Om die doel te bereik is ʼn indiepte agtergrond studie en eksperimente uitgevoer op gewone beton en LV-VVB. Die drie meganismes benodig vir PSK is: 1→ Kapillêre druk tussen die deeltjies van die beton is die hoof bron van krimping. 2→ Lugindringing in die beton wat krake inisieer. 3→ Inklemming van die beton is noodsaaklik vir kraakvorming. Die eksperimente het die volgende noemenswaardige bevindinge opgelewer: PSK is slegs moontlik indien al die bloeiwater van die beton oppervlakte verdamp het en indien lug die beton ingedring het. Die kritiese periode waar die meerderheid van die PSK plaasvind is tussen die aanvanklike en finale set van die beton. Enige voorsorgmaatreël vir PSK is mees effektief gedurende die periode. Die bloei eienskappe van ʼn meng het ʼn noemenswaardige effek op die PSK. Die byvoeging van ʼn lae volume polimeriese vesels tot beton verminder die PSK deur die addisionele weerstand wat die vesels bied teen die toename in kraakwydte. Die weerstand vergroot noemenswaardig vanaf die begin van die kritiese periode. Die fundamentele kennis wat in die studie opgedoen is, kan gebruik word vir die ontwikkeling van ʼn praktiese model vir die ontwerp en verhoed van PSK in gewone beton en LV-VVB

Cracking of Plastic Concrete in Slab-Like Elements

Thesis (DEng)--Stellenbosch University, 2016.ENGLISH ABSTRACT: The cracking of plastic concrete involves two cracking types namely: plastic settlement cracking which is caused by differential settlement of the concrete and plastic shrinkage cracking which is caused by evaporation of free concrete pore water. These cracks are mainly a problem for slab-like elements exposed to conditions with high evaporation rates and typically occur within the first few hours after the concrete has been cast. The early occurrence of these cracks greatly reduces the durability and service life of a concrete structure. These cracks remain a problem in the construction industry even though there are several effective, but mostly neglected, precautionary measures. The reasons these cracks remain a problem are due to the complex nature of the cracking as well as the lack of a unified theory or model that can account for all the complexities involved. With this in mind, this study aims to fundamentally understand both plastic settlement and plastic shrinkage cracking in slab-like elements individually and combined as well as to determine the tensile material properties of plastic concrete. Once the cracking is fundamentally understood the final objective is to develop a model that can simulate the cracking of plastic concrete using a finite element method approach. The fundamental understanding of these cracks was obtained by conducting various tests on different mixes at various climates and in various moulds. The tests showed that both crack types can occur separately, where plastic settlement cracking occurs first in the form of multiple cracks at the surface as well as shear induced cracks beneath the surface, followed by plastic shrinkage cracking in the form of a singular, well defined crack. In addition, a significant deviation from the individual cracking behaviour was observed when combining these cracks, highlighting the shortfall of most available literature where these cracks are seldom researched in tandem. From all the tests, six different cracking behaviours were identified depending on the potential severity for each cracking type. The test also showed worryingly that both these cracks can be present internally without being visible at the concrete surface where they act as weak spots for future crack growth. The practically challenging tensile testing of plastic concrete was conducted with a newly built direct tensile test setup, which provided stress-strain curves that were used to determine the tensile material properties of plastic concrete such as: Young’s modulus, tensile strength, strain capacity and fracture energy. This included tests at different temperatures as well as cyclic tests. The results showed that the tensile material properties develop significantly faster, the greater the ambient temperature surrounding the concrete as well as the resilient nature of a still plastic concrete which proved to be capable of withstanding cyclic loading without failure, while a solid but still weak concrete could not. The tensile material properties together with the measured strains of plastic concrete were combined to provide both an analytical and numerical estimation of the cracking behaviour of plastic concrete. The analytical estimation was more simplistic and required a few crude assumptions, while the numerical estimation used finite element methods to create a model that accounted for the major complexities involved such as time-dependency of material properties and anisotropic volume change of plastic concrete. Both the analytical and finite element model gives adequate representation of the cracking behaviour for extreme climates but not for normal climates, with the size discrepancy between the interior and surface cracks during experiments as well as the relaxation of stresses in plastic concrete being provided as the main reasons for the poor correlation. The finite element model was further used to conduct a parameter study, where the settlement and shrinkage strains were shown to govern the size of the final crack, while the material properties only influence the time of crack onset and rate of crack widening. Finally, the finite element model was successfully applied to a large scale example of a concrete slab, indicating that the model can be a helpful tool to simulate the cracking of plastic concrete without the need to perform timely experiments.AFRIKAANSE OPSOMMING: Die kraak van plastiese beton betrek twee kraak tipes naamlik: plastiese versakkings krake wat deur differensiële versakking veroorsaak word en plastiese krimp krake wat veroorsaak word deur die verdamping van vry porie water in beton. Die krake is hoofsaaklik ʼn probleem vir vloer-tipe elemente wat blootgestel word aan kondisies met hoë verdampings tempo’s en gebeur tipies binne die eerste paar ure nadat die beton gegiet is. Die vroeë voorkoms van die krake verlaag die duursaamheid en dienslewe van ʼn beton struktuur drasties. Die krake bly ʼn probleem in die konstruksiebedryf al is daar verskeie effektiewe, maar meestal geminagte, voorsorgmaatreëls. Die rede hoekom die krake ʼn probleem bly is weens die komplekse natuur van die krake sowel as ʼn gebrek aan ʼn algehele aanvaarde teorie of model wat al die kompleksiteite in ag neem. Na aanleiding van bogenoemde beoog die studie om fundamentele kennis te ontwikkel van beide plastiese versakkings en plastiese krimp krake in vloer-tipe elemente individueel en gekombineer sowel as om die trek materiaal eienskappe van plastiese beton te bepaal. Sodra die krake fundamenteel verstaan word, is die finale doel om ʼn model te ontwikkel wat die kraak van plastiese beton kan simuleer deur gebruik te maak van ʼn eindige element metode benadering. Die fundamentele kennis van die krake was verkry deur verskeie toetse te doen op verskillende mengsels by verskeie klimate in verskillende vorms. Die toetse het gewys dat beide kraak tipes afsonderlik kan plaasvind, met plastiese krimp krake wat eerste plaasvind as meervoudige krake op die oppervlakte sowel as skuif geïnduseerde krake onder die oppervlakte, gevolg deur plastiese krimp krake as goed gedefinieerde enkel kraak. Verder is ʼn drastiese verskil in individuele kraak gedrag opgemerk wanneer die krake gekombineer word. Dit beklemtoon die tekortkoming van meeste beskikbare literatuur waar beide kraak tipes selde in tandem ondersoek word. Uit al die toetse is ses verskillende tipes kraak gedrag geïdentifiseer afhanklik van die omvang van elke kraak tipe. Die toetse het ook kommerwekkend gewys dat beide kraak tipes intern teenwoordig kan wees sonder dat dit op die oppervlakte van die beton sigbaar is en dus kan dien as swakplek vir verdere kraking. Die praktiese uitdagende trek toetsing van plastiese beton was uitgevoer met ʼn nuut geboude direkte trektoetsopstelling, wat spanning-vervormings kurwes gelewer het vir die bepaling van die trek materiaal eienskappe van plastiese beton soos: Young’s modulus, trek sterkte, vervormings kapasitiet en fraktuur energie. Dit sluit in toetse by verskillende temperature sowel as sikliese toetse. Die resultate het gewys dat die trek materiaal eienskappe merkwaardig vinniger ontwikkel by hoër temperature sowel as die veerkragtige natuur van plastiese beton wat gesien kan word as ʼn materiaal wat sikliese belasting kan hanteer sonder faling, in vergelyking met ʼn meer soliede beton wat faling ondergaan. Verder is die trek materiaal eienskappe saam met die gemete vervormings van plastiese beton gekombineer om beide ʼn analitiese en numeriese voorstelling van kraak gedrag in plastiese beton te gee. Die analitiese voorstelling is meer simplisties en benodig ʼn paar gru aannames terwyl die numeriese voorstelling van eindige element metodes gebruik gemaak het om ʼn model te skep wat al die belangrike kompleksiteite in ag neem soos die tyd afhanklikheid van materiaal eienskappe en anisotropiese volume verandering. Beide die analitiese en eindige element model gee voldoende voorstelling van die kraak gedrag vir uiterste klimate, maar nie vir normale klimate nie. Die kraak grootte verskil tussen die interne krake en oppervlak krake tydens toetse sowel as die ontspanning van spannings in plastiese beton is gegee as redes vir die swak korrelasie. Die eindige element model is verder gebruik vir ʼn parameter studie wat gewys het dat die versakking en krimp vervormings die finale grootte van die krake bepaal, terwyl die materiaal eienskappe slegs die tyd van eerste kraakvorming en die tempo van kraakopening beïnvloed. Laastens is die eindige element model suksesvol aangewend om ʼn grootskaalse voorbeeld van ʼn betonvloer te analiseer, wat aantoon dat die model ʼn handige instrument kan wees vir die simulasie van krake in plastiese beton sonder die nodigheid om tydsame eksperimente uit te voer

Behaviour of Steel Fibre Reinforced Concrete Pavements on A Single Fibre Level

Concrete is a popular construction material used all around the globe. It is strong in compression and weak in tension. To mitigate this tensile weakness, various reinforcement methods have been used over the years, of these, fibre reinforced concrete (FRC) has gained popularity. Fibres not only improve the tensile strength of the concrete matrix, but also control crack propagation through crack bridging action. The goal of this study is to investigate the long-term behaviour of FRC pavements through conduction of static and fatigue meso-scale level tests using different steel fibres. To achieve this goal, the influence of different fibre geometries and embedment angles on the pull-out behaviour of fibres in FRC was investigated first. This was carried out by examining 60mm Dramix 3D steel and 5D steel fibres at single fibre level. The embedment depth was kept constant at half the fibre length, and the fibre embedment angles were tested at 0°, 15° and 30°. The average maximum pull-out load, Amax, was determined first through static tests then fatigue tests were carried out using 85% of the Amax value on pre-damaged samples. These tests indicated that an increase in fibre embedment angle and number of hooks leads to additional anchorage of the fibre. Fibre pull-out was found to be the dominant fibre failure mechanism for the static single fibre pull-out tests (SFPTs)

The tensile deformation and capillary pressure build up in fresh concrete

During the plastic state of concrete, any hindrance or resistance of the free volume change in plastic concrete induce tensile stresses and or strains in the concrete element. Crack formation is expected to occur if the tensile stress and strain is greater than the capacity of the concrete. Investigation into the tensile properties and relaxation behaviour of plastic concrete was carried out using a direct tensile testing machine. The capillary pressure was measured during the tensile tests in low evaporation conditions, as well as in a climate controlled chamber where the concrete was exposed to high evaporation conditions. Most of the measured strength gain (tensile capacity) of the concrete is due to the capillary pressure in the pores of the fresh concrete which keeps the particles together by means of free water in the concrete during the early stiffening phase of the concrete. Later the hydration products bridge the pores which provides strength to the concrete. The capillary pressure results indicate how the rate of hydration influence the interconnectivity of the pores, and the contribution to the measured strength gain of the fresh concrete. The capillary pressure measurements during tensile tests revealed that the mechanism behind relaxation is the negative capillary pressure build-up induced by the mechanical tensile strain. The results also showed a correlation between the build-up of the capillary pressure in the concrete and the tensile deformation of the fresh concrete where the capillary pressure increased as the tensile load increased

Modelling the cracking of fresh concrete

The cracking of fresh concrete, while still in a plastic state, includes both plastic settlement and plastic shrinkage cracking, which starts once the concrete is cast to around the final setting time. The cracking process is complex and is influenced by numerous factors which include the climate, mix proportions, element geometry and construction procedures. Preventing these cracks therefore remains a problem in practice. One of the reasons for this is the lack of a model that can be used to determine the location, timing and severity of the cracking before the cracking occurs. The main challenges with such a model are the testing of the fresh concrete to determine the tensile material properties, the appropriate constitutive law needed, and the time dependency of material properties as well as the anisotropic volume change. This paper presents a finite element model that uses a total strain smeared cracking approach and accounts for both the time dependency of material properties and the anisotropic volume change. The model gives an adequate representation of the cracking behaviour of fresh concrete for extreme climates but not for normal to moderate climates, mainly due to the size discrepancy between the interior and surface cracks during experiments as well as the relaxation of stresses that are not accounted for in the model. A parameter study showed that both the settlement and shrinkage strains significantly influence and therefore govern the size of the final plastic crack, while the material mechanical properties only influence the time of crack onset and rate of crack widening

The influence of temperature on the cracking of plastic concrete

High early age concrete temperatures can lead to many problems such as an increased rate of cement hydration, as well as an increased rate of moisture loss from fresh concrete which can ultimately lead to the occurrence of plastic shrinkage cracking. Concrete is also batched and cast at various ambient temperatures which greatly influences the temperature development of the concrete after placement. There is a need to understand the influence of concrete temperature on the plastic cracking of concrete. This study investigated the temperature development over the thickness of a concrete slab when exposed to different initial concrete and ambient temperatures as well as the effect these factors have on plastic shrinkage cracking. This was achieved by experiments on concrete samples at varying temperatures while measuring the concrete temperature, pore water evaporation, shrinkage, settlement, setting times and plastic shrinkage cracking magnitude. These tests were conducted in a climate controlled chamber. It was concluded that exposure to higher initial concrete and ambient temperatures significantly increases the average temperature over the thickness of the concrete. The evaporation of pore water was higher when exposed to higher evaporation conditions. The plastic shrinkage, settlement and plastic shrinkage cracking were more severe in the presence of higher initial concrete and ambient temperatures even though the critical period and setting times were reduced due to an increase in the concrete temperature. Finally, the surface temperature of the concrete as tested in slabs up to 100 mm in thickness can be used as a good indication of the temperature development in the lower layers of the concrete

A textile reinforcement method for 3D printed concrete

The reinforcement of 3D printed concrete elements has proven to be a significant challenge that needs to be addressed. However, before reinforcement can be applied, the behaviour of the consequent composite materials must first be studied. This study, therefore, investigates the macromechanical behaviour of AR glass fibre textile material as reinforcement in terms of its flexural performance, to determine whether it is a feasible solution. During this study, elements, consisting of four layers, are printed flat on the surface bed of the printer and are reinforced by a textile mesh in between subsequently printed layers. The specimens are reinforced at different locations, with different geometrical orientations as well as different filament orientations, and are then compared. The flexural performance is quantified by conducting four-point bending tests 28 days after printing. The results of the tests show that by adding this specific mesh, the average flexural strength of the elements increases significantly. Furthermore, elements with the warp yarns aligned with the printed filament have increased flexural strength. During the testing, it is also discovered that voids form underneath the textile mesh when applied between the layers and that these voids influence on the performance of the elements

X-Ray Micro Tomography of Water Absorption by Superabsorbent Polymers in Mortar

Superabsorbent Polymers (SAP) have been recently subject of investigation as smart admixtures for cement-based materials. The properties of these polymers enable their use for internal curing, increasing freeze/thaw resistance, boosting autogenous self-healing and providing a crack self-sealing effect in cementitious composites. Except for the earliest application, the functioning of these beneficial effects invloves the absorption by the polymers of ingress water in the hardened cementitious matrix and later release, as well as their capacity to complete multiple absorption/desorption cycles. In this work, the absorption of water in mortar with superabsorbent polymers is monitored during the first 60 min of absorption through micro-CT. The experimental series included the presence of cracks. The registration and differentiation of sub-minute (18 s) scans enabled the individuation of bulk water content distribution in the mortar with a resolution of 55 μm. The swollen volume of SAP could also be quantified and studied in time. The results point out that although embedded SAP absorb water from the matrix, this absorption is slow and reduced with respect to water absorption during mixing for the used SAP. Same effect is observed for SAP in the cracks.</p