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Effect of admixtures on the mechanism of plastic shrinkage cracking in self-compacting concrete
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.ORCID iD: 0000-0001-8586-2651
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering. (Byggmaterial)
(English)In: ACI Materials Journal, ISSN 0889-325X, E-ISSN 1944-737XArticle in journal (Refereed) Accepted
Abstract [en]

This research studies the effect of retarder, accelerator, stabilizer, air-entraining agent, and shrinkage reducing admixture (SRA) on plastic shrinkage cracking in self-compacting concrete (SCC). The main objective is to identify the dominant cracking cause, i.e. plastic settlement or plastic shrinkage in a SCC containing a particular admixture. Crack-free concretes were achieved by adding air-entraining agent and SRA, while accelerator and retarder increased the crack area. The impact of admixtures on the cracking mechanism is identified, by comparing the respective vertical and horizontal deformations. It was observed that the crack-free concretes had moderate settlement and horizontal shrinkage, while the cracked specimens exhibited significant deformation either vertically or horizontally.

Keywords [en]
Self-compacting concrete, Cracking, Plastic shrinkage, Admixtures, Settlement, Horizontal deformation.
National Category
Civil Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-73169OAI: oai:DiVA.org:ltu-73169DiVA, id: diva2:1295614
Available from: 2019-03-12 Created: 2019-03-12 Last updated: 2020-01-30
In thesis
1. Plastic Shrinkage Cracking In Concrete: Mitigation and Modelling
Open this publication in new window or tab >>Plastic Shrinkage Cracking In Concrete: Mitigation and Modelling
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Plastiska krympsprickor I betong : begränsning och modellering
Abstract [en]

Early-age (up to 24 hours after casting) cracking may become problematic in any concrete structure. It can have a negative influence on the aesthetics of the concrete structure, as well as decreasing the durability and serviceability by facilitating the ingress of harmful materials into the concrete bulk. Moreover, these cracks may expand gradually during the member’s service-life due to long-term shrinkage and/or loading. Early-age cracking is caused by two driving forces: 1) plastic shrinkage cracking which is a physical phenomenon and occurs due to rapid and excessive loss of moisture, mainly in form of evaporation, 2) chemical reactions between cement and water which causes autogenous shrinkage. In this PhD project only the former is investigated.

Rapid evaporation from the surface of fresh concrete causes negative pressure, known as capillary pressure, in the pore system. This pressure pulls the solid particles together and decreases the inter-particle distances, causing the whole concrete element to shrink. If this contraction is hindered in any way, the induced tensile stresses may exceed the low tensile strength of the concrete and cracking starts. The phenomenon, occurring shortly after casting while the concrete is still in the plastic stage, is mainly observed in elements with high surface to volume ratio such as slabs and pavements.

Many parameters may affect the probability of plastic shrinkage cracking. Among others, effect of water/cement ratio (w/c), fines, admixtures, geometry of the element, ambient conditions (i.e. temperature, relative humidity, wind velocity and solar radiation), etc. has been investigated previously. In the presented research, in addition to studying the influence of various parameters, i.e. w/c, cement type, coarse aggregate content, superplasticizer dosage, admixtures, and steel fibres, effort is made to reach a better and more comprehensive understanding about the cracking governing mechanism. Evaporation, capillary pressure evolution and hydration rate are particularly investigated in order to identify their relationship.

This project started with extensive literature study which is summarized in Paper I. Then, the main objective was set upon which series of experiments were defined. The utilized methods, material, investigated parameters, and results are presented in Papers II-IV. A model was, then, proposed in Paper V, to estimate the cracking severity of the plastic concrete.

It has been observed that evaporation is the driving force behind the plastic shrinkage crackingin concrete. However, a correlation between evaporation, rate of capillary pressure development and the duration of dormant period governs the severity of the phenomenon. Among other things, the results show that rapid capillary pressure development in the pore network accompanied by slower hydration significantly increases the cracking risk.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2019
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Civil Engineering Other Materials Engineering
Research subject
Structural Engineering
Identifiers
urn:nbn:se:ltu:diva-73351 (URN)978-91-7790-344-4 (ISBN)978-91-7790-345-1 (ISBN)
Public defence
2019-05-10, F1031, Luleå, 13:00 (English)
Opponent
Supervisors
Available from: 2019-04-02 Created: 2019-03-28 Last updated: 2019-10-28Bibliographically approved

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