The long-term deflections of reinforced concrete members under sustained loads are mainly due to
1. differential shrinkage
2. creep under sustained loading
3. temperature effects
Select the correct answer.

Question

The long-term deflections of reinforced concrete members under sustained loads are mainly due to

1. differential shrinkage

2. creep under sustained loading

3. temperature effects

Select the correct answer.

Prepp Team · Accepted Answer

Understanding Long-Term Deflection in Reinforced Concrete

Reinforced concrete members, like beams and slabs, experience deflections under applied loads. These deflections can be categorized into immediate deflections and long-term deflections. Immediate deflections happen as soon as the load is applied. Long-term deflections develop over time under sustained loading. The question asks about the primary causes of these long-term deflections.

Factors Contributing to Long-Term Deflections

Several factors influence the long-term deflection of reinforced concrete structures. The main ones under sustained loads are shrinkage and creep.

Shrinkage: As concrete dries, it shrinks. If this shrinkage is uniform throughout the member, it primarily causes a change in length. However, if shrinkage is different across the cross-section (e.g., different moisture levels or reinforcement distribution), it can cause bending and thus deflection. This is known as differential shrinkage.
Creep: Concrete is a viscoelastic material. Under a sustained load, it continues to deform slowly over time. This time-dependent deformation under constant stress is called creep. Creep increases the strains in the concrete, leading to increased curvature and, consequently, increased deflection over time. Creep is significantly affected by the magnitude of the sustained load, the age of the concrete at loading, temperature, and humidity.
Temperature Effects: Temperature changes cause concrete to expand or contract. Daily or seasonal temperature cycles lead to thermal movements. While temperature gradients across a section can cause some deflection, the primary long-term deflection under sustained load is typically dominated by creep and shrinkage, not temperature effects, which are often more transient or cyclic. Extreme or sustained high temperatures can influence creep rates, but temperature changes themselves are not usually listed as the main *direct* cause of long-term deflection under sustained load compared to creep and shrinkage.

Analyzing the Given Factors

Let's look at the factors listed in the question:

Differential shrinkage: As explained, differential shrinkage causes bending and is a significant contributor to long-term deflection.
Creep under sustained loading: This is a major time-dependent phenomenon in concrete under load, directly increasing deflection over time. It is a primary cause of long-term deflection.
Temperature effects: While temperature variations cause deflections (thermal expansion/contraction), they are generally not considered the main cause of *long-term deflection under sustained loads* compared to creep and shrinkage. Long-term effects of temperature are more about durability or influencing creep/shrinkage rates rather than being the direct mechanism of the primary long-term deflection itself.

Conclusion on Main Causes

Based on the analysis of shrinkage, creep, and temperature effects, the main reasons for the long-term deflections of reinforced concrete members under sustained loads are differential shrinkage and creep under sustained loading. Temperature effects, while relevant for structural behavior, are not the primary long-term deflection mechanism in the same way creep and shrinkage are under sustained load conditions.

Therefore, the factors contributing mainly to long-term deflections under sustained loads are 1 (differential shrinkage) and 2 (creep under sustained loading).

Revision Table: Long-Term Deflection Factors

Factor	Contribution to Deflection	Type of Deflection	Relevance to Long-Term Deflection under Sustained Load
Shrinkage	Causes contraction; differential shrinkage causes bending.	Time-dependent	Significant (Differential shrinkage is a primary cause).
Creep	Time-dependent deformation under stress; increases strain and curvature.	Time-dependent	Major (A primary cause of increasing deflection over time).
Temperature Effects	Causes expansion/contraction; gradients cause bending.	Thermal/Cyclic	Less significant as a primary long-term mechanism compared to creep/shrinkage under sustained load.

Additional Information: Controlling Long-Term Deflections

Controlling long-term deflections is crucial for the serviceability of concrete structures. Several measures can be taken during design and construction:

Increased Member Depth: A deeper section is stiffer and deflects less.
Adequate Reinforcement: Providing sufficient tension and compression reinforcement (especially compression reinforcement) helps restrain creep and shrinkage strains, reducing long-term deflection.
Concrete Mix Design: Using concrete with lower water-cement ratio, appropriate aggregates, and potentially admixtures can reduce shrinkage and creep.
Curing: Proper curing reduces shrinkage by allowing the concrete to hydrate more fully before drying begins.
Load History: The magnitude and duration of sustained loads significantly impact creep.
Environmental Conditions: High temperatures and low humidity increase shrinkage and creep.

Engineers use codes like IS 456 or ACI 318 which provide methods and factors to estimate long-term deflections based on the effects of creep and shrinkage.