Concrete Mix Design Proportioning of Concrete

RECAP OF PREVIOUS DISCUSSION • SOME IMPORTANT IDEAS• WATER-SLUMP RELATIONSHIP• CHARACTERISTIC AND TARGET STRENGTHS FORPROPORTIONING CONCRETE MIXES• w/c— STRENGTH RELATIONSHIP• WHAT IS s/a

Air in concrete* Entrained• Entrapped

Entrained air isintentionally put inplace and particlesare very fine anduniformly distributedwithin concrete

Entrapped air ispresent'unintentionally', andis generally muchlarger is size ofindividual particles

AIR CONTENT IN CONCRETE

FRESH STATE - shows behaviour similar to that of a fluid, can be pumped, acquires the shape of the mold in which it is poured and little resistance to deformation.

HARDENED STATE - solid material with substantial compressive strength and so on.

Functionally, the concrete should satisfy laid downcriteria for:

a) Fresh state (e.g. have required workability, aircontent, etc.)

b) Hardened state (e.g. have adequate strength)

c) Durability (in terms of restrictions on parameterssuch as w/c, cement content)

d) Other properties, such as temperature rise duringsetting, setting time, etc.

To find a suitable combination of relative amounts of sand, water, coarse aggregate and cement, so that the concrete,

* has the required properties in the fresh and hardened state.

* meets durability and other requirements depending on the structure and the environment.

Objective

CONCRETE AS A MULTIPHASE COMPOSITE

PROPORTIONING OF CONCRETE

Hypothetical Model

1.7”

0.56”

0.80”

1.16”

1.44”CA

FA

CEMENT

WATER

Air

Fixed volume of the pitcher.The proportioning exercise isreally to find the quantities ofdifferent constituents so that:a) The pitcher is FILLED.b) The properties of the concrete satisfy required conditions.

We will work with a onecubic meter (1000 liters)pitcher and determine thequantities in kgs.

Kgs better than volumeVol determined by Gs

How much of each of these constituent materials ??

The quantity shouldbe in kgs/m3 (exceptfor air !!); but theexercise is to quite anextent volumetric

Fundamental basis

•For a given concrete mix,• Slump is determined primarily by the unit water content.

• Strength is determined primarily by the water-cement ratio.

1. The str in mix 1 and mix 3 are same as w/c ratio is same 2. The slump of mixes 2 and 3 is the same as W% is the same

GOVERNING CONSIDERATIONS

(c) A balance between the amounts ofFA and CA is required for “good”concrete, expressed through thes/a, which varies with the shapeand size of CA% Fineness Modulus of FA, amount of cement in the mix, etc.

(d) Durability requirements• W/C not exceeding 45%* Use a cement content of atleast 400kg,

Required input1. Air content2. Slump - unit water content relationship3. Compressive strength - water to cement ratio relationship4. Required s/a (or mortar content) *5. Material properties (specific gravity of cement, fine aggregate and coarse aggregate)

* Will require other parameters (eg. particle size distribution of fine and coarse aggregates. their fineness modulus, etc.)

AIR CONTENT IN FRESH CONCRETE

As mentioned the air contentin fresh concrete is an inputand assumed to be present.

The remaining volume thus isto be appropriatelyapportioned.

1. Required slump fromconsiderations such asdetails of structure, etc.

2. Determine the unit watercontent based on theslump-unit water

content relationship

WATER CONTENT FOR REQUIRED WORKABILITY-

Slope depends upon aggregate properties etc

We now know air and water vol

By specific gravity of water vol can be determined

1. Required target strengthfrom characteristic strength,likely standard deviation, etc.

2. Determine the (required) water-cement ratio based on the strength-water to cement ratio relationship

3. Calculate the cementcontent (mass) and find thevolume of cement, using itsspecific gravity.

Cement content

Please note that at timesother criteria such asdurability or heat ofhydration may govern thecement content.

Strength vs w/c

Fine and coarse aggregate content• Remaining volume needs to be 'divided' between fine and Coarse

aggregate More sand means more mortar content in the matrix.•Mortar should be able to cover all coarse aggregate. s/a may

be used to 'control' the sand volume in concrete.• Value of s/a needs to be adjusted depending on size and type

of aggregate. For smaller coarse aggregate, higher s/a areneeded.• Once the sand content is known, the coarse aggregate content

is automatically fixed. Convert to respective masses usingspecific gravity of fine and coarse aggregate.

Vol measurement

Wt measurement

CA, Coarse aggregate

BASIC STEPS IN PROPORTIONING NORMAL CONCRETESSteps ProcedureStep 1 Air content

Step 2 Water content for required workability

Step 3 Cement content using required w/c ratio

Step 4 Sand content within the total aggregate volume

Step 5 Coarse aggregate content

MIX PROPORTIONS - A NUTSHELL

• Start with a 'target' slump and air content and compressivestrength. Since determination of compressive strength takestime, only the former are discussed here.•Proportioning can, in principle, be carried out once the air

content (Air), unit water content (W), water-cement ratio(w/c), and the sand-aggregate ratio ( s/a) along with thespecific gravities of the constituent materials are known.•Obtained results could always be different, requiring 'minor

adjustments’ to be made in the proportions.

EXAMPLE

Reference for initial mix

• Typical value for w/c = 55%and slump = 8cm, using Fineness Mod = 2.8 (sand).• A, s/a in volume, W in kg/m3.

• Values may be different when using HRWR, AE agents, etc• Adjust for type of aggregate

Size mm

AE: Air Entrained

AS the size of aggregate increases amount of water reqd for same workabilityof 8 cm is much lower.

• In the discussion so far, we went through theprocedure for proportioning a normal concrete mix and learnt what kind of information or input data isrequired to he able to carry out the exercise.

• We are now ready to work with numbers and try tosee how this process actually) yields a 'design mix’.

BASIC INPUT FOR ILLUSTRATIVE EXAMPLE1. Air content (05% )2. Slump: 80mm3. Characteristic strength : 25 MPa4. Expected standard deviation in strength : 03 MPa5. s/a for given aggregate characteristics : 0.366. Material properties (specific gravity)•Cement 3.10• Fine aggregate 2.61•Coarse aggregate 2.63

OTHER INPUT REQUIRED ON THE BASIS OF THE BASIC DATA

•From the slump - unit water content relationship,and the required slump for the job, w = 160 kgs/m3.

•Target strength of the concrete is 25 + 1.65x3 = 29.95 (say 30 MPa), and,

•From the strength - w/c relationship, the water-cementratio required for the required target strength = 50%

Step I : A = 50 litersGiven that air content is5% (volume)

Step 2: w= 160 litersGiven that this is the unit water content for the required slump

Step 3: a) Given that w/c =50%, C = 320 kgs.b) Given that the Sp Gr of cement is 3.10, the volume is (320/3.10), i.e.103.2 liters

Remaining volume, i.e. sum offine and coarse aggregate. Is (1000-50- 160- 103.2) or686.8 liters

Given that s/a ratio is 0.36.

Step 4 : Vol of FA = 0.36 * 686.8= 247.2 liters

Step 5 : Vol of CA = 0.64 *686.8= 439.6 liters (OR, 686.8 - 247.2)

Multiply by Specific Gravity

With this we come to an end of our discussion. We set out to go through an algorithm to carry out proportioning of concrete mixes, which we have done.

We also went through a numerical example of how the procedure is to be implemented.

We should remember dial we set out with the beliefthat the proportions that we decide will yield a concrete that will have the desired properties (in terms of slump, air and strength), but that may not always happen!!

For such cases, we need to revise the mix proportions— and just how we can do that, we will see the nexttime.

Food for thought• Indian codes (IS 456, 2000) talks in terms of nominalmixes and design mixes. Find out more about these.•Study specifications of concrete which lay downrequirements of a maximum water-cement ratio andcement content. •How is the workability requirement determined for areinforced concrete construction?•Study more about some other methods of proportion inconcrete mix.

Kind of app of concrete restrictions imposed

Slump Reqmt vs Density of Rft