The mechanical properties of concrete, together with a low cost of raw materials, flexibility in shaping, durability and ease of production, have enabled it to become the most used construction material of the twentieth century. The remarkable compressive strength of concrete is highly suitable for structural elements subjected to compressive stresses, such as piers. However, the low-tensile and flexural strength of concrete prevent it from being used in horizontal structures subjected to vertical loads. In such situations, the structural elements should resist stresses which are in most cases higher than the tensile strength. In order to widen the use of concrete, it was merged with steel bars which formed what has been conventionally termed reinforced concrete (RC). Moreover, for certain applications steel bars have been complemented with what is commonly known as active reinforcement in the form of steel wires and strands. Such a combination is called pre-stressed concrete.
This profuse use of concrete has been subsequently followed by a rise in the production and use of cement. As is widely reported, [1
] cement production generates almost a ton of CO2
per ton of cement manufactured. Consequently, cement production was responsible for an amount from 5% [2
] to 7% [3
] of the global industrial production of CO2
in 2010. Regarding the production of steel, it should be noted that as production has increased in the last decades the contribution to global CO2
production has also grown. For instance, Chinese crude-steel production has reached 683.3 million tons, accounting for 45.9% of world steel production [4
]. Although impressive progress has been made, this industry still has low resources, low levels of energy efficiency, and heavy environmental pollution [5
Regarding the durability of a reinforced concrete element, there are certain pathologies that may lead towards the total failure of the concrete piece, with some of them being mainly associated with degradation of the steel bars caused by potentially hazardous environments [6
]. This is the case of marine environments, where saline mist or even salt content of water might accelerate the corrosion velocity of the steel bars and lead to premature failure [7
]. If such an event takes place, the financial and environmental cost of rehabilitation, refurbishment or substitution of the infrastructure by a new one, increases the overall impact. This situation has been extensively explained by some published authors who have determined the financial cost of the options previously mentioned [8
Some authors have tried to minimise the socio-economic and environmental impact of the construction of concrete infrastructure by optimising some characteristics of the raw materials that form it. For instance, in order to reduce the amount of cement used and improve the mechanical properties, some by-products of other industries have been employed. Blast-furnace slag has been added even in marine environments, obtaining beneficial effects both in mechanical properties and durability [10
]. Some other products and by-products, such as contaminated marine sediment, have been employed, in the latter case as raw material for cement production [12
]. It has been shown that they improve the mechanical properties if they are compared with a CEM II/A-LL 32.5 that contains a proportion of limestone similar to the sediment substitution. Fly ash has been another possibility when a reduction of the impact of cement production is sought. It has been shown that high fly-ash replacement (>75% by mass) of cement is possible for all aging times and slump ranges for controlled low-strength applications [13
]. Regarding the use of aggregates, the main tendency has been to recycle debris from dismantled concrete structures. This practice has become a subject of major priority in several countries in the world [14
]. The Structural Concrete Code EHE-08 in force in Spain [15
] enables the use of proportions of such aggregates of up to 20% substitution in concretes with a characteristic compressive strength no greater than 40MPa. Nevertheless, there are other published studies where such proportions have risen up to 100% [16
]. In general, compressive, splitting and flexural strength of recycled aggregate concrete mixes obtained slightly lower results than a conventional concrete.
Although these approaches have been used on several occasions, the positive influence that the addition of fibres might have not only on the mechanical properties, but also on the durability of the material is still being studied [17
]. The latter is of significant importance because on many occasions the impact of the total life cycle of the infrastructure is neglected, ignoring the high impact of the cost of maintenance and refurbishment. Moreover, the present codes and recommendations have introduced in their last versions the requirements for FRC in order to consider the contribution of the fibres in the mechanical design of concrete structures. Such a contribution might, in certain cases, enable reduction of the amount of steel bars used in the concrete element and save not only money, but also contribute to reducing the impact of concrete in the natural environment. Although this issue might be questioned in the case of the construction costs, it should be noted that the influence of the eventual maintenance, repair or even re-building of the infrastructure ought to be taken into account if the entire life cycle is considered. The contribution of the fibres reduces the width of the cracks, hampering the entrance of deleterious chemical substances in the concrete matrix. Several of the aforementioned substances, such as chlorides or sulphates, cause overall damage to concrete which ultimately may imply socioeconomic and environmental costs of several orders of magnitude greater than the manufacturing and construction costs. Recently, it has been pointed out that the addition of polyolefin fibres might be apt as an alternative to substitute the traditional usage of steel fibres in FRC. In previous studies, polyolefin fibres added in certain amounts to a vibrated conventional concrete or even to a self-compacting concrete, have been able to meet the requirements set by some of the most relevant recommendations which enable to reduce the amount of steel bars in RC [19
]. Analysing the cited contributions it was decided to take as reference a concrete formulation with the addition of 10 kg/m3
of polyolefin fibres. Moreover, polyolefin fibres are not metallic and therefore do not suffer from corrosion when subjected to environments with high concentrations of chloride or sulphate ions. This characteristic of polyolefin fibres might contribute to enhance the durability of the material and consequently the life span of the structure.
Following this rationale, this contribution seeks to apply the MIVES formulation to a common example of bridge typology. The analysis starts by performing the structural analysis on a single span multi-girder bridge and determining the reduction of the steel bars that can be achieved by substituting the conventional concrete by FRC manufactured with 10 kg/m3
of polyolefin fibres (PFRC10). The application of PFRC10 will be limited to the slab of the bridge, designing the beams with conventional reinforced concrete in the form of prefabricated girders. Once this step is completed, the MIVES analysis will evaluate the socioeconomic and environmental costs that the aforementioned options imply [21
]. Lastly, some recommendations will be offered in order to serve as a reference for future applications and potential structural designs.
3. Structural Design Methodology
When designing with RC, the amount of reinforcing bars is computed by considering the ultimate calculation strength, while the compression at concrete is taken as a homogenous stress reduced by a fatigue coefficient. The boundary condition to be met in the section is that the sum of all axial forces should be zero, balancing the tensile forces that appear in the steel bars with the compressive force that appears in concrete. This is carried out by estimating the depth of the compressive zone until the aforementioned balance is obtained.
In order to perform the structural design through use of PFRC10, some assumptions should be made. Instead of using the sectional strain, the crack width has to be employed as the constitutive relations of FRC relate the crack width with the stresses that the material is able to bear. Moreover, a more sophisticated analysis has been performed where a moment-curvature diagram is prepared to evaluate the flexural capacity of a one-width cross-section with a determined amount of steel reinforcement.
The moment-curvature diagram is carried out in terms of mean values, not by using characteristic ones. Consequently, the reductions in strength are applied on the final structural strength. To consider the post-cracking tensile stresses, the crack width is evaluated at the level of the reinforcing bars which is suitable for accounting the crack-width limit in the service limit state (SLS). In the case of FRC, since the tension force provided by fibres is much more reduced than that provided by steel bars, it can be concluded that the main reinforcement continues to dominate the flexural response. Consequently, an overall safety factor equivalent to the partial coefficient of the steel is assumed (γs
= 1.15). It should be highlighted that this approach deviates from the partial coefficients adopted in the Model Code 2010 [22
] in order to make the calculation consistent. This approach corresponds to those used in other North American standards such as those provided by the American Association of State Highway and Transportation Officials (AASHTO) or the American Concrete Institute (ACI).
The moment-curvature diagram is obtained from a discrete-point method. As this contribution is not focused on the structural design process, it has not been included in the manuscript. However, it should be highlighted that the ultimate bending moment is determined as the previous point of the diagram before the failure of any material of the cross-section occurs. This failure occurs when any of the following situations in PFRC10 is reached:
The maximum compressive strain in concrete greater than compressive ultimate strain: εcmax > εcu = 0.35%.
The tensile strain in main reinforcement greater than the ultimate strain of steel: εs > εsu = 1.00%.
The crack width in concrete greater than the ultimate crack width: w > wu.
The dimension of shear reinforcement is also developed by following the guidelines of the Model Code 2010 [22
], which proposes a specific modification to the conventional formulation to consider the fibre-reinforcing effect. The shear strength is computed as the sum of several components: concrete bond, aggregate interlocking, dowel action, vertical shear reinforcement, longitudinal reinforcement contribution and crack bridging by fibres.
Adding fibres to the matrix allows concrete to bear tensions in the shear cracks thanks to the action of the fibres sewing both crack surfaces. That effect is analogous to that of the longitudinal steel reinforcement, except that the fibres are randomly distributed. Therefore, the fibres contribute by means of normal tensile stresses and depend on the shear crack width. As an agreement to define this tensile stress, an ultimate crack width wu = 1.5 mm is taken. In addition, unlike the flexure crack, it is considered that the surfaces on both sides of the crack are parallel so that the crack width is constant.
The use of a multi-criteria decision-making method based on the value function concept and the seminars delivered by experts such as MIVES has been useful tool in assessing the sustainability of a flyover bridge built with RC or PFRC10.
The MIVES approach to the decision-making process has identified the differences between the options considered, considering not only socioeconomic but also environmental aspects. The evaluation of parameters aside from the economic ones has been a key factor in choosing the PFRC option over the conventional RC one.
While the economic evaluation of the two options differs only by 5%, the environmental and social scores show differences of 33% and 25%. The partial environmental and social scores have shown that slightly higher maintenance costs represent an important impact on the environment and on society if the disturbances generated are considered throughout the life cycle of the infrastructure.
The development and availability of enhanced data basis and life-cycle analysis of construction materials and procedures may supply more accurate results. In addition, the continuous use of MIVES could also supply accepted rules for the seminars provided by experts.