Opinion of:

Claudio Burgazzi

Claudio Burgazzi Design studio

From Realtà Mapei International n° 84 - 3/23/2021

Strengthening and seismic upgrading work for Villa Alba

Strengthening and seismic upgrading work for a private villa in Northern Italy.

Villa Alba is a residential building in Piacenza (Northern Italy) with three storeys above ground level (one of which is an attic) and one below ground level. The structure of the building is made from load-bearing, “two-header”, solid brick masonry walls and lime mortar. The building was found to be pretty vulnerable from a seismic point of view, in that the existing structure was unable to guarantee a “box action” behaviour, as recommended by Italian Technical Standards for Constructions for masonry structures. Following an in-depth survey of the building itself and a study of its recent history, it was possible to identify when and how the original static arrangement of the structure had been modified over the years. The surveyors found that work had been carried out several times on Villa Alba since its construction between 1956 and 1958 and that the dimensions of the load-bearing walls had been gradually reduced, leaving the structure more vulnerable to static loads and seismic activity. It was also found that some of the building’s vertical elements (such as two small masonry pillars in the main inside walls) were overloaded. Also, the masonry was generally in good condition in the areas where there were no flues or cable and pipe chases. From the tests carried out on the structure, it emerged that the 16 cm thick skimmed, self-supporting floors did not comply with current regulations; apart from being deformable along their plane, the effect of gravitational loads had not been verified and the way they were connected to the outer walls was inadequate.

It was decided, therefore, to intervene on the structure to upgrade its overall seismic capacity, thereby enabling the client to qualify for Superbonus 110%, an Italian Government’s initiative aimed at relaunching and promoting growth in the Italian economy and, above all, a chance for the country’s heritage buildings to be upgraded and made more safe in the event of an earthquake.

The structural work described in this article, along with other more traditional work carried out on the building, has led to a reduction in the building’s seismic vulnerability and to its risk category being upgraded by three levels, according to conventional testing methods, with respect to its situation prior to carrying out the work.

 

 

STRENGTHENING THE FLOOR-SLABS AND AND PERIMETRAL WRAPPING
To upgrade the static capacity of the floor-slabs, improve the box action behaviour of the building in the event of seismic activity and, at the same time, prevent the onset of localised mode I mechanisms
(overturning and out-of-plane vertical bending), the self-supporting reinforced brick floors (SAP) without structural screed were strengthened and stiffened by adding a 3 cm-thick, HPFRC structural screed made from high-performance, fibre-reinforced micro-concrete, connected to the perimeter walls with anchors made from rebar grouted into the masonry to form a floor diaphragm resistant to seismic loads.

In correspondence with each floor, a uniaxial, carbon fibre band running around the outside of the building was applied to counteract the tensile stresses in the kerb of each floor diaphragm and to counteract the onset of the mechanisms mentioned above.

To strengthen the floor-slabs, once all the various layers on the surface of the extrados of the SAP brick floor-slabs had been removed, their substrate was prepared for application of the structural screed by removing all loose material and roughening the surface with a power-scarifier to create a surface roughness of at least 5 mm. Lengths of B450C steel were then anchored in the outer walls with MAPEFIX EP 385 pure epoxy, resin-based chemical anchor.

After consolidating the substrates with PRIMER 3296, a 3 cm-thick layer of PLANITOP HPC FLOOR 46 was applied to form the structural screed.

The effectiveness of this solution was tested and validated by the University of Brescia (Northern Italy) by carrying out a series of in-situ load tests, through which it was possible to verify an absence of slip at the interface between the PLANITOP HPC FLOOR 46 structural screed and the extrados of the floor-slab and a three-fold increase in the slab’s flexural stiffness, against an increase in thickness of just 3 cm.

To form the strengthening wrapping for each floor, the first step was to remove the render from the walls. The sharp corners of the existing joists were then rounded off to a radius of 30 mm, the surfaces were levelled off and any damaged concrete was integrated with PLANITOP SMOOTH&REPAIR R4 after cleaning the surface of the exposed rebar and applying MAPEFER to prevent corrosion. The strengthening band was then formed using MAPEWRAP C UNI-AX 600 W uniaxial, high modulus, high tensile carbon fibre fabric, particularly recommended for seismic upgrading applications.

The fabric was applied using a special Mapei epoxy resin system, consisting of MAPEWRAP PRIMER 1 as a primer, MAPEWRAP 11 thixotropic epoxy putties for levelling, and MAPEWRAP 31 adhesive for impregnating and bonding.

The surface was then broadcast with dry QUARTZ 1.2 quartz sand while the resin was still wet to create a good rough surface for the skim coat. To make the strengthening system even more effective, special ties made from pieces of MAPEWRAP C FIOCCO carbon fibre cord impregnated with MAPEWRAP 21 super-fluid epoxy resin were applied transversally in correspondence with any areas where there were changes in direction, to the corners of the masonry and at every two metres along the straight parts of the strengthening band, and were anchored with MAPEFIX VE SF chemical styrene-free vinylester anchor in holes drilled into the joists.

 

 

CONFINEMENT OF MASONRY PILLARS
To strengthen the masonry pillars, which have to withstand mainly axial loads, it was decided to confine them with a continuous wrapping
  with MAPEWRAP C UNI-AX 300 W unidirectional, high-modulus, high-tensile carbon fibre fabric. The first step was to remove the render, round off the edges to a radius of 30 mm and then smooth over the surface with PLANITOP HDM MAXI two-component, pre-blended, high-ductile, fibre-reinforced mortar. The fabric was again applied in these areas using a special Mapei epoxy resin system. The last step was to broadcast the resin while still wet with dry QUARTZ 1.2 sand to create a good rough surface for the skim coat.

 

 

STRENGTHENING THE MASONRY
For the in-plane strengthening of the old masonry PLANITOP INTONACO ARMATO was applied
, a two-component, cement-free, pre-blended, high-ductile, natural hydraulic lime (NHL) and Eco-Pozzolan-based mortar reinforced with glass fibres, particularly recommended for increasing the strength and deformation capacity of wall panels in the event of seismic activity, without having to resort to the application of strengthening mesh. Once all the old render had been stripped off, localised areas of the damaged masonry were demolished and then re-built using the “patching” technique in corre-  spondence with the flues, recesses and cable and pipe chases. After applying a coat of PRIMER 3296 to consolidate the substrate, a layer of PLANITOP INTONACO ARMATO was applied with a rendering machine designed for spraying two-component products, equipped with a separate mixer unit.

 

Claudio Burgazzi. Claudio Burgazzi Design studio

 

 

Load tests at Villa Alba

Load tests were carried out on the floor-slab both before and after applying the strengthening system. Both series of tests were carried out by applying concentrated loads and measuring the amount of deflection. The loads were simulated by applying bags of cement positioned at a distance of L/4 from the supports with a footprint measuring 0.65 x 0.85 m. The test set-up is shown in Figure 1.

The amount of deflection was measured using centesimal displacement indicators on telescopic poles. Three indicators were positioned at the mid-point, two were positioned directly below the load points and two were placed near to the supports. Figures 2 and 3 show the position of the dial indicators and the loads.

In the pre-strengthening phase the load was applied in seven stages, reaching a maximum load of 10 kN (distributed over two points) and the maximum deflection, taken at the mid-point, was found to be 0.71 mm.

After carrying out the strengthening work, the load was applied in eight stages, reaching a maximum load of 16 kN (distributed over two points). 

In this case, the amount of deflection, taken again at the mid-point, was found to be 0.28 mm.


By comparing the amount of deflection before and after the strengthening work, it was found that the PLANITOP HPC FLOOR 46 micro-concrete structural screed had increased the floor-slab’s flexural stiffness by around 300%. As a result, the amount of deflection measured after applying the strengthening system was much lower (by 2.53 times). Also, the maximum load applied was 60% higher.

While carrying out the tests on the strengthened floor-slab, dial indicators with an accuracy of one thousandth of a millimetre were used to measure any slip at the interface between the PLANITOP HPC FLOOR 46 structural screed and the surface of the extrados of the SAP floor-slab. The amount of slip detected, 0.003 mm, may be considered negligible and is a demonstration of the excellent level of adhesion between the existing substrate and the fibre-reinforced microconcrete.



Prof. Fausto Minelli and eng. FabiolaIavarone.
University of Brescia (Northern Italy)

 

Products mentioned in the article

MAPEFER
MAPEFER
Two-component, anti-corrosion cementitious mortar for steel reinforcement rods. TECHNICAL DATA: Maximum dimension of aggregate: 0.5 mm.…
MAPEFIX EP 385/585
MAPEFIX EP 385/585
Pure epoxy, resin-based chemical anchor for structural loads. Certified for threaded bar, construction bars, core-drilled holes and C2…
MAPEFIX VE SF
MAPEFIX VE SF
Styrene-free, hybrid vinyl resin-based chemical anchor for structural loads. Certified for threaded bar, construction bars and C1 seismic…
MAPEWRAP 11
MAPEWRAP 11
Two-component, normal-setting, thixotropic epoxy grout for evening out concrete surfaces and for structural bonding. TECHNICAL DATA: Mixing…
MAPEWRAP 21
MAPEWRAP 21
Two-component, super-fluid epoxy resin for impregnating MAPEWRAP fabrics using the “damp system”. TECHNICAL DATA: Mixing ratio: comp. A :…
MAPEWRAP 31
MAPEWRAP 31
Two-component, medium-viscosity epoxy adhesive for impregnating MAPEWRAP fabrics using the “dry system”. TECHNICAL DATA: Mixing ratio:…
MAPEWRAP B UNI-AX
MAPEWRAP B UNI-AX
Unidirectional, high strength basalt fibre fabric. TECHNICAL DATA: Weight: 400-600 g/m². Equivalent thickness of dry fabric: 0.143-0.215…
MAPEWRAP C FIOCCO
MAPEWRAP C FIOCCO
High-strength carbon fibre unidirectional cord to be impregnated with MAPEWRAP 21 (two-component, super-fluid epoxy resin) to make…
MAPEWRAP C UNI-AX SYSTEM
MAPEWRAP C UNI-AX SYSTEM
Structural strengthening system consisting of high-strength high-modulus unidirectional carbon fibre fabric and epoxy resins to impregnate…
MAPEWRAP PRIMER 1
MAPEWRAP PRIMER 1
Two-component epoxy primer specifically formulated for the MAPEWRAP system. TECHNICAL DATA: Mixing ratio: comp. A : comp. B = 3 : 1.…
PLANITOP HDM MAXI
PLANITOP HDM MAXI
Two-component ready-mixed, high ductility pozzolan-reaction, fibre-reinforced mortar for structural “reinforced” strengthening work, when…
PLANITOP HPC FLOOR 46
PLANITOP HPC FLOOR 46
Ultra-high performance, high ductility, free-flowing micro-concrete reinforced with metal fibres.
PLANITOP INTONACO ARMATO
PLANITOP INTONACO ARMATO
Two component, ready-mixed, cement-free, high-ductility fibre-reinforced natural hydraulic lime (NHL) and ECO-POZZOLAN mortar, containing…
PLANITOP RASA & RIPARA
PLANITOP RASA & RIPARA
Quick-setting, fibre-reinforced, compensated-shrinkage, thixotropic cementitious mortar for repairing and smoothing concrete, applied in…
PRIMER 3296
PRIMER 3296
Acrylic primer in water dispersion with high penetration, consolidating and anti-dust properties for screeds. TECHNICAL DATA: Consistency:…
QUARTZ 1.2
QUARTZ 1.2
Calibrated silica sand to improve the bond on resin or epoxy primers. TECHNICAL DATA: Colours: grey - beige. Grain size: 0.7-1.2.…
Realtà Mapei Tags: #research #strengthening

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