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CFRP Strengthening of Cut-Out Openings in Concrete Walls – Analysis and Laboratory Tests
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.ORCID iD: 0000-0001-9423-7436
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Redesigning buildings to improve their space efficiency and allow changes in use is often essential during their service lives to comply with shifts in living standards and functional demands.This may require the introduction of new openings in elements such as beams, walls, and slabs,which inevitably reduces their structural performance and hence requires repair or strengthening.However, there are uncertainties regarding both the effects of openings and the best remedial optionsfor them. Traditionally, two methods have been used to strengthen reinforced concrete (RC) walls with openings, these being either to create a frame around the opening using RC/steel membersor to increase the cross-sectional thickness. Currently, intervention in existing buildings must be minimal in order to minimise inconvenience caused by limiting the use of the structure during repairs. One option is to use externally-bonded fibre-reinforced polymers (FRPs).

In this study, the author reports on an experimental investigation of the effectiveness of carbonFRP (CFRP)–based strengthening for restoring the axial capacity of a solid reinforced concretewall after cutting openings. Nine half-scale specimens, designed to represent typical wall panels in residential buildings with and without door-type openings, were tested to failure. The walls were tested in two-way action and subjected to axial loading with low eccentricity (defined as one sixth of the wall’s thickness) along the weak axis to represent imperfections due to thickness variation and misalignment of the panels during the construction process. An extensive instrumentation scheme was used to monitor the specimen’s behaviour during the loading cycles. In addition to classical approaches for measuring strains and displacements, optical 3D measurements were also acquired using the digital image correlation (DIC) technique. These provided better overviews of the failure mechanism by recording the crack pattern development and deformation of the walls throughout the loading history.

Reducing the cross-sectional area by cutting out openings i.e. 25% (hereafter referred to as small opening) and 50% (hereafter referred to as large opening) led to 36% and 50% reductions in peak loads, respectively. In both situations the failure was brittle due to crushing of concrete with spalling and reinforcement buckling. The CFRP strengthening increased the axial capacity of walls with small and large openings by 34 – 50% and 13 – 27%, respectively. This partially restored theircapacities to 85 – 95% and 57 – 63% of their precutting capacity (i.e. solid wall), respectively. A procedure based on a rigid-plastic approach for evaluating the ultimate load of walls with cut-out openings that have been strengthened with FRPs was also proposed in this study. Predictions made using the proposed method agree closely with experimental results.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2017. , 159 p.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Building Technologies
Research subject
Structural Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-61515ISBN: 978-91-7583-794-9 (print)ISBN: 978-91-7583-795-6 (electronic)OAI: oai:DiVA.org:ltu-61515DiVA: diva2:1066546
Public defence
2017-02-23, F1031, Luleå, 10:00 (English)
Opponent
Supervisors
Note

Examining Committee: Professor Karin Lundgren, Division of Structural Engineering, Department of Civil and Environmental Engineering, Chalmers University of Technology, Gothenburg, Sweden

Professor Henrik Stang, Section for Structural Engineering, Department of Civil Engineering, Technical University of Denmark, Lyngby, Denmark

Professor Mats Oldenburg, Division of Mechanics of Solid Materials, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå, Sweden

Available from: 2017-01-20 Created: 2017-01-18 Last updated: 2017-02-02Bibliographically approved
List of papers
1. Concrete walls weakened by openings as compression members: A review
Open this publication in new window or tab >>Concrete walls weakened by openings as compression members: A review
2015 (English)In: Engineering structures, ISSN 0141-0296, E-ISSN 1873-7323, Vol. 89, 172-190 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this paper is to review the advances that have been made in the design of monolithic and precast reinforced concrete walls, both with and without openings, subject to eccentrically applied axial loads. Using the results of previous experimental studies, a database was assembled to enable statistical assessment of the reliability of existing design models. Several design aspects are highlighted, including the size and position of openings, and the roles of boundary conditions and geometric characteristics. In addition, the performance of fiber-reinforced polymers in strengthening wall openings is discussed. Overall it is found that design codes provide more conservative results than alternative design models that have been proposed in recent studies. Research into the strengthening of walls with openings is still in its early stages, and further studies in this area are needed. The paper therefore concludes by highlighting some areas where new investigations could provide important insights into the structural behaviour of strengthened elements.

Keyword
Concrete walls, Openings, Axial load, Eccentricity, Out-of-plane behaviour, FRP, Strengthening, Civil engineering and architecture - Building engineering, Samhällsbyggnadsteknik och arkitektur - Byggnadsteknik
National Category
Infrastructure Engineering
Research subject
Structural Engineering
Identifiers
urn:nbn:se:ltu:diva-10356 (URN)10.1016/j.engstruct.2015.02.006 (DOI)928a2b3f-2a36-49a4-af0a-a30ce79fc9a7 (Local ID)928a2b3f-2a36-49a4-af0a-a30ce79fc9a7 (Archive number)928a2b3f-2a36-49a4-af0a-a30ce79fc9a7 (OAI)
Note
Validerad; 2015; Nivå 2; 20150218 (cospop)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-06-27Bibliographically approved
2. Effect of cut-out openings on the axial strength of concrete walls
Open this publication in new window or tab >>Effect of cut-out openings on the axial strength of concrete walls
2016 (English)In: Journal of Structural Engineering, ISSN 0733-9445, E-ISSN 1943-541X, Vol. 142, no 11, 4016100Article in journal (Refereed) Published
Abstract [en]

Old structures are frequently modified to comply with current living standards and/or legislation. Such modifications may include the addition of new windows or doors and paths for ventilation and heating systems, all of which require openings to be cut into structural walls. However, effects of the required openings are not sufficiently understood. Thus, the objective of the work reported here was to analyze openings’ effects on the axial strength of large concrete wall panels. Three half-scaled walls with two opening configurations, corresponding to “small” and “large” door openings, were subjected to a uniformly distributed axial load with a small eccentricity. The results indicate that the 25% and 50% reductions in cross-sectional area of the solid wall caused by introducing the small and large openings reduced the load-carrying capacity by nearly 36% and 50%, respectively. The failure progression was captured using digital image correlation technique and the results indicated involvement of a plate mechanism rather than uniaxial behavior as adopted in current design codes. Using a simplified procedure, the load-carrying capacity was predicted using existing design models found in the research literature and design codes.

National Category
Infrastructure Engineering
Research subject
Structural Engineering
Identifiers
urn:nbn:se:ltu:diva-13543 (URN)10.1061/(ASCE)ST.1943-541X.0001558 (DOI)000386366600008 ()2-s2.0-84991677088 (Scopus ID)cc593011-a98a-471f-9ff8-ca86152f99d9 (Local ID)cc593011-a98a-471f-9ff8-ca86152f99d9 (Archive number)cc593011-a98a-471f-9ff8-ca86152f99d9 (OAI)
Note

Validerad; 2016; Nivå 2; 2016-11-08 (andbra)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-06-27Bibliographically approved
3. Concrete walls with cutout openings strengthened by FRP confinement
Open this publication in new window or tab >>Concrete walls with cutout openings strengthened by FRP confinement
2017 (English)In: Journal of composites for construction, ISSN 1090-0268, E-ISSN 1943-5614, Vol. 21, no 3, 04016106Article in journal (Refereed) Published
Abstract [en]

Redesigning buildings to improve their space efficiency and allow changes in use is often essential during their service lives tocomply with shifts in living standards and functional demands. This may require the introduction of new openings in elements such as beams,walls, and slabs, which inevitably reduces their structural performance and hence requires repair or strengthening. However, there are uncertaintiesregarding both the effects of openings and the best remedial options for them. Here, the authors report on an experimental investigation ofthe effectiveness of fiber-reinforced polymer (FRP)–based strengthening for restoring the axial capacity of a solid RC wall after cutting openings.Nine half-scale specimens, designed to represent typical wall panels in residential buildings with and without door-type openings, were testedto failure. It was found that FRP-confinement and mechanical anchorages increased the axial capacity of walls with small and large openings(which had 25 and 50% reductions in cross-sectional area, respectively) by 34–50% and 13–27%, to 85–94.8% and 56.5–63.4% of their precuttingcapacity, respectively. 

National Category
Infrastructure Engineering
Research subject
Structural Engineering
Identifiers
urn:nbn:se:ltu:diva-2760 (URN)10.1061/(ASCE)CC.1943-5614.0000759 (DOI)000400524800008 ()2-s2.0-85017517559 (Scopus ID)071b8226-9b6c-41f9-9efc-499979742502 (Local ID)071b8226-9b6c-41f9-9efc-499979742502 (Archive number)071b8226-9b6c-41f9-9efc-499979742502 (OAI)
Note

Validerad; 2017; Nivå 2; 2017-04-25 (andbra)

 

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-05-29Bibliographically approved
4. Tests on reinforced concrete slabs with cut-out openings strengthened with fibre-reinforced polymers
Open this publication in new window or tab >>Tests on reinforced concrete slabs with cut-out openings strengthened with fibre-reinforced polymers
2014 (English)In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 66, 484-493 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents the results of experimental investigations on reinforced concrete slabs strengthened using fibre-reinforced polymers (FRP). Eight tests were carried out on four two-way slabs, with and without cut-out openings. Investigations on slabs with cut-outs revealed that the FRP can be placed only around the edges of the cut-out when retrofitting the slabs whereas, in the situation of inserting cut-outs combined with increased demands of capacity, it is necessary to apply FRP components on most of the soffit of the slab. The proposed strengthening system enabled the load and deflection capacities of the FRP-strengthened slabs, in relation to their un-strengthened reference slabs, to be enhanced by up to 121% and 57% for slabs with and without cut-outs respectively.

Keyword
Civil engineering and architecture - Building engineering, Samhällsbyggnadsteknik och arkitektur - Byggnadsteknik
National Category
Infrastructure Engineering
Research subject
Structural Engineering
Identifiers
urn:nbn:se:ltu:diva-10489 (URN)10.1016/j.compositesb.2014.06.008 (DOI)94dd8eba-eb71-4369-b4dd-cbfc03582655 (Local ID)94dd8eba-eb71-4369-b4dd-cbfc03582655 (Archive number)94dd8eba-eb71-4369-b4dd-cbfc03582655 (OAI)
Note
Validerad; 2014; 20140616 (cospop)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-06-27Bibliographically approved
5. Assessment of RC walls with cut-out openings strengthened by FRP composites using a rigid-plastic approach
Open this publication in new window or tab >>Assessment of RC walls with cut-out openings strengthened by FRP composites using a rigid-plastic approach
2017 (English)In: Engineering structures, ISSN 0141-0296, E-ISSN 1873-7323, Vol. 150, 585-598 p.Article in journal (Refereed) Published
Abstract [en]

Building refurbishment works frequently require the cutting of new openings in concrete walls. Cutting new openings weakens the overall response of such elements, so they usually require strengthening. However, current design codes offer little guidance on strengthening walls with openings, and less still on the use of non-metallic reinforcements such as FRP (Fibre Reinforced Polymers) to ensure sufficient load bearing capacity. This paper proposes a new procedure based on limit analysis theory for evaluating the ultimate load of walls with cut-out openings that have been strengthened with carbon-FRP (CFRP). First, the approach is verified against transverse (out-of-plane) and axial (in-plane) loading for unstrengthened specimens. These loading types result in different failure mechanisms: transverse loading leads to failure due to yielding/rupture of the steel reinforcement while axial loading leads to failure by concrete crushing. Second, the proposed method is further developed for CFRP-strengthened specimens under axial loading. It accounts for the contribution of CFRP indirectly, by updating the concrete model with an enhanced compressive strength as a result of confining the piers. Predictions made using the new method agree closely with experimental results.

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Infrastructure Engineering
Research subject
Structural Engineering
Identifiers
urn:nbn:se:ltu:diva-9022 (URN)10.1016/j.engstruct.2017.07.069 (DOI)2-s2.0-85026533028 (Scopus ID)794c01ea-1ee7-4cdf-8f59-1d088954f9bf (Local ID)794c01ea-1ee7-4cdf-8f59-1d088954f9bf (Archive number)794c01ea-1ee7-4cdf-8f59-1d088954f9bf (OAI)
Note

Validerad; 2017; Nivå 2; 2017-08-15 (andbra)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-08-15Bibliographically approved

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Citation style
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Output format
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