Environmental analysis of concrete deep foundations: Influence of prefabrication, concrete strength, and design codes

Journal Pre-proof Environmental analysis of concrete deep foundations: Influence of prefabrication, concrete strength, and design codes Ester Pujadas-Gispert, David Sanjuan-Delmás, Albert de la Fuente, S.P.G. (Faas) Moonen, Alejandro Josa PII:

S0959-6526(19)33621-2

DOI:

https://doi.org/10.1016/j.jclepro.2019.118751

Reference:

JCLP 118751

To appear in:

Journal of Cleaner Production

Received Date: 29 May 2019 Revised Date:

28 August 2019

Accepted Date: 5 October 2019

Please cite this article as: Pujadas-Gispert E, Sanjuan-Delmás D, de la Fuente A, Moonen SPG(F), Josa A, Environmental analysis of concrete deep foundations: Influence of prefabrication, concrete strength, and design codes, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/ j.jclepro.2019.118751. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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Environmental Analysis of Concrete Deep Foundations: Influence of

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Prefabrication, Concrete Strength, and Design Codes

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Ester Pujadas-Gisperta, David Sanjuan-Delmásb, Albert de la Fuentec, S.P.G. (Faas) Moonena, Alejandro Josac

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a

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b

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c

Department of the Built Environment, Eindhoven University of Technology (TU/e), Eindhoven, Netherlands Envoc Research Group, Green Chemistry and Technology, Ghent University, Coupure Links 653, 9000 Ghent, Belgium Department of Civil and Environmental Engineering (DECA), School of Civil Engineering (Escola de Camins), Universitat Politècnica de Catalunya (UPC), Barcelona, Spain

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Abstract

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There is great potential to reduce the environmental impact of the building sector, which is now an area of immense importance, through the optimisation of construction materials and components. This study assesses both the design and the construction of Concrete Deep Foundations (CDFs), which are widely used in construction, from an environmental perspective considering the following variables: (i) grade of prefabrication, i.e., fully cast in situ, partly prefabricated, and fully prefabricated; (ii) compressive strength of cast-in-situ concrete; and (iii) building design codes, i.e., current Spanish codes (EHE-08 and CTE), Eurocode with the Spanish annexes, and Eurocode with the United Kingdom annexes. In addition, the results of Dynamic Load Tests (DLTs) and the economic cost of several CDFs are evaluated. Geotechnical and structural designs of CDFs are carried out along with their life-cycle assessment. Some of the main findings include: (i) partially and fully prefabricated CDFs and conducting DLTs reduced the environmental impact in most categories (by up to 44% for global warming emissions) compared to the fully cast-in-situ CDFs, although they were 12– 37% more expensive; (ii) changing the compressive strength of the concrete (in piles and cap) in fully cast-in-situ CDFs from 25 to 35 MPa reduced the environmental impact by up to 14–17% in all categories and economic costs by up to 12%; and, (iii) CDFs with bored piles resulted in the lowest environmental burden when designed with Eurocode and UK annexes (11–31% less impact), as did CDFs with driven piles designed with current Spanish codes (11–18% less impact). The study variables and sensitivity analysis showed a significant effect on the results and should be considered in future construction, research, and building codes.

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Keywords: LCA, pile, EHE-08, Eurocode, CTE, economic.

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Word count: 8,000. 1

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1.

Introduction

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1.1.

Background

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The building and construction sector must decarbonize by 2050 (IPCC, 2018) to meet

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the targets of the Paris Agreement on Climate Change (EC, 2018). The sector accounts

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for nearly 40% of total energy-related CO2 emissions and 36% of final energy use

45

worldwide (GABC, 2018). In addition, both population growth (UN, 2019) and the

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increasing purchasing power of emerging economies and new companies will

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foreseeably produce a 50% increase in energy demand by 2060 (IEA, 2017).

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It is commonly expected that the operational phase of a building (e.g., cooling,

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heating ventilation, lighting, appliances) will govern the life-cycle impact (UNEP,

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2009). Nonetheless, several studies have highlighted the growing importance of

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embodied impacts (e.g., extraction and processing of raw materials, manufacturing of

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products, construction and demolition) (Bionova Ltd, 2018; de Klijn-Chevalerias and

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Javed, 2017), which are sometimes not correlated with the operational impacts (Hoxha

54

et al., 2017).

55

Life-Cycle Assessment (LCA) is a tried and tested technique to examine sustainable

56

outcomes for buildings (Ingrao et al., 2018). However, it is important to minimize its

57

calculation-related uncertainties and to provide reliable benchmarks for assessing

58

buildings (Hoxha et al., 2017; König and De Cristofaro, 2012). The foundation, which

59

is the structural element of the building that transfers the loads to the ground, is rarely

60

environmentally assessed despite the impact of its materials that can be significant

61

(Estokova et al., 2017; Sandanayake et al., 2016a).

62

Foundations can be broadly classified into either shallow or deep foundations.

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Shallow foundations transfer loads near the surface of the ground (Fig. 1a), while deep

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foundations (Fig. 1b) transfer the load onto a deeper and more resistant soil layer (Fig. 2

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1b) (Tomlinson and Woodward, 2014). Deep foundations have a greater environmental

66

impact compared to shallow foundations mainly due to their larger use of materials

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(Ay-Eldeen and Negm, 2015; Bonamente and Cotana, 2015). The optimization of deep

68

foundations in terms of the environment can reduce the impact of buildings and

69

constructions, thereby contributing to slowing down global warming.

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Fig. 1. Simplified layouts of (a) shallow and (b) deep foundations.

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1.2.

Concrete deep foundations

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There are several types of deep foundations (e.g., piles, caissons, drilled shafts) made

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of different materials (e.g., reinforced concrete (RC), timber, steel) and techniques (e.g.,

77

prefabricated, prestressed). Two common types of concrete deep foundations (CDFs)

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are precast driven piles and cast-in-situ bored piles. Driven piles are prefabricated in a

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factory (typically subject to high-quality standards) and subsequently installed on site.

80

Conversely, bored piles are built directly into the previously excavated ground, which

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hinders visual inspections of previously installed piles, leading in some cases to

82

unexpected economic (Brown, 2005) and environmental costs. 3

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Piles normally work in group by means of a pile cap (Fig. 2), a raft or a beam that

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distributes the load of the building into them and, in turn, to the ground. Pile caps are

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normally built on site (cast in situ), even if the piles are precast, because of the difficulty

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of fitting a prefabricated cap onto piles that can deviate from its exact position (EN

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1992-1-1, 2004). Semi-precast pile caps, built partly in a factory and finished on site,

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can withstand higher loads and are quicker to install on site (Chan and Poh, 2000).

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Fig. 2. Geometry and parameter definitions for concrete thick three-pile caps: strut (s); tie (t); distance from the external face of the pile to the edge of the cap (b); axial building load (N); distance between piles (e); pile-cap depth (h); and, angle at which pile caps spread the axial force from the superstructure through the piles (α) and pillar side (a).

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The design of a CDF involves both a geotechnical and a structural design. In Europe,

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Eurocode 7 (EN 1997-1, 2004) regulates the geotechnical design, while Eurocode 2 (EN

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1992-1-1, 2004) governs the concrete structural design. Moreover, the Eurocode

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national annexes, which adapt the Eurocode to each country, are mandatory.

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Nonetheless, the Eurocode and its annexes are not yet compulsory in Spain and instead 4

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the Technical Building Code (CTE-SE-C, 2008) and the EHE-08 Structural Concrete

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Code (EHE-08, 2008) are applicable.

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The geotechnical design of piles can be conducted using diverse methodologies. In

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Spain and the UK, among other countries, piles are designed based on ground

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parameters (EN 1997-1, 2004), and load tests are sometimes used to verify these pile

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resistances. This is partly explained by tradition, ground conditions, and economic

106

costs. In addition, Dynamic Load Tests (DLTs) are frequently carried out on driven

107

piles to assume a better pile resistance on calculations (Raison and Egan, 2016) and, in

108

consequence, to design shorter and thinner piles.

109

1.3.

Environmental assessment of concrete deep foundations

110

Materials are the major contributors to the GHG emissions of CDFs (Sandanayake et

111

al., 2017; Zhang and Wang, 2016) followed by equipment usage and transportation,

112

with piles and a raft (Sandanayake et al., 2016a, 2016b) and with only piles (Luo et al.,

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2019). However, it is the piles that account for most GHG emissions in the construction

114

of a CDF (Ay-Eldeen and Negm, 2015).

115

Several factors, among which the selection of the type of CDF, can reduce the

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economic cost of a CDF. Giri and Reddy, (2014) indicated that caissons could reduce

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environmental damage by 60% and economic costs in comparison with pipe piles.

118

Yeung (2015) indicated that high-strength prestressed concrete piles can approximately

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halve both GHG emissions and economic cost compared to steel H-piles. Misra and

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Basu (2011) demonstrated that driven piles had a better environmental performance in

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sandy soils than drilled shafts, although their environmental performance in clayey soils

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depended on the design load. Focusing on the same topic, Luo et al. (2019) showed that

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driven piles accounted for higher GHG emissions compared to bored piles. It also 5

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highlighted that materials and transport were points to be optimized for driven piles,

125

while the installation of bored piles needed optimization. In addition, the use of more

126

sustainable materials (concrete with fly-ash or blast furnace slag) in both types

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decreased around 3-7% of the total GHG emissions. Finally, Lee and Basu (2016, 2018)

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evaluated several design methods in driven piles and drilled shafts and concluded that

129

decisions made during the design process can potentially imply substantial and long-

130

term environmental impacts.

131

However, the environmental impacts associated with using other concrete compressive

132

strengths (compatible with the structural and durability requirements) and by

133

conducting DLTs have yet to be investigated in depth. In addition, both an economic

134

and an environmental based-optimization approach is required to obtain practical

135

conclusions (Luo et al., 2019) and to ease its implementation in the industry (Pujadas,

136

E., de Llorens, J.I., Moonen, 2013; Pujadas-Gispert, 2016). In a previous study on

137

concrete shallow foundations (Pujadas-Gispert et al., 2018), the combination of the

138

variables related to prefabrication, type, and design codes showed reductions of 40–60%

139

for all impact indicators. They have therefore been included here, although type and

140

prefabrication have been addressed together (i.e. precast driven piles).

141

1.4.

Objectives

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The main goal of this research is to analyse CDFs from an environmental perspective,

143

considering the variables of level of prefabrication (cast-in-situ or precast), concrete

144

compressive strength (25, 30, 35 MPa), and the building design code in use (EHE-08

145

and CTE, Eurocode with Spanish annexes and Eurocode with the United Kingdom

146

annexes). In addition, a sensitivity analysis was performed, to evaluate the influence of

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using DLTs on the results of CDFs with driven piles, and an economic study was

148

conducted to assess the foundations designed with Spanish regulations.

149

The specific objectives are (1) to conduct a structural analysis of equivalent structural

150

alternatives, in order to determine the required amounts of concrete and reinforcing

151

steel; (2) to calculate, analyse, and compare the environmental impacts using LCA and

152

the economic costs of alternatives; and, (3) to assess the influence of the variables and

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the sensitivity analysis of the environmental burdens (and economic cost) of CDFs and,

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by doing so, to define specific design conclusions and recommendations.

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2.

Materials and methods

157

2.1.

Selection of alternatives

158

Building loads tend to be lower than other construction loads (e.g., bridges).

159

Therefore, buildings’ pile caps tend to be composed of few piles. This study considers a

160

three pile cap because that design is a minimum arrangement for 3D stability. In

161

addition, the method considered for installing bored piles was the continuous flight

162

auger (CFA), which is a common cost-effective method in sufficiently uniform soil

163

conditions that are sufficiently homogeneous (Brown, 2005). Finally, thick (or deep)

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pile caps were designed, which are preferable in practice to slender caps (CPH, 2014).

165

The study variables and the abbreviations of the alternatives are listed in Table 1.

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Table 1. Abbreviations used in the study for CDFs. Variables

Abbreviations

1

Pile type

Bored pile (B), Driven pile (D)

2

Pile concrete compressive

25, 30, 35 MPa (Cast in situ); 40 MPa (Precast)

3

strength Pile cap type

Cast in situ (I) (concrete is poured on site), Precast (P)

4

Pile cap concrete compressive

(concrete is poured in a specialised facility) 25, 30, 35 MPa (Cast in situ); 40 MPa (Precast)

5

strength Building design code

EHE-08 and CTE (ES), Eurocode with Spanish annexes (SP), Eurocode with United Kingdom annexes (UK)

7

6

Performance of DLTs

Yes (*), No ( )

167

Example: D40/I25/SP*—Concrete deep foundation composed of 3 Driven piles (with concrete of

168

compressive strength of 40 MPa) and a cast-In-situ concrete cap (with concrete of 25 MPa), designed in

169

accordance with Eurocode with SPanish national annexes and conducting DLTs.

170

2.2.

Case study

171

The case study is a concrete modular housing building located in the Barcelona area

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(Spain). The vertical load to the CDF is 2,300 kN (N in Fig. 2), and the bending

173

moments are 6 kNm and 15 kNm around axis X and axis Y, respectively (all these

174

values are unfactored). The dimensions of the square-shaped pillar that transmits the

175

loads to the CDF are 0.45 × 0.45 m (a in Fig. 2).

176

The soil (Table 2) is composed of an upper stratum of 15 m of soft clay placed over a

177

stratum of compacted sand. Because this research is aimed at obtaining general results

178

regarding the environmental and economic impacts of CDFs, certain elements that are

179

highly dependent on each case study, such as the presence of water, seismicity, negative

180

skin friction, and chemical action, were omitted from the scope of the study.

181

Table 2. Parameters and characteristics of the case study soil. Ground stratum

Parameter

Abbreviation

Value

Unit

Soft clay

Undrained shear strength Weight density

cu γn

15 17

kN/m2 kN/m3

Compacted sand

Angle of shearing resistance Weight density

ϕ' γn

39 18

º kN/m3

182 183

2.3.

Functional unit

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The Functional Unit (FU) considered in this analysis is a CDF consisting of an RC

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thick pile cap and three RC piles with different levels of prefabrication and concrete

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compressive strengths, designed using different building design codes; furthermore,

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DLTs were also conducted on driven piles. All the alternatives were designed for a

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service life of 50 years.

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2.4.

System boundaries

190

In Fig. 3, the LCA phases and elements considered in each stage are shown. Life-

191

cycle phases run from the extraction of materials until the completion of on-site

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construction. Moreover, each phase includes the impact of transportation. Nonetheless,

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the vibration and pumping of concrete were not considered, as a preliminary analysis

194

showed no significant environmental impacts. The use phase was excluded, because

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well-designed foundations require neither maintenance nor repairs. Similarly,

196

decommissioning was excluded, because foundations are usually left installed at the

197

end of their life, implying an absence of any significant impacts.

198 199

Fig. 3. Life-cycle diagram and system boundaries of the construction of a CDF.

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2.5.

Data sources

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Several data sources were consulted to calculate the environmental burdens of CDFs,

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all of these considering a proper degree of uncertainty, so that possible contingencies of

203

the nature of each source and their elements can be covered. The amounts of resources

204

were retrieved from the Construction Technology Institute of Catalonia (ITeC, 2019).

205

Nonetheless, whenever certain items were not found from those sources, the 9

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information was requested from the manufacturers. Keller Cimentaciones (Keller

207

Cimentaciones, 2017) provided the diesel for building the piles and performing the

208

DLTs and estimating the average distances to transport piling machines. In addition,

209

concrete dosages were provided by the Spanish National Association of Ready-Mixed

210

Concrete Manufacturers (ANEFHOP, 2019). LCA processes were retrieved from the

211

Ecoinvent v.3.0.3.0 database (Swiss centre for life cycle inventories, 2013). The

212

transport distances and their sources are summarised in Table 3. Further information

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can be found in Supplementary Material 1.

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Table 3. Transport distances used for calculation. Transportation Item

Distances (km)

Retrieved from

From

To

Cement

Place of production

Concrete plant Precast concrete plant

75

Aggregates

Place of production

Concrete plant Precast concrete plant

40

Steel reinforcement

Place of production

Construction site Precast concrete plant

130

Concrete

Place of production

Construction site

Soil

Construction site

Landfill sites

30

Waste

Construction site

Waste management facility

30

Additives

Place of production

Concrete plant Precast concrete plant

100

(Mendoza et al., 2012)

Piling machine

Previous construction site

Construction site

500

Facilitated by companies

Precast units

Precast concrete plant

Construction site

150

(The Concrete Centre, 2009)

30

(Pujadas-Gispert et al., 2018; SanjuanDelmás et al., 2015)

215

216

Most data for the economic assessment were retrieved from ITeC (2018), although

217

some specific costs that were not found there were provided by manufacturers

218

(transport of piling and driving machines and DLTs).

10

219

220

2.6.

Quantitative models

The quantitative models for assessing the cradle-to-gate environmental impact (per

221

category) and economic cost of the FU (i.e., CDF) are shown below.

222

2.6.1. Quantitative model for environmental impact category calculation

223

=∑

∑



(1)

224

where, EI is the environmental impact category (e.g., global warming, cumulative

225

energy demand) of the FU;

226

of environmental intervention (e.g., material, transport, consumption from equipment

227

usage); qij is the quantity of environmental intervention in an FU phase; i is

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environmental intervention; n is the total number of environmental interventions; j is

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the phase of the life cycle (e.g., raw material extraction, production); and, m is the total

230

number of phases in the life cycle (see section 2.4).

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2.6.2. Quantitative model for economic cost calculation

232

=∑

×

is the characterization factor of impact category per unit

(2)

233

where, C is the economic cost of the FU;

is the current unit cost of item I; qi is the

234

quantity of item i in the FU over time t; r1 is the discount rate per unit of time t for item i

235

of the FU; t1 is time for item i of the FU (t=0, as we consider from raw material

236

extraction to construction); i is an item in the FU; and, n is total number of items in the

237

FU.

238

2.7.

Geotechnical design

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Several best practices from UK and Spain were followed for the geotechnical design

240

of study alternatives. First, the distance between piles (e in Fig. 2) was considered to be

241

three times the pile’s diameter and were embedded a minimum of six diameters in the 11

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firm soil to guarantee an optimal transfer of the building’s loads to the ground (CTE-

243

DB-SE-C, 2008). In addition, the geotechnical values for the base and shaft pile

244

resistances were set at 20,000 kN/m2 and 120 kN/m2, respectively (CTE-DB-SE-C,

245

2008). Furthermore, pile resistances were calculated by factoring strengths and actions

246

when they were designed with ES and SP, whereas a global safety factor was used

247

instead with CTE to guarantee structural reliability (BS EN 1997-1:2004+A1:2013,

248

2004; EN 1997-1, 2004; Gepp et al., 2014; UNE-EN 1997-1, 2016). Finally, the

249

foundation had 60 piles and it was considered that 5 DLTs were conducted which meets

250

both usual Spanish and English practice (ACHE, 2004; Raison and Egan, 2016).

251

2.8.

Structural design

252

CDFs are made of concrete and steel. The compression strengths selected for cast-in-

253

situ concrete were 25, 30, and 35 MPa; 40 MPa was selected for precast concrete. These

254

strengths are representative on an international scale, although some are more common

255

than others depending on locations. The steel used for reinforcement was B-500-S, and

256

partial factors for concrete (1.5) and steel reinforcement (1.15) were selected according

257

to EHE-08 (2011), and EN 1992-1-1 (2004). Moreover, the surrounding environment

258

was classified as XC2 (wet, rarely dry; corrosion induced by carbonation) by EC-2 and

259

IIa (high humidity; corrosion of different origin than chlorides) by EHE-08, which

260

conditions the width of the concrete covers and hence the durability of the concrete

261

structures.

262

The codes used to calculate the amounts of concrete and steel reinforcement in the

263

CDFs are listed in Table 4. In addition, the structural approach used to calculate pile

264

caps was the strut-and-tie approach (ACHE, 2003; Goodchild et al., 2014) proposed by

265

(Ritter, 1899), with the tension elements provided by reinforcement (t in Fig. 2) and the

266

concrete acting as struts (s in Fig. 2). This approach is commonly used according to 12

267

(Miguel-Tortola et al., 2018) and suitable (EHE-08, 2011; Miguel et al., 2007; Souza et

268

al., 2009) for designing thick pile caps, although Eurocode (EC-2, 2004) states that

269

flexural-based methods are also applicable.

270

Table 4. Building actions, geotechnical, and structural design codes. Design

ES

SP

UK

Actions

(CTE-DB-SE-AE, 2009)

(UNE-EN 1991-1-1:2019, 2019)

(BS EN 1990:2002+A1:2005, 2002) (NA to BS EN 1990:2002+A1:2005, 2004)

Geotechnical

(CTE-DB-SE-C, 2008)

(UNE-EN 1997-1, 2016)

(BS EN 1997-1:2004+A1:2013, 2004) (NA+A1:2014 to BS EN 19971:2004+A1:2013, 2007)

Structural

(EHE-08, 2011)

(UNE-EN 1992-1-1:2013/A1:2015, 2015)

(BS EN 1992-1-1:2004+A1:2014, 2004) (NA+A2:2014 BS EN 1992-11:2004+A1:2014 UK, 2005)

271

Terminology: EHE-08 and CTE (ES); Eurocode with Spanish annexes (SP); Eurocode

272

with United Kingdom annexes (UK).

273

Bored piles tend to be reinforced only in the uppermost meters when there are no

274

lateral forces or seismicity. Herein, 6 m has been considered for this length, according

275

to (NTE, 1977; Raison and Egan, 2016). Conversely, driven piles are reinforced over

276

their entire length, and have an extra (stirrup) reinforcement at the top and bottom 500

277

mm and 200 mm, respectively, according to BS EN 12794:2005 (2005) and UNE-EN

278

12794:2006+A1:2008 (2008). This is because driven piles must resist extra stresses

279

during handling and driving. Moreover, all piles, bored or driven, were reinforced

280

according to the minimum amounts of steel required for each code.

281

2.9.

Life-cycle assessment

282

An LCA was used to determine the environmental impacts. This method is well

283

established in scientific literature and has been standardised through global documents

284

(ISO 14040:2006, 2006; ISO 14044:2006, 2006). The software SimaPro 8.4.0.0 (PRé

285

Consultants, 2017) was used to implement the LCA, along with the calculation method

286

ReCiPe 2016 Midpoint, Hierarchist version (Huijbregts et al., 2016).

13

287

The impact categories under consideration were selected on the basis of the product

288

specifications and construction product standards (EN 15804:2012+A1:2013, 2013)

289

and the authors’ expertise: Global Warming (GW, kg CO2eq), Ozone Depletion (OD, kg

290

CFC-11eq), Terrestrial Acidification (TA, kg SO2eq), Freshwater Eutrophication (FE, kg

291

Peq), Photochemical Oxidant Formation (POF, kg NMVOC), Mineral Depletion (MD,

292

kg Feeq), Fossil Depletion (FD, kg oileq), and Cumulative Energy Demand (CED, MJ).

293

294

3.

Results and discussion

295

3.1.

Preliminary remarks

296

An important element to consider when addressing the environmental and economic

297

impacts of CDFs is the angle (α in Fig. 2) at which the pile cap spreads the axial force

298

(N in Fig. 2) from the superstructure through the piles, which determines the depth of

299

the pile cap (h in Fig. 2). This angle is typically 45º (Jones, 2013), but might range from

300

21.8º to 45º (UK), and from 26.6º to 63.4º (ES and SP). In a preliminary analysis, that

301

angle (α in Fig. 2) (40º, 45º, 50º) and the smallest angle that the concrete strut could

302

withstand were considered, as well as the angular constraints. CDFs with the shallowest

303

angles (i.e., smaller cap depths) showed better environmental results in most impact

304

categories, because they had smaller concrete volumes. Nevertheless, these alternatives

305

showed higher impacts in the highly steel-dependent categories (FE and MD), because

306

they had more steel (the decrease in the depth of the CDF was compensated with more

307

steel reinforcement).

308

Similarly, bored and driven piles were calculated with the smallest possible cross

309

sections to save concrete in the piles and cap. Driven piles resulted in smaller cross

310

sections than bored piles as the concrete for driven piles was more resistant. 14

311

Nevertheless, driven piles with sides smaller than 235 mm were not considered to

312

prevent the pile from breaking in this type of soil (in accordance with standard

313

practice).

314

Regarding the economic comparison, it only takes into account the CDFs designed

315

with ES and SP as they use the same sources (ITeC, 2019). Costs for UK CDFs were

316

also calculated, but were not included in this paper, as difficulties over establishing a

317

comparison between CDFs were observed when retrieving data from different

318

databases, especially as each database makes its own assumptions.

319

3.2.

320

3.2.1. Prefabrication

321 322

Environmental performance of CDFs according to study variables

The main characteristics of piles and caps considered for the CDFs alternatives are shown in Supplementary Material 2.

323

Concrete and steel play an important role in the environmental burden of CDFs. They

324

contributed up to 75–95% in most impact categories. In this regard, concrete and steel

325

accounted for 80–90% of the GW emissions in all CDFs.

326

Figure 4 compares the GW emissions of CDFs with bored and driven (precast) piles,

327

with a cast-in-situ cap and calculated with SP. In CDFs with driven piles, concrete

328

showed lower impact percentages (around 20% less) than CDFs with bored piles,

329

because they required less concrete. Conversely, higher percentages were found in steel

330

(approximately 20% more) because they are all length reinforced, in contrast to bored

331

piles, which have only the uppermost meters reinforced (according to study load

332

combination). In addition, precast products require transportation to the site and

333

installation (piling, pile cutting, excavation, and precast installation), each of which

334

account for up to 8% of the GW emissions.

15

100% 90%

Relative impact (GW)

80% 70% 60% 50% 40% 30% 20% 10% 0% B25/I25/SP

B25/P40/SP

D40/I25/SP

D40/P40/SP

Concrete Deep Foundations Concrete Piling Precast installation Transport of soil

Blinding Pile cutting Transport of waste Precast transport

Steel reinforcement Excavation Transport of piling machine

335 336 337 338 339 340

Fig. 4. Relative GW emissions of the construction of CDFs designed with SP. Terminology: Bored pile (B); Driven pile (D); Concrete compressive strengths: 25, 40 MPa; Cast in situ (I); Precast (P); Eurocode with Spanish annexes (SP); Global Warming (GW).

341

The results show that the piles are of greater importance to the environmental

342

performance of CDFs than the pile caps, because the piles contain more concrete.

343

Driven piles can significantly reduce these materials compared to bored piles, because

344

they are more resistant for the same section, as higher concrete strengths are used. In

345

addition, the codes assume that driven piles can withstand higher loads than bored

346

piles, because they are produced in a factory under rigorous quality-control protocols

347

and uncertainties are reduced.

348

The use of a smaller cross section of piles also leads to smaller caps, because the size

349

of the piles conditions the width of the cap. It must be emphasised, however, that when

350

the pile cross section is smaller, either the depth of the cap or the compressive strength

16

351

of concrete in the strut should be increased (s in Fig. 2) to withstand compressive

352

forces.

353

Nevertheless, it should be remembered that prefabricated concrete has a greater

354

environmental burden for the same volume of concrete. In this study, prefabricated

355

concrete (40 MPa) accounted for 30–40% more GW emissions compared to cast-in-situ

356

concrete (25 MPa). This discrepancy is because prefabricated concrete tends to be of

357

higher strength, which requires larger amounts of cement or the use of cements with

358

higher strength. Cement is the component with higher environmental impacts in the

359

mixture although it is in a small proportion. Cement accounted for 75–80% of GW

360

emissions of the concrete (including transport of concrete components). Furthermore,

361

transport and on-site assembly of precast products resulted in additional burdens.

362

Figure 5 compares the environmental impacts of CDFs and bored and driven piles

363

designed with SP. For the sake of simplicity, the environmental categories of OD

364

( GW) and TA, POF, FD ( CED) are not shown. CDFs were designed by applying

365

the minimum amounts of concrete and reinforcement that are specified in each code.

366

CDFs with driven piles accounted for smaller values in most categories (up to 15%),

367

although they obtained higher results in the FE and MD categories (19–33%), because

368

they contained more steel. In addition, the environmental impact of CDFs with a

369

precast cap was up to 7% higher than CDFs with a cast-in-situ cap. This difference is

370

because the extra strength of prefabricated concrete was not fully used in some cases,

371

as codes limit the minimum depth of the cap. Furthermore, the slight reduction in depth

372

was compensated with additional steel bar reinforcements, which has a higher burden

373

by volume than concrete.

17

100%

90%

Relative impact

80%

70%

60%

50%

40% GW

FE

MD

CED

Impact categories B25/I25/SP

D40/I25/SP

B25/P40/SP

D40/P40/SP

374 375 376 377 378 379

Fig. 5. Relative impact of CDFs designed with SP, with a cast-in-situ or precast cap and with bored piles or driven piles. Terminology: Bored pile (B); Driven pile (D); Concrete compressive strength: 25, 40 MPa; Cast in situ (I); Precast (P); Eurocode with Spanish annexes (SP); Global Warming (GW); Freshwater Eutrophication (FE); Mineral Depletion (MD); Cumulative Energy Demand (CED).

380

CDFs tend to be oversized in reality. For instance, companies and designers try to

381

standardise products and solutions to save money in the design. They normally prefer to

382

invest this money in oversizing construction and enhancing safety. Furthermore,

383

construction on site can sometimes be unpredictable in terms of the amount of resources

384

and waste. For example, concrete leakage (particularly during construction) is likely

385

when bored piles are built in highly permeable ground (e.g., sand, silt, low plasticity

386

soils). It would also be interesting to account for all the unforeseen events during

387

construction (which can be relevant), though they are specific to each building project

388

and some are difficult to quantify. Obviously, these unforeseen events must be predicted

389

and avoided as much as possible.

390

3.2.2. Concrete compressive strength

18

391

Figure 7 presents the results obtained when concrete of different compressive

392

strengths (25 MPa, 30 MPa, and 35 MPa) were tested for bored piles and cast-in-situ

393

caps designed with SP. The results of the OD, TA, POF, and FD impact categories are

394

not shown, as they showed a similar trend to CED. Changing the compressive strength

395

of concrete in bored piles from 25 to 35 MPa reduced the environmental impacts of

396

CDFs from 18 to 24%. The amounts of concrete and steel decreased considerably,

397

which completely counteracted the higher environmental burdens associated with higher

398

concrete strengths. As can be seen, this difference is more important between bored

399

piles with concrete strengths of 25 MPa and 30 MPa, than between those with 30 MPa

400

and 35 MPa. Conversely, no significant difference in the environmental burdens (up to

401

7% more) was found between CDFs with caps of different concrete strengths. There are

402

several possible explanations for this result. First, the cap has a low impact on the

403

environmental burdens of the CDF, as it accounts for a small part of it. In addition,

404

structural design codes fix the minimum depth of the cap, impeding its environmental

405

optimisation in some cases. Similar results were obtained for CDFs designed with SP

406

and UK.

19

100%

Relative impact

90%

80%

70%

60%

50%

40% GW

FE

MD

CED

Impact categories B25/I25/SP B35/I30/SP

407

B30/I25/SP B25/I35/SP

B35/I25/SP B30/I35/SP

B25/I30/SP B35/I35/SP

B30/I30/SP

408 409 410 411 412 413

Fig. 6. Relative impact of CDFs with bored piles, a cast-in-situ cap and different concrete compressive strengths designed with SP. Terminology: Bored pile (B); Concrete compressive strength: 25, 30, and 35 MPa; Cast in situ (I); Eurocode with Spanish annexes (SP); Global Warming (GW); Freshwater Eutrophication (FE); Mineral Depletion (MD); Cumulative Energy Demand (CED).

414

3.2.3. Codes

415

This section compares the environmental impacts of CDFs with bored and driven

416

piles using the ES, the SP, and the UK annexes. CDFs were designed with the

417

minimum allowed amounts of steel and concrete, to establish a fair comparison

418

between these codes. In addition, the compressive concrete strength for cast-in-situ

419

piles and caps was set up at 30 MPa. There is no case of CDF with driven piles and a

420

cast-in-situ cap with a concrete strength of 25 MPa for the UK in this study, as the

421

concrete struts could not withstand the compression loads.

422

CDFs with bored piles designed with the UK codes resulted in 11–31% lower impacts

423

compared to those calculated with ES, because less concrete was used and because ES

20

424

and SP include an upper limit for the compressive strength of concrete piles. This limit

425

is particularly restrictive for bored piles, because of the uncertainties linked to their

426

construction process. In this sense, ES is even more restrictive than SP (CTE-DB-SE,

427

2009; Gepp et al., 2014; UNE-EN 1997-1, 2016). In addition, a smaller concrete pile

428

cross section requires less steel to comply with the codes, because the amounts of steel

429

are dependent on the pile cross section (a larger area indicates more steel); it is also

430

easier to fulfil the minimum distances between steel reinforcing bars specified in the

431

codes. In this regard, UK reinforces piles the most (for cast-in-situ and driven piles) for

432

the same pile section, whereas ES reinforces them the least.

433

CDFs with driven piles designed with ES yielded the lowest impacts: FE (18% SP;

434

30% UK), MD (22% SP; and 38% UK), and other categories (7–12% SP, 11–18%

435

UK). A result that is explained by the minimum amounts of steel specified in ES that

436

are lower than the other regulations for the same pile cross section.

437

A higher design value for the concrete compressive strength of the cap struts is

438

specified in ES for pile caps (s in Fig. 2), which permits a shallower pile cap depth (h in

439

Fig. 2), with a subsequent increase in steel reinforcement. Conversely, ES sets a larger

440

distance from the external face of the pile to the edge of the cap (b in Fig. 2) in which

441

larger amounts of concrete and steel are specified, in comparison with SP and UK.

21

100%

90%

Relative impact

80%

70%

60%

50%

40%

Bored

Driven

DrivenGWbored

Bored

Bored Driven MD

Driven FE

Bored Driven CED

Impact categories B30/I30/ES

442

B30/I30/SP

B30/I30/UK

D40/I30/ES

D40/I30/SP

D40/I30/UK

443 444 445 446 447 448 449

Fig. 7. Relative impact of CDFs designed with ES, SP and UK with bored and driven piles and a cast-in-situ cap (30 MPa). Terminology: Bored pile (B); Driven pile (D); Concrete compressive strength (30),(40) MPa; Cast in situ (I); EHE-08 and CTE (ES); Eurocode with Spanish annexes (SP); Eurocode with United Kingdom annexes (UK); Global Warming (GW); Freshwater Eutrophication (FE); Mineral Depletion (MD); Cumulative Energy Demand (CED).

450

3.3.

Sensitivity analysis in relation to DLT applications

451

It is fairly common to carry out DLTs when it is expected that their economic benefit

452

will be higher than their cost. In this study, DLTs were considered, to validate the

453

geotechnical calculations. Figure 8 compares the environmental impacts of CDFs with

454

driven piles and a cast-in-situ cap (30 MPa) in relation to whether DLTs are conducted.

455

For the sake of simplicity, the impact categories of OD, TA, POF, and FD are not

456

depicted because they had a similar trend to CED.

457

Conducting DLTs on driven piles reduced the environmental impacts of CDFs: 9–

458

14% ES, 11–13% SP, and 4–5% UK. On the one hand, the piles under study were

459

designed with the smallest permissible cross-section in the codes (see section 3.1). As 22

460

the pile cross-section was fixed, DLTs reduced the length of the driven piles,

461

decreasing the amounts of both concrete and steel and, hence, the environmental

462

impact. Nevertheless, those reductions were different, depending on the code due to the

463

specific structural response model accepted in each of them. In that sense, UK allows

464

less reduction on these amounts compared to ES and SP. Moreover, the amounts on the

465

caps remained unaltered, as the dimensions depended on the pile cross-sections (that

466

remained unchanged). On the other hand, DLTs had a minor impact on the

467

environmental performance of CDFs (around 1-2%). Therefore, the optimisation of the

468

quantities of concrete and steel completely counteracted the environmental impact of

469

the DLTs. 100% 90% 80%

Relative impact

70% 60% 50% 40% 30% 20% 10% 0% ES

SP GW GW

UK

ES

SP FE FE

UK

ES

SP MD MD

UK

ES

SP CED CED

UK

Impact categories D40/I30/ES

D40/I30/ES*

D40/I30/SP

D40/I30/SP*

D40/I30/UK

D40/I30/UK*

470 471 472 473 474 475 476 477

Fig. 8. Relative impact of CDFs with driven piles and a cast-in-situ cap with and without DLTs. Terminology: Driven pile (D); Concrete compressive strength: 30, 40 MPa; Cast in situ (I); EHE-08 and CTE (ES); Eurocode with Spanish annexes (SP); Eurocode with United Kingdom annexes (UK); Dynamic Load Tests (*); Global Warming (GW); Freshwater Eutrophication (FE); Mineral Depletion (MD); Cumulative Energy Demand (CED).

23

478 479

3.4.

Environmental and economic discussion of several relevant alternatives

480

Table 5 shows the environmental and economic costs of several representative CDFs

481

designed with the current Spanish codes (ES) and the Eurocode with Spanish Annexes

482

(SP).

483

A CDF with bored piles and a cast-in-situ pile cap (with concrete 25 MPa) is a

484

conventional solution in Spain. The results show that this conventional solution could

485

be optimised by changing the strength of the concrete in the piles and cap from 25 MPa

486

to 35 MPa. In this way, the environmental results could be diminished by 14–17% in all

487

categories and economic costs by 11% (SP) and 12% (ES).

488

Using driving piles (conducting DLTs and a cast-in-situ cap) resulted in 27% (SP) and

489

44% (ES) lower impacts compared to the fully cast-in-situ CDF (bored piles and a cast-

490

in-situ cap). Even the fully prefabricated CDF (driven piles conducting DLTs and a

491

precast cap) resulted in 24% (SP) and 40% (ES) lower impacts in most categories than

492

the fully cast-in-situ CDF, although the percentages in the FE and MD categories were

493

lower. Furthermore, the prefabrication of part (driven piles conducting DLTs and a

494

cast-in-situ cap) or all of the CDF (driven piles conducting DLTs, a precast cap and

495

DLTs) was more expensive than building it entirely in situ: by up to 33% (SP), 12%

496

(ES); and 37% (SP), 24% (ES), respectively.

497

Moreover, conducting DLTs on driven piles ultimately reduced the environmental

498

impacts of CDFs by 13–16% in all categories and reduced their costs by 12% (SP), and

499

6% (ES).

500

The economic costs were mostly retrieved from a Spanish construction reference

501

database (ITeC, 2019). Nevertheless, economic costs are highly influenced by the

502

factors of each building project (units, construction company, location, etc.). In 24

503

addition, driven piles are a common solution when there are large loads, aggressive or

504

weak soils, etc. In this respect, there are several circumstances that can make them a

505

cost-effective solution, but they are beyond the scope of this study. For example, the

506

use of wider piles can reduce the costs of the CDF because the number of piles is

507

reduced (e.g., from four to three) and the cap and the length of the piles become smaller

508

(it is more expensive to increase the length of the pile than to select a wider pile).

509

511 512 513 514 515 516 517 518

B25/I25/SP

B25/I35/SP

D40/I25/SP*

D40/P40/SP*

B25/P40/SP

B25/I25/ES

B35/I35/ES

D40/I25/ES*

D40/P40/ES*

B25/P40/ES

Table 5. Relative impact of several relevant CDFs from the study.

Categories

510

GW OD TA FE POF MD FD

80% 80% 79% 78% 81% 73% 80%

81% 80% 80% 81% 82% 78% 81%

59% 62% 69% 84% 69% 93% 67%

61% 65% 73% 90% 74% 100% 70%

82% 83% 83% 81% 84% 76% 83%

94% 92% 92% 93% 93% 88% 93%

80% 77% 78% 78% 79% 74% 78%

52% 56% 60% 67% 60% 70% 58%

56% 61% 66% 75% 68% 80% 63%

100% 100% 100% 100% 100% 94% 100%

CED

80%

81%

68%

71%

83%

93%

78%

58%

64%

100%

COST

73%

73%

97%

100%

77%

79%

70%

89%

98%

85%

Terminology: Bored pile (B); Driven pile (D); Concrete compressive strength: 25, 35, 40 MPa; Cast in situ (I); Precast (P); EHE-08 and CTE (ES); Eurocode with Spanish annexes (SP); Dynamic Load Tests (*); Global Warming (GW); Ozone Depletion (OD); Terrestrial Acidification (TA); Freshwater Eutrophication (FE); Photochemical Oxidant Formation (POF); Mineral Depletion (MD); Fossil Depletion (FD); Cumulative Energy Demand (CED); Economic Cost (COST). Percentages are calculated in each impact category in relation to the case with the highest impact, set at 100%. Cell colours reflect these percentages.

519 520

4. Conclusions

521

An assessment has been presented, from an environmental (and economic)

522

perspective, of the construction of Concrete Deep Foundations (CDFs) composed of

523

three piles and a thick pile cap, according to a number of variables. These include the 25

524

level of prefabrication, the compressive strength of concrete, the building design code,

525

and conducting or not DLTs. Some of the main conclusions of the study are

526

summarised below.

527

o Steel and concrete accounted for 75–95% of the impact in most environmental

528

categories and 80–90% of GWP emissions. A result that underlines the

529

importance of optimizing the impact of these materials when designing CDFs.

530

o Driven piles are preferred over bored piles from an environmental perspective,

531

because they require less concrete and therefore have a lower impact despite

532

using a larger amount of steel. However, this was not the case for the impact

533

categories of MD and FE, for which steel has a strong influence, meaning a

534

better performance for bored piles.

535

o The prefabrication of pile caps is not recommended for the conditions

536

considered in this study, because they account for up to 7% more impact, as the

537

reduction in concrete does not compensate for the higher burdens of

538

prefabrication (cement, transportation, installation, etc.)

539

o Changing the compressive strength of concrete in bored piles from 25 to 35

540

MPa is recommended, as impacts are reduced from 18 to 24% in the CDF for all

541

categories; however, it is not recommended in only pile caps, as the impact of

542

CDFs increases by up to 7%.

543

o The Eurocode with United Kingdom annexes is preferable from an

544

environmental perspective when designing CDFs with bored piles; its impacts

545

are 11–31% lower in most categories compared to the EHE-08 and CTE, while

546

these codes perform better for driven piles with 11–18% less impact in most

547

categories compared to the former. It must be highlighted that these CDFs were

26

548

designed with the minimum amounts of concrete and steel reinforcement

549

specified by the codes, in order to ensure fair comparisons between each one.

550

o Concrete compressive strength of 35 MPa (in piles and cap) is preferred to build

551

fully cast-in-situ CDFs as it reduces the environmental impact from 14% to 17%

552

and economic costs by up to 12% in comparison with 25 MPa (ES and SP).

553

o The partial or full prefabrication of CDFs and conducting DLTs have been

554

shown to reduce the environmental impact of CDFs in most categories (by up to

555

44% in GW) compared to those built entirely on site. Nevertheless, the

556

prefabrication of CDFs resulted in 12–37% more costs (ES and SP).

557

o Conducting DLTs on driven piles reduced the environmental impact of CDFs

558

between 13% and 16% in all categories and costs by up to 12% in this case

559

study (ES and SP).

560

The study variables have shown a significant effect on the environmental results. It is

561

therefore recommended to consider them in future constructions and codes.

562

Nonetheless, the findings in this study are restricted to the case study selected and

563

might be subject to other factors that depend on each case and can be difficult to

564

quantify. Moreover, further research might investigate CDFs and other RC structures

565

considering additional study variables and using recycled aggregates, other types of

566

reinforcement, biomaterials, and optimised cements to improve their environmental

567

burdens.

568

Acknowledgments

569

The authors are grateful for the help and support of Prof. Arch. Josep Ignasi de Llorens,

570

Ing. José María Díaz, Ing. José Ángel Alonso, Antonia Navarro, Xavier Reverter, Ing.

571

José Estaire, Ing. Chris Raison, Ing. César Bartolomé, Dr. Ing. José María Vaquero,

572

Ing. Theo Salet, Ing. S.N.M. (Simon) Wijte, Ing. Rijk Blok, Ing. Omar Diallo, Arch.

573

Marc Sanabra, Dr. G. P. Hammond, Arch. Ramon Badell, Marina Mañas, Dr. Anna 27

574

Petit-Boix, Antony Ross Price,

and Dr Esther Sanyé-Mengual. They thank the

575

following companies and associations: Keller Cimentaciones, Spanish National

576

Association of Ready-Mixed-Concrete Manufacturers (ANEFHOP), Hanson Hispania

577

S.A., Hormiconsa, Studies and Experimentation Centre of Public Works (CEDEX),

578

Spanish Institute for Cement and its Applications (IECA), Construction Technology

579

Institute of Catalonia (ITeC), Wineva (Arch. Ramon Sastre), and Computers and

580

Structures, Inc. (SAP2000). They also wish to express their gratitude to the Spanish

581

Ministry of Economy, Industry and Competitiveness for economic support through

582

project SAES (BIA2016-78742-C2-1-R).

583 584

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585 586

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587 588

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The prefabrication of Concrete deep foundations (CDFs) reduces environmental impact and increases the economic cost Increasing the compressive strength of concrete reduces environmental impact and cost Conducting Dynamic Load Tests on driven piles reduces environmental impact and cost CDFs with bored piles are environmentally better designed with Eurocode with UK annexes CDFs with driven piles are environmentally better designed with current Spanish codes