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- Carlswärd, Jonas, et al.
(författare)
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Avoiding undesirable end results of bonded steel fibre concrete overlays : observations from tests and theoretical calculations
- 2014
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Ingår i: Nordic Concrete Research. - 0800-6377. ; 49, s. 93-112
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Tidskriftsartikel (refereegranskat)abstract
- Due to a high degree of damages and undesirable final results of bonded overlays, research has been conducted to develop recommendations on design and execution. Laboratory and half scale tests as well as theoretical analyses have been carried out including e. g. base and end restraint tests on overlays with various reinforcement, concrete qualities, substrate preparing and curing. Also, analytical and numerical calculations have been performed. Results reveal that the bond between overlay and substrate is the most critical parameter for a successful final result. Other key parameters are shrinkage and curing, while fibre and bar reinforcement generally proved to be less significant. Theoretical models work well on this case and will be further developed
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| 2. |
- Carlswärd, Jonas, et al.
(författare)
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Prediction of stress development and cracking in steel fiber-reinforced self-compacting concrete overlays due to restrained shrinkage
- 2010
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Ingår i: Fiber-Reinforced Self-Consolidating Concrete. - : American Concrete Institute. - 9780870313981 ; , s. 31-49
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Konferensbidrag (refereegranskat)abstract
- Shrinkage cracking of self-compacting concrete (SCC) overlays with and without steel fibres has been assessed through laboratory testing and theoretical analysis. Test results verified that steel fibre reinforcement has a crack width limiting effect. However, the contribution in case of fibre contents up to 0.75 vol% was not found to be sufficient to distribute cracks in situations where bond to the substrate was nonexistent. Thus, even higher steel fibre contents (or other types of fibres) are required in order to control cracks. A distributed pattern of fine cracks was however obtained even for unreinforced SCC within bonded areas of the overlays. This implies that steel fibres, or other crack reinforcement, are not required if high bond strength is obtained. An analytical model, proposed to assess the risk of cracking and to predict crack widths in overlays, was found to give reasonable correlation with experimental results
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| 6. |
- Carlswärd, Jonas
(författare)
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Shrinkage cracking of steel fibre reinforced self compacting concrete overlays: test methods and theoretical modelling : test methods and theoretical modelling
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Steel fibres are often applied as crack reinforcement in overlays even though methods for the design with respect to crack widths are not yet available. Thus, the intention was for the research to form a basis for future design of overlays with respect to cracks. A main ambition was to give guidance on e.g. type of steel fibre reinforced concrete (SFRC) required to limit crack widths and how to prepare the substrate surface in order to produce a high and even base restraint. Test methods and theoretical analysis has been applied to fulfil the aims. The experimental part consisted of end-restrained shrinkage tests, bond tests and half scale overlay tests. End-restrained shrinkage and half scale overlay tests were conducted in order to evaluate the effect of steel fibres for different restraint situations while the bond tests were intended to give information regarding the appropriateness of different substrate treatments from a bond strength perspective. Test results showed that the situation of restraint has a significant influence on the cracking response. A single crack developed if the overlay was restrained only at the ends for both un-reinforced concrete as well as for SFRC. However, the crack width was found to be reduced due to the addition of steel fibres. In case of a continuous restraint provided by bond to the substrate on the other hand, numerous well distributed, fine cracks were observed. For this situation there was no measurable influence of fibres on the width and distribution of cracks. A conclusion is thus that reinforcement is not required in case of thin overlays (depths of 50 mm have been studied) if a high and even bond strength is obtained. Test results verified that high bond strength can be achieved by pre-moistening the substrate prior to overlaying, in combination with thorough cleaning. However, the substrate should be allowed to dry back prior to overlaying, as a wet surface was shown to be deteriorating for the bond strength. Caution is also recommended if the overlay is cast onto a dry substrate. In case of insufficient bond strength, with an apparent risk for partial debonding, i.e. parts of the overlay debond while other parts are still bonded, reinforcement is however required to limit crack widths. A theoretical method has been proposed for the design of SFRC for such situations and predictions have been verified through comparisons with experimental results.
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| 7. |
- Carlswärd, Jonas, et al.
(författare)
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Shrinkage cracking of thin concrete overlays
- 2014
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Ingår i: Nordic Concrete Research. - 0800-6377. ; 50, s. 355-359
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Tidskriftsartikel (refereegranskat)abstract
- Due to a high degree of damages and undesirable end results of bonded overlays, research is conducted to develop recommendations on design and execution. Laboratory and full scale tests as well as theoretical analyses have been carried out including e. g. base and end restraint tests on overlays with various reinforcement, concrete qualities, substrate preparing and curing. Moreover, analytical and numerical calculations have been performed. Results reveal that the e. g. bond between overlay and substrate is a critical parameter for a successful end result. Another key parameter is sufficient curing, while reinforcement generally proved to be less significant. Theoretical models works well on this structural situation and will be further developed.
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| 8. |
- Carlswärd, Jonas
(författare)
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Steel fibre reinforced concrete toppings exposed to shrinkage and temperature deformations
- 2002
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work presented in the thesis primarily focuses on problems that are characteristic for thin layers of concrete exposed to imposed loads. More specific, the work aims at investigating, or rather finding ways to evaluate the efficiency of steel fibres as regards the limitation of crack widths in concrete under restrained conditions. To fulfil the ambitions a test method was developed in which concrete toppings were placed on the upper face of a bottom slab that constituted a stiff foundation. The toppings were then exposed to temperature loads that successively resulted in both the formation of vertical cracks in the concrete as well as horizontal cracks along the interface to the substructure. A total number of eight tests divided into two series were then performed using the proposed technique of testing. At each such occasion two toppings were tested simultaneously, one Plain Concrete (PC) and one Steel Fibre Reinforced Concrete (SFRC) specimen. In this way a comparative evaluation of the influence of steel fibres was facilitated. Results showed that end hooked steel fibres, in amounts of 30 to 60 kg/m3, in most cases reduced the maximum widths of appearing cracks substantially although the reinforcement ratios were not sufficient to get completely crack free structures. It is also clear from the experiments that the effect to some extent was influenced by the properties of the interface in the sense that the fibre contribution seemed to increase as the bond quality became poorer. A series of restrained shrinkage tests were also performed primarily for the sake of verification. Within the frames of this study a total of eight half- scale toppings, four with a depth of 6 cm and four with 12 cm depth, were cast on the surface of an old concrete floor. Both plain and steel fibre reinforced concrete specimens were included in the series. Also, for comparison reasons some conventional steel bar reinforced toppings were produced as well. After an initial curing period of three days the specimens were exposed to one-sided drying that successively resulted in the development of visible cracks. From a crack limiting point of view it was concluded that steel fibres were at least as effective as the steel bar mesh, although it was clear that none of the alternatives were adequate for the purpose of achieving crack-free structures. However, the main reason as to why the effect of reinforcement was not as pronounced as anticipated was believed to be that the bond proved to be insufficient. In particular, considering that the ends of the toppings were fastened to the floor by means of expanding bolts this resulted in a too severe load situation. Regarding the influence of the depth it was shown that cracks appeared at a considerably earlier stage for the thin toppings. At the end of the measuring period the cracks were also considerably wider. This was explained as being a result of the considerably faster rate of desiccation experienced for a thin layer. A simple linear elastic Finite Element analysis was also conducted to verify the effect of the sectional depth in addition to the influence of the concrete creep on the development of stresses in the concrete. Results from this study showed that stresses are substantially reduced due to creep effects. It was further shown that the progress of tensile stresses in the concrete is somewhat slower for a thicker section, mainly due to the slower shrinkage strain development.
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