Search

Plastic cracking is probably one of the earliest defects for concrete structures, and is capable of reducing a structure’s durability and service life if not prevented. Over the last 60 years, researchers have attempted to unravel the complex nature of mechanisms leading to plastic cracking of concrete. The main driving mechanisms of plastic cracking have been found to be settlement of solid particles, bleeding, evaporation, capillary action, as well as surface finishing. These, in combination lead to a 3-dimensional volume contraction which if restrained, causes plastic cracking of concrete. Several methods, tests, and models have been developed to enumerate the effect of the respective mechanisms leading to the plastic shrinkage and cracking of concrete. Plastic cracking has thus been uniformly understood to mean both plastic settlement cracking and plastic shrinkage cracking. Several mitigation strategies of plastic cracking have been proposed, such as fogging, using less amount of fines, use of fibers, among others. This state-of-the-art paper traces the extensive development of research on the subject matter, and updates it with what is considered to be state-of-the-art and up-to-date.

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.

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.

Settlement and shrinkage cracking occurs in plastic concrete once cast up to around the final setting time. This paper reports on the influence of element depth and surface finishing operations on the cracking of plastic concrete. Deeper concrete elements are shown to have less severe shrinkage cracking when no settlement cracking is present, while when combined with settlement cracking at similar cover depths the cracking is more severe in deeper concrete. Surface finishing operations are shown to only close the surface of plastic cracks and not the crack below the surface, therefore temporarily hiding the true severity of the cracking.

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.

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.

Sugarcane bagasse ash (SCBA) has been pitched as a feasible supplement that can ease the sustainability issues associated with cementitious materials. In this respect, SCBA has been well researched with over 100 journal publications. However, there is a scarcity of comprehensive reviews detailing the research developments so far. Therefore, this study is a holistic review with a large scope, content and coverage that emanated from a comprehensive dissection of the literature. The study systematically reviewed the SCBA's origin of pozzolanicity, processing approaches, sustainability potentials, and performance in cementitious materials. The performance entailed characterisation, pozzolanic activity, microstructure refinement, fresh and rheological properties, mechanical and durability properties. SCBA processing is key to its reactivity and performance; hence, recommendations were made on the best processing approach. Performance ranging from 80 to 160% of the control (without SCBA) exist in the literature. Such performance emanates from a conservative use of about 20–25% or further processing that yields optimum contents up to 50%. This study serves as an up-to-date appraisal of data and researches related to the use of SCBA in cementitious materials.

Extensive research has been conducted during the past three decades to include waste plastic, rubber and glass in concrete. Recent interest to include these materials in concrete can be ascribed to the growing need for innovative waste disposal, as well as to minimise raw material usage. However, this research domain and application has not yet gained popularity and urgency in countries with an abundance of aggregates, such as South Africa. For these materials to be a viable option to be used as a concrete constituent in these countries, concretes produced using these materials should retain satisfactory properties. This study investigates various fresh, mechanical and durability properties, utilising waste ground Low-Density Polyethylene (LDPE) plastic, crumb tyre rubber, and crushed clear flat glass as a partial fine aggregate replacement, using materials sourced in South Africa. Two replacement contents are investigated: 15% and 30%, replacing fine aggregate by volume. The results show that plastic, rubber and glass decrease concrete workability, and increase air content. It also demonstrates that glass improves the mechanical and durability properties of concrete. Concrete containing low plastic content has promising mechanical properties but performs poorly in durability. Rubber significantly impacts the mechanical and durability properties of concrete negatively.

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.

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.