MATURITY of CONCRETE

 👉 The strength of concrete depends upon both the time as well as temperature during the early period of gain in strength.

👉 The maturity of concrete is defined as the summation of product of time and temperature 

                                                 Maturity = ∑ (time x temperature)

👉 Its units are ℃ hr or ℃ days.

👉The temperature is reckoned from -11 ℃ as origin in the computation of maturity, since hydration continues to take place up to about this temperature.

👉 Example 1: A sample of concrete cured at 18℃ for 28 days is taken to be fully matured which is equal to ?

      Maturity (M28 days) = 28 x 24 x [ 18 - (-11)] = 19488 ℃ hr.

                                         OR

     Maturity (M28 days) = 28 x [ 18 - (-11)] = 1092 ℃ days.

👉 The relationship between maturity and strength of concrete is shown in figure below:

👉 The maturity concept is very useful for estimating the strength of concrete at any other maturity as a percentage of strength of concrete of known maturity by using the formula:

  Percentage of strength at maturity of 19800 ℃ hr = A + B log maturity/1000

👉Plowman has given the following values of constants A and B

     28 day strength at 18 ℃                                             Coefficients

   ( M = 19800 ℃ hr/kg/cm^2)                                       A              B

                < 175                                                              10             68

          175 - 350                                                              21             61

          350-525                                                                32             54

          525- 700                                                               42            46.5

Example 2: The strength of a fully matured concrete sample is found to be 500 kg/cm^2. Find the strength of identical concrete at age of 7 days when cured at an average temperature of 20 ℃ in day and 10 ℃ in night?

Solution: Maturity of concrete at the age of 7 days = ∑ (time x temperature)

                                     = 7 x 12 x [ 20 - (-11)] + [ 7 x 12 x (10-(-11)]

                                     = 4368 ℃ hr

 Now A = 32 , B = 54

Percentage of strength of concrete at maturity of 4368 ℃ hr = A + B log (4368/1000) 

                                                                                                = 32 + 54 x log (4368/1000)

                                                                                                 = 66.5%

Strength at 7 days = 500 x (66.5/100) = 332.5 kg/cm^2

Foundation Engineering

 👉 The Primary objective of soil exploration is:

• Determination of the nature of the deposits of soil, depth and thickness of various soil strata.
• Location of Ground water table and obtaining soil and rock samples from the various strata.
•  The determination of the engineering properties of the soil and rock strata that affect the performance of the structure.
• Determination of the in-situ properties by performing field

Methods: Test Pits

                 Boring

                 Standard Penetration Test (SPT) [N Value]

                Cone Penetration Test (CPT): Static cone penetration test (SCPT)[qc value]

                 Dynamic cone penetration test (DCPT)

                 Pressuremeter Test (PMT)

                 Dilatometer Test (DMT)

                 Vane Shear Test (VST)

                Geophysical Exploration: Seismic reflection survey

                                                         Seismic refraction survey

                                                         Seismic Cross-hole survey

                                                         Resistivity Survey

              Plate Load Test

              Pile Load Test

  

👉 Standard Penetration Test (SPT): IS 2131-1981

• The Standard Penetration Test (SPT) is widely used to determine the parameters of the soil in-situ. The test consists of driving a split-spoon sampler into the soil through a bore hole at the desired depth.
•  The split-spoon sampler is driven into the soil a distance of 450 mm at the bottom of the boring
• A hammer of 63.5 kg weight with a free fall of 750 mm is used to drive the sampler.
• The number of blows for a penetration of last 300 mm is designated as the “Standard Penetration Value” or “Number” N.
• The test is usually performed in three stages. The blow count is found for every 150 mm penetration.
• The blows for the first 150 mm are ignored as those required for the seating drive.

• Number of blows for the first 150 mm penetration is disregarded due to the disturbance likely to exist at the bottom of the drill hole
• The test can be conducted at every 1m vertical intervals (Not more than 1.5 m)

👉The refusal of test when

• 50 blows are required for any 150 mm increment.

• 100 blows are obtained for required 300 mm penetration.

• 10 successive blows produce no advance.

👉Two corrections due to:

(a) Overburden pressure (granular soil)

(b) Dilatancy (for saturated fine sands and silts)

👉The corrected N value is given by (overburden correction)

                  

                                   N′ = (CF). N

                 where N′ = corrected value of observed N

                CF = correction factor for overburden pressure

👉Dilatancy Correction

                         

                            N′′= 15+0.5(N′-15)       if N′ > 15

Cone Penetration Test (CPT)

There are two types of CPT

(i) Dynamic cone penetration test (DCPT)                 (ii) Static cone penetration test (SCPT)

Static Cone Penetration Test

• The Static cone penetration test, which is also known as Dutch Cone test, has been standardized by “IS: 4968 (Part-III)-1976 - Method for subsurface sounding for soils - Part III Static cone penetration test”.

• The equipment consists of a steel cone, a friction jacket, sounding rod, mantle tube, a driving mechanism and measuring equipment.

• The cone have an apex angle of 60° ± 15′ and overall base diameter of 35.7 mm giving a cross-sectional area of 10 cm2.

• The friction sleeve should have an area of 150 cm2 as per standard practice.
• The sounding rod is a steel rod of 15 mm diameter which can be extended with additional rods of 1 m each in length.
• The driving mechanism should have a capacity of 20 to 30 kN for manually operated equipment and 100 kN for the mechanically operated equipment.

Dynamic Cone Penetration Test

• The dynamic cone penetration test is standardised by “IS: 4968 (Part I) – 1976 - Method for Subsurface Sounding for Soils-Part I Dynamic method using 50 mm cone without bentonite slurry”.

• The equipment consists of a cone, driving rods, driving head, hoisting equipment and a hammer.

• The hammer used for driving the cone shall be of mild steel or cast-iron with a base of mild steel and the weight of the hammer shall be 640 N (65 kg).

• The cone shall be driven into the soil by allowing the hammer to fall freely through 750 mm each time.

• The number of blows for every 100 mm penetration of the cone shall be recorded.

• The process shall be repeated till the cone is driven to the required depth.

Design of Steel Structure

 👉 Steel's modulus of elasticity is about 3 times that of Aluminium.

👉 Coefficient of thermal expansion for aluminium is about twice that for steel.

👉 Aluminium requires less maintenance than steel as steel prone to rusting

👉Strength to unit weight ratio of aluminium is higher than that for steel.

👉As per IS 800-1984 Cl 9.1.3 members subjected to heavy impact and fatigue shall not be designed on basis of plasticity theory, as it makes full use of materials strength beyond elastic limit.

👉 Truss carries axial load and that can cause shortening.

👉Beam carries force by bending.

👉Column resist force as axial load which can cause buckling of column.

👉Shaft carries force by twisting.

👉According to IS 800-1984 Following are the limits for slenderness ratios

• For compression member carrying dead and super imposed loads =180
• For members carrying compressive loads due to wind or seismic forces only =250
• For members carrying tension but in which the reversal of stress occurs due to wind and seismic forces = 350

👉Permissible axial tensile strength = 0.60 fy

👉Permissible shear stress = 0.45 fy

👉Permissible bearing stress = 0.75 fy

👉Permissible stress in slab base = 185 MPa for all steels

👉Allowable maximum average shear stress = 0.40 fy.

                                                            HAPPY LEARNING

Engineering Hydrology

 👉The hydrological budget equation is:

                          P-R-E-T-G = △S

Where, P = total precipitation

             R = Net runoff = R2-R1 = surface runoff flow- surface runoff inflow

            E = Total evaporation

            T = total Transpiration

          G = Net ground water flow

         △S = Total storage increase

 👉 Hydrological cycle is the cycle in which water is transported from the oceans to the atmosphere as vapours, from the atmosphere to the land as precipitation and back from land to oceans as runoff.   

👉Drizzle:  A fine sprinkle of numerous water droplets of size less than 0.5 mm and intensity less than 1 mm/h

Rain: The precipitation of liquid water with drops of sizes larger than 0.5 mm 

Hail: It is showery precipitation in the form of irregular pellets or lumps of ice of size more than 5 mm.

👉 Average rainfall over a catchment area can be estimated as

1. Arithmetic mean method: 

• every rain gauge station is given equal weightage regardless of its location.
• Rainfall is almost uniformly distributed over the whole catchment area which rarely occurs in nature.
• method is fast but does not give accurate results.

2. Theissen polygon method: 

• In this method weightage to the various rainfall datas based on area close to the rain gauge station 
• Method is fast when once the weights are known
• It does not take care of the variability in rainfall due to elevation difference(topographical influence are not take care of)
• New polygon is required to be drawn when due to addition or deletion of raingauges to the network, weight of each station changes.

3. Isohyetal method :

• This is most accurate method
• It utilizes all relevant data's and properly interpret them.
• method is slow and laborious

4. Normal ratio method: 

• It is used to estimate missing rainfall data.