Rapporten diskuterar tidsberoende deformationer i anläggningskonstruktioner av armerad betong. Deformationerna kan bero på faktorer som uttorkning och fuktförhållanden (krympning) och/eller laster (krypning). I två bilagor ges en presentation av metoder för modellering (Bilaga 1) och av beräkning av spännkraftförluster i spännarmering (Bilaga 2). Områden för fortsatta studier rekommenderas.

The report discusses time-dependent deformations in civil engineering structures made of concrete. These deformations can depend on factors such as drying and moisture conditions (shrinkage) and/or loads (creep). Two appendices provide state-of- the-art reports  on modelling (Appendix 1) and on prestress losses in reinforcement (Appendix 2). Further studies are recommended.

The transition to low-carbon concrete requires a rapid implementation of new alternative binders also in infrastructure projects. The Swedish Road Administration requires analysis of early cracking risk in projects with massive structures (bridges, tunnels, etc.). This analysis needs determination of material parameters at a young age using e.g. the temperature-stress testing machine (TSTM) which are very time consuming. Today, these tests are performed in Sweden without a standard on limited laboratory resources, which was identified as the most critical hinder for rapid implementation of new cements on the market. This study explores how young concrete material parameters can be derived more easily, quickly and in a more verifiable way. This paper analyses the literature concerning the testing methods for mechanical properties and free deformation of concrete at early age and presents findings from a study on determination of maturity function and activation energy by isothermal calorimetry and mortar tests. The latter presents promising results that could be used to test those basic binder properties and extrapolate them into concrete level. 

Self-Compacting Concrete (SCC) offers favourable properties which help accelerate the casting time, especially in congested reinforced structures but when casting with SCC uncertainty remains a challenge on the behaviour of its formwork pressure. Researchers have introduced several design models to predict pressure and its behaviour. This research aims to assess the design models that have been reported in the literature. The assessment was carried out through a series of rigorous laboratory tests and the results from the tests served as input for the mathematical model evaluation. Twelve concrete columns with 2 m height were cast in the laboratory to study the effect of varying the input parameters in the existing design models. The formwork pressure was documented by a pressure monitoring system, with the capacity to produce instant results for real-time remote monitoring of the pressure development during and after concrete casting. The formwork pressures were calculated according to the current design models and were compared with pressure data acquitted from the laboratory tests. The results showed that the pressure predicted by the design models was typically greater than the pressure observed during the laboratory tests. The DIN18218 design model showed a relatively close approximation of the pressure distribution over the formwork height and casting time. The limitation of the models is observed when the casting rate varies, and models are sensitive to the input parameters. Thus, additional development of the current design models is needed to enable reliable estimations of the pressure, for example, in the case of low and high casting rates. The laboratory tests also showed that high casting rates and high slump flows generate higher pressures whereas higher thixotropy results in faster pressure reduction during construction.

Despite the advantageous benefits offered by self-compacting concrete, its uses are still limited due to the high pressure exerted on the formwork. Different parameters, such as those related to concrete mix design, the properties of newly poured concrete, and placement method, have an impact on form pressure. The question remains unanswered on the degree of the impact for each parameter. Therefore, this study aims to study the level of impact of these parameters, including slump flow, T500 time, fresh concrete density, air content, static yield stress, concrete setting time, and concrete temperature. To mimic the casting scenario, 2 m columns were cast at various casting rates and a laboratory setup was developed. A pressure system that can wirelessly and continuously record pressure was used to monitor the pressure. Each parameter’s impact on the level of pressure was examined separately. Casting rate and slump flow were shown to have a greater influence on pressure. The results also demonstrated that, while higher thixotropy causes form pressure to rapidly decrease, a high casting rate and high slump flow lead to high pressure. This study suggests that more thorough analysis should be conducted of additional factors that may have an impact, such as the placement method, which was not included in this publication.

Plastic shrinkage cracking in cementitious materials is caused mainly by rapid and excessive moisture loss during mixture’s early ages, before sufficient tensile strength is gained. A novel model has been previously developed by the authors to estimate the severity of plastic shrinkage cracking in concrete. This paper presents findings of a series of full-scale experiments carried out to validate the accuracy of the proposed model. The experiments included investigating the impact of cement type, water-cement ratio (w/c), and admixtures (i.e., accelerator, retarder, and superplasticizer). The tests were performed in three rounds under similar ambient conditions using 3 slabs (3 m × 2 m) and 3 ring test moulds at each round. The results confirm the accuracy of the model in anticipating/comparing the cracking severity of the tested concretes.

This paper investigates and compares the effect of steel fibres obtained through recycling waste tyres (known as RTSF), and a commercially available hooked steel fibre (HSF), on plastic shrinkage cracking in self-compacting concrete. The volumetric deformations of the specimens, bleeding, and the mass loss have been quantified. Mixtures containing 2.5, 5, 7.5 and 10 kg/m3 of RTSF, and 5 and 7.5 kg/m3 of HSF have been tested. The results show that an almost similar reduction of the crack area can be attained if HSF is replaced by a slightly higher amount of RTSF. However, the former seems to be more effective in restraining plastic shrinkage. Both fibres decreased the volumetric shrinkage and the bleeding capacity of the specimens.

The maximum amount of lateral formwork pressure exerted by self-compacting concrete is essential to design a technically correct, cost-effective, safe, and robust formwork. A common practice of designing formwork is primarily based on using the hydrostatic pressure. However, several studies have proven that the maximum pressure is lower, thus potentially enabling a reduction in the cost of formwork by, for example, optimizing the casting rate. This article reviews the current knowledge regarding formwork pressure, parameters affecting the maximum pressure, prediction models, monitoring technologies and test setups. The currently used pressure predicting models require further improvement to consider several pressures influencing parameters, including parameters related to fresh and mature material properties, mix design and casting methods. This study found that the maximum pressure is significantly affected by the concretes’ structural build-up at rest, which depends on concrete rheology, temperature, hydration rate and setting time. The review indicates a need for more in-depth studies.