•Plastic-Sleeve Test (PST) method allows for the measurement of free shrinkage.•PST method provides results similar to the corrugated tube test method.•PST method allows the accurate measurement of ...early-age autogenous shrinkage.•PST method offers reduced water loss compared to standard prismatic test methods.•PST method allows the measurement of drying shrinkage paste, mortar and concrete.
The Plastic-Sleeve Test (PST) method of molding cylindrical specimens in plastic sleeves enables the measurement of autogenous and total shrinkage of pastes, mortars, and concretes. Photogrammetric tests confirmed that the linear support of the specimens with a temporary bottom support mold does not restrain deformation. Tests were performed to compare the PST versus corrugated tube methods and similar relative humidity and autogenous shrinkage results were obtained. The PST shows comparable CoV values to those of the corrugated tube test. The method presented provides solid circular cross-section samples, enabling control over entrapped air content, specimen shape, and bleeding effects, thereby ensuring the measurement method’s high repeatability. The PST showed a tighter seal than prismatic samples molded for total shrinkage testing according to EN 1367-4. Results indicate that the PST can effectively be used for molding samples intended for total shrinkage measurement. It is concluded that the PST method is an effective method to measure autogenous shrinkage and total shrinkage of cementitious materials.
To solve the cracking and interface debonding problems of ultra-high-performance concrete (UHPC) induced by high shrinkage in confined spaces or under strongly restrained conditions, a series of ...laboratory experiments was performed to evaluate the use of a CaO-based expansive agent (EA) in combination with a super absorbent polymer (SAP) and/or a shrinkage-reducing agent (SRA) to obtain expansive UHPC. The mechanisms underlying these techniques were also investigated. The test results showed that sufficient expansion was obtained by using a high dosage of CaO-based EA in UHPC, but the samples exhibited cracks owing to excessive and delayed expansion. The addition of SAP significantly enhanced the expansion effect of EA in UHPC. Autogenous shrinkage after 3 d and drying shrinkage within 180 d were effectively reduced by using SRA. Moreover, expansive UHPC was achieved by the combined use of EA, SAP, and SRA, in which excessive and delayed expansion was avoided. This was mainly attributed to i) increased initial expansion by using SAP for the reaction of CaO-based EA and ii) reduced later-age autogenous and drying shrinkage by adding SRA.
Abstract
Im Herbst dieses Jahres wurde die Überarbeitung der EN 1992‐1‐1:2004 (Eurocode 2) abgeschlossen und damit der Entwurf prEN 1992‐1‐1:2020 vorgelegt. Er stellt der Praxis zahlreiche neue und ...bessere Konzepte und Modelle zur Verfügung. Dies gilt auch für die Modelle zum Kriechen und Schwinden von Konstruktionsbeton, die zum Teil erhebliche Veränderungen erfahren haben. In diesem zweiteiligen Aufsatz (Teil 1 Schwinden, Teil 2 Kriechen) werden die neuen Modelle zur Prognose des Verformungsverhaltens von Beton vorgestellt. Er beleuchtet auch die Hintergründe, die zur Entwicklung der verbesserten Ansätze geführt haben.
Dieser Beitrag befasst sich mit dem Schwinden von Beton. In einem Überblick werden zunächst die wesentlichen Schwindarten und ihre Merkmale dargestellt. Danach werden die normativen Entwicklungen aufgezeigt, die zum heute gültigen Schwindmodell in DIN EN 1992‐1‐1:2011 geführt haben. Im Mittelpunkt steht die Analyse der Zuverlässigkeit und Genauigkeit dieses Modells. Sie ergab erhebliche Defizite, die Anlass für eine wesentliche Korrektur und Weiterentwicklung im Zuge der Erarbeitung der prEN 1992‐1‐1:2020 waren. Zahlreiche Untersuchungsergebnisse untermauern den neuen Ansatz, der hier vorgestellt und begründet wird. Statistische Betrachtungen zur Zuverlässigkeit des neuen Modells und die Darstellung eines Konzepts für den Einbezug von Laborversuchsdaten zur Verbesserung des Modells bilden den Abschluss des Beitrags.
Abstract
Creep and shrinkage prediction models for concrete
This autumn the revision of EN 1992‐1‐1:2004 (Eurocode 2) was finished and the draft of prEN 1992‐1‐1:2020 was released. Hereby the practice of design will be faced by several new and improved concepts and models. This holds also true for the models for creep and shrinkage of structural concrete, which have partially been completely revised. This article presents in two parts (part 1 Shrinkage, part 2 Creep) the new models for the deformation prediction of concrete and indicates also the related background.
This paper deals with shrinkage of concrete. The different types of shrinkage and major characteristics are given at a glance. The developments in codes leading to the currently applied DIN EN 1992‐1‐1:2011 are indicated. Focus is put on the analysis of the reliability and accuracy of this model. Considerable weaknesses became evident and led to essential improvements being included in prEN 1992‐1‐1:2020. Many test data confirm the new approach, which is presented here as well. Statistical considerations on the reliability of the new model and a concept how to integrate experimental laboratory data for the improvement of the model are finally presented.
•The shrinkage mechanisms and shrinkage-mitigating strategies of alkali-activated slag (AAS) systems were discussed.•The effects of various shrinkage-mitigating methods on the mechanical behavior and ...shrinkage of AAS systems were compared.•The shrinkage of AAS systems is closely associated with the characteristics of the raw materials and the hydration process.
Alkali-activated binders are considered as promising alternatives for ordinary Portland cement (OPC) binders due to their positive environmental effects and excellent mechanical properties and durability. However, significant shrinkage of alkali-activated composite systems, especially alkali-activated slag (AAS) systems, induces nonuniform deformation across the material to form tensile stresses and then causes the formation of harmful cracking. These harmful cracks can create pathways for various corrosive substances to penetrate the composites, seriously decreasing the load-bearing capacity and affecting the durability of concrete structures. Therefore, this paper presents a literature review on the shrinkage of AAS composites and systematically summarizes the shrinkage characteristics, shrinkage mechanisms, and shrinkage-mitigating strategies of AAS systems. The effects of various shrinkage-mitigating methods, i.e., reducing alkali activator modulus and alkaline content, incorporating mineral admixtures, adding chemical additives, toughening with fibres, introducing porous aggregates, and optimizing curing conditions, on the mechanical behavior and shrinkage of AAS systems were compared and analyzed. It is concluded that the shrinkage-reducing mechanisms obtained by applying these methods are mainly attributed to densifying the microstructures of the pastes, coarsening the pore structures, lowering the surface tension of the pore solution, and increasing the formation of crystalline phases (e.g., calcium hydroxide and ettringite).
Im Herbst dieses Jahres wurde die Überarbeitung der EN 1992‐1‐1:2004 (Eurocode 2) abgeschlossen und damit der Entwurf prEN 1992‐1‐1:2020 vorgelegt. Er stellt der Praxis zahlreiche neue und bessere ...Konzepte und Modelle zur Verfügung. Dies gilt auch für die Modelle zum Kriechen und Schwinden von Konstruktionsbeton, die zum Teil erhebliche Veränderungen erfahren haben. In diesem zweiteiligen Aufsatz (Teil 1 Schwinden, Teil 2 Kriechen) werden die neuen Modelle zur Prognose des Verformungsverhaltens von Beton vorgestellt. Er beleuchtet auch die Hintergründe, die zur Entwicklung der verbesserten Ansätze geführt haben.
Dieser Beitrag befasst sich mit dem Schwinden von Beton. In einem Überblick werden zunächst die wesentlichen Schwindarten und ihre Merkmale dargestellt. Danach werden die normativen Entwicklungen aufgezeigt, die zum heute gültigen Schwindmodell in DIN EN 1992‐1‐1:2011 geführt haben. Im Mittelpunkt steht die Analyse der Zuverlässigkeit und Genauigkeit dieses Modells. Sie ergab erhebliche Defizite, die Anlass für eine wesentliche Korrektur und Weiterentwicklung im Zuge der Erarbeitung der prEN 1992‐1‐1:2020 waren. Zahlreiche Untersuchungsergebnisse untermauern den neuen Ansatz, der hier vorgestellt und begründet wird. Statistische Betrachtungen zur Zuverlässigkeit des neuen Modells und die Darstellung eines Konzepts für den Einbezug von Laborversuchsdaten zur Verbesserung des Modells bilden den Abschluss des Beitrags.
Creep and shrinkage prediction models for concrete
This autumn the revision of EN 1992‐1‐1:2004 (Eurocode 2) was finished and the draft of prEN 1992‐1‐1:2020 was released. Hereby the practice of design will be faced by several new and improved concepts and models. This holds also true for the models for creep and shrinkage of structural concrete, which have partially been completely revised. This article presents in two parts (part 1 Shrinkage, part 2 Creep) the new models for the deformation prediction of concrete and indicates also the related background.
This paper deals with shrinkage of concrete. The different types of shrinkage and major characteristics are given at a glance. The developments in codes leading to the currently applied DIN EN 1992‐1‐1:2011 are indicated. Focus is put on the analysis of the reliability and accuracy of this model. Considerable weaknesses became evident and led to essential improvements being included in prEN 1992‐1‐1:2020. Many test data confirm the new approach, which is presented here as well. Statistical considerations on the reliability of the new model and a concept how to integrate experimental laboratory data for the improvement of the model are finally presented.
This study aims to develop a drying shrinkage model for recycled aggregate concrete (RAC) using both fine and coarse recycled aggregate (termed FRA and CRA, respectively). An experiment was conducted ...to measure the drying shrinkage of RAC with varying replacement ratios of CRA (0%, 50%, and 100%) and two types of FRAs (0%, 50%, and 100%). Test results show that: (1) both FRA and CRA have significant influence on the development of drying shrinkage, (2) the influence of FRA on the development of drying shrinkage decreases with increasing content of CRA, and (3) the influence of FRA on the final drying shrinkage is almost independent of the CRA. For example, the use of 100% FRA results in an increase of 23%–41% for concrete with 0% CRA. Similarly, 100% FRA results in 29%–38% increase for concrete with 100% CRA. Based on the results, a new model with two influence factors is proposed accounting for the influence of recycled aggregate on the development of drying shrinkage and on the final value of drying shrinkage of RAC; the expression was validated using a database of RAC shrinkage test data.
•Autogenous shrinkage mitigation of ultra-high performance concrete was investigated.•Total shrinkage mitigation of ultra-high performance concrete was investigated.•Coupled effect of lightweight ...sand and shrinkage mitigating admixtures was evaluated.
This paper evaluates the efficiency of various shrinkage mitigation approaches in reducing autogenous and drying shrinkage of ultra high performance concrete (UHPC). This included the use of various contents of CaO-based and MgO-based expansive agents, shrinkage-reducing admixture, and pre-saturated lightweight sand. Workability, compressive strength development, autogenous and drying shrinkage were evaluated for UHPC mixtures subjected to moist curing periods of 1, 3, and 7 d. Test results indicate that the use of lightweight sand was shown to be more effective in mitigating shrinkage than enhancing compressive strength. The replacement of natural sand by 60% of lightweight sand, by volume, was found to reduce autogenous shrinkage from 530 to 35 µm/m at 91 d. The coupled effect of using 60% lightweight sand and either CaO-based expansive agent, MgO-based expansive agent, or shrinkage-reducing admixture can reduce autogenous shrinkage at 91 d by up to 600 µm/m and drying shrinkage by up to 700 µm/m. In some combination, the use of shrinkage mitigating admixtures was found to reduce the 91-d compressive strength ranging from 8 to 20 MPa, even when 60% lightweight sand was employed. The initial moist curing period had positive effect on mitigating total shrinkage. The incorporation of 10% CaO-based expansive agent in UHPC with 60% lightweight sand subjected to 7 d of moist curing exhibited the best overall performance with 91-d autogenous shrinkage of 110 µm/m in expansion and 91-d total shrinkage (autogenous shrinkage after 1 d plus drying shrinkage) of 580 µm/m (also in expansion).
This paper comprehensively reviews the factors influencing the autogenous shrinkage and drying shrinkage of recycled aggregate concrete (RAC). Compared with natural aggregate concrete (NAC), RAC has ...a lower autogenous shrinkage because of the internal curing effect of recycled concrete aggregate (RCA). However, the internal curing efficiency of RCA is less than that of the lightweight aggregate because of its lower porosity and different pore structure. The drying shrinkage of RAC is larger than that of NAC, which is mainly caused by the mortar attached to the original virgin aggregate (OVA). Therefore, the methods of strengthening adhered or removing adhered mortar can reduce the drying shrinkage of RCA effectively. Some methods of strengthening adhered mortar are more environmentally friendly than removing adhered mortar. Finally, the existing autogenous shrinkage and drying shrinkage prediction models of RAC are discussed. The models obtained by seeing RCA as the two-phase composite has a higher degree of accuracy. This paper can support a better and more comprehensive understanding of the autogenous and drying shrinkages of RAC and how to reduce the cracking risk of RAC, which provides a theoretical base for future research and applications of RAC in the construction industry.
Low water to cement ratio (
w/
c) concrete exhibits increased occurrences of early-age shrinkage cracking. These occurrences have spawned innovative developments in concrete mixture design that ...reduce the risk of cracking. One such development is internal curing, which utilizes the inclusion of saturated lightweight aggregate (SLWA). The SLWA supplies additional moisture to cement paste as it hydrates thereby counteracting the effects of self-desiccation. This paper presents results from a study that examined mortar systems with different volumes of SLWA under sealed and unsealed conditions. Specifically, the results indicate the influence of SLWA volume and type on the internal humidity, autogenous shrinkage, and restrained shrinkage cracking behavior. The performance of each system is shown to be related to the cavitation of vapor-filled space in the paste, the SLWA spacing, and the pore structure of the paste in relation to the LWA.
The problem of excessive drying shrinkage in alkali-activated concrete (AAC) is well-documented in the literature. The magnitude of drying shrinkage is often three or more times that in portland ...cement concrete. This study investigates the effects of binder type, activator concentration, strength, age, and curing method on the manifestation of drying shrinkage in alkali-activated fly ash and slag cement concrete. Early-age shrinkage strains in excess of 1200με (0.12 percent strain) are observed in AAC. This is attributed to delayed hydration, microstructure refinement, and strength development. The resulting damage is far more significant than in portland cement concrete. Shrinkage and resulting damage are greatly reduced when specimens are dried at later age and after heat-curing. Alkali-activated slag cement concrete is more sensitive to water loss than portland cement or alkali-activated fly ash concrete. This results from a finer pore structure in alkali-activated slag binders.
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