Aleksandra Radlińska

Shrinkage Mitigation Strategies in Cementitious Systems: A Closer Look at Differences in Sealed and Unsealed Behavior

Shrinkage reducing admixtures (SRAs) and saturated lightweight aggregates (LWAs) are increasingly used to reduce shrinkage cracking of concrete mixtures. While both methods show great potential, to obtain the full anticipated benefits of either SRA or LWA the boundary conditions of the concrete element must be carefully considered and understood. Addressed are shrinkage and shrinkage cracking behavior of concrete with sealed and unsealed boundaries. The sealed concrete undergoes self-desiccation, while the unsealed concrete simultaneously experiences both self-desiccation and external drying. The research work presented provides a theoretical and experimental demonstration of the differences in the shrinkage behavior of mixtures containing SRA and LWA. Data are provided from experiments that demonstrate the benefits of SRA and LWA under sealed and unsealed conditions. Theoretical considerations explain the influence of boundary conditions on shrinkage and cracking of concrete. This has important implications for selecting an adequate shrinkage mitigation strategy. In addition, it is demonstrated that the experimental results are consistent with the theoretical predictions.

Shrinkage Characteristics of Alkali-Activated Slag Cements

AbstractRecent interest in creating green construction materials has sparked the development of portland cement–free binders. Alkali-activated slag (AAS) concrete has a low embodied energy and comparable or superior strengths to ordinary portland cement (OPC) concrete. However, one factor limiting AAS usage is its durability performance, specifically its susceptibility to shrinkage. Before declaring AAS a marketable product, the mechanisms behind its volumetric instability need to be understood. This paper presents a preliminary study of the shrinkage deformations of various AAS mixtures, wherein four unique AAS mortars were designed and tested for autogenous, chemical, and drying shrinkage; time of setting; and compressive strength. All results were compared to those obtained for a control OPC mortar. Alkali-activated slag mixtures with comparable strength to OPC show a higher autogenous and drying shrinkage. A lower elastic stiffness, higher degree of saturation, and potentially higher chemical shrinkag...

A Review and Comparative Study of Existing Shrinkage Prediction Models for Portland and Non-Portland Cementitious Materials

This paper reviews shrinkage prediction models for cementitious materials and presents analysis of selected published data utilizing the aforementioned models. The main objective of this review is to revisit and reexamine the primary shrinkage mechanisms, that is, capillary pressure theory, Gibbs-Bangham shrinkage, and withdrawal of disjoining pressure in Portland and non-Portland cement. In particular, the theoretical basis for current shrinkage models is elaborated on and its soundness and applicability to explain the published experimental data are discussed. Additionally, a specific comparison was made among high water-to-cement (w/c) ratio ordinary Portland cement (OPC), low w/c OPC, and alkaline activated slag.

Effect of Drying Rate on Shrinkage of Alkali-Activated Slag Cements

The volumetric instability of alkali-activated slag (AAS) binders has raised concerns and impeded the acceptance of this Portland cement-free material. The objective of this article is to characterize the influence of drying rate on drying shrinkage behavior of AAS mortars to better understand the mechanisms responsible for its large shrinkage deformation. A series of four AAS mortar mixtures with varying activator composition, as well as a reference Portland cement mortar, was cast and dried at different relative humidities, that is, 30, 50, 70, and 85% RH. Drying took place inside nitrogen-purged environmental chambers for the purpose of eliminating the contribution of carbonation to the total volumetric change of AAS. The shrinkage and corresponding mass loss of 1.27 cm × 1.27 cm × 12.7 cm prisms were measured as a function of time. The results show that shrinkage of AAS varies largely depending on the drying rate, that is, ambient RH. Interestingly, even though the drying mass loss increases with reducing the RH, the magnitude of shrinkage is the largest for samples stored at 50 and 70% RH, depending on the mixture type. Possible causes of these irregular behaviors are discussed. It is concluded that the drying rate has a much more significant influence on AAS than on ordinary Portland cement (OPC), which implies a more complicated shrinkage mechanism for AAS samples stored at various relative humidities.

Comments on the Interpretation of Results from the Restrained Ring Test

The restrained ring test has recently been standardized (ASTM C1581-04) as a test method to assess the restrained shrinkage cracking susceptibility of a concrete mixture. Unfortunately, there is currently a lack of published information regarding the repeatability of the test. This paper quantifies the variability of the ASTM C1581-04 ring test procedure. To determine the repeatability of the standard test, 24 restrained ring specimens were cast from the same mortar mixture and stored in carefully controlled environmental conditions. Results from the restrained ring test were assessed using the same mixture proportions. The results indicate that a standard deviation of approximately 6 μe can be expected in the strain measurements from the same mixture and 8 μe from different batches of the same mixture. The total coefficient of variation observed was less than 12 % (within single mixture and between different batches). A probabilistic approach was used to describe the variability associated with the age of cracking and to provide a tool that allows cracking prediction, especially when only a fraction of the ring specimens crack.

Methodology for Determining the Timing of Saw Cutting in Concrete Pavements

Stresses develop in portland cement concrete pavements (PCCP) shortly after placement when the volume changes associated with temperature change, moisture loss, or chemical reaction are restrained. These stresses may become large enough to cause microcracking around the aggregates or random cracking through the depth of the pavement. To reduce the potential for microcracking and to control crack formation, saw cuts are placed in PCCP shortly after placement. Although the idea of saw cutting is relatively straightforward, determining the timing of the saw cut can be complicated. Sawing too early can result in raveling, whereas sawing too late can result in microcracking or random cracking. Even though saw cutting is done on a daily basis in practice, the timing of saw cut placement is often determined based only on the saw cutting operators experience. Opportunities exist to provide construction crews with more reliable methods to determine when the cut should be placed. This study introduces a method to determine the timing of saw cutting based on the concrete strength and the stress that develop. Toward this end, a strength reduction factor was introduced. A finite element model was used to determine the influence of a variety of factors, including material properties (e.g., strength, elastic modulus, moisture migration, heat capacity), environmental conditions (relative humidity and wind speed), and geometric factors (pavement thickness and crack depth), on the behavior of concrete pavements. The introduction of the saw cut and the development of a localized crack are discussed. The implications of construction variability are also discussed.

Material Properties of Structurally Viable Alkali-Activated Fly Ash Concrete

AbstractAs the concern for the environment and need for sustainable construction practices continues to grow, development of portland cement–free binders is gaining wide interest in the concrete research and engineering community responsible for design and construction of civil infrastructure. In this work, microstructural (as determined by quantitative X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy techniques) and material properties of alkali-activated fly ash concrete were evaluated to verify the material’s performance and structural viability. Results show that the heat-cured products of reaction of fly ash in an aqueous alkaline solution form a concrete binder with adequate design properties and promising durability aspects.

Measuring the Chemical Shrinkage of Alkali-Activated Slag Cements Using the Buoyancy Method

Recent incentives to reduce the carbon footprint of Portland cement (PC) concrete have sparked the research and development of eco-friendly products such as alkaliactivated slag (AAS) concrete. AAS is a promising product capable of developing compressive strengths comparable to PC concrete; however, its large drying and autogenous shrinkage (the latter being the focus of this paper) have impeded its marketability and use in the construction industry. The mechanisms and parameters that control the self desiccation of AAS are relatively unknown. Chemical shrinkage, the driving force of autogenous shrinkage, has never been measured for AAS systems; however, it has been theoretically estimated to be much higher than that of PC. The first step in determining why the autogenous shrinkage of AAS is so large is to investigate its chemical shrinkage. This research focuses on proper measurement of the chemical shrinkage of AAS and the parameters that affect the results.

Long-Term Field Performance of Pervious Concrete Pavement

The work described in this paper provides an evaluation of an aged pervious concrete pavement in the Northeastern United States to provide a better understanding of the long-lasting effects of placement techniques as well as the long-term field performance of porous pavement, specifically in areas susceptible to freezing and thawing. Multiple samples were taken from the existing pavement and were examined in terms of porosity and unit weight, compressive and splitting tensile strength, and the depth and degree of clogging. It was concluded that improper placement and curing led to uneven pavement thickness, irregular pore distribution within the pervious concrete, and highly variable strength values across the site, as well as sealed surfaces that prevented infiltration.

Causes of Early Age Cracking on Concrete Bridge Deck Expansion Joint Repair Sections

Cracking of newly placed binary Portland cement-slag concrete adjacent to bridge deck expansion dam replacements has been observed on several newly rehabilitated sections of bridge decks. This paper investigates the causes of cracking by assessing the concrete mixtures specified for bridge deck rehabilitation projects, as well as reviewing the structural design of decks and the construction and curing methods implemented by the contractors. The work consists of (1) a comprehensive literature review of the causes of cracking on bridge decks, (2) a review of previous bridge deck rehabilitation projects that experienced early-age cracking along with construction observations of active deck rehabilitation projects, and (3) an experimental evaluation of the two most commonly used bridge deck concrete mixtures. Based on the literature review, the causes of concrete bridge deck cracking can be classified into three categories: concrete material properties, construction practices, and structural design factors. The most likely causes of the observed early-age cracking were found to be inadequate curing and failure to properly eliminate the risk of plastic shrinkage cracking. These results underscore the significance of proper moist curing methods for concrete bridge decks, including repair sections. This document also provides a blueprint for future researchers to investigate early-age cracking of concrete structures.