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The study sets out to evaluate the thixotropy of conventional concrete containing rheology modifiers using various methods of rotational rheometry. Five mixes were examined: the control, two mixes with viscosity modifying agent (VMA) and superplasticiser (SP) respectively, a mix with combined viscosity modifying agent and superplasticiser (VMA + SP) and the last mix with an increase in water content. These mixes were formulated to achieve concrete with varying rheological properties. Tests were carried out to examine the evolution of rheology parameters (static yield stress, dynamic yield stress and plastic viscosity), thixotropy index, shear thinning and thickening, flocculation properties, torque decay pattern (with a subsequent drop in apparent viscosity), and hysteresis loop. Results show that conventional concrete known to be a shear thinning and thixotropic material, can possess anti-thixotropic (rheopexy) and shear thickening properties depending on the condition pre-history, resting time, shearing rate and inclusion of rheology modifiers. These factors also influence the magnitude of the thixotropy but not necessarily the evolution of thixotropy. Inclusion of VMA in conventional concrete had only slight influence (both increasing and decreasing depending on the method of measurement) on its thixotropic behaviour while the inclusion of SP, VMA + SP and increased water considerably influenced concrete’s thixotropic behaviour in a similar pattern. However, each rheology modifiers had its unique influence. VMA significantly improves the evolution of concrete’s viscosity; inclusion of SP tends to result in a rheopectic concrete; addition of VMA + SP and pre-shearing of concrete may result in an unstable thixotropy, while SP can initially dominate the thixotropic behaviour of concrete containing both VMA and SP, VMA’s influence may later surpass that of SP; more water in concrete tends to result in a lesser rate of thixotropic structure breakdown.

Plastic shrinkage cracking (PSC) is one of the earliest possible defects that can potentially ruin the performance of concrete structures with large exposed surfaces. PSC is mainly attributed to tensile stresses arising in concrete due to a combination of capillary pressure, and restraints provided by reinforcement and formwork. The adoption and use of several admixtures in modern-day concrete has been known to alter properties of both the fresh and hardened concrete. In this research, the adopted admixtures include a minimum and maximum dosage of a glucose-based retarder, calcium chloride-based accelerator, chloride free air entraining agent, lignosulphonate plasticiser, shrinkage-reducing admixture (SRA), poly carboxylate ether (PCE) based super-plasticiser, and a sulphonated melamine formaldehyde (SMF) based super-plasticiser. The experimental tests were conducted in a climate chamber with an ambient temperature of 40 °C, relative humidity of 10% and a wind speed of 20.2 km/h. By understanding the phenomenological and fundamental relationships of PSC when various admixtures are used in conventional and high-flow concretes, it was found that admixtures cause a substantial reduction in PSC severity. The respective admixtures led to a reduction in measured crack areas, capillary pressures, evaporation amount/rate, as well as settlement/shrinkage behaviours in both types of concrete. Compared to conventional concrete, high-flow mixes showed a greater reduction in the severity of PSC. The underlying working mechanism of these admixtures largely relates to the surface tension reduction ability.

The cracking of plastic concrete remains a problem in the construction industry due to its complexity and incomplete understanding. One of the reasons for this is the lack of knowledge regarding the tensile material properties of the plastic concrete. This paper describes and builds the appropriate test setup needed for the challenging tensile testing of plastic concrete as well as discusses the measured tensile properties. The tensile strength and Young's modulus increase exponentially near the final setting time and initial setting time respectively. The strain capacity and initial fracture energy decrease significantly between the initial and final setting times. The tensile properties develop faster and with higher values the greater the ambient temperature. Plastic concrete proved to be resilient and capable of withstanding multiple loading and ceased loading cycles, while a solid but still weak concrete could not. The tests also highlighted the need for further research regarding the influence of hydrostatic pressure and the relaxation of stresses in plastic concrete.

In fibre reinforced concrete (FRC), energy is dissipated in the wake of the crack tip through the actions of fibre bridging and fibre pull out. This is the main mechanism which inhibits crack growth, thus increasing the load carrying capacity of FRC by providing post–cracking ductility. Furthermore, the same mechanism is present when FRC undergoes fatigue loading. Typical applications for FRC which undergo significant fatigue loading during their service life include paving applications such as bridge decks, highways and industrial floors. The continuous exposure to cyclic loading results in a decrease in apparent stiffness of the material, which may lead to fatigue failure [1, 2]. Fatigue failures are almost always unexpected, and can have a catastrophic outcome [3, 4]. Thus, the fatigue characteristics become vital performance and design parameters [1]. In this paper, the mechanisms of fatigue failure of pre-damaged hooked-end steel fibre reinforced concrete (SFRC) are investigated at a single fibre level. An initial pre-damage was applied to the fibres before the cyclic loading commenced. The pre–pull out ranged from 0.6 mm to 2.5 mm, and the cyclic loading was applied at 70% and 85% of the maximum static pull out capacity of the fibre embedded in the concrete.

The major processes that influence and constitute the plastic cracking behaviour of concrete include bleeding, evaporation, settlement, shrinkage, and capillary pressure. These underlying processes mainly occur during the plastic phase of concrete, during which time concrete has considerable rheological properties. Potentially, there should be interactions between these phenomena. The goal of this experimental study is in two phases – showcase the plastic cracking behaviour of concrete and reveal the nature of its interactions with the rheological properties. A background to the envisaged role of rheology is provided, the plastic cracking and rheological behaviours of five concrete mixes were independently investigated, and the relationship between them established. Emphases are placed on the cracking underlying process (settlement, shrinkage, capillary pressure) and the initial/evolving rheo-physical parameters. The plastic cracking behaviour is shown to have three unique stages observable on the plastic cracking behaviour plot. The settlement process is responsible for the interconnectivity with rheology and is interwoven with the plastic shrinkage and capillary pressure. Hence, the rheological interdependence is mainly valid during the settlement period. An initial settlement period is directly linked to the permeability properties while the later period is inversely influenced by the yield stress and its evolution (structuration). The plastic shrinkage and capillary pressure build-up are directly linked to the yield stress and thixotropy. The study suggests that the stiffness evolution of concrete and its suspending mortar in terms of storage modulus does not influence the plastic cracking underlying processes.

This research attempts to further add onto the existing knowledge-base of the utilisation of nano-silica in concrete by understanding its influence on plastic shrinkage in concrete, a subject with hardly any specific literature available. The results of extensive experimentation reveal that the adoption of nano-silica in concrete mixes should be cautiously undertaken, as it behaves in such a way to cause acceleration of the mechanisms of plastic shrinkage. This research has further been done in tandem with the combined plastic shrinkage crack mitigation methods, such as the use of synthetic fibres and super absorbent polymers (SAP) in concrete. The combined use of nano silica with SAP can be effective in alleviating the associated plastic shrinkage cracking. Synthetic micro-fibres too are useful in lessening plastic shrinkage cracking in nano silica mixes, but are not as effective as SAP. The mechanism of acceleration of plastic shrinkage cracking by nano silica is attributed to the finer particles of the material, causing reduced bleeding amounts in mixes in which it is incorporated.

The plastic period in conventional concrete is dominated by two volume changes namely, plastic settlement and plastic shrinkage, which if restrained can result in cracking. Although both volume changes and the resulting cracking have been well documented cracking during the plastic period remains a problem. One reason is the complex interaction between these cracks. This paper shows the necessity of considering the combined effect of plastic settlement and plastic shrinkage cracking when investigating the cracking of plastic concrete. This is achieved by isolating both cracking types individually, followed by the interaction between these cracks. Plastic settlement cracking shows multiple tensile surface cracks and shear induced cracks below the surface. Plastic shrinkage cracking shows a single well defined crack pattern which forms suddenly throughout the entire depth of the concrete. When combined significant crack widening can occur long before normally expected due to the negative synergy between these two crack types.

Providing additional water to the hydrating cementitious particles is essential to achieve the optimal mechanical performance of the low w/b concrete mixes preferred for 3D printing. This study incorporates superabsorbent polymers (SAP) and additional water in 3D printed concrete (3DPC) to promote the hydration process through delayed internal water release. The study shows that a retentive SAP modifies the rheological development by absorbing the pore fluid for a short period after printing. The absorption-induced stiffening increases the thixotropy and buildability by 49% and 25%, respectively. A retentive SAP increases the flexural strength and interlayer adhesion by 19% and 10%, respectively. This is due to the internal water release that promotes hydration. Evaporation of the interlayer moisture during the pass time has the opposite effect— evaporation reduces the interlayer adhesion. Based on this assumption, an analytical model is proposed. The model accounts for the pass time, bleeding, and the environmental evaporation rate to estimate the surface moisture and predict the lack of interlayer adhesion. In this study, the model accurately (RMSE = 2.5%) predicted an interlayer adhesion reduction from 30% to 50%. The interlayer adhesion results of other studies could also be predicted.

3D printed concrete exhibits a high magnitude and rate of plastic shrinkage (early age drying-induced shrinkage) compared to conventional cast concrete. This study investigated if restraining this shrinkage adversely affects the long-term durability and mechanical performance. Shrinkage was restrained by inserting reinforcement-like rods into the fresh concrete and exposing the specimen to a moderate evaporation rate . The interlayer bond strength, permeability and ion penetration of the unrestrained and restrained specimens were then compared. The results show that shrinkage causes shear strain concentrations at the interlayers due to a lack of particle interlocking and non-uniform pore water evaporation from the undulated evaporation face. Consequently, discrepancies in the transfer of shrinkage between layers occur, a phenomenon coined as interlayer slip. Evidence is presented of a 12% reduction in interlayer bond strength, a 70% higher permeability and 17% higher chloride ion penetration. These adverse consequences were due to interlayer slip and microcracking. Nonetheless, the durability of uncracked printed concrete is satisfactory, even with localised interlayer slip and microcracking.

The advancement of concrete technology such as 3D concrete printing has rekindled research interest in the early-age mechanical properties of concrete. However, the non-adaptability of early-age (plastic) concrete to classical mechanical measurements causes the quest for alternative approaches. This study is a new and unique approach, using rheological methods, aimed at investigating the early development of shear and viscoelastic properties of concrete from the time of mixing up to the initial setting time. Dynamic shear rheometry (coupled with rotational tests), taking advantage of the normal stress measurement, was used to evaluate the shear and viscoelastic properties. Viscosity modifying agent (VMA), superplasticiser (SP) and increased water content were incorporated to achieve a robust test program. The Mohr-Coulomb shear parameters, shear yield parameters, shear modulus, creep and stress relaxation properties were evaluated from rheometry tests. Theoretical and experimental analyses of a vane-in-cup rheometry reveal that the resultant normal stress, during an elastic response, originates from the self-weight of the sample and is dependent on the sample's weight distribution. The results, therefore, show that it is not sufficient to estimate the self-weight of the concrete for normal stress evaluation and that the shear and creep properties of concrete mainly develop within half-time of the initial set while stress relaxation is inherent and remains fairly unchanged. Challenges and prospects of this approach were also discussed.

Concrete suffers from brittle failure due to its low tensile strength. This drawback can be compensated for by adding reinforcement bars and/or steel fibres, and more recently, macro synthetic fibres. When mixing concrete with these fibres the aggregates could damage the fibres. This paper presents work done on the effect of mixing on the performance of macro synthetic fibre reinforced concrete. Single-fibre pull-out tests were conducted on various fibres in both the original and mixed state. Furthermore, flexural tests were performed to investigate the influence of mixing time and mixer type on the performance. It can be concluded that mixing is beneficial for flat type fibres, but the performance of crimped or embossed fibres remains the same. Furthermore, longer mixing times (> 10 min) in a pan mixer are detrimental to the performance, while the performance in a tilting drum mixer remains unchanged even after a mixing time of 60 min.

Plastic shrinkage cracking (PSC) in concrete is related to the amount and rate of free pore water loss due to evaporation. This study investigates the influence of solar radiation on evaporation, the concrete temperature, plastic shrinkage and cracking. Literature suggests that exposure to solar radiation can either: increase the concrete temperature and, consequently, increase the rate of tensile strength gain resulting in less severe PSC or can increase the concrete temperature, resulting in a higher amount and rate of pore water loss to produce more severe PSC. The results of this study indicate that exposure to solar radiation significantly increases the amount and rate of pore water loss as well as plastic shrinkage and severity of PSC. To study PSC, many authors have estimated the evaporation rate from wind speed, air and concrete temperature and humidity which would result in significantly underestimating the actual evaporation rate if the concrete is exposed to moderate or high solar radiation. The accuracy of several radiation-related evaporation estimation models was evaluated by comparing the models to the actual rate of evaporation from specimens placed in the sun. A more accurate model was identified to estimate the evaporation in concrete specimens when exposed to solar radiation.

In the past, industrial waste and by-products have successfully been used to improve the properties of concrete. Used engine oil is a waste product which is burdensome to discard of and, due to frequent replacement, is produced in high quantities in the construction industry. The utilisation of used engine oil in concrete has shown potential as an admixture by reducing slump and increasing air-content. The main disadvantage is a reduction in long term compressive strength. This study investigates used engine oil (UEO) and used hydraulic oil (UHO) as admixtures to concrete, focusing on its effect on the rheological properties. Slump, air-content, compressive strength and rheometer tests are conducted for concrete containing different dosages of UEO and UHO. Adding low dosages of UEO and UHO have no noteworthy effect on the compressive strength, although increasing air-content and altering the rheological properties significantly. UEO and, to a lesser extent UHO, reduced the energy required to initiate flow (static yield stress) as well as decreased the plastic viscosity. Adding UEO has a similar effect on the static yield stress and plastic viscosity as increasing water content or substituting cement with a proportion of fly-ash. In conclusion, UEO shows potential as an air-entrainer or water-reducing admixture.

The influence of different supplementary cementitious materials (SCM) and superplasticisers on the rheological properties of concrete was investigated to identify potential compatibility issues. Superplasticisers and SCM often have unexpected interaction with certain cementitious compounds, resulting in concrete that is difficult to place in the fresh state due to poor rheological properties. Various mixes were designed containing different superplasticisers and/or SCM in different quantities. Slump, slump flow and concrete rheometer tests were conducted to determine the yield stress, plastic viscosity and thixotropic behaviour of the concrete. Obtained results showed that the specific Sulphonate Naphthalene Formaldehyde (SNF) and Polycarboxylic Ethers (PCE) superplasticisers used, reduced the yield stress, thixotropic behaviour and plastic viscosity of concrete. Modified Acrylic Polymer (ACR) superplasticiser showed a similar effect except for the plastic viscosity which increased at higher dosages. The addition of fly ash and slag to concrete containing superplasticiser had little effect on the rheology and showed similar results as mixes only containing superplasticiser. The use of superplasticiser in conjunction with silica fumes caused a decrease in yield stress and thixotropic behaviour while plastic viscosity increased. The use of superplasticiser in conjunction with higher than normal dosages of gypsum also caused a decrease in yield stress and thixotropic behaviour but had negligible effect on plastic viscosity. It was also found that the use of PCE superplasticiser in conjunction with gypsum, used to control the set of concrete, can cause potential slump loss issues.

In concrete, early age pore water evaporation results in volumetric shrinkage that, if restrained, can cause plastic shrinkage cracking (PSC). 3D printed concrete (3DPC) is vulnerable to PSC due to a lack of formwork, minimal bleeding water, low aggregate to binder ratio and high quantities of fines in the mixture. A novel experimental method was developed to determine the PSC risk, evaluating the efficacy of PSC prevention measures and understanding the behaviour of PSC in early age 3DPC. This study evaluated the free shrinkage of 3DPC specimens as well as identified and systematically introduced sources of restraint to induce PSC. The free shrinkage results showed a rate of strain gain and peak strain significantly higher than commonly found in ordinary concrete. Severe cracking was observed when the shrinkage was restrained under a moderate evaporation rate. Cracks formed within the first 2 h after printing, earlier than in ordinary concrete. The proposed method was employed to study the fundamental behaviour of crack formation and propagation in 3DPC. The unique filament interlayer plane has a notable effect on the transfer of shrinkage deformation in the specimen. Differential horizontal deformation in consecutive layers and consequential interlayer slip was observed when the shrinkage was restrained. It is reasonable to conclude that interlayer slip caused by early age shrinkage has the potential to reduce the long-term interlayer bond strength and the durability of 3DPC.

The self-settlement phase of plastic cracking is majorly shear dominated and can be characterised by shear rheometry. This phase is susceptible to microstructural damage/cracking that can be rapidly propagated by the later occurring plastic shrinkage. This study experimentally determined the material properties and behaviour of rheologically modified concrete mixes using shear rheometry and measured their plastic cracking. Von-Mises and Hencky, and Bresler-Pister theories were then used as failure criteria with hypothetically fully restrained plastic strain/stress analyses to model the occurrence of plastic cracking. The incorporated rheology modifiers influenced the shear properties, viscoelastic abilities and plastic cracking of the concrete. The predictions of the model revealed that the plastic settlement is indeed mainly shear-related, the viscoelastic behaviour influences plastic cracking, and microcracking can occur before the initial setting time. It was also concluded that plastic concrete damage tends to be strain-oriented with pressure-insensitive form of ductile failure.

The viscoelasticity of fresh fine cement-based material can be investigated by rheological means using dynamic shear rheometry, hence, the term rheo-viscoelasticity. However, readily available classical rheometers are limited to the study of paste due to the aggregate size associated with mortar and concrete. This study, therefore, sets out to experimentally investigate the rheo-viscoelastic behaviour of cement-based materials containing rheology modifiers, by progressively evaluating the viscoelasticity of cement paste-mortar-concrete. The tests carried out include strain sweep, shear rate sweep, creep and creep recovery, stress relaxation and three interval thixotropy test (3ITT). The results show a trend of improved linear viscoelastic properties of the control fresh cement-based materials due to progressive inclusion of aggregate (increasing volume fraction and size). However, the rheology modifiers widely varied this trend, tending to make it difficult to approximate the rheo-viscoelastic behaviour of mortar and concrete from that of the cement paste as generally suggested in literature. The increase in the coarse volume fraction improved the stability of the cement-based materials microstructure to varying shearing rates. The study suggests that the liquid phase has a role in improving the creep recovery and stress relaxation of the mixes. Furthermore, pseudo-strain hardening was observed for the cement-based materials and flocculation-driven structuration was found to be reduced for the cement paste due to the progressive inclusion of aggregates leaving hydration as the main driving source of concrete’s structuration.

Plastic shrinkage cracking in concrete is due to evaporation, which is aggravated by harsh environmental conditions. By controlling the conditions in a laboratory to cause high evaporation rates, plastic shrinkage cracks can be induced and then tested. However, in order to control these conditions (wind, temperature, humidity) a large and expensive climate controlled chamber is required. Super absorbent polymers (SAP) are designed to absorb water and could possibly be used to mimic evaporation and thus induce plastic shrinkage cracks. In this study, by testing varying amounts and types of SAP, as well as different concrete mixes, the validity of using SAP as a novel method for testing plastic shrinkage cracking is investigated. The test results indicated that by using the method proposed and placing SAP on the surface of the concrete, plastic settlement and shrinkage occurs in the concrete. The results showed that higher amounts of SAP placed on the surface of the concrete resulted in more plastic shrinkage and ultimately larger plastic shrinkage cracks. Larger plastic shrinkage cracks also formed when SAP with a higher and faster absorption was used. Based on the results of the tested proposition, the proposed method of placing SAP on the surface of the concrete can be utilised to induce plastic shrinkage cracks in concrete. If the same SAP is used internationally for this purpose, all tests could be compared using the same test procedure.

The degradation of sisal fibre in a cement-based matrix due to the highly alkaline environment reduces the strength of the composite. In this research, to avert this degradation, alkaline treatment and acetylation were respectively performed with sodium hydroxide (NaOH) and Acetic Acid or Acetic Anhydride to improve the resistance of the fibre to alkaline attack. In addition, single fibre pull-out (SFP) tests were performed to evaluate the influence of chemical treatment on fibre strength, fibre-matrix interaction and also determine the critical fibre length. Specimens were tested in indirect tension (flexure) at 28 days to determine the strength of the composite. Additional ageing tests by extended curing in water at 24 °C, lime saturated hot water at 70 °C, and alternate cycles of wetting and drying were done. Aged samples were further tested at 90 days to evaluate the durability of the fibre. It was found that a combination of alkali treatment and acetylation was the most effective treatment condition, followed by that of alkali treatment at low concentrations of sodium hydroxide. At higher concentrations of sodium hydroxide, a significant reduction in strength was observed. Chemical treatment improves the durability of sisal fibres in concrete, albeit slight degradation still occurs.

Plastic shrinkage cracking (PShC) is one of the earliest forms of cracking in concrete as it occurs within the first few hours after the concrete has been cast. Concrete elements with large exposed surfaces are especially vulnerable to PShC. Many researchers have proposed models to simulate plastic shrinkage, bleeding and to predict the occurrence of PShC. In this paper a model to predict the degree of PShC is proposed. This model, the so called PShC Severity Model, is based on the volume of water that evaporates from the concrete between the placing and the initial setting time of the concrete. This model was verified using a large number of PShC test results. A further study was also done on the effect of the addition of low volumes of synthetic micro fibres on the degree of PShC. It was found that the addition of the fibres effectively reduces the PShC Severity value.