Mix design 3

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Mix Design-III

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So far, we went through a numerical example ofhow a simple algorithm, based on

A) SOME EXPERIMENTSB) PROPERTIES OF THE CONSTITUENT MATERIALS, AND,C) DESIRED PROPERTIES OF CONCRETE•CAN BE IMPLEMENTED, TO DETERMINE THEPROPORTIONS OF A CONCRETE MIX.

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Our discussion today will focus on

• SOME ANALYSIS OF THE PROPORTIONS OF INGREDIENTS WE OBTAINED IN THE LAST CLASS

•HOW DO WE ADJUST OR REVISE A PREDETERMINEDPROPORTION , SHOULD THERE BE A NEED.

•PROPORTIONING OF CONCRETE WITH CHEMICAL ANDMINERAL ADMIXTURES

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ANALYSIS OF THE PROPORTIONS OF INGREDIENTS WE OBTAINED

IN THE LAST CLASS

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We had set out to proportion an air-entrained concrete with(a) A = 5%; (b) W = 160 kg, (c) w/c = 50%, and (d) s/a = 36%

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#Unit weight of concrete 160+320 + 1178 = 2303kg/m3

*Assuming that the air is a part of the mortar

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HOW DO WE ADJUST OR REVISE A PREDETERMINED

PROPORTION: If Reqd

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Such a need can arise• IF THE CONCRETE MADE DOES NOT HAVE THE PROPERTIES WE 'ASSUMED’ OR 'TARGETED'• IF THE PROPERTIES OF THE MATERIALS BEING USED CHANGE• IF THE FINENESS MODULUS (FM) OF THE SAND

CHANGES.• THERE IS A CHANGE IN THE PROPERTIES OF

CEMENT OR THE CHEMICAL ADMIXTURE, ETC.

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At the outset we have the following in mind

We need to make an adjustment in the mix because :a) The concrete has a lower slump (1 cm)b) The concrete has a lower air content (1%)

Reference for minor adjustments

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For the example we worked with,

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Property Target Actual Slump (cm)

8 7

Air (%) 5 4

Item Correction in s/a Correction in wFor 1 cm increase in slump None +1.2 %For 01 % increase in air content -(0.5 to 1.0 %) -3%

Adj for s/a wSlump (+1) None 1.2 %

Air Content (+1%)

-1.0% -3.0 %

Net -1.0% -1.8 %

W = 160 - 2.9 = 157.1 (say 157 kgs) s/a = 36 - 1 = 35%

No change in W/C

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1. An appropriate adjustment will he needed in the dosage of the chemical admixture to correct the air content.

2. Concrete mix will he apportioned afresh.

We had set out to proportion an air-entrained concrete with(a) A = 5%; (b) W = 160 kg, (c) w/c = 50%, and (d) s/a = 36%

Now we have to proportion an air-entrained concrete with(a) A = 5%; (b) W = 157kg, (c) w/c = 50%, and (d) s/a = 35%

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With sp gr as before.Cement 3.10Fine aggregate 2.61Coarse aggregate 2.63

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•WITH THE REVISED MIX WE CAN ONLY HOPE THAT DESIRED SLUMP AND AIR CONTENT (08 CM AND 05 % IS MET) •THEN THERE IS UNCERTAINTY ABOUT THE PROPERTIES OBTAINED IN A TRIAL RUNS BE REPLICATED IN THE FIELD.

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• Summmary of case studies. Further trials may need to be carried out.

• Keeping records even forunsuccessful mixes helps infuture works.

Use more stringent criteria during trial mixes as suggested.

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Other than the air, w/c, s/a, there could be other ways of Specifying a concrete mix.

Example 2Air - 5% ; W =175kg/m3, w/c = 50% ; CA = 48%

MORTAR CONTENT (INCLUDING AIR CONTENT) - 52%|

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PROPORTIONING OF CONCRETE WITH CHEMICAL

ANDMINERAL ADMIXTURES

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ExampleFor an application a(normal) concrete isproportioned with thefollowing conditions:Air content: 5%Unit water content: 180kgWater - cement ratio : 45%C A content: 40%

Other conditions dictate thatcement content cannot exceed320kg. Assume that followingadmixtures are available:•A water reducer that can he usedto reduce the water demand by20%. (Chemical Admix)•A Mineral Admixture with anefficiency of 75%

Carry out the proportioning exercise to meet the above conditions

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For reference, let us first quickly carry outthe proportioning of the 'normal' concrete.

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The mix does not meet the 'other, criteria as the cementcontent (which exceeds the maximum Permissible)

The cement content can he brought down by:

Either using a chemical admixture to reduce the water content (For the same slump), and the cement content can be reduced (keeping w/c at the old level)OR Using a mineral admixture to directly replace cement.The amount of admixture will depend on its efficiency.

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Option I: Chemical Admixture

• Now, since the maximum (unit) cement content is 320 kgs, & the w/c has to he kept at 45%, the (unit) water content cannot he allowed to exceed 144 kgs.• In other words, We have to use a chemical admixture that will reduce

the (unit) water content from 180kgs to 144 kgs. • This is a reduction of 36kgs (20%).• The problem is reduced to identifying an admixture and its dosage,

and that will require trial mixes !!!

Slump cant be compromised

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An appropriate doseof a suitable chemical admixture in part of this mix

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Option II: Mineral Admixture

• The other option is to proportion the concrete using only the mineral admixture, which directly replaces cement. • Ill the present case. 80 kgs (i.e. 400-320) need to be replaced,

This is a reduction of 36kgs (20%).•And given that the mineral admixture available has an

efficiency of 75%, 106.7 kgs of MA will need to he added to the concrete (on a per cubic meter basis).

Strength cant be compromised

Earlier- 400 kg of cementNow- 320 kg of cement + 106.7 kg of mineral admixture

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In all the cases. having kept the coarse aggregate content at40%, we have kept the total mortar content in the mix at 600l

There is a difference, however is in the paste content. Compared to (1) . It is much lesser in (2) and the additional space (to keepthe mortar at 60%) , is filled with fine aggregate.

• Does Admixers Contribute to Paste

• Yes- If fine like cement• No-If Coarse

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FOOD FOR THOUGHT• WHAT ARE THE BASIC ASSUMPTION IN ARGUMENT PUT

FORWARD IN WORKING WITH OPTIONS I AND 2 OUTLINED INTHE DISCUSSION WHEN TRYING TO REDUCE THE CEMENTCONTENT IN A CONCRETE MIX.• LOOK AT THE COMPARISON OF THE THREE MIXES (BASIC. ONLY WITH

CA AND ONLY WITH MA).• WORK OUT AN ALTERNATIVE MIX USING BOTH CA AND MA.• HINT: TRY WITH BRINGING DOWN THE UNIT WATER CONTENT BY 10%

AND ACHIEVING THE REMAINING REDUCTION (IN CEMENT) THROUGHUSE OF AN MA.

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Thank You

Engineering

Mix design 3