Normativa de Durabilidad Del ACI y Su Impacto en La Normativa Latinoamericana (E)

7/31/2019 Normativa de Durabilidad Del ACI y Su Impacto en La Normativa Latinoamericana (E)

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ACI Durability Standards and its impact in Latin America

Summary

The recommendations stated in ACI-318 have been for many years the base for the standarization

in most of the Latin American countries. In particular, many structural design codes have adopted

the recommendations of this important document. By doing so, they have adopted, without a deepanalysis of its convenience, the durability recommendations presented in the northamerican

document, where some of the phenomena are specific for countries with marked seasons. This has

two inmediate effects: ACIs recommendations have no application in the tropics, and secondly,

common phenomena in the tropics do not get covered in the recommendations, which makes the

document meaningless. It is a must that when adopting foreign standards, material engineers be

invited to the discussion to establish any necessary actualization of the local standards with the aim

of adapting them to the specific conditions of the point of use.

Chapter 4 of the ACI-318-08

In several latin american countries the latest update of the referenced north american code was

been expected inpatiantly, as based on it the local codes would be updated (for example the NSR-10 in Colombia). Even for the material engineers, who considered that chapter 4 Durability

requirements from ACI-318-05 was quite poor when applied its recommendations to tropical

environments, the changes greatly announced, predicted a new way of confronting the durability

specifications of structures in Latinamerica.

The surprise was major as when analyzing the new material the only thing that was found was a

different way of saying the same thing, and with the exception of a few valuable contributions, which

will be mentioned later, instead of a progress in durability specifications actually the contrary was

presented. Enough to just mention the introduction of the 17.5 MPa strength for some

environments and exposure conditions, which does not make sense when the intention is to specify

durability. To point out that in Europe, a vulnerable concretepresents strengths between 10 and 20

MPa, as shown in tables 1 and 2.

% of pozz. addition % of pozz. addition

Durable 50 MPa 10% pozzolan, Silica

Fume or slag

10% pozzolan or

Silica Fume or slag

65% slag

Resistant 35-45 MPa

10% pozzolan, Silica

Fume


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C3A in anhydrous

CementPozzolans (%) Pozzolans (%)

Durable 50 MPa 5%

20% pozzolan or

Silica Fume or slag

65% slag

20% pozzolan or

Silica Fume or slag

65% slag

Resistant 35-45 MPa 5%

10% pozzolan or

Silica Fume

35% slag

10% pozzolan or

Silica Fume

35% slag

Normal 25-30 MPa 8% No restriction No restriction

Attackable 10-20 MPa No restriction No restriction No restriction

Concrete

Class

Strength Level

Damage caused by

leakageDamage caused by expansion

Table 2. Classification of concretes with regard to the risk of deterioration due to expansion or

lixiviation.

On the other hand, being rigorous, it is necessary to clarify that the durability specifications by the

ACI consider Northamerican conditions, with special climatic conditions and for the most common

and serious structural attacks in that country, and not for other climatic conditions or other type of

attacks more common in other countries. It cannot be expected that the ACI legislates for us. Theproblem lays with whoever adopts a foreign standard without the adequate analysis done by

experts in materials and durability, where the need of a possible aclimatization or adaptation of a

chapter gets defined, before putting it in use. It is also important to mention that in some countries

these standards become local law, which makes its application mandatory and hence the

importance that they are adequate for the local conditions, that they are applicable.

It seems that in all latitudes there exists a division within the engineering between the experts in

structural design and the experts in materials. Both have the same desire: to obtain resistant and

durable structures. But there is nobody working to unite their efforts for a common target, and it is

actualy rare that the structural experts invite the materials experts when discussing durability and

when minimum requirements are being established to face the different types of exposition.

Attacks suffered by reinforced concrete

Let us observe in a graphical way the main internationally recognized attacks undergone by

reinforced concrete structures:

Electrochemical

Chlorides

Carbonation

Sulfates

Freezing and ThawingPhysical

Permeability(H

umidity)

pH(water, soil)

CO2,Ammonia,Magnesium

ASR

Dry Residue

Chemicals

Type of Attack

Abrasion


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Figure 1: Main attacks undergone by reinforced concrete structures

As can be seen, attacks can be group in those of electrochemical nature that lead to the well-known

phenomena of corrosion, which has chemical and electrical components, those of net chemical

origin and those of physical origin.

Reviewing the types of exposition presented in chapter 4 of the ACI-318-08 standard and adjusting

the previous outline, some coincide while others do not; let us see:

Figure 2: Types of exposure as from chapter 4 from the ACI-318-08 standard

A brief evaluation of this diagram immediately shows some differences. First, the ACI standard

does not mention the effect of the attack by carbon dioxide, worldwide recognized as causing

carbonation and responsible of the depassivation of the reinforcement in environments where the

corrosion is not originated by salts. This includes all inland cities away from shores, industrial

centers, and in any place where no deicing salts are used, like in our tropics..

When analyzing the type of exposure C from chapter 4 of the ACI-318-08 standard (Reinforced and

prestressed concrete exposed to conditions that require special protection against corrosion), we

realize that it only covers the attack by chlorides as shown in table 3. Excluding the attack due to

carbonation, because it is less severe, would be like excluding shear stresses and consider only

flexural ones.

A second important difference is with regard to moisture. In many of the phenomena that cause

deterioration of reinforced concrete structures, water is the vehicle that transports the ions (case of

chlorides and sulfates), or it is an element without which the phenomena does not occur (corrosion,

reactivity alkali-aggregate, expansion by sulfates, and even deterioration due to freeze-thaw). In

figure 1, moisture appears covering all the common attacks, like a participating factor, and also as

an essential element in some of them. And the goal of showing it as such is to make it clear that in

order to produce structures resistant to many of these attacks, watertight or low-permeability

structures are the first factor in order to obtain the required or specified service life.

The same does not happen with the ACI-318-08 standard, where a separate category (P) of

structures that would require to we watertight or not, which would lead a inexperienced specifier to


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the conclusion that the other exposure conditions (freeze-thaw, chlorides, and sulfates for example)

do not require a concrete resistant to the transport of water (in other words, impermeable).

Now, reviewing the minimum requirements of the concrete for the type (P) exposure, the only

specifications are the water/cement ratio and the strength, ignoring other factors not less important

when producing a watertight concrete, like the use of reactive pozzolans (fly ash, slag and

microsilica), and the air entrainment of the mix.

Something similar happens with the type of exposure (C). Even though the use of pozzolanic

additions is recommended, minimum addition levels are not specified, wasting this way the

enormous power of these materials in the elaboration of concretes of low permeability to chlorides,

and only water/cement and strengths are specified. Here it is important to remember that a series

of mixes with the same water/cement ratio, and the same strength, but with different type of binder

systems can produce very different results with respect to the passage of chloride ions, for example

in the RCPT test (Rapid Chloride Permeability Test), governed by the ASTM C-1202-97 standard,

and which measures the permeability towards chloride ions as a function of the charge in coulombs

that passes through the specimen, as can be seen in Figure 3.

Chargepassed(Coulomubs)

Water/cementious material

Figure 3. Results obtained with the RCPT for different mix designs with and without pozzolanic

additions (2).

Graph 3 shows not only the beneficial influence of pozzolanic additions when aiming for a concrete

with lower chloride permeability, but also differences in performance between the available

additions, and even that a ternary mix (cement + addition 1 + addition 2) can perform even better.

This is the case of mixes, for example with a blend of cement, microsilica, and slag used as binder.

It is important to clarify that a low permeability to chlorides is obtained when the passing charge isless than 2000 coulombs.

This situation shows once again, the urgent need to start specifying performance when discussing

durability of structures.

Unfortunately, while the ACI does not take the initiative, and as in Europe, the specifiers of large

projects stop including mix designs (which with the variety of cements and the usual moderate

strength values of many projects, may or may not work) and start including performance values of


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the concrete mixes that pretend to classify for a given project, not much will change. A decision of

this type would be beneficial for both sides, as it would not only force the concrete producer to

evaluate the performance of their products against common aggressors, but it would allow to

extend the service life of infrastructure projects, which in many of our countries are already being

subjected to rehabilitation processes even before 20 years of service, situation which is further

negatively impacted due to the non-existence of project service-life requirementsin the majority of

important infrastructure projects in developing countries, like ours.

Current situation of latin american countries with regard to durability standardization

Those countries that follow literally the recommendations of chapter 4 of the ACI-318-08 standard

will have an curious situation when trying to specify the durability requirements of structures away

from marine environments, in particular when trying to extend the durability by stopping corrosion

processes.

A quick look to the types of exposure for an important structure located inland, in the tropics (for

example a building with exposed concrete) would give the results shown in table 3.

ExposureCategorie

Description Exposure Class Severity

C Corrosion C0, C1 N/A, Moderate

S Sulfates S0 N/A

F Freezing + Thawing F0 N/A

P Permeability P0 N/A

Table 3: Types of exposure applicable as by ACI-318-08 for a structure with exposed

concrete in inland regions.

It is at least disturbing that the majority of the inland cities in many of our tropical countries, and

even some beyond where winter is moderate and where there is no freezing, would have, based on

the classification given in chapter 4 of the ACI-318-08 standard with a exposure class ceroor

maximum one, when analyzed based on exposure class (C) that judges the risk of corrosion.

The result can be anticipated: the minimum requirements applicable to the concrete are really

minimal. Minimal strength of 17.5 Mpa, any water/cement ratio (NA), additional requirements:

none! It is important to remember that a concrete with a compressive strength of 17.5 Mpa has a

water/cement ratio of the order of 0.8, which is a real sieve. Its porosity can easily reach 25%. And

we ask ourselves, how can then, a structure with such requirements be durable, if we are practicly

giving the contractor the freedom to do whatever he wants?

What we expected to see in chapter 4 of the ACI-318-08 standard

Some thoughts have been entertaining researchers in structural durability, which they would like tosee reflected in a next, not update but rework of the chapter 4 of the ACI-318-08 standard.

To redefine the calculation of the water/binder ratio. It is not possible to keep adding

cement plus additions in the denominator (see 4.1.1 of the ACI Code), unless we are

convinced that 1 kg of fly ash or slag is equivalent to 1 kg of Portland cement. The

europeans calculate the water/binder ratio as:

w/cm = water/ (cement+ kslag* slag weight + kfly Ash* Fly Ash weight)


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Where K is the equivalency factor. While the k of the Fly ash and the slag vary between 0.4

and 0.7, the k of microsilica is of the order of 2.

Prohibit once and for all the use of chloridecontaining admixtures. All admixture

manufacturers already have accelerators innocuous to the reinforcing steel of the

structures. Europe prohibited the use of chlorides over 20 years ago.

To include carbonationas an depassivating element of the reinforcing steel and to generateminimum requirements to face it, which could start by limiting the use of reactive pozzolanic

additions to 10% in concretes where the expected critical aggressor is carbon dioxide from

the atmosphere. Bridges, silos, large structures with exposed concrete, etc., located in

large cities or in industrial sites can be typical examples of this clase of exposure.

To promote the use of larger cement quantitiesrequiring larger strengths. The large

majority of the concretes placed in structures in our latin american countries correspond to

the 21 Mpa class. Currently they are manufactured with binder contents of 240 kg (cement

+ additions), based on the desire of the manufacturers to obtain one MPa of strength for

each 10 kg of binder. A mix without cement is an anemic mix, with reference to

durability, even when it complies with the required strengths level. Let us give the cement

back to the concrete as it really benefits it.

Contribution of chapter C.4 of the ACI-318-08 standard

Next, some progress of the new chapter 4 of theACI-318-08 standard that have to be

recognized:

Due to the unavailability of type V cement in some markets, the use of pozzolanic additions

is stipulated to make a concrete resistant to sulfates, which uses them in certain dosages,

besides a low water/cement ratio and high strengths. Typically levels of microsilica

between 7 and 10%, or fly ash or slag around 30% are specified. It is important to note

that the nature of the slag can result determining of a good or bad result in the control of

expansion by sulfates. On the other hand, the recommendation of controlling the attack of

sulfates by using reactive pozzolans results positive as it also controls the expansion

caused by the Thaumasite, which can not be controlled by managing the C3A content of

the cement.

The inclusion of the table that relates the maximum expansions and the ages at which the

expansions are evaluated when pozzolanic additions are used instead of using low C3A

cements to control the expansion by sulfates, is quite useful.

Table 4. Expansion limits when puzzolanicadditions are used to control the attack by

sulfates


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BIBLIOGRAPHY

1. Helene, Paulo Roberto Do Lago, Proyectar para la durabilidad, publicacin del ACI,

Captulo Colombia, 2001.

2. Howland, Juan Jos, Tecnologa de Hormigonado y Durabilidad, La Habana, 2005.3. ACI 318-08 Cap 4 Durability Requirements

Author

Carlos Arcila Lpez

Civil Engineer from the National University of Colombia. Specialization courses in Rehabilitation and

Structural Pathology in Instituto Eduardo Torroja (Spain). 25 years of experience in the field of concrete,

particularly in the area of concrete structures durability. Member of the Colombian Concrete and Mortar

Standardization Committee. Cathedra Professor at the Civil Engineering Department of Universidad

Javeriana. He has work as consultant in the survey of more than 300 structures in Colombia and

Caribbean countries. His principal research activities involve high performance concrete, durability

specifications and the concrete corrosion and its control.

[email protected]
mailto:[email protected]:[email protected]

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Normativa de Durabilidad Del ACI y Su Impacto en La Normativa Latinoamericana (E)