Concrete. Chapter 11 Durability of Concrete & Mix Design. Materials of Construction-Concrete 1. Wikipedia.org

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1 Wikipedia.org Concrete Chapter 11 Durability of Concrete & Mix Design Materials of Construction-Concrete 1

2 Durability of hardened concrete Materials of Construction-Concrete 2

3 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-silicatehydrate 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. Materials of Construction-Concrete 3

4 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. Materials of Construction-Concrete 4

5 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 calciumhydroxide in the structure of the hardened cement paste. 2. Ettringite (C 6 AS 3 H 32 ) is formed deu to the reaction of the gypsum with the calcium-alumino-monosulfohydrate that is present within the hardened cement paste. Materials of Construction-Concrete 5

6 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. Materials of Construction-Concrete 6

7 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 C 2 S and C 3 A should be used! (Because C 2 S produces high amount of calciumhydroxide and C 3 A 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!!!! Materials of Construction-Concrete 7

8 Sulfate attack ACI : American Concrete Institute Materials of Construction-Concrete 8

9 Carbonation Carbondioxide (CO 2 ) 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 CaCO 3. 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 cm of the surface is under the effect of carbonation. Materials of Construction-Concrete 9

10 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. Materials of Construction-Concrete 10

11 Alkali-aggregate reaction 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. Materials of Construction-Concrete 11

12 Alkali-aggregate reaction The rocks containing reactive forms of silica: Opaline cherts Chalcedonic cherts Siliceous limestones Rhyolites Rhyolitic tuffs Dacites Andesites Materials of Construction-Concrete 12

13 Alkali-aggregate reaction Mechanism: ASR starts with the attack on the siliceous minerals in the aggregate by the alkaline hydroxides derived from the alkalies (Na 2 O and K 2 O) 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. Materials of Construction-Concrete 13

14 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. Materials of Construction-Concrete 14

15 Freezing and thawing Materials of Construction-Concrete 15

16 Freezing and thawing Materials of Construction-Concrete 16

17 Microstructure of hydrated cement paste Capillary Pores Gel Pores Gel particles: Dimension is around 90 A, CSH + CAH + Ca(OH) 2 + Unhydrated cement particles + voids Materials of Construction-Concrete 17

18 Void structure of hydrated cement paste Void diameter (mm) micro voids capillary voids macro voids 10 2 Voids due to poor consolidation Capillary voids Air voids Important for durability 1x10-4 Gel voids 1x10-6 Materials of Construction-Concrete 18

19 CONCRETE MIX DESIGN (Proportioning of concrete mixes) Materials of Construction-Concrete 19

20 Procedure for selection of mix proportions Materials of Construction-Concrete 20

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30 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. Materials of Construction-Concrete 30

31 An Example of Mix Design Materials of Construction-Concrete 31

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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? Sieve size (mm) Sand Retain (g) Coarse aggregate Retain (g) PAN

39 Answer (Aggregate grading for concrete production) Materials of Construction-Concrete 39