Specification faced with durability to meet superior level of performance in structural elements buried

ABSTRACT

The performance standard NBR 15575-1: 2013 [1] states that to achieve the higher level of performance for durability, buried concrete structures must achieve a minimum service life (VUP) of 75 years. However, the NBR 6118:2014 [2] does not specify the minimum project properties for the structure fulfill this VUP. The objective of this study is to provide recommendations for structural designers to meet higher performance requirements of NBR 15575:2013 [1] in durability. It was conducted a research of national and international standards and reference publications about the durability and VUP 75 years for structural elements buried. The paper was divided into three parts: (1°) the analysis of the Brazilian and international legal system, as well as reference publications in the area; (2°) discussion of service life prediction models; and (3°) specifications to meet the superior level of durability of structural elements buried.

Keywords:
performance standard; superior performance; service life; burried structures

RESUMO

A ABNT NBR 15575-1:2013 [1] estabelece que, para atingir o nível de desempenho superior em termos de durabilidade, as estruturas de concreto armado enterradas devem atender a uma vida útil de projeto (VUP) mínima de 75 anos. Contudo, a norma ABNT NBR 6118:2014 [2] não apresenta recomendações de projeto para que esta VUP seja cumprida. O objetivo deste trabalho é determinar alguns parâmetros projetuais de durabilidade para estas estruturas, visando atender ao requisito de desempenho superior da norma. Para tanto, realizou-se uma pesquisa em normas nacionais e internacionais, além de publicações que remetam a especificações de projeto para vida útil de 75 anos para elementos estruturais enterrados. O trabalho dividiu-se em três partes: (1º) a análise do sistema normativo (nacional e internacional) e publicações referência da área; (2º) discussão de modelos teóricos de previsão de vida útil; e (3º) definição das especificações de projeto para elementos estruturais enterrados para o nível superior de desempenho.

Palavras-chave:
norma de desempenho; desempenho superior; vida útil de projeto; estruturas enterradas

1. Introduction

According to the performance standard for residential buildings, ABNT NBR 15575-1:2013[¹], a structural system must remain safe, stable and efficient throughout its intended service life. Its constituents must be designed and built so that, under pre-established use conditions, they keep their functional capability throughout its intended service life, which is 75 years for superior performance.

ABNT NBR 15575-1:2013[¹] provides that the method for assessing the durability of a structure can be to determine whether it complies with the requirements set forth in applicable Brazilian standards or, in the absence thereof, specific international standards addressing the matter.

The purpose of this paper is to provide design recommendations for durability, which may be adopted to size and verify the elements in contact with the ground - such as foundations - in order to achieve the ABNT NBR 15575:2013 [¹] superior performance level requirement. In such environments, major aggressive agents are chloride ions, sulfate and humidity.

However, the Brazilian standards for structural design and structural system durability mentioned in ABNT NBR 15575-1:2013 [¹], ABNT NBR 6118:2014 [²] and ABNT NBR 12655:2015 [³] do not provide recommendations on how to achieve the durability requirement for buried elements, particularly superior service life. Thus, other references should be sought to meet this purpose, both normative and non-normative.

Therefore, this study was carried out in three stages: (1) analysis of the national and international normative system; (2) review

Table 1
Service life for minimum, intermediate and superior levels for residential structures

• Source: ABNT NBR 15575-1:2013, edited by the authors

Table 2
Environmental aggressiveness classes

• Source: ABNT NBR 6118:2014

of theoretical models for service life prediction; and (3) definition and proposal of design recommendations for buried structural elements to meet the service life requirement of 75 years. In the first stage, the applicable Brazilian and international standards are presented, as well as relevant technical publications addressing the minimum design requirements to be in compliance with the standard. In the second stage, these normative design parameters are tested through theoretical models. In the third stage, specific design specifications are recommended, integrating that used as reference and the results of the service life prediction models.

2. Analysis of the brazilian and international normative system

2.1 Brazilian standards

ABNT NBR 15575-1:2013 [¹] provides that the design of service life for structural elements of residential buildings must be 50, 63 or 75 years to meet the minimum, intermediate and superior durability levels, respectively, as shown in Table 1.

In reinforced concrete structure designs, environmental aggressiveness should be classified according to the criteria used by ABNT NBR 6118:2014 [²], which stipulates that these structures must be

Table 3
Correspondence between environmental aggressiveness class and nominal coverings thickness

• Source: ABNT NBR 6118:2014

Table 4
Correspondence between environmental aggressiveness class and concrete quality, for reinforced concrete structures

• Source: ABNT NBR 12655:2015

Table 5
Requirements for exposed concrete of sulfates solutions

• ^*
  Low w/c ratio or high strength could be necessary to obtain low concrete permeability or steel corrosion protection or freezing and thawing; **See water; ***For severe aggressiveness is necessary to use sulfate resistant cements.
• Source: ABNT NBR 12655:2015

designed and built so that, under the environmental conditions established at the time of design, they remain safe, stable and efficient throughout its intended service life. Table 2 shows the criteria for environmental aggressiveness classification proposed by this standard.

Depending on the environmental classification, ABNT NBR 6118:2014 [²] presents recommendations regarding reinforcement cover thickness. This cover thickness is intended to ensure physical, chemical and mechanical protection of the reinforcement, as shown in Table 3.

According ABNT NBR 12655:2015[³], when the requirements provided in ABNT NBR 6118:2014 [2] are met, structural durability will depend on the characteristics of the concrete constituents. Therefore, this standard defines the maximum water/cement ratio, minimum compressive strength, and minimum cement consumption, as shown in Table 4. It also shows the ground aggressiveness classification depending on the amount of water-soluble sulfate (SO4) in soil, as shown in Table 5.

The recommendations provided by these standards are assumed to be for a service life of 50 years. ABNT NBR 8681:2004 [⁴] determines that for actions that vary over time (variable actions) a reference period is required for them to be equaled or overcome. These values were defined by consensus, having a 25%-35% probability that they will be exceeded during 50 years.

Table 6
Recommendations for high severe aggressiveness zones

• Source: Indian Standard IS -456, adapted by the authors

2.2 International standards and reference literature

According to ^{Tutikian, Isaía e Helene (2011}) [⁵], the durability of concrete structures depends on both extrinsic factors (presence of salts, sea spray, acid rain) and intrinsic factors (cement type, water/cement ratio, additives additions, etc.). This paper focused on the assessment of intrinsic factors required to have a service life of 75 years. The intrinsic factors recommended by the national and international normative system for durability are (a) minimum cement consumption; (b) minimum reinforcement cover thickness;

Table 7
Specifications to attend service life of 100 years in buried reinforced or pretensed concrete elements

• Source: adapted from BS 8500-1:2006

and (c) maximum water/cement ratio. Concrete compressive strength is also mentioned but, as it is related to water/cement ratio, it does not require further specification. The type of cement used could also affect the durability of these structures by chemical deterioration, but it is rarely described in the standards.

2.2.1 Minimum cement consumption

Cement consumption is often related to parameters such as mechanical strength, durability, fluidity, setting time, and others. According to BRE (2005) [⁶], penetration of sulfate ions occurs in porous alkaline solutions resulting from cement hydration, leading to damage such as expansion and cracking. By using sulfate-resistant cement, the level of tricalcium aluminate is kept at a minimum, thus reducing the impact of such reaction on the structure.

Regarding structures embedded in sulfate environments, the recommendations provided in IS 456:2000 [⁷] should be noted. This regulation has a qualitative classification by zone of potentially aggressive environments to concrete structures. The Indian standard mentions buried structures, classifying them under aggressiveness zone number 4. For structures in these circumstances, the standard determines that minimum cement consumption should be 370 kg/m³, as shown in Table 6.

BS EN 206-1:2006 [⁸] recommendations for cement consumption are similar to those provided by IS 456:2000 [7]. Similarly to Brazilian regulations, these standards do not have a description about the service life used. For both ACI 318-11:2002 [⁹] and ABNT NBR 6118:2014 [²], service life is assumed to be 50 years, based on the time of return of the design variable actions. These recommendations are more conservative than those adopted by ABNT NBR 6118:2014 [²] for buried structural elements.

2.2.2 Nominal cover to reinforcement

Concrete cover is used to provide the reinforcement with chemical and physical protection in order to ensure that the superior performance level is obtained, that is, a minimum service life of 75 years.

Figure 1
Effect of w/c ratio in chloride penetration (Source: JAEGERMANN, 1990 apud POLITO, 2006)

Table 8
ACEC classification for natural soils

• Source: adapted from BRE Special Digest 1:2005

Table 9
DC class and APM number for concrete elements in zones with hydraulic gradient less than or equal to 5: concrete for general use in situ

• Source: adapted from BRE Special Digest 1:2005

Table 10
Concrete quality for resistance to chemical attack: concrete for general use in situ

• Source: adopted from BRE Special Digest 1:2005

Table 3, taken from ABNT NBR 6118:2014 [²], specifies the minimum reinforcement covers for a service life of 50 years; for more strict criteria, additional information is required.

According to Neville & Brooks [¹⁰], it is possible for structures to have a service life of 100 years if specific nominal covers are adopted based on the type and exposure condition of structures, in addition to the concrete strength classification. This is the principle adopted in ABNT NBR 6118:2014 [²] and ABNT NBR 12655:2015 [³]. Within this context, Table 7 shows the BS 8500-1:2006 [11] specifications for a service life of 100 years, focusing on chloride ion attack.

Thus, for wet and rarely dry conditions in environments potentially prone to corrosion by chloride, reinforcement covers should be at least 55 mm, with of 40 MPa, maximum water/cement ration of 0.40, and minimum cement consumption of 380 kg/m3. BS 8500-1:2006 [¹¹] uses concrete cover, compressive strength, water/cement ratio and cement consumption to ensure structural durability, similarly to ABNT NBR 6118:2014 [²], ABNT NBR 12655:2015 [³] and other international standards.

2.2.3 Water/cement ratio

As the thickness of reinforcement covers, maximum water/cement ratio will also depend on environmental aggressiveness. In order to determine the ratio for a service life of 75 years for buried structures, attack by chloride and sulfate should be considered, as well as soil humidity.

According to CCAA (2009) [¹²] and ^{Figueiredo (2011}) [¹³], concrete resistance against chloride attack depends on the material porosity, particularly size, distribution and interconnectivity of paste pores. This variable is directly proportional to the water/cement ratio. Figure 1 shows the effect of such variable on chloride ion penetration (^{POLITO, 2006} [¹⁴]). Table 8 shows the BS 8500-1:2006[¹¹] specification for maximum water/cement ratio allowed in designs for areas having this type of environmental aggressiveness.

ACI 318-11:2002 [⁹], similarly to Brazilian standards, does not discriminate the intended service life by adopting design parameters (it is assumed to be 50 years), according to ASCE 7-05 (2005) [¹⁵]. However, similarly to the Brazilian standard, ACI 318-11:2002[⁹] adopts an environmental classification based on aggressiveness. Considering a structure exposed to humidity and external chloride sources (class 2), a maximum water/cement ration of 0.4 is recommended, consistent with the BS 8500-1:2006 [¹¹] recommendation.

Another hypothesis to be considered when analyzing the durability of buried structures is attack by sulfates in the ground, particularly in industrial areas. ^{Mehta & Monteiro (2014}) [¹⁶] note that magnesium sulfate, sodium and potassium are commonly found in groundwater.

According to ^{Isaia (2011}) [¹⁷], sulfate attack occurs by chemical reaction of the sulfate ion with the aluminate components of hardened concrete. ^{Mehta & Monteiro (2014}) [¹⁶] argue that concrete degradation can occur as expansion and cracking. Concrete expansion can cause cracking, which increases concrete permeability, facilitating the penetration of other aggressive agents in the structural element.

BRE (2005) [⁶] proposes a methodology to select sulfate-resistant concrete. The starting point is to determine the ACEC (Aggressive Chemical Environment for Concrete) class of the ground, as shown in Table 9. This classification depends on the total potential for sulfate and on the pH of the groundwater. After that it is possible to determine the DC (Design Chemical) class of concrete for a service life of 100 years (Table 10Erro! A origem da referência não foi encontrada.). With this it is possible to determine the maximum water/cement ratio, cement type and consumption (Table 9).

For comparison purposes with the water/cement ratio recommendations for chloride attack, the ground aggressiveness classification is considered to be AC-5, the most severe level condition of exposure to sulfates. In this class, for a service life of 100 years the concrete class is DC-4. Accordingly, a water/cement ratio of 0.35, 0.4 or 0.45 is recommended (values associated with other concrete characteristics).

Table 11
Classification of concrete strength against the risk of reinforcement corrosion

• Source: ^{Medeiros, Andrade e Helene, 2011}

2.2.4 Cement type

ABNT NBR 5736:1999 [¹⁸] allows adding up to 50% of pozzolan to clinker, calcium sulfate and carbonate material to produce CPIV. Pozzolanic cement, due to the reaction of the pozzolan with the calcium hydroxide produced by cement hydration, has increased durability, because the calcium hydroxide, which is highly soluble and leachable, is combined as hydrated calcium silicate, more resistant to aggressive agents. The same occurs with CPIII, which, according to ABNT NBR 5735:1991 [¹⁹], is a mixture of clinker, calcium sulfate and carbonate material with 30%-70% of blast-furnace slag.

These types o cement are therefore recommended for applications exposed to flowing water and aggressive environments. They are less permeable, more durable, having higher long-age compressive strength as compared to concrete produced from Portland cement with lower levels of additions. They have lower heat of hydration during the setting period because their reactions are slower, being recommended for high volume applications such as foundation blocks, for example.

Also, the alkali-aggregate reaction, which can cause expansion of the finished structure, is mitigated by adding pozzolan, according to ABNT NBR 15577:2008 [²⁰]. ^{Nagesh (2012}) [²¹] states that application of this type of cement is recommended for the production of structural elements potentially prone to this kind of attack, such as those in contact with the ground.

ABNT NBR 6118:2014 [²] defines carbon dioxide and chloride ions as the main aggressive agents for concrete structures, and they should be anticipated and prevented in the design and use stage of the building. This standard also refers to attack by leaching processes and expansion reactions, such as that caused by alkali-aggregate and sulfates. In the absence of experimental test values, ^{Medeiros, Andrade e Helene (2011}) [²²] recommend adopting a guiding specification, as shown in Table 11 and Table 12, using pozzolanic additives.

Therefore, in order to analyze the durability of buried structures in terms of design, the chemical composition (type) and minimum cement consumption should be assessed, among other factors.

2.3 Service life prediction models

Theoretical models for service life prediction are useful to understand the propagation rate of aggressive agents in concrete and to determine the required reinforcement cover and other means of protection, such as reduced water/cement ratio, argue Bolina e Tutikian [²³]. Interpreting these theoretical models, which are usually developed from experimental tests, helps to understand the rate of chemical deterioration of concrete constituents.

To determine the durability of concrete structures, service life prediction models must be analyzed for chloride ion penetration, which will basically depend on the material porosity and reinforcement cover, in the worst scenario, as penetration is rather quick. The most adequate theoretical models are by ^{Helene (1993}) [²⁴] and ^{Bob (1996}) [²⁵].

2.3.1 ^{Helene (1993}) [24]

This model determines the penetration depth of chloride ion according to equation 1.

Where t is time, in years, and k is the chloride ion diffusion coefficient. ^{Helene (1993}) [²⁴] suggests that chloride concentration in a structural element should be expressed in relation to the cement mass (), directly influenced by the chloride concentration in the environment . This correlation is expressed in equation 2.

Table 12
Classification of concrete strength against the risk of deterioration by leaching or by formation of expansive compounds

• Source: ^{Medeiros, Andrade e Helene, 2011}

Where is cement consumption, in kg/m³, Ab is maximum water absorption in concrete, expressed in %, and ɣ is the bulk density of concrete, in kg/m³.

2.3.2 ^{Bob (1996}) [²⁵]

After verifications based on long-term experiments data, ^{Bob (1996}) [26] observed that the modeling of chloride penetration could be expressed by equation 3.

Where xm is the average chloride penetration depth, in millimeters, fc is the characteristic compressive strength of concrete, in MPa, c is the fixing ability of chlorines, k1 is the influence of temperature on the model, k2 is the influence of humidity and d is the ratio between critical concentration and surface concentration of chlorides in the structure.

3. Results and discussion

Based on international standards and reference publications in the area, minimum parameters were proposed to ensure a service life of 75 years for buried structures, reaching the superior level of the Performance Standard. These parameters build on standard recommendations that provide specifications for this kind of structures, such as ACI 318-1:2002 [⁹], EN 206-1:2000 [⁸], IS 456:2000 [⁷],

Table 13
Service life prediction model for elements subjected to severe aggression by chlorides, according (a) ^{Helene (1993}) [²⁴] e (b) ^{Bob (1996}) [²⁵]

ABNT NBR 15577-1 [²⁶] and BS 8500-1:2006 [¹¹]. These durability parameters, similarly to ABNT NBR 6118:2014[²], are minimum compressive strength, maximum water/cement ratio, minimum cement

Table 14
Recommendations for specification of concrete to meet service life of 75 years for buried structures

consumption, cement type and nominal reinforcement cover.

From these prediction models design parameters were derived to yield a service life of 75 years for buried structures in slightly aggressive, moderately aggressive and highly aggressive environments, as shown in Table 13. The models of ^{Helene (1993}) [²⁴] and ^{Bob (1996}) [²⁵] were assessed to find an average value. As these models represent chloride ion attack, a critical value was obtained for buried elements.

Table 14 shows the design properties to meet such requirement, where all parameters are numerically supported by and in line with national and international references.

Specifications for water/cement ratio followed BS 8500-1:2006 [¹¹]. This standard also provides for variations of water/cement ratio and cover, which was not adopted in this paper.

It was used the aggressiveness classification of Brazilian standards, such as ABNT NBR 12655:2015 [³], and international standards, such as ACI 318-11:2002 [⁹], EN 206-1:2000 [⁸] e IS 456:2000 [⁷], where environments are categorized as slightly aggressive, moderately aggressive and severely aggressive. It should be noted that for a slightly aggressive class a nominal cover of 45 mm is required, which, according to service life prediction models, will protect the structure for 75 years. If the structure is located in a moderately aggressive environment, nominal cover should be 55 mm. And for severely aggressive environments the specification if for nominal cover of 70 mm. Use of fibers or cover reinforcement is recommended when cover exceeds 50 mm (IS 456:2000 [⁷]).

4. Conclusions

ABNT NBR 15575-2:2013 [¹] provides unprecedented durability parameters, not yet seen in Brazil. As it is a performance standard, it is based on prescriptive rules of the Brazilian normative system; however, it provides no recommendation as to how to comply with the higher performance requirement. For instance, there is a gap on how to achieve superior performance for buried reinforced concrete structures, such as foundation blocks and stakes. In order to fill this gap, a review of the existing standards in the world and other relevant publications was carried out to support the decision-making. Confirmation through theoretical models for service life prediction was required to validate the comparison. It was concluded that to ensure a service life of 75 years, a buried concrete structure can be made of cement with additions, providing increased durability when in contact with contaminated areas or materials, thus preventing propagation of a potential alkali-aggregate reaction. Concrete dosage parameters were also determined in relation to compressive strength, water/cement ratio and cement consumption, in addition to nominal cover to steel bars, to ensure a service life of 75 years, superior level of the Performance standard.

References

• ¹
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• ²
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• ³
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• ⁵
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Publication Dates

• Publication in this collection
  Apr 2016

History

• Received
  09 May 2015
• Accepted
  27 Jan 2016