Durability of concrete structures in Bangladesh against some environmental related deterioration .

Presented by : Engr. Md. Arafat Hasan

A PRESENTATION ON

Structural Engineer at Parent Construction & Consultancy Ltd.

Introduction:

Basic definition of durability: “Durability is the ability of a

Material or Structure to withstand its design service

conditions for its design life without significant

deterioration.”

According to ACI 201.2R-08 “Durability of hydraulic-cement

concrete is determined by its ability to resist weathering action,

chemical attack, abrasion, or any other process of deterioration.”

Objectives :

Main objectives of our study are:

(1) to identify some common causes of deterioration of

concrete structure in Bangladesh;

(2) to find out the possible solution to make concrete more

durable to resist these common deterioration problems.

Durability problems of concrete structures in case of Bangladesh

Reinforced concrete structures have the potential to be very durable

and capable of withstanding a variety of adverse environmental

conditions. However, failures in the structures do still occur as a result

of premature reinforcement corrosion which are Durability problems related

to environmental causes deterioration. Some common causes of deterioration

of concrete structures in Bangladesh are (A) Carbonation induced corrosion

of steel bar, (B) Chloride induced corrosion, (C) Sulphate attack.

A. Carbonation induced corrosion of steel bar

It is a process by which CO2 from the air penetrates into

concrete and reacts with calcium hydroxide to form calcium

carbonates in presence of water [2]. Thus reduce pH value,

finally breaking the passivation film of concrete which leads to

corrosion of steel.

Chemical Reactions

The first reaction is in the pores where carbon di oxide (CO2) and water (H2O)

react to form carbonic acid (H2CO3):

CO2 + H2O H2CO3

carbonic acid then reacts with the calcium phases: 

H2CO3 + Ca (OH)2 CaCO3 + 2•H2O

Once the Ca(OH)2 has converted and is missing from the cement paste, hydrated CSH

(Calcium Silicate Hydrate - CaO•SiO2•H2O) will liberate CaO which will then also produce

carbonate: H2CO3 + CaO CaCO3 + H2O

Why it occurs

If clear cover depth is not sufficient carbonation induced

corrosion will occur shortly after construction.

Lack of Cover

Formation of a thin passive film at a high pH protects from corrosion.

The passive oxide film can be broken with the reduction of pH.

Steel bar in concrete.

Steel Bar

Why steel bars in Concrete are not Corroded?

Steel bar in atmosphere

No protection from corrosionConcrete

Carbonation-induced Corrosion

OHCaCOCOOHCa H23

022

2)(

CO2

pH in Concrete drops significantly. The passive film is lost. Corrosion continues depending on the moisture condition, oxygen permeability through the cover concrete, etc.

Moist Concrete

Carbonation Depth

Corrosion cells formation:

Rust has a lower density than steel. Hence, as corrosion takes place, the volume

increases and since the expansion is restrained by the surrounding concrete, bursting

stresses are induced, resulting in cracking, spalling or delamination of the concrete

cover.

Figure : Corrosion Cells formation After Carbonation Figure: Volumetric Expansion due to the Oxidation of Fe

EFFECT OF CARBONATION

It can cause soft surface, dusting and color change

It reduces quality concrete

It reduces the concrete ability to protect reinforcement from

corrosion (in an exposed environment).

It will result in additional shrinkage in carbonated region.

DETECTING CARBONATION

Depth of carbonation can be detected using an indicator.

A chemical such as Phenolphthalein sprayed on to freshly broken

concrete.

Areas remaining alkaline will turn in a bright purply-pink color.

Carbonated areas of concrete will remain unchanged in color.

CARBONATION TEST

Test Cylinder After Phenolphthalein Indicator Applied.

PREPARATION OF TEST SURFACE: By breaking a piece of concrete from the main area and spraying the underlying surface immediately

METHODOLOGY OF CARBONATION TEST (Phenolphthalein Test)

100ml of 1% Phenolthalein solution

Generally Phenolphthalein indicator is used which indicate the carbonated concrete. The uncolored layer is considered to be carbonated.

Field Investigation Method of Measuring Carbonation Depth :

By drilling a hole into the concrete either at a given depth or in small increments  at a constant speed throughout the operation. The first powder coming out is collected at the bottom of the picker and the last at its top

PHASES OF THE INVESTIGATION : PHASE 1 – CHOOSING A TEST POINT PHASE 2 – COLLECTING THE SAMPLE :

PHASE 3 – MEASURING HOLE AND SAMPLE :

Using the ruler with the graded scale, one establishes the exact length of the collected powder sample, and this is compared with the measurement of the actual depth of the hole in the item.

PHASE 4 – ANALYSIS WITH PHENOLPHTHALEIN :

PHASE 5 – MEASURING CARBONATION :

the graded scale ruler allows to measure to the millimeter the length of the carbonatated part, distinguishing it from the non degraded part. Comparing the measurement with the scale ratio established during Phase 3, one can calculate the actual value of the depth of the carbonation front at the point of the item being analyzed.

PHASE 6 – CLOSING THE HOLE :

At the end of the test, the hole made to carry out the collection is closed. Using the quick drying cement plaster.

Where it occurs

Carbonation mainly occurs preferentially on prism faces of calcium hydroxide crystals [3].

Also other hydration phases with greater surface

areas, such as calcium silicate hydrates, have

carbonated

How to prevent corrosion due to Carbonation

Use good quality concrete air-entrained with a w/c of 0.40 or less.

Use a minimum concrete cover of 1.5 inches .

Increase the minimum cover to 2 inches for deicing salt exposure and

to 2.5 inches for marine exposure.

Chloride enters the concrete from cement,water,aggregets and sometimes

from admixtures. Chloride attacks the reinforcement rather than Concrete.

Nothing happens to concrete in the beginning except the reduction in ph.

Chloride induced corrosion:

Chloride helps to destroy the passivation (protection) film over the steel bar that

prevent it from corrosion.If chloride level over the steel bar reach the threshold level

(0.4 % of cement or 1.2 kg/m3) the protection film will be broken.After broken down the

passivation film, corrosion over steel bar will start → Rust Formation (6 ~ 7 times

volume increase) → cracking → Spalling of concrete.

Progress of Corrosion

Passive Steel

Break Down of the Passive Film and Initiation of Corrosion

Cracking Spalling

Cl-, CO2,..

Control of chloride attack :

1.Extra concrete cover.2.Well curing .3.Impermebilty .

Sulfate attack is a chemical breakdown mechanism where sulfate ions attack components of the cement paste. The compounds responsible for sulfate attack are water-soluble sulfate-containing salts, such as alkali-earth (calcium, magnesium) and alkali (sodium, potassium) sulfates that are capable of chemically reacting with components of concrete.Delayed Ettringite Formation:Deleterious formation of ettringite.Late formation causes expansion distress.Expansion causes cracking –premature deterioration

3CaO.Al2O3.CaSO4.18H2O + Sulfate → 3CaO. Al2O3. 3CaSO4.32H2O (monosulfate) (Ettringite)

o If concentration of sulfate is high, the sulfate will consume CSH of concrete and

will reduce the strength of concrete significantly.

(i) MgSO4 + Ca(OH)2 + 3CaO.Al2O3.CaSO4. 18 H2O → 3CaO.Al2O3. 3CaSO4.32H2O + Mg(OH)2

(Ettringite) (Brucite)

↓

volume expansion and hence crack and strength reduction

Main factors affecting sulphate attack:1.Cement type and content:The most important mineralogical phases of cement that affect the intensity of sulphate attack are: C3A, C3S/C2S ratio and C4AF.2.Effect of w/c ratio :

Control of sulfate attack:

The quality of concrete, specifically a low permeability, is the best

protection against sulfate attack.

Adequate concrete thickness

High cement content

Low w/c ratio

Proper compaction and curing

Conclusions: So in sum up we found there are two fundamental ways to make concrete durable by addressing the properties of the concrete like proper mix design with low w/c ratio, etc. Second by providing protective systems external to the concrete surface and by providing enough cover of concrete on reinforcement.

 

References: [1]. ACI 201.2R-08, Guide to Durable Concrete.

[2]. Notes on concrete durability by DR. M NAGESH. Professor and Head Civil Engg. Dept. Government Engineering College.

[3]. Journal on Carbonation as an Indicator of Crack Age by ACI member Dipayan Jana and Bernard.4. Carbonation of concrete by DR. AYUB ELAHI professor of UNIVERSITY OF Engineering and Technology Taxila.5. Durability of Reinforced Concrete Structures, Theory vs Practice by Albert K.H. Kwan and Henry H.C. Wong Department of Civil Engineering, The University of Hong Kong, Hong Kong.6. PPT on Concrete Durability Design by Donald Meinheit, WJE (retired) on Minnesota Concrete Council .