10.5
Effects of Creep
Creep in concrete, the gradual deformation under prolonged stress, significantly impacts the integrity of structures. For reinforced concrete beams, i…
Reinforced concrete beams subjected to creep undergo increased deflection. This may result in further bending of the beam, widening existing cracks, and forming new cracks.
In the case of eccentrically loaded, very slender reinforced concrete columns, creep induces deflection, which leads to column buckling.
For indeterminate structures, such as at the junction of concrete columns and beams, stress concentrations induced by shrinkage, temperature variations, or movement of supports may be relieved by creep in concrete, resulting in fewer cracks.
On the contrary, in mass concrete, such as dams, the chances of crack formations in the interior of the structure increase due to concrete's creep.
In high-rise buildings, the outer and inner columns may experience differential creep, causing shifts in the connecting partition panels and resulting in cracks within these panels.
Similarly, the external cladding rigidly affixed to a reinforced concrete column that undergoes creep is prone to develop cracks.
Lastly, prestressed concrete beams made without high tensile steel, when subjected to creep, tend to lose their prestressing forces over time, leading to failure.
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Q1: How does creep affect reinforced concrete beams?
Creep causes reinforced concrete beams to experience increased deflection under prolonged stress. This additional deflection can further bend the beam, widen existing cracks, and form new cracks. Understanding creep is vital for beam design, as engineers must often implement additional design measures to accommodate these gradual deformations and maintain structural integrity.
Q2: Why can creep cause column buckling in slender reinforced concrete columns?
In slender reinforced concrete columns under eccentric loads, creep induces deflection that compromises stability. This deflection can trigger column buckling, a critical failure mode. The combination of eccentric loading and gradual creep deformation creates conditions where the column loses its load-carrying capacity, making slender columns particularly vulnerable to this phenomenon.
Q3: Can creep be beneficial in concrete structures?
Yes, creep can be beneficial in indeterminate structures like column-beam junctions. Stress concentrations induced by shrinkage, temperature variations, or support movements can be relieved by creep, resulting in fewer cracks. This stress mitigation effect demonstrates that creep's impact varies depending on structural configuration and loading conditions.
Q4: What problems does creep cause in mass concrete structures like dams?
In mass concrete structures such as dams, creep increases the chances of crack formation in the interior. Constraints during temperature cycles from hydration heat and subsequent cooling, combined with creep deformation, create conditions that promote cracking. This makes creep a significant concern in massive concrete design and maintenance planning.
Q5: How does differential creep affect high-rise buildings?
In high-rise buildings, outer and inner columns may experience differential creep, causing shifts in connecting partition panels and resulting in cracks within these panels. Additionally, external cladding rigidly affixed to columns undergoing creep is prone to developing cracks. These differential movements create stress concentrations that compromise both partition and cladding integrity.
Q6: What happens to prestressed concrete beams when they undergo creep?
Prestressed concrete beams made without high-tensile steel tend to lose their prestressing forces over time due to creep, potentially leading to failure. This prestress loss was once so significant it prompted the introduction of high-tensile steel to counter the effects. Creep-induced force loss remains a critical consideration in prestressed concrete design for bridges and other applications.
Q7: What design provisions are necessary to accommodate creep in concrete structures?
Provisions for accommodating potential movement and stress changes due to creep are essential in concrete structure design and maintenance. Engineers must account for increased deflection in beams, potential buckling in slender columns, differential movements in high-rise buildings, and prestress loss in prestressed elements. Understanding types of non structural cracks in concrete helps identify creep-related damage patterns.