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.

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.

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.

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 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.

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.