A multi-disciplinary approach to the characterization and assessment of early reinforced concrete structures

 

Concrete is one of the three most common materials (together with timber and steel) used worldwide for civil engineering works, thanks to its characteristics of mechanical strength and versatility. After the first experiments performed during the 19th century, the use of concrete showed its full potential at the beginning of the 20th century with the development of the ‘reinforced concrete system’ by François Hennebique. However, the outstanding performance of this material was not accompanied by research for adequate durability; nowadays, most of the RC structures built in the 20th century show clear signs of decay, mostly related to the phenomenon of reinforcement bars corrosion. The problem of materials durability is accompanied by more specific structural problems. The reuse of RC structures, older than 50 years, often implies changes in use; moreover, since 2003, the whole Italian territory has become seismic. It results in a significant increase of expected loads and changes in the required safety levels, according to the importance classes of buildings, strategic or not. The classification depends on the consequences of collapse for human life, on their importance for public safety and civil protection in the immediate post-earth- quake period, and on the social and economic consequences of collapse. The evaluation of their vulnerability entails the identification of significant features that affect the structural behavior, such as materials properties and construction quality, characteristics and structural details of the main structural elements: beams, columns, slabs.

Aims and methods
This study is aimed at providing a reference framework for static and seismic assessment of first generation reinforced concrete structures, based on the data collected from historical research, onsite investigation campaigns and laboratory tests. The case-studies are early RC structures mainly located in the Veneto Region (North-Eastern Italy). The period of construction spans between 1900 and 1960. The case-studies are: the Civic Theatre of Schio (Vicenza) (1907), the Northern Wing of the former Carraresi castle in Padova (1910), Villa Girasole in Marcellise (Verona) (1935), the Department of Pharmacy of the University of Padova (1937), ex-Trap Shooting Building in Venice (1949), the Victory Monument in Bolzano (1929), the former Cooling Station in the former General Market area in Verona (1930), two industrial buildings in the former Agrimont area in Porto Marghera (1930s), the Maternity ward (1945-50) and the Pediatric ward of the Padova hospital (1954), the former Zweifel Embroidery roof in Verona (1960s), the former cattle market in Padova (1960s) (figure 1).

The first approach in the analysis of all existing structures is the knowledge phase, generally performed through the collection of original design drawings, pictures of construction phases and documents which show transformations, both structural and in use. Then, a careful on-site investigation of the asbuilt details is advised. Inspection planning must pursue minimum obtrusiveness promoting crosschecking of results among tests at different invasiveness levels. Visual inspection is often the first qualitative step to identify the most significant elements on which tests shall be carried out to characterize vertical and horizontal structures. A thorough knowledge phase should take into account multiscale and multi-physic quantitative tests on materials and structural elements. Before extracting cores (figure 2), an electromagnetic covermeter (figure 3) is used to locate reinforcement bars and avoid them. Concrete cores are, then, extracted in key portions of RC structures and used for laboratory mechanical tests of compression, splitting and identification of elastic modulus values. After mechanical tests, concrete pieces are used to study the composition, the minero-petrographic and chemical characteristics of conglomerate (figure 4). Non-destructive onsite tests with a Rebound Hammer (figure 5) are, then, performed on a large number of RC elements to recover compressive strength values and assess concrete quality over the entire structure. A good practice is to carry out non-destructive tests where corings were extracted for a consequent crosschecking among results of tests at different invasiveness and reliability levels. Regarding structural details, the most effective technique for checking the correspondence between the built structure and the original drawings or simulated design outcomes (in case of unavailable original documents) is performing local scarifications of concrete cover (figure 6) to identify cover thickness, reinforcement bars quantity and distribution. The results are then employed to perform static and seismic assessments on the buildings, functional for the execution of strenghtening interventions to improve their seismic behaviour.

Results
All test data concern the 1900 - 1960 period and refer to 86 concrete cores and 78 pieces of rebars (figure 7) sampled from public and private buildings within a wide framework of structural typologies, such as residences, public buildings, industrial buildings and monuments.
Variations in concrete composition and in compressive strength were observed in different structural element from the same structure. The compressive strength values on slabs are comprised within the ranges 5÷10 N/mm2 and 15÷20 N/mm2, highlighiting the great variability in the production process of conglomerate. The compressive strength values of beams are mainly concentrated in the 20÷25 N/mm2 range, whereas concrete cores from columns show values of compressive strength in the 10 ÷15 N/mm2 range. These results can be analysed considering the different use of buildings. In public buildings built between 1920 and 1950, concrete compressive strength is lower compared to private/industrial buildings built in the same period and to public structures built between 1900 and 1920. These results suggest that in the very early period (1900-1920) of RC system applications there was a special attention to material properties, probably to compensate for uncertainty factors related to the novelty of this construction system. After this period, the two World Wars caused economic restrictions and the need to ‘save’. This logicled to saving, especially, in materials quality. In particular, concrete mix design and, then, its compressive strength worsened during the realization of structural elements with low stress levels. On site concrete production favored this negative practice. Generally, concrete mix design was characterized by high porosity and inadequate aggregate size. As for cement content, it is generally attested at values around 300 kg/m3, close to the prescriptions of the first Italian code for RC structures (1907), reaching in some cases higher values (350 ÷ 400 kg/ m3). Low values (e.g. under 200 kg/m3) are observable only in concretes characterized by compressive strength lower than 10 N/mm2. W/C ratio is a very variable parameter: in the concretes of the Carraresi Castle the ratio is lower than 0.5, whereas in the Victory Monument in Bolzano (1929) and in the ex-Embroidery in Verona (1960) values range from 0.7 to 0.8. Furthermore, porosity values vary from >20% to <10% in the case of Hennebique concrete.
Regarding reinforcement, results from tensile tests showed that more than half of the samples have a yield strength value between 300 and 350 N/mm2 and a failure strength between 450 and 500 N/mm2. These values underline a higher homogeneity of steel reinforcement with respect to concrete. Therefore, in RC historic structures the main issue concerning rebars is their distribution (details, mainly of beam-column joints), their diameters and surface properties.
Regarding static assessment, in case of existing structures, the Italian buildings code allows to perform safety assessments and intervention design only according to Ultimate Limit States (ULS), with very few exceptions. Static analyses performed according to ULS underlined a satisfactory level of safety for all structural elements, also considering an increase of live loads due to a new use. In the past, the use of high safety factors, the consequent oversizing of structures, and the limited exploitation of concrete compressive strength and steel tensile strength, allowed to satisfy static assessment, also according current codes.

All the case-studies analyzed in this work were designed considering only gravity loads. When horizontal actions are considered, linear analyses showed that more than half of the structural elements are not verified. In particular, for shear assessment of beams, current seismic codes require closed stirrups with 135° hooks and, therefore, no bent-up bars (figure 8) can be considered as transverse reinforcement. This requirement strongly limits shear resistance of RC elements. Furthermore, the main problems for columns are related to the combined action of axial force and bending moment. This is mainly related to the fact that only gravity loads were considered during the design phase of such elements.

Conclusions
Nowadays, Italian RC structures built at the beginning of the 20th century need repair or strengthening interventions due to material degradation related to abandon, change in use and current seismic rules which extended seismic hazard to the whole Italian peninsula.
The main conclusions of this research may be summarized as follows:
a_even if variations in concrete composition and in compressive strength were observed in different structural elements even from the same structure, the compressive strength of concrete cores dated 1900-1920 is on average +30% than the compressive strength of concrete of the period 1920-1950;
b_the good quality reinforcement (fy,av=365 MPa and ft,av=493 MPa) was noticed in all the investigated structures, with a variability of mechanical properties more limited compared to concrete;
c_high safety factors applied to materials as required by codes in the early 20th century allow to satisfy static assessment according to current building standards;
d_seismic assessments showed columns deficiencies under the action of combined axial force and bending moment and poor shear resistance for beams.
Apart from the specific technical outcomes, this study underlined the necessity to start considering the cultural values of early reiforced concrete buildings, conjugating structural interventions with pure restoration activities necessary to preserve the aesthetic features of these modern works of art.

[by Claudio Modena, Full Professor, University of Padua]