Introduction
Construction activities lead to remarkable impacts on the global economy (Ahmad et al. 2019; Wells 1985), environment (Huang et al. 2018), and society (Zhang et al. 2019). Accordingly, these impacts have often been assessed from the holistic point of view of sustainability since this concept was defined as the coexistence of economic, environmental, and social issues (Kaur and Garg 2019). Sustainability and component performance are expected to be enhanced if the construction processes are optimized and uncertainties diminished (Okema 2000; Salimi et al. 2018). These uncertainties pose a challenge in the case of underground infrastructure, such as tunnels and foundations (Pujadas-Gispert et al. 2018, 2020) and especially, in deep foundations (Buyle-Bodin and Madhkhan 2002).
With respect to the latter, piles are widely utilized in building construction and can be classified depending on the following characteristics: (1) interaction with other piles—isolated single piles and groups of piles that are spaced closely or apart; (2) materials—onsite reinforced concrete, precast concrete, steel, timber, and composite; (3) mechanical features—end bearing piles and friction piles; (4) cross-section shape—circular, polygonal, laminated profiles, and rectangular diaphragm walls; (5) diameter—micropiles (
Ø≤300 mm
), conventional diameter (
300<Ø<800 mm
), and large diameter (
≥800 mm
); and (6) construction process—driven and bored piles (Tomlinson and Woodward 2008). This last classification category is the most used, and each of its two types have several subtypes. The subtypes of driven piles are precast or poured onsite with a cylindrical shaft and steel bottom plate or gravel plug and poured onsite drilled-in displacement micropiles (Armour et al. 2000). Conversely, the subtypes of bored piles are poured onsite with temporary shaft, permanent shaft, bentonite slurry and no shaft, segmental flight auger and no shaft, and continuous flight auger (CFA), all of which are currently used in practice (Bersan et al. 2018; Hosny et al. 2018).
This study focuses on CFA steel-cage reinforced concrete piles (RCPs), which are a cost- and time-efficient solution that is well established in the building construction sector because it is widely accepted to be the quickest pile type for inhabited areas, with a speed three times that of its first competitors (Brown et al. 2007). The construction process (Fig. 1) comprises the following steps: (1) a CFA that performs the excavation without a shaft or slurry; (2) concrete is poured through the hollow stem and the auger is withdrawn ensuring that the bottom always remains within the poured concrete; and (3) the reinforcement cage is pushed or vibrated into the freshly poured pile (Brown et al. 2007). Thus, RCPs are a cost-effective system for inhabited areas because no hammering is required and also has the advantage that the excavated soil is visible (Rajapakse 2016). However, CFA piles require the soil to be consolidated and the water table to be below the pile unless the soil is very cohesive and the water does not circulate (Brown et al. 2007). CFA piles present several disadvantages: (1) slower process than displacement piles; (2) uncertainty in relation to the pile bearing capacity owing to driving and inspection difficulties; (3) boring process decompresses the soil and encounters difficulties when the pile has to be embedded in particularly hard ground; and (4) concrete must have high workability to avoid clogging and to facilitate the reinforcement embedding process (Brown et al. 2007). This last point represents a challenge and has resulted in the following limitations: (1) reinforcement depths less than 12 m, frequently not more than 6.0 m; and (2) the requirement to use additional equipment such as a bulldozer arm to push the steel cage down with the risk of causing damage to the rebars and/or large deviations in both geometry and position. These are known drawbacks associated with reinforcement placement. Thus, to mitigate the potential structural implications of these known shortcomings, large safety factors must be considered in the design process to guard against the potential lack of reinforcement either because the concrete covers are not guaranteed, which could result in corrosion of the reinforcement, and/or the reinforcement does not reach the required depth, and as a consequence, part of the pile is assumed to be unreinforced. This ultimately leads to larger cross sections that demand both larger excavated volumes and concrete consumption and therefore higher costs and greater environmental impacts. It is also noteworthy that on the construction site, a yard is temporarily required to stack the steel cages, which creates a challenge in terms of mobility and space management in dense urban areas.
Structural fibers that have emerged as a suitable alternative to the traditional steel cage for concrete reinforcement are known as fiber-reinforced concrete (FRC). Accordingly, the acceptance of FRC as a structural material in the fib Model Code 2010 (MC-2010) (fib 2013) has accelerated the use of FRC, predominantly with steel fibers in structural applications: (1) ground-supported (Meda et al. 2004) and column-supported slabs (de la Fuente et al. 2019), (2) sewerage and drainage pipelines (de la Fuente et al. 2012b, 2013), (3) earth-retaining systems (de la Fuente et al. 2011), and (4) hydraulic and metro tunnel linings (Chiaia et al. 2009; de la Fuente et al. 2012c; Liao et al. 2015a, b; Nogales and de la Fuente 2020; Plizzari and Tiberti 2006; Rinaldi and Zila 2017). The Model Code is a structural concrete design guideline written by the fib (Fédération Internationale du Béton), which is intended to be a guidance document for future codes. This document is issued, tentatively, every 10 years with the purpose of incorporating the latest advances in the design of concrete structures. It must be remarked that the fib MC-2010 emphasizes the need of dealing with sustainability through the whole design process and proposes criteria and methods to assess the sustainability performance. Likewise, synthetic fibers are being introduced into the flooring (Alani and Beckett 2013), pipeline (Al Rikabi et al. 2018; Ashley and Ali 2014; de la Fuente et al. 2013; Lee et al. 2019; Park et al. 2014), and tunneling (Conforti et al. 2017, 2019) sectors due to the improvements in the mechanical properties inherent with this type of fiber. Accordingly, synthetic fibers have proven to be inert to the aggressive environments that lead to the corrosion and deterioration of the steel reinforcements (Richardson 2004; Hannant 1998), and thus, higher durability and service life with higher reliability can be guaranteed.
Fiber-reinforced concrete piles (FRCPs) have already attracted the attention of researchers. Accordingly, several experimental programs have been conducted to consider steel fiber-reinforced concrete (SFRC) as a structural material (Akdag and Özden 2013; Buyle-Bodin and Madhkhan 2002; Ozden and Akdag 2009; Sterin et al. 1984). The purpose of these studies was to prove the postcracking, ductility, and fatigue performances of FRCPs; these properties are required (and mandatory) in deep foundations in soil with low cohesion, in seismically active zones (Ozden and Akdag 2009). The use of synthetic (polymeric) fiber-reinforced concrete (PFRC) in FRCPs has also been explored in marine environments (Sadiqul Islam and Gupta 2016) to enhance durability. Finally, the technical feasibility of piles that comprises of a steel profile embedded into a PFRC was also investigated (Zyka and Mohajerani 2016).
Since different technically viable reinforcement alternatives are available for FRCPs, each of which have different economic, environmental, and social impacts, this study aims to assess the sustainability of CFA-RCPs focusing on the use of traditional steel cages and structural fibers for concrete reinforcement. The construction sector’s sustainability awareness is increasing, and its stakeholders are searching for assessment tools to evaluate and improve the impacts their building processes create (Pons and Nikolic 2020). However, to the best of the authors’ knowledge, this is the first definition and application of a holistic sustainability assessment model for CFA, preceded by environmental analysis of deep foundations (Giri and Reddy 2014; Pujadas-Gispert et al. 2020) as well as specifically environmental piles (Misra and Basu 2011) and eco-efficient assessments studies (Saravanan 2011) some starting incorporating neighborhood nuisances (Misra and Basu 2012). Hence, a sustainability-driven multicriteria decision-making approach is proposed for the assessment of CFA-RCPs, and a case study is presented. The proposed approach and the outcomes of this research are expected to be useful in the stakeholders’ decision-making processes.
Sustainability Assessment of Foundation Piles Based on Integrated Model for Assessing the Sustainability Value of Structures
Integrated Model for Assessing the Sustainability Value of Structures and Delphi Approach
The integrated model for assessing the sustainability value of structures (MIVES) is a multicriteria decision-making (MCDM) model that supports the sustainability analysis of any type of product and construction process. MIVES was designed to minimize the subjectivity associated with the indicators involved, particularly those related to environmental and social requirements, and permit the derivation of an integrated sustainability index (
Is).
The method defines (1) the system boundaries that determine the scope of the analysis, (2) the decision-making tree that gathers the requirements (R), criteria (C), and indicators (I) involved in the decision-making process, (3) the value functions (Alarcon et al. 2011) to convert the attributes or physical units of each indicator into a satisfaction unit that ranges from 0 to 1, and (4) the weights’ sets.
The entire procedure involved experts, chosen from a group of representative stakeholders, using the Delphi method (Hallowell and Gambatese 2010), which is explained in detail in Section 2.5. The Delphi method was applied to select the experts as well as to manage the research survey and assign the weights’ set following the schema established by del Casanovas-Rubio and Armengou (2018).
The suitability of MIVES for the types of analysis dealt with in this study has been previously confirmed in other areas, such as underground (del Casanovas-Rubio et al. 2019; de la Fuente et al. 2017; Ormazabal et al. 2008), hydraulic (de la Fuente et al. 2016; Pardo-Bosch and Aguado 2015) and electric-power generation (Cartelle Barros et al. 2015; de la Fuente et al. 2017) infrastructure, and building (Josa et al. 2020; Pons and Aguado 2012; Pons and De La Fuente 2013; Reyes et al. 2014; Lombera and Rojo 2010; Lombera and Aprea 2010; Sánchez-Garrido and Yepes 2020) and postdisaster reconstruction (Hosseini et al. 2016a, b). A number of researchers (del Caño et al. 2016, 2012) have also developed methods intended to treat the uncertainties related to the input data.
System Boundaries
This study aims to assess the sustainability index of the technically feasible reinforcement alternatives for CFA piles: traditional steel-cage RCP and steel fiber-reinforced concrete (SFRCP) or polypropylene fiber reinforced concrete (PFRCP). The fibers considered should be structural macrofibers capable of providing a postcracking concrete residual strength according to fib MC-2010 (fib 2013).
The functional unit is 1.0 CFA pile of length
land diameter
Φconsidering the life cycle from its instigation to the end of its service life (
≥50 yearsfor buildings). The study focuses on the pile but excludes the pile cap because its geometry and reinforcement steel bars are independent of the reinforcement configuration of the pile. However, the reinforcement of the pile top is included with the indicators’ quantification. The considered life cycle analysis (LCA) stages were (1) extraction, transport and processing of the constituent materials of the piles including the concrete components (cement, aggregates, water, and admixtures), and reinforcing concrete products (steel bars and fibers); (2) soil boring with a CFA; (3) concrete production; (4) concrete transport and pouring, (5) reinforcement embedding; and (6) pre- and operational stages throughout which repairs during construction and maintenance may be required.
Decision-Making Tree and Elements
During the experts’ seminars, the decision-making tree presented in Table 1 was established, relying on these experts’ knowledge, expertise, and information from numerous real projects such as the case study and the extend-related technical literature presented in “Introduction” section. This encompasses the economic, environmental, and social requirements (R) according to UN (2005). These Rs are divided into seven criteria (C) and ten indicators (I) which are selected for the decision under consideration, with regard to the type of reinforcement, after a filtering procedure in which the representativeness and independency, with no overlapping, between indicators was guaranteed.
Table 1. Requirements’ tree with weights for sustainability analysis of CPs
Table 1. Requirements’ tree with weights for sustainability analysis of CPs
| Requirements | Criteria | Indicators | Units |
|---|---|---|---|
|
R1. Economic (43.8%) |
C1. Costs (68.8%) |
I1. Direct costs (41.5%) |
k€ |
|
I2. Nonacceptance costs (20.7%) |
points | ||
|
I3. Durability costs (37.8%) |
points | ||
|
C2. Construction time (31.2%) |
I4. Time (100%) |
points | |
|
R2. Environmental (28.7%) |
C3. Resource consumption (52.4%) |
I5. Energy consumption (58.1%) |
GJ |
|
I6. Water consumption (41.9%) |
m3 |
||
|
C4. Emissions (47.6%) |
I7. CO2emissions (100%) |
TonCO2-equivalent |
|
|
R3. Social (27.5%) |
C5. Occupational risks (50.1%) |
I8. ORI index (100%) |
weighted person-hours |
|
C6. Third-party effects (26.3%) |
I9. Building site space (100%) |
points | |
|
C7. Innovation (23.6%) |
I10. New solutions (100%) |
points |
The economic requirement (
R1) consists of two criteria: costs (
C1) and construction time (
C2). The former encompasses three indicators: (1) direct costs (
I1) related to materials and construction processes, including labor; (2) nonacceptance costs (
I2) caused by disconformities associated with the material properties and/or the piling process; and (3) durability costs (
I3) which involve those costs caused by materials’ repair due to deterioration—for example, corrosion of the steel reinforcement. The latter is represented by the time (
I4) required to construct a pile (without disconformities).
The environmental requirement (
R2) involves two criteria: resources consumption (
C3), which is represented by both energy (
I5) and water (
I6) consumption, and emissions (
C4) of
CO2(
I6). The latter three indicators are evaluated considering the LCA phases described in the section “System Boundaries.” For the assessment of indicators
I5and
I7, the local inventory (ITEC 2019) was considered, and international inventories (Circular Ecology 2019; Wuppertal 2014) were used as reference. Nonrenewal resources consumption other than water—for example, aggregates for cement and concrete—were discarded since the concrete component and proportions are essentially the same and independent of the reinforcement alternative, except for slight variations in the admixture quantities that should be redefined to guarantee the workability of the FRCPs. These variations could be taken into consideration in indicator
I1.
Finally, the social requirement (
R2) considers three criteria: (1) occupational risks (
C5) by means of the occupational risk index (ORI) (
I8) developed by del Casanovas et al. (2014) to identify and quantify the potential risks the workers are subjected to during construction; (2) the third-party effects (
C6) are taken into account considering the building site space (
I9) required for stacking the concrete reinforcement materials; and (3) innovation (
C6) through the indicator new solutions (
I10) for reinforcing concrete. This last indicator was incorporated to encourage research and development of new reinforcing technologies. Steel cages for RCPs have been satisfactorily utilized worldwide for more than a century (Tomlinson and Woodward 2008), but fibers are a promising technically feasible alternative. Still, the building sector tends to react slowly and with reticence to changes; thus, the introduction of improvements, which could attract initial reservations, should be motivated by using MCDM approaches that also recognize and reward the innovation.
Other indicators attributed to the requirement
R2could also have been included, such as recyclability potential, which was disregarded since it was found to have a minor impact on the sustainability index. Foundation piles can often be subsequently reutilized if they are found to be in sound structural condition.
Value Functions
The aforementioned value functions (Alarcon et al. 2011) were assigned by experts to each indicator (
Iind). Following MIVES, these functions were mathematically expressed by Eq. (1) in the case of indicators
I1and
I5–
I7(Appendix I) and followed simpler equations for the other indicators because the relation between the value and satisfaction of the indicators followed specific patterns described in detail at the end of this section (Table 2). Thus, there is an allowed computation of the value of each indicator (
VIind) and
Isof the RCP that is subject to evaluation.
Isranges from 0.0 to 1.0 and is obtained by multiplying
VIindby the corresponding indicator weight and summing the result with those obtained for the same criterion. The same process is repeated upward (from indicators to requirements) to derive the
Is
(1)
VIind(Xind)=A+B[1−e−K(|Xind−Xmin|C)P]
where
Ais the value of
VIindfor
Xmin;
Xminis the minimum abscissa value of the indicator interval assessed;
Xis the abscissa value for the indicator assessed;
Piis a shape factor that defines whether the curve is concave (
P<1), convex (
P>1), linear (
P=1), or S-shaped (
P>1);
Capproximates the abscissa at the inflexion point;
Ktends toward
VIind(Xind)at the inflexion point;
Bis the factor that prevents the function from exceeding the range (0, 1) according to Eq. (2); and
Xmaxis the abscissa value of the indicator that gives a response value of 1 for increasing value functions
(2)
B=[1−e−Ki(|Xmax−Xmin|Ci)Pi]−1
Table 2. Value functions and respective constitutive parameters
Table 2. Value functions and respective constitutive parameters
| Indicator | Equation | Function |
Xmax |
Xmin |
C | K | P |
|---|---|---|---|---|---|---|---|
|
I1. Direct costs |
(1, 2) | DS | 1.25 | 0.75 | 1.00 | 20 | 1.93 |
|
I2. Non-acceptance costs |
6≤lp≤12; 0.75≤VI2≤0.50 |
MLD (RCP) | — | — | — | — | — |
|
12<lp≤16; 0.25≤VI2≤0.00 |
— | — | — | — | — | ||
|
VI2=0.75 |
L (FRCP) | — | — | — | — | — | |
|
I3. Durability costs |
RCP:
VI3=0.50; SFRCP: VI3=0.75; PFRCP: VI3=1.00 |
— | — | — | — | — | — |
|
I4. Time |
6≤lp≤12; 0.75≤VI4≤0.50 |
MLD (RCP) | — | — | — | — | — |
|
12<lp≤16; 0.50≤VI4≤0.00 |
— | — | — | — | — | ||
|
VI4=1.00 |
L (FRCP) | — | — | — | — | — | |
|
I5. Energy consumption |
(1, 2) | — | 1.25 | 0.50 | 1.3 | 2.6 | 1 |
|
I6. Water consumption |
(1, 2) | DCx | 1.25 | 0.50 | 1.3 | 2.6 | 1 |
|
I7. CO2emissions |
(1, 2) | — | 1.25 | 0.50 | 1.3 | 2.6 | 1 |
|
I8. ORI index |
VI8=−0.4ORIRCPOR+1 |
DL | — | — | — | — | — |
|
I9. Building site space |
RCP:
VI9=0.50; FRCP: I9=1.00 |
— | — | — | — | — | — |
|
I10. New solutions |
RCP:
VI10=0.50; SFRCP: VI10=0.75 |
— | — | — | — | — | — |
| PFRCP:
VI10=1.00 |
The value function allows physical units of each indicator—for example, €,
kgCO2— to transform into dimensionless values also ranging from 0 to 1. These values represent the sustainability or satisfaction of each indicator. Table 2 presents the equations, shapes, and constitutive parameters of the value functions for the 10 indicators of this study presented in the previous section.
Indicators
I1and
I5–I7are referred to as RCPs:
Xind=Xalt/XRCP,
Xalt, and
XRCPbeing the argument of the indicator (Eq. 1), the magnitude for the alternative (FRCP), and the magnitude of the reference (RCP), respectively. In this experts-based method definition, the following criteria were assumed for defining the value functions’ constitutive parameters:
| • | Direct costs (
I1) assess the construction costs including material, labor, machinery and equipment, and auxiliary elements. Relying on the competitiveness of the RCP solution due to its widespread usage and considering previous related research projects (de la Fuente et al. 2019), Eqs. (1) and (2) with the parameters depicted in Table 2 were defined. The Xminwas the reference satisfaction value of 0.75 that was set for this RCP solution. A VI1=1.0is achieved for a 25% cost reduction in comparison to RCP, whereas VI1=0.0is achieved for an increase of 25%. The transition is simulated with an S-shape function (Appendix I) with a remarkable sensitivity to increasing/decreasing costs to emphasize the market behavior. |
||||
| • | Non-acceptance costs (
I2) evaluate the magnitude of the costs from unsuccessful construction processes. In RCPs, nonconformities may be caused in the case of steel cage misalignment, such as insufficient concrete cover and/or depth. In that scenario, the longer the pile is, the higher the likelihood of geometric deviations. Accordingly, and considering again the RCP as the reference widespread but improbable solution, a decreasing multilinear function shape is assigned to this indicator for RCPs. For pile lengths ( lp) below 6.0 m, VI2=0.75was considered, while VI2decreases to 0.0 for lp≥16.0 m. For lp>12.0 m, the longitudinal steel bars should be connected by welding and/or lapping to guarantee continuity of the reinforcement; this connection renders the cage more prone to nonconformities. For FRCPs, a VI2=0.75was assigned since it is assumed that both the concrete admixtures and concrete-pumper pipe diameter are selected to meet workability requirements equivalent to the RCP solution. In the case where a large quantity of fibers is used, the probability of the occurrence of technical issues increases, and this can be accounted for by reducing the satisfaction value. |
||||
| • | Durability costs (
I3) assesses the potential durability of the different alternatives. Firstly, the degradation risks of the steel-based reinforcing alternatives due to potential corrosion during the service life were contemplated, and consequently, the risk of reducing the bearing capacity in the elements that are difficult to inspect was considered. The corrosion mechanisms and reduction of the bearing capacity, in the case when this occurs, can be less severe for SFRCPs ( VI3=0.75) than for RCPs ( VI3=0.50). Conversely, synthetic fibers do not corrode and are resistant to most chemical attacks expected in underground environments—for example, contamination of the phreatic water and marine soils; consequently, VI3=1.00was considered since no retrofit/repair costs associated with durability issues are expected. |
||||
| • | Time (
I4) evaluates the average time devoted to concrete reinforcing tasks. FRCPs ( VI4=1.00) present the quickest construction process since the fibers are directly introduced in the concrete mixer. RCP requires more time to introduce the steel cage into the excavation filled with concrete and to previously weld the two consecutive parts of the steel cage in the event the cage is longer than 6 meters. VI4follows the same pattern as VI2for RCPs. |
||||
| • | Energy consumption (
I5), water consumption ( I6) and CO2emissions ( I7) are assessed with the same value functions that, to encourage environmentally sustainable solutions, assume a value of 0.60 for the reference RCP, and maximum (1.00) and minimum satisfactions (0.00) are obtained for a decrease of 50% and an increase of 25% of Xind, respectively, through a convex function (Appendix I). |
||||
| • | ORI index (
I8), previously introduced in Section 2.3, is defined as the sum of all the risks of the activities performed during the building process (del Casanovas et al. 2014). The risk of an activity is assessed by ranking the probability (P) of the occurrence of an accident multiplied by the level of severity of its most probable consequence (C) and by the exposure (E) of the workers to the risk, expressed in time (h). |
||||
| • | Building site space (
I9) evaluates the satisfaction related to the onsite space required for stacking the reinforcement materials. A VI9=1.00was assigned to FRCPs and VI9=0.50for RCPs. With regard to the latter, constructors have already accepted and integrated the space requirements, and consequently, null satisfaction would be unrepresentative. Accordingly, in the case of long steel cages and/or high demands on space for stacking, such as a significant number of piles to be constructed, VI9could be reduced accordingly. |
||||
| • | New solutions (
I10) consider the integration of new technologies into the construction sector, which in this case are reinforcement alternatives for concrete foundation piles and awards the level of innovation. The standard RCP solution was assigned with VI10=0.5, while values of VI10=0.75and 1.00 were assigned to SFRCPs and PFRCPs, respectively. Synthetic fibers are being used in structural elements as mentioned in Section 1; nevertheless, to the best of the authors’ knowledge, no previous studies have reported their use in piles. Thus, the maximum satisfaction is assigned to PFRCPs. |
||||
Weight Assignment with Delphi Method
To select the experts, determine the weights of the requirements, criteria and indicators of the requirement tree, the Delphi method, as presented in (Hallowell and Gambatese 2010), was fully adhered to throughout this entire procedure. In this study, 28 qualified experts were identified and invited to participate in the surveys, 23 of whom initially accepted and 17 participated, which is more than the minimum number of panelists recommended for this method. The participants were from various backgrounds including academia, construction industry, public administration, and civil engineering and architecture. This diversity of backgrounds provided a wider vision and enriched the developed method. The weights were assigned by the direct assignment method.
As proposed by the Delphi method (Hallowell and Gambatese 2010), the median absolute deviation, as defined in Eq. (3), is used to determine the consensus of the panelists. According to the procedure, the consensus is reached when the median absolute deviation is
<1/10of the range of possible values. As the range of the weights is 0%–100%, the consensus is reached when the median absolute deviation is
<10%, as assumed by Casanovas-Rubio & Armengou (del Casanovas-Rubio and Armengou 2018)
(3)
Median absolute deviationi=∑j=1n|wij−mediani|n
where
iis the requirement, criterion or indicator considered;
jis a panelist;
nis the total number of panelists (17 in this study);
wijis the weight assigned to the requirement, criterion, or indicator
iby the panelist
j; and the
medianiis the median of the weights assigned by the panelists for the requirement, criterion, or indicator
i.
Two rounds of surveys were conducted to reach a consensus on the weights. In the first round, the experts were asked to assign weights to the requirements, criteria, and indicators according to their preferences, with the total sum of the requirements and each independent set of criteria and indicators being 100%. In the second round of surveys, the panelists were provided with the results of the first round (the mean values) and asked to adjust their assigned weights, if possible, to the results of the first round while retaining their preferences at the same time. They were asked to provide reasons for the weights that deviated by more than 10% from the mean of the first round. The results of rounds one and two are presented in Appendixes II and III, respectively.
As established in Delphi, to reduce judgement-based bias, which comprises collective unconscious, contrast, Von Restorff, myside bias, recency, primacy, and dominance effects, the following controls were implemented: (1) randomized question order, (2) iteration and anonymity, and (3) reporting of means as feedback. Reasons for outlying responses were included as part of the feedback of the third round; nonetheless, a consensus was reached in the second round.
As presented in Table 1, the resulting weights show that the economic requirement (43.8%) is the most important when selecting the type of pile, whereas the environmental (28.7%) and social (27.5%) requirements have similar weights. This weights’ set reflects that the decision making is still driven by economics, while the environmental and social requirements are of moderate importance. This could be a symptom of construction stakeholders becoming more sensitive to the potential impacts of both requirements.
Within the economic requirement, the importance of cost (68.8%) is approximately double that of the execution time (31.2%). The importance of possible nonacceptance costs (20.7%) represents half that of the construction process costs when selecting the type of pile (41.5%), and these costs notably lower than the durability aspects (37.8%). Resource consumption (52.4%) is considered to be slightly more important than emissions (47.6%), whereas energy consumption (58.1%) is more relevant than water consumption (41.9%). Occupational risks during construction (50.1%) resulted in being the most important criterion within the social requirement, whereas third-party effects (26.3%) and technology innovation (23.6%) have similar weights.
Finally, it is worth noting that the value functions and weights proposed in this study might be representative of a competitive market mainly driven by costs and with remarkable sensitivity toward the environmental and social indicators presented in Table 1. Nevertheless, should other stakeholders’ preferences be considered, these functions and weights could be properly calibrated as this level of flexibility is permitted by the model.
Case Study: School Building in Canovellas (Spain)
Description of Structure
The case study is on the new public elementary educational center of Canovellas, Barcelona, Spain. This center was designed by the Beta Architecture studio (Camps and Felip 2014) and built in 2018 by the Department of Education (Pegenaute 2018). The database used in this study relies on the construction documents and as-built drawings (Camps and Felip 2016) as well as the onsite experience of the professionals involved. This building had a cost of 3.3 M€, of which the foundations represented 10%. The total surface area of
3,430 m2is divided into a basement of
14×17 m, a ground and first floor of
17×100 m, and an attic of
4×100 m. The underground level is the location of an archaeological site from the Neolithic, the ground and first floor accommodate all the school premises (Brković et al. 2015), and the attic services the educational space.
The building has a reinforced concrete (RC) framed structure distributed in a grid of maximum size
5.5×8.2 m, with onsite columns of
25×90 cmand beams of 55 cm in height, as well as precast TT slabs 45 cm in height and a poured onsite topping layer of 5-cm thickness. Deep foundations were required to transmit the heavy service loads from the structure to a solid formation consisting of clay and sandy clay with a resistance of the base (unit end bearing) of
qb=2.775 MPaand a resistance of the shaft (unit side friction) of
fp=0.019 MPa. The foundations are composed of 187 CFA piles with
Ø=45 cmand
length=15.9 m, as shown in Fig. 2. These piles work individually or are grouped in caps from two to six elements. As per project, the reinforcement consisted of steel cages of 6-m depth with 6Ø16 and 1eØ8c/24 as depicted in Fig. 3. Fig. 1 presents the construction process with a flight auger.
Alternatives Analyzed and Data
The three main alternatives for reinforcing CFA piles introduced and described in the first section and illustrated with a representative case study in the previous section were considered: (1) the previously described RC constructed solution (RCP) with a 6 m rebar cage and two alternative solutions consisting of (2) SFRCP, and (3) PFRCP. These alternatives were subsequently studied in terms of sustainability for future similar projects.
Both the geometry and mechanical properties of the piles were established based on the loads to be resisted and transmitted to the soil. This information and the mechanical properties of the soil was extracted from the reference project. The mechanical performance (i.e., flexural residual strength) of the fiber-reinforced concretes was established by means of sectional analysis (de la Fuente et al. 2012a) and the design recommendations for FRC structures proposed by the fib MC-2010.
A
500 N/mm2yield strength steel type B-500S was considered for the cage production. Steel fibers for concrete reinforcement can exhibit different geometries and mechanical performances. For this study, steel macrofibers have an aspect ratio of
60≤λf≤80,
λf=lf/Φf;
lf
isthe length, and
Φfis the diameter of the fiber with a tensile strength (
ffu) ranging from
1,000 to 1,200 N/mm2. For the synthetic fibers, macrofibers with
40≤λf≤60,
500≤ffu≤650 N/mm2and
5≤Ef≤9 MN/mm2were included in the analysis. Table 3 presents the main features considered regarding the reinforcement alternatives.
Table 3. Main features considered for each reinforcement alternative
Table 3. Main features considered for each reinforcement alternative
| Type of pile | Reinforcement type | Amount | Cost |
|---|---|---|---|
| RCP | Steel-cage (6.0 m) | 6Ø16 + stirrups Ø[email protected] |
1.17 €/kga |
| SFRCP | Steel macrofibers |
30 kg/m3 |
1.25 €/kgb |
| PFRCP | Synthetic macrofibers |
6 kg/m3 |
3.50 €/kgb |
Regarding the information presented in Table 3, firstly, the 6.0-m-long steel cage considered for the RCPs guarantees a minimum reinforcement and continuity, whereas the remaining 9.9 m is unreinforced. This type of partially reinforced RCP is permitted in the standards applicable in Spain (MV 2008), among other countries (Brown et al. 2007; Johnson 2013), due to the technical difficulties related to embedding a reinforcement throughout the full pile depth. Conversely, fibers provide a continuous reinforcement, and these can substitute the steel cage in this project provided that the residual strength of the FRC is sufficient for (1) controlling cracks due to thermohygral phenomena such as concrete shrinkage and (2) resisting minor bending moments, which is inferior to the cracking bending moment of the cross section. Based on this and using the fib (2013) as a design guideline for FRC alternatives, quantities of 30 and
6 kg/m3of steel and synthetic fibers, respectively, were found necessary to fulfill these two requirements. If the loads, soil conditions, and pile cross section had been different, other FRC strength requirements would have dominated, which would have resulted in a requirement for different quantities of fibers.
For concrete, a characteristic compressive strength (
fck) of
25 N/mm2after 28 days was considered, with a plasticizer admixture to guarantee the fluid consistency, a maximum aggregate diameter of 20 mm, and cement content higher than
375 kg/m3. The same concrete composition was also assumed for the SFRC and PFRC; however, this should be slightly modified by increasing the quantity of the admixture (superplasticizer) to compensate for the reduction in consistency owing to the addition of fibers. These modifications, however, can be disregarded in this analysis since they do not have a significant impact on the economic and environmental indicators.
Quantification of Indicators
Quantification of the indicators mainly relied on the construction documentation (Camps and Felip 2016) and the local database (ITEC 2019), especially for the cost (
I1–I4) and environmental (
I5–I7) indicators. Environmental indicators are also based on other European databases (Circular Ecology 2019; Wuppertal 2014) and specific studies regarding steel (Alberti et al. 2018) and polypropylene fibers (Yin et al. 2016). Table 4 summarizes the quantification of the environmental indicators (
I5–I7) for each alternative along with the references used, considering the phases and the boundaries described in Sections 2.2 and 2.3, respectively.
Table 4. Quantification of the environmental indicators
Table 4. Quantification of the environmental indicators
| Quantification | Units | References | |||
|---|---|---|---|---|---|
| RCP | SFRCP | PFRCP | |||
| CFA drilling and pouring concrete | (m) | 2973 | 2973 | 2973 | Camps and Felip (2016) |
| Pile cap preparation | (m) | 178 | 178 | 178 | |
| CFA machinery | (unit) | 1 | 1 | 1 | |
| Piles rebars | (kg) | 14119 | — | — | |
| Piles steel macrofibers | (kg) | — | 14187 | — | fib (2013) |
| Piles synthetic macrofibers | (kg) | — | — | 2837 | |
| Piles caps rebars | (kg) | 1176 | 3530 | 3530 | Camps and Felip (2016) and fib (2013) |
|
I5. Energy consumption |
(MJ) | 3320023 | 3410662 | 3151588 | Circular Ecology (2019) and ITEC (2019) |
|
I6. Water consumption |
(l) | 4114965 | 4257314 | 3524724 | Wuppertal (2014) and Yin et al. (2016) |
|
I7. CO2emissions |
(
kgCO2-equivalent) |
399963 | 376963 | 370177 | Alberti et al. (2018), ITEC (2019), and Yin et al. (2016) |
Risks during the building process were analyzed using the ORI (
I8) in which the activities performed during the construction of the foundation piles were identified for the three alternatives and are presented in Table 5. The values of W (PxC/1000) were taken from (del Casanovas et al. 2014), as the technology and safety management practices of the case study match with those for which the guidance values of that research were obtained. The following hypothesis was created:
| • | The journey by a
6 m3mixer truck from the concrete plant to the site (8.8 km, real distance in the Barcelona, Spain, case study context) takes 9 min. A total of 497 m3of concrete are needed for the 187 piles. |
||||
| • | The journey by a truck hauling 18,000 kg of rebars from the plant to the site (38.9 km, real distance in the case study) takes 35 min. The total amount of steel needed for the rebars of the 187 piles is 15,295 kg and 3,530 kg for the RC and FRC (only for pile caps) alternatives, respectively. | ||||
| • | The installation of reinforcing bars takes 2 min by two operators per pile only for the RC. | ||||
| • | The welding of the rebars for the RC takes 5 min per pile by one welder. | ||||
| • | The drilling of a pile takes 10 min, and the concreting takes 5 min. The pile rig moves around the construction site 5% of the time required for drilling and concreting (15 min), and two workers could be involved. | ||||
| • | The concrete mixer truck moves around the construction site 5% of the time dedicated to concreting while two workers are performing tasks in the vicinities. | ||||
| • | The concrete pump truck moves around the construction site for a total of 5 min for the construction of 187 piles while two workers can be required. | ||||
| • | The excavator is working during the concreting time (5 min per pile), and two workers could be involved. | ||||
| • | The crane handles the steel reinforcement for 5 min for the RC pile and 1 min for the FRC alternatives while two workers are performing tasks in the vicinities. | ||||
Table 5. Resulting ORI and components for each pile alternative
Table 5. Resulting ORI and components for each pile alternative
| Risk—activity | W | ||||||
|---|---|---|---|---|---|---|---|
| RC | SFRC | PFRC | RC | SFRC | PFRC | ||
| Traffic accident–transport of concrete to construction site | 0.040 | 24.9 | 24.9 | 24.9 | 0.996 | 0.996 | 0.996 |
| Traffic accident–transport of steel rebars to construction site | 0.030 | 1.2 | 1.2 | 1.2 | 0.035 | 0.035 | 0.035 |
| Blows to upper and lower limbs–manual load handling: installation of reinforcing bars | 0.021 | 12.5 | 0.0 | 0.0 | 0.262 | 0.000 | 0.000 |
| Burns–welding | 0.007 | 15.6 | 0.0 | 0.0 | 0.109 | 0.000 | 0.000 |
| Collision with or running over by heavy equipment or heavy-goods vehicles–work with CFA pile rig | 0.068 | 4.7 | 4.7 | 4.7 | 0.318 | 0.318 | 0.318 |
| Collision with or running over by heavy equipment or heavy-goods vehicles–work with concrete mixer truck | 0.068 | 1.6 | 1.6 | 1.6 | 0.106 | 0.106 | 0.106 |
| Collision with or running over by heavy equipment or heavy-goods vehicles–work with concrete pump truck | 0.068 | 0.2 | 0.2 | 0.2 | 0.011 | 0.011 | 0.011 |
| Collision with or running over by heavy equipment or heavy-goods vehicles–work with excavator | 0.068 | 31.2 | 31.2 | 31.2 | 2.119 | 2.119 | 2.119 |
| Collision with or entrapment by moving load due to its movement or detachment–mechanical load handling with crane | 0.065 | 31.2 | 6.2 | 6.2 | 2.026 | 0.405 | 0.405 |
| ORI | 5.982 | 3.991 | 3.991 |
The magnitudes quantified for each measurable indicator (designed as measurable) are presented in Table 6. Based on the results presented in Table 6, the following can be noted:
| • | Direct costs increase by 1.9% and decrease by 2.4% for the SFRC and PFRC alternatives, respectively, in comparison to the RC traditional solution according to the material costs presented in Table 3. The FRC piles are reinforced throughout their entire length. In Section 5, other associated costs for the reinforcement are analyzed to allow quantification of the effect of this variable. | ||||
| • | Regarding the environmental indicators, energy consumption increases by 2.8% for the SFRC alternative, whereas it decreases by 5.1% when PFRC is considered as the reinforcement. This is related to the manufacturing processes of each type of fiber. A similar tendency was found for the water consumption, for which a 3.6% (SFRC) increase and a 14.5% (FRC) decrease of water consumption was estimated. For contextualizing purposes, the reduction in water consumption in the case where polymeric fibers were used would be as much as
600 m3(600,000 L) of the water consumption savings. Finally, it was found that the CO2-equivalent ( CO2-eq) emissions could be reduced by 5.1% (SFRC) and 7.5% (PFRC) with respect to the RC solution. |
||||
| • | A 34.4% reduction to the risk exposition (ORI-based quantification) was observed for the FRC alternatives. Besides the social implication, this reduction could be directly considered as an economical benefit (or saving) when establishing the cost of insurance. | ||||
Table 6. Quantification of measurable indicators (per pile) according to reinforcement alternative
Table 6. Quantification of measurable indicators (per pile) according to reinforcement alternative
| Indicators | RC | SFRC | PFRC |
|---|---|---|---|
|
I1. Direct costs (k€) |
0.86 | 0.88 | 0.84 |
|
I2. Nonacceptance costs (points) |
— | — | — |
|
I3. Durability costs (points) |
— | — | — |
|
I4. Time (points) |
— | — | — |
|
I5. Energy consumption (GJ) |
17.75 | 18.24 | 16.84 |
|
I6. Water consumption ( m3) |
22.0 | 22.8 | 18.8 |
|
I7. CO2emissions ( TonCO2-equivalent) |
2.14 | 2.03 | 1.98 |
|
I8. ORI (weighted person-hours/pile) |
0.032 | 0.021 | 0.021 |
|
I9. Building site space (points) |
— | — | — |
|
I10. New solutions (points) |
— | — | — |