The New Boundaries of Structural Concrete

During the last 30 years, concrete technology has made considerable advances and provided new and high performance materials suitable for satisfying the requirements of designers and construction companies. In the beginning, research achievements mainly concerned compression strength. The increase of the concrete strength was made possible due to a reduction of the water-cement (w/c) ratio up to a value of about 0.40. Below this value, it was not possible to assure the workability of the mixture for practical applications. Later on, with the aim of reducing the placement costs, superplasticizers have been developed, which also allowed to further reduce the w/c ratio with the consequent benefits in terms of strength. The development of new generations of superplasticizers has then allowed for the production of self-compacting concrete (SCC), which often represents the only solution suitable for the realization of earthquake-resistant RC joints, where the reinforcement density makes the placement of ordinary concrete very difficult, because of the reduced spacing among rebars. Research efforts in last few years mainly focused the attention on material and structural durability which is a critical issue for the sustainability of the constructions. Actually, concrete durability represents one of the revolutionary novelties of the new Italian building code (D.M. 14-1-2008) that, for the first time, introduces the concept of “nominal life” of a structure in a mandatory document, defined as the “number of years in which the structure, although subjected to ordinary maintenance, has to accomplish the function to which is devoted”. A durable structure requires the use of materials having an enhanced quality. The choice of durable materials is also a convenience’ choice as the increase of costs for improving the material performance is generally minimal if compared with the construction costs. In addition, it is further convenient when considering the maintenance costs not to say the lawyer expenses when the controversy goes into a court. In order to increase the “nominal life” of RC structures (and so, to enhance their durability), the concrete porosity has to be reduced so that a better protection of steel reinforcement is assured. This implies the reduction of the w/c ratio which should be very low when the external environment is very aggressive. The lower w/c ratio required for structural durability implies a higher concrete strength which, for the new Italian structural code, can have a class strength up to C70/85, characterized by a characteristic value of the compressive strength (measured on cubic specimens) not lower than 85 N/mm2. Once a lower concrete porosity is assured, protection of steel reinforcement also requires a control of cracks occurring in the structure. This can be addressed by means of a correct placement of the rebars and, if necessary, also by adding fibers to the concrete matrix; special types and contents of fibers have to be chosen according to the required performance. Fiber Reinforced Concrete (FRC) is one of the materials that, together with the alloys of aluminum, the alloys of copper, the fiber reinforced polymer materials, the panels with collaborating polyurethane or polystyrene, non-traditional wall materials, the structural glass and the non-metallic reinforcement, has been mentioned in the Italian structural code. For this reason, with the preliminary approval of Central Technical Service of the Infrastructure Department, FRC can be used for the structural members. The Department of Infrastructure has under preparation specific Guidelines on the use of fiber-reinforced concrete for which the “Guide for the Design and Construction of Fiber-Reinforced Concrete Structures” - edited by the National Research Council - is already available (CNR DT 2004). A new concrete performance which will have an increasing importance in the design regulations is represented by shrinkage, aimed at reducing the cracking of the structure. This need is already a priority in industrial concrete pavements and will become more relevant for other structural elements since the increase of concrete strength made the autogenous shrinkage significant. The new Fib Model Code – reference document for several international structural codes – includes specific design regulations for the new materials. In summary, it is evident that the research and technological development have allowed concrete to be keep up with time, by continuing to represent an optimal choice for the realization of constructions more and more demanding in terms of safety, durability and sustainability. However, this modern know-how has not been completely understood by many structural designers that, often, make still use of “out-of-date” concretes. This anomaly is mainly due to the lack of regular meetings between concrete technologists and structural engineers. This is one of the main motivations which encouraged to organize the first Workshop on “The new boundaries of structural concrete” held at the University of Salerno in April 22-23, 2010. The Workshop has been organized through a joint collaboration between the Department of Civil Engineering of the University of Salerno and the American Concrete Institute Italy Chapter. Experts in concrete technology and structural engineering from universities, industry and construction companies joined this event. This volume collects the papers presented at the workshop, divided in the following four sessions: Session A – Performance and life-cycle costs of new concrete structures Session B – Controlled-performance concrete Session C - New scenarios for concrete Session D – Concrete quality control on site Four invited lectures opened the two workshop days, while each session was preceded by a General Lecture dealing with the specific topic of the session. Two “Honorary Members” of ACI International, namely Professors Mario Collepardi and Surendra Shah, were present at the workshop and gave invited lectures. The Workshop aimed to put together experts in concrete technology and structural engineering to discuss about the future of concrete, which is more and more devoted to the improvement of the performances, durability and care for the environment, without significantly affecting the costs. Several scientific contributes on the use of recycled aggregates in concrete – obtained by demolition or industrial waste (such as pneumatics) – have been also presented at the workshop. Other papers have dealt with the problem of the structural rehabilitation by following the new earthquake-resistance’s requirements of the constructions