Test: Loss Due to Creep of Concrete - Civil Engineering (CE) MCQ

The sustained prestress in the concrete of a prestressed members results in creep of concrete which effectively reduces the stress in high tensile steel and the progressive inelastic strains due to creep in a concrete are likely to occur under the smallest sustained stresses at ambient temperatures, shrinkage and creep of concrete are basically similar in origin, being largely the result of migration of water in the capillaries of cement paste.

Creep is a time-dependent permanent (plastic) deformation under a certain applied load.
Creep deformation has three stages: 

• Primary creep starts rapidly and slows down with time. 
• Secondary creep progresses at a relatively uniform rate. 
• Tertiary creep has an accelerated rate of deformation which terminates when the material fails (breaks or ruptures). 

Important Point
In practice, drying shrinkage and viscoelastic behavior such as creep usually take place simultaneously.
Considering the various combination of loading, restaining, and humidity conditions, the following terms are defined:

• True or Basic Creep: It is defined as the creep that occurs under conditions that there is no drying shrinkage or moisture movement between concrete and ambient environment.
• Specific Creep: It is defined as a creep strain per unit of applied stress.
• Drying Creep: It is the additional creep that occurs when the specimen under load is also drying.
• Creep Coefficient: It is defined as the ratio of creep strain to the elastic coefficient.

Under sustained compressive loading, deformation in concrete increases with time even though the stress level remains constant.
This time-dependent component of strain is called creep strain.
Creep occurs due to:

• Internal movement of absorbed water
• Moisture loss
• Sliding between gel particles
• Growth in microcracks.

Hence creep is caused due to dead load and sustained live load. It depends on the stress level.
Important Point
Defects of concrete

• Crazing:- It is a development of fine random cracks on the concrete surface due to difference in shrinkage between surface and interior.
• Segregation:- It is a separation of coarse aggregate from fine aggregate, paste from coarse aggregate or water from the mix and hence reducing the strength.
  There are 5 types of shrinkage:-
  a) Chemical shrinkage, b) Plastic shrinkage, c) Drying shrinkage, d) Autogenous shrinkage, and e) Carbonation shrinkage
• Shrinkage:- The reduction in the volume of concrete when it changes phase from plastic to solid-state. The shrinkage strain is independent of stresses.
• Bleeding:- It is an autogenous flow of mixing water within or emergence to the surface from freshly prepared concrete usually due to excessive vibrations imparted to concrete to achieve full compaction. It leads to formation of pores inside the concrete.

As per IS 1343(1980): Code of Practice for Prestressed Concrete 19.5.2.1
Loss of prestress due to creep of concrete:

• The loss of prestress due to the creep of concrete under load shall be determined for all the permanently applied loads including the prestress.
• The creep loss due to live load stresses, erection stresses, and other stresses of short duration may be ignored.
• The loss of prestress due to creep of concrete is obtained as the product of the modulus of elasticity of the prestressing steel and the ultimate creep strain of the concrete fiber integrated along the line of center of gravity of the prestressing tendon at the section at which creep loss is being calculated for bonded tendons.
• In the case of unbonded tendons, the creep loss is a product of modulus of elasticity of prestressing steel and creep strain calculated by integrating and averaging creep stress along the line of center of gravity between the anchorage points.
• The total creep strain during any specific period shall be assumed for all practical purposes, to be the creep strain due to sustained stress equal to the average of the stresses at the beginning and end of the period. 

The loss of stress in steel due to creep of concrete can be estimated if the magnitude of ultimate creep strain or creep coefficient is known and the values of creep coefficient which is the ratio of ultimate creep strain to the elastic strain is 2.2 at 7 days of loading, 1.6 at 28 days and 1.1 when the age at loading is 1 year.

The value of creep coefficient ‘f’ depends upon various factors such as humidity duration of load applied, age of loading and effective section thickness and the effective section thickness is defined for uniform sections as twice the cross sectional area divided by the exposed perimeter, it can be assumed about 4060 and 80 percent, respectively of the final creep develops during the first, six and 30 months under load when concrete is exposed to conditions of constant relative humidity.

The creep coefficient varies from a minimum value of 1.5(for wet conditions) to a maximum value of 4.0(for dry condition), for design purposes it is convention to differentiate between deformation due to externally applied stress generally referred as creep and the deformation which occurs without externally applied stresses referred as shrinkage as the increase in strain under a sustained stress is several times larger than the strain on loading, it is of considerable importance in prestressed structural members.

Creep coefficient is generally low for post tensioned members and high for pre tensioned members, the various factors influencing creep of concrete are relative humidity, stress level, strength of concrete, age of concrete at loading, duration of stress, water cement ratio, and type of cement and aggregate in the concrete, for stress up to half of the crushing strength of concrete.

The magnitude of the creep coefficient ϕ varies depending upon the humidity, concrete quality, duration of applied loading and the age of concrete when loaded,
Creep coefficient = (creep strain / elastic strain), ϕ = ℇ_c/ℇ_e, ℇ_c = ϕ ℇ_e = ϕ(f_s/E_c),
Loss of stress in steel = ℇ_cE_cϕE_s = ϕ(f_s/E_c)E_s = ϕf_cα_e.

The loss of stress in steel due to creep of concrete is ℇcc fc Es, ℇcc = ultimate creep strain for a sustained unit stress, fc = compressive stress in concrete at the level of steel, Es = modulus of elasticity of steel.