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Materials of Construction-Concrete 1

Concrete

Chapter 11

Durability of Concrete

&

Mix Design

Wikipedia.org

Durability of hardened

concrete



Leaching and efflorescence

‘Leaching’ is the dissolving out of the

calcium hydroxide (and various salts)

which take place in hardened concrete

under the effect of percolating water.

Cement + water C-S-H + Ca(OH)2

Calcium-silicate-

hydrate is the main

hydration product

and it is essentially

insoluble.

Calcium hydroxide is

another hydration

product and it is not

resistant to the

dissolving effect of

water.


Leaching and efflorescence

Water from various sources (rain water,

melting snow, industrial waters, ground

water) may penetrate the concrete and lead

to dissolving of the calcium hydroxide.

When leaching of the Ca(OH)2 and other salts (such as the sulfates and

carbonates of sodium, potassium or calcium) occurs, the water that

contains the dissolved materials moves upward by capillary action.

When this water evaporates, a salt solution, usually white, is formed on

the surface of the concrete.

The salt deposite formed on the surface of the concrete is called

‘efflorescence’.

This causes an aesthetic problem.


Sulfate attack

Sulfates are often present in

groundwaters (when high potions of clay are present in the soil)

sea waters

rain water (from air pollution)

sewage water (because of biological growths)

When sulfate-containing waters seep into hardened concrete:

1. Gypsum is formed deu to the reaction of the sulfates with the calcium-

hydroxide in the structure of the hardened cement paste.

2. Ettringite (C6AS3H32) is formed deu to the reaction of the gypsum with

the calcium-alumino-monosulfohydrate that is present within the

hardened cement paste.


Sulfate attack

The formation of ettringite in

the hardened cement paste

or concrete leads to very

large volume expansion, and

generates accompanying

internal stresses leading to

cracking.


Sulfate attack (Recommended precautions)

Using low water/cement ratio A lower w/c ratio will decrease the the penetration of sulfate-containing

waters into the concrete.

Using the proper type of cement The cement types containing relaively smaller amount of C2S and C3A

should be used! (Because C2S produces high amount of calcium-

hydroxide and C3A produces high amount of ettringite among the other

compounds of cement!)

The use of portland-pozzolan or slag cements is highly recommended.

(Because the calcium-hydroxide in these cements is decreased by its

taking part in the pozzolanic reaction.

Using finely divided puzolanic admixtures Think about the reason !!!!


Sulfate attack

ACI : American Concrete Institute

www.aci.org

Carbonation


Carbondioxide (CO2) is present in the atmosphere: about 0.03 % in rural

air, 0.3 % in large cities.

When concrete is exposed to the atmosphere (or when groundwaters

that contain some carbon dioxide seep into concrete) a reaction takes

place between the carbondioxide and the calcium hydroxide of the

hydrated cement paste leading to the formation of CaCO3.

Carbonation is a slow process.

It usually starts on the surface of the concrete and proceeds toward the

inner portions.

The concrete that is within approximately 2.5 – 3 cm of the surface is

under the effect of carbonation.


Carbonation

When carbonation occurs, concrete loses some of its

calcium hydroxide and water. Therefore carbonation is

accompanied by shrinkage of the concrete. This type of

shrinkage is called carbonation shrinkage.

Since the calcium hydroxide of the concrete present near

the surface is reduced by carbonation, the alkalinity of the

concrete in those carbonated sections is reduced. In this

way, carbonation makes the steel reinforcement more

vulnerable to corrosion.


Alkali-aggregate reaction


(AAR or alkali-silica

reaction ASR) is the

reaction that takes place in

the hardened concrete

between the alkali of the

cement and reactive silica

minerals of the aggregate.

It causes distributed cracks

on the concrete element.



The rocks containing reactive forms of silica:

Opaline cherts

Chalcedonic cherts

Siliceous limestones

Rhyolites

Rhyolitic tuffs

Dacites

Andesites



Mechanism:

ASR starts with the attack on the siliceous minerals in the

aggregate by the alkaline hydroxides derived from the

alkalies (Na2O and K2O) in the cement.

The alkali gel that forms as a result of this reaction attracts

water by absoprtion or by osmosis.

This gel is of the unlimited swelling type!

The gel formation may take weeks, months and even years.

In order to avoid the formation of ASR gel either the

aggregates should not contain reactive silica, or the

cement should not contain excessive amounts of alkalis.

Freezing and thawing


The water in the capillary pores of hardened cement paste

freezes when the temperature is cooled to below 0 ºC.

A 9% volume increase occurs as water turns to ice.

When the capillary pores are more than 91% full of water and

freezing in such a condition, expansion takes place.






Capillary Pores

Gel Pores

Gel particles: Dimension is around 90 A°, CSH + CAH + Ca(OH)2 + Unhydrated cement particles + voids

Microstructure of hydrated cement paste


Void structure of hydrated cement paste

Gel voids

Capillary

voids

Air voids

Voids due to poor

consolidation

0.01

1

102

Important for

durability

Vo

id d

iam

ete

r (m

m)

1x10-4

1x10-6

mic

ro v

oid

s

capill

ary

void

s

macro

void

s

CONCRETE MIX DESIGN

(Proportioning of concrete mixes)



Procedure for selection of mix

proportions











The grading of the combined

aggregates (the combination

of fine and coarse aggregates)

should be fall into grading

limits which are given in ASTM

standards.


An Example of Mix Design







38

Example

(Aggregate grading for concrete production)

Find the mix percentage of fine and coarse

aggregates (by weight) in concrete design

given in the previous example.

Plot the grading curves of fine, coarse, and

combined aggregates if the sieve analysis is

given below.

Check the combined aggregate grading in

terms of being in conformity with the ASTM

standard limitations given below?

Sand Coarse

aggregate

Sieve size

(mm)

Retain (g) Retain (g)

31.5 0 0

16 0 2450

8 0 1450

4 37 1100

2 1300 0

1 1114 0

0.5 1077 0

0.25 706 0

0.125 594 0

PAN 371 0


Answer

(Aggregate grading for concrete production)

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