CONCRETE MIX DESIGN

1.   INTRODUCTION

Concrete is a composite material having various ingredients like cement, fine aggregate (sand), coarse aggregate & water.

Concrete is popularly used in various constructions for its various qualities such as:

  • Its economy over other
  • Good durability.
  • Ease in
  • Very good
  • Its plasticity when it is green (wet) & subsequent hardening & gaining
  • Being an engineering material, concrete is also required to design for its various

The strength, durability, workability & cohesiveness of the concrete changes with the properties of various ingredients. The properties of concrete also depend upon the properties of the ingredients. The requirements of the properties of concrete also change with the requirement of the structure to be constructed.

For all the above reasons, concrete mix is required to be designed suitably. Therefore it is a science in itself, to design a mix of concrete.

The relative properties of ingredients are decided for the desired properties without compromising the quality in the most economical way. This is known as procedure of “CONCRETE MIX DESIGN.”

 

2.   INGREDIENTS OF CONCRETE WATER

Water used for mixing of concrete should be free from alkalis and organic materials. Water to cement ratio (W/C ratio) is the most important factor governing the strength of concrete. Strength of concrete depends upon, W/C ratio rather than the cement content. Higher the water cement ratio lower is the strength of concrete. As a thumb rule every 1% increase in quantity of water added, reduces the strength of concrete by 5%. A water cement ratio of only 0.38 is required for complete hydration of cement. Water added for workability over and above this water cement ratio of 0.38, evaporates, leaving cavities in the concrete. These cavities are in the form of thin capillaries. It reduces the strength and durability of concrete. Hence it is very important to control the water cement ratio on site.

CEMENT

Cement is the core material in concrete, which acts as a binding agent and imparts strength to the concrete. From durability consideration, cement content should not be reduced, below 290Kgs/M3 , considering moderate conditions of exposure. IS 456 recommends higher cement contents for more severe conditions of exposure or weathering agents to the concrete. It is not necessary that higher cement content would result in higher strength. Higher cement content helps in getting the desired workability at a lower water cement ratio.

AGGREGATES

Aggregates are of two types.

Coarse aggregate (Metal): These are particles retained on standard IS 4.75mm sieve. 

Fine aggregate (Sand): These are particles passing through standard IS 4.75mm sieve.

Proportion of fine aggregates to coarse aggregate is decided with the help of the gradation curves.

 

Seive Size

Cumulative Passing %
Fine Aggregate (30%)Coarse Aggregate 10 mm (25%)Coarse Aggregate 20 mm (45%)Combined Grading
20 mm100100100100
10 mm100921559.75
4.75 mm977030.85
2.36 mm740022.2
1.18 mm560076.8
600 μ33009.9
300 μ12003.6
150 μ2000.6

CALCULATION FOR COMBINED GRADING– 97 x 0.3 + 7 x 0.25 + 0 x 0.45 = 30.85

The coarse aggregates of 20mm & 10mm are generally used for concreting. 20mm coarse aggregate is defined as the one that passes through 20mm sieve and is retained on 10mm sieve. While 10mm metal shall pass the 10mm IS sieve and should be retained on 4.75mm sieve. Some common problems of metal available in Pune are as follows: –

The Metal has considerable proportion of over-size aggregate varying from 20mm to 40mm size. Generally when the sand is fine, smaller proportion of it is enough to get a cohesive mix; while coarser the sand, greater has to be its proportion in the concrete. Excess sand increases the water requirement of concrete and reduces the strength, while under-sandy mix results in segregation and honeycombed concrete.

USE OF ADMIXTURES

Admixtures can change the properties of concrete. Types of admixtures used are :-

  • Plasticizers
  • Retarders
  • Accelerators
  • Air Entraining Agents
  • Shrinkage compensating Agents
  • Water proofing Agents

3.   IMPLEMENTATION OF MIX DESIGN ON SITE

Items necessary on site for Mix Design

  1. Set of 12 Cube moulds of size 150 x 150 x 150 mm
  2. A slump cone, tamping rod and a
  3. Weighing balance of 5Kg, with accuracy of 2gms and corresponding weights. Electronic Balance is highly
  4. Spring balance of 100kgs
  5. Liquid Measuring cans of 5,2,1 liter capacity for addition of
  6. Measuring cylinder of 250ml capacity for silt
  7. Set of sieves as under:

40mm, 25mm, 20mm, 12.5mm, 10mm, 4.75mm, 2.36mm, 1.18mm, 600 microns, 150 microns 90 microns & 75micron along with a pan

  1. Conical cans of 5 liter and 1 liter capacity for

4.   IMPORTANT POINTS FOR IMPLEMENTATION OF MIX DESIGN:

  1. Brand of cement to be used shall be standardized in advance
  2. All materials should arrive at-least one week in advance on site to enable proper testing prior to their use.
  3. Make regular corrections on the proportions of mix design for,
    • Moisture in sand
    • Absorption by metal
    • Bulk density of sand
    • Changes in fineness of sand
  4. Regularly check the weight of cement Average weight of at least 5 bags should be taken each day prior to concreting. If the average weight of cement bags is below 49 Kgs, proper correction should be made by reducing the weight of aggregates in order to maintain the cement consumption. Cement should not be used if average weight falls below 46 Kgs.
  5. The water cement ratio shall be strictly controlled, by maintaining the slump as per the design specifications.
  6. The slump readings should be taken after 25 bags of The slump readings should be specially taken for the batches from which casting of test samples (cubes) is done.
  7. Cement to be used shall not be older than one Cement used shall not have lumps and shall give a cool feel on hand.
  8. Extremely coarse sand or silty sand shall not be

Note: If any of the above conditions are not satisfied, a trial mix shall be conducted in the laboratory, to determine fresh proportions.

5.   STANDARD CORRECTION FOR IMPLEMENTING MIX DESIGN

  • CORRECTION FOR SURFACE MOISTURE IN SAND

Sand contains some fraction of water as surface moisture. This moisture affects the water cement ratio of concrete. Water Cement ratio takes into account the total water available for hydration of cement.

Calculating Surface moisture of sand

Take 500gms. of sand and heat it in a tray gently till it dries and starts flowing freely. Dry sand is weighed again to find the loss in weight.

Surface moisture = W / (500-W)%

NOTE: The sand shall not be overheated.

To get a rough estimate of surface moisture, following thumb rule may be used when sand is not extremely wet.

Moisture in SandBulkage
 Medium SandCoarse Sand
1%8%6%
2%16%12%
3%22%15%
4%27%17%
5%29%18%

Procedure to decide the quantity of water to be added in each batch of concrete.

Water to be added per batch = (Water cement ratio x 50) – (surface moisture x Wt of sand per batch) If moisture content                                          = 4%

Wt. Of sand                                  = 119kgs Water cement ratio   = 0.45

Water to be added per batch                = 0.45 x 50 – 4% x 119

= 22.50  – 4.76             = 17.74 ltrs

Wt. Of sand per batch should be increased correspondingly equivalent to the weight of surface moisture in sand e.g. 4.7 kgs.

Moisture content in sand will vary from day to day and also with different lots of sand. It is therefore very important to make correction for moisture in sand to maintain w/c ratio. If slump of concrete is kept under control, the w/c ratio is automatically controlled.

Calculating Aggregate absorption

For any mix design, the coarse aggregate is considered to be in saturated surface dry condition. Where as, the metal on the site is often in bone-dry condition. (Especially during summers). This dry aggregate often absorbs the water added in concrete and reduces the workability of concrete. Correction for surface absorption is necessary to maintain the water cement ratio.

A correction of 1% to 2% of weight of coarse aggregate may be made. In case aggregate is 192kgs then Aggregate absorption                                    = 0.02 x 192

= 3.84 liters

Hence quantity of water added (From previous example) = 17.74 + 3.84

= 21.58 Liters

  • VOLUME BATCHING

Every mix is designed by considering weights of different ingredients In case volume batching is done on site; the weights of aggregates need to be converted into volume. This is done with the help of bulk densities of aggregates.

 
  

Bulk density   =          weight of material in kgs

Volume in liters

Bulk density can be measured on site by filling a standard 35 liters ‘Form’ with aggregate and then weighing the aggregate filled in the form.

e.g. Wt. Of sand in 35 liters form                       = 58kgs.

Bulk density                  = 58/35              = 1.66. Kg/liters.

 
  

Volume of sand per batch                                   =        Weight Of aggregate per batch

Bulk density

If wt. Of sand per batch = 119kgs.

Vol. Of sand = 119/1.66 = 71.7litrs. Say 70litrs

Hence, use 2 forms of 20 litres. And one forms of 30litres.

Bulk density of sand changes with Bulkage. Hence correction for bulk density of sand should be made every day.

Bulk density of metal is between 1.4 Kg/Lit to 1.5 Kg/Lit where as bulk density of sand varies and usually lies between 1.6 to 1.8 Kg/Lit.

  • CORRECTION FOR BULKAGE

This is done in case of volume batching of concrete. When sand is moist it ‘bulks’ i.e. it occupies greater volume. Hence, less weight of sand is taken in the given volume. In other words, the bulk density of sand changes. If correction for bulk density is done, effect of bulkage is taken care of & correction for bulkage need not be done. Otherwise, volume of sand needs to be increased in proportion of bulkage.

Measuring Bulkage on site

Take 100 ml. of sand in 250 ml. beaker and add water so as to completely submerge the sand. Shake the beaker well and the keep it steady for some time. Note the level to which it settles.

%bulkage        = (Original level – New level) x 100 New Level

  • CHECK THE SILT CONTENT

Excessive silt in sand affects the bond between cement and sand. This reduces the strength of concrete. This problem is quite severe during monsoon. Therefore a sand, containing silt, shall not be used beyond a certain proportion. Therefore, it is necessary to measure the silt content in sand.

Measuring silt content on site

Take 50ml. of water in 250ml beaker and add some salt to it. Add sand to it till 100ml, mark is reached. Again add water up to 150 ml. Stir the sand well to wash it in salt water. Keep the beaker still for 3 hours.

Observe the thickness of the silt layer.

 
  

%silt content = Height of silt layer (ml) x 100

Height of sand (ml)

Sand with silt content greater then 7% shall not be used for concreting.

  • CORRECTIONS FOR CHANGES IN GRADATION OF SAND

Sand shows lot of variation in gradation, because of change in source of supply. The fineness modulus of sand in Pune varies from 3.2 to 3.8 The IS specifies a limit of 2.2 (fine sand) to 3.2 (coarse sand) for fine aggregate. This means that, sand available in Pune is extremely coarse. This is partly because of absence of fines in the sand. 10% to 15% of the sand is retained on 4.75mm sieve as chal or shingle. Hence, in case of extremely coarse sand, use of stone dust up to 10ltrs. is recommended. If sand available on site is coarser than the one used for mix design, following correction may be made.

  1. WEIGH BATCHING

Increases the weight of sand up to 15kgs and reduce the weight of metal by equal quantity. Total weight of aggregate (fine + coarse) per batch should remain constant

If the mix does not improve with above correction, replace up to 15kgs of sand by dust in the weights mentioned on mix design.

  1. VOLUME BATCHING
  2. Increase the volume of sand by 5 litres and reduce volume of metal by 5 litres. The above correction may be made up to 10
  3. If the mix does not improve with above correction, replace 10 litres of sand in mix design proportion by 10 litres of

The new mix should be tested for compressive strength before being used at site..

Following precautions should be taken when stone dust is to be used.

Stone dust should not be too fine powdery (FM. should be between 2.0 to 2.5) It shall be used in limited quantity as mentioned above, unless higher quantity is recommended by the laboratory / consultants.

Strict control on water cement ratio should be kept. Stone dust reduces the workability; hence, the water cement ratio is likely to increases. The slump may be maintained 2cms below the value given in the mix design.

Note: – If above corrections in the proportions do not yield satisfactory mix, fresh trials shall be conducted in the laboratory for a revised Mix design.

6.   CASTING AND TESTING OF CUBES

Test sample:- Cubes of concrete shall be casted to estimate the strength of concrete.Concrete for filling the cubes shall be taken from the middle discharge of concrete mixer. Concrete from beginning or at the end of discharge should be avoided. Cubes should be filled in 3 equal layers. Each layer should be uniformly tamped at least 35 times with a 16mm blunt rod. Cubes should be properly levelled and finished using hand trowel. Cubes should be kept in shadow properly covered with wet gunny bags.

Cubes should be demoulded after 24 hours and immersed in water for curing. It is important to keep the cubes away from the shocks or vibrations especially for initial 3 days of casting.

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