Maturity Concept of concrete & Strength test of concrete

*Maturity Concept of concrete:-

·         The strength of concrete not only depends upon the time, but also upon the temperature during hydration.

·        
Maturity of concrete is defined as the product of time & temperature.

Maturity of concrete = Time * Temperature = °C Hours or °C days

·         A datum temperature of (-) 11°C is taken for majority calculation because hydration starts at this temperature.

·         A sample of concrete cured at 18°C for 28 days is taken as fully matured concrete.

·         Maturity at 28 days = 28*24[18-(-11)] = 19488°C-hr.

*Yield of concrete:

·         If proportion of concrete is a: b: c then resulting concrete will have volume of 2/3(a+b+c) where a is volume of cement, b is volume of sand, c is volume of coarse aggregate.

·         If w, a, b, c are absolute volumes of water, cement, sand & coarse aggregate respectively then w+a+b+c=1.

*Methods of proportioning of concrete mixes:

                I.            Fuller’s maximum density method:

·        
p=100√d/D

·         P=% by weight of matter finer than diameter.

·         D=maximum size of aggregate.

              II.            Abram’s water-cement law:

·         For any given condition of the test, the strength of the workable concrete mix is dependent only on water cement ratio.

·         The quantity of water used in concrete is 30% of the weight of cement = 5% of the weight of aggregate.

·         By the use of compaction vibrator, the quantity of water can be reduced 20%.

·         According to the law, the strength of the mix increased with decrease in water content.

·         Strength of concrete increases with age in the following way.

Month                                                   Age Factor 

                                     1                                                                      1.00

                                     3                                                                      1.10

                                     6                                                                      1.15

                                    12                                                                    1.20

           III.            Minimum Voids Method:

·         Voids in coarse aggregate should be filled by sand & voids in sand should be filled by cement.

·         Generally actual practice quantity of sand used in the mix is kept 10% more than the voids in the coarse aggregate & quality of cement is taken as 15% more than the voids in sand.

*Water Cement Ratio:

·         It is the mass of ‘free water’ (excluding that absorbed by aggregate) to that of cement in the mix.

·         The ratio generally lays b/w 0.35 to 0.65.

·         It is mainly responsible for porosity of the hardened cement paste.

·         For durable concrete W/C=0.8. If W/C > 0.6, Porosity increase and strength decrease.

·         Due to the presence of 2% oil in water, the strength of concrete is reduced to 20%.

·         Sea water in the world oceans has a salinity of approximately 3.5% or 35 parts per thousand

·         The constituent of sea water reacts chemically with constituents of cement concrete causing damaged to concrete structure.

·         Sea water corrodes the reinforcement of RCC thus affecting durability of structure.

·         Sea water due to presence of CaCl₂ accelerates the setting time of cement.

·         The sea water reduced the strength of concrete 10 to 20%.

·         The sea water cannot be used for pre-stressed concrete.

·         The strength of concrete cube made from normal water should not be less than 90% of strength of concrete cube made from distilled water.

 

Exposure conditions and requirements for building construction works with normal aggregates of 20mm nominal size.   

Exposure	Description	Nominal Cover(mm)	Min grade	Min cement Content(kg/m³)	Max free w/c ratio
Mild	Normal	20	M20	300	0.55
Moderate	Expose to Rain	30	M25	300	0.50
Severe	Immersed in sea water	45	M30	320	0.45
Very Severe	Exposed to sea spray	50	M35	340	0.45
Extreme	Tidal Zone	75	M40	360	0.40

 

                                       

  

            

            

            SARALCIVIL TRICK:

                                                

 

      Strength of Concrete:-

·        It is designated in terms of numbers preceded by the letter ‘M’ where M refers to the ‘Mix’ & number represents the characteristics strength.

·        Tensile strength = 10% of compressive strength.

·        Bending strength = 15% of compressive strength.

·        Shear strength = 20% of compressive strength.

·        

·        Characteristics strength is the strength of material below which not more than 5% of the test result are expected to fall.

fm = fck + 1.65σ

·         Generally samples are tested for 7 days or 28 days but IS code only suggests 28 days strength only.

·         7 days strength= 2/3 of 28 days strength of concrete.

·         Average of 3 values is taken as compressive strength of concrete, provided the individual variation is not more than ±15% of the average.

1.      Compressive strength test

2.      Flexural Tensile strength test

3.      Split Tensile strength test

 

1. Compressive Strength Test:

·       Size of coarse aggregate up to 38mm.

·       Size of concrete cube – 150*150*150 mm & Size of cylinder – 150 mm dia, 300mm height.

·       Cube mould filled in 3 layers, tampered 35 times per layer with tampering rod of 16 mm dia & 600 mm length.

·       Rate of loading in Compression testing machine = 14 N/mm²/minute.

·       Stored at temp. Of 27±3°C at 90% humidity for 24 ±1/2 hour.

·       Then immersed in water for 7 days or 28 days.

·       Cube strength = 1.25*Cylinder strength.

2. Flexural tensile strength:

·       Tensile strength of concrete in flexure is called flexure strength.

·       IS: 516-1959, Specifies two points loading.

·       Flexural strength is used to determine the onset cracking or the loading at which cracking starts in a structure.

3. Splitting tensile strength:

·       It is measured by testing cylinders under diametrical compression.

·       

·       Splitting tensile strength (fct ) = 2P/πDL

·       Since concrete does not take any tensile strength, it is taken as zero. But, IS code recommends the tensile strength to be calculated using fcr =0.7√fck N/mm².

4. Direct tensile strength:

·       It is measured by testing rectangular specimens under direct tension.

·       In the absence of test results, the cod recommends to use an estimate of the flexural tensile strength from the compressive strength by the following equation-

        fcr = 0.7√fck.

·         Where fck = Characteristic compressive strength of cubes in N/mm².

·         fcr = Flexural tensile strength is N/mm².

·            fcr = 0.7√fck.

·         fct = 0.66 fcr.

·         & Splitting = [0.5-0.625] fcr

·         Where fct = Splitting tensile strength

·         fcr = 0.7√fck, fct = 0.66 fcr == 2P/πDL

·         fcr  (Flexural> Splitting> Splitting)

*Factors affecting strength of concrete:

1.     Size of specimen: Size decreases then strength increases.

2.     Age of cement: Strength reduces as it absorbs moisture from atmosphere.

3.     Moisture in specimen: Reduces the strength

4.     Rate of loading: Strength increasing with increase in rate of loading.

5.     Cement aggregate ratio: If cement is increases then ultimate strength will increase.

6.     Size of aggregate: Larger aggregate size reduces strength.

7.     Type of aggregate: Rounded or cubical aggregate give higher compressive strength (but poor binding).

8.     Air Voids: They decrease the strength considerably.

9.     Degree of compaction: 5% air voids decreases strength by 30% where as 10% air void decrease strength by 60%.