10.4
Factors Affecting Creep
In normal-weight aggregate concrete, the hardened cement paste is the primary contributor to creep, whereas the aggregates, being stiffer than the cem…
In concrete mixes, the hardened cement paste primarily contributes to creep, while the aggregates in the mix restrain creep.
Aggregates with a high modulus of elasticity, being stiffer, reduce creep. Hence, the greater their volume in the concrete, the lower the creep.
Creep linearly increases with the stress applied within design limits. Beyond the design limits, creep increases non-linearly with the applied stress.
Consequently, for concrete mixes with similar cement paste content, lower water-to-cement ratio mixes exhibit higher strength and less creep.
Moreover, as the age of load application increases, the creep decreases as the concrete gains strength with time.
Curing concrete at elevated temperatures before loading enhances its strength, further reducing creep.
Early-loaded members with high-early-strength cement have less creep than those with ordinary Portland cement.
Admixtures in concrete that increase the volume of fine pores holding moisture lead to increased creep.
Lastly, the concrete member's size and the relative humidity of the surrounding air affect creep. The higher the relative humidity, the lower the creep, and larger members exhibit less creep at constant relative humidity.
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Q1: Why does the water-to-cement ratio affect creep in concrete?
Lower water-to-cement ratios increase concrete strength, which directly reduces creep. Stronger concrete at the time of loading exhibits less creep because the material is more resistant to stress-induced deformation. This relationship is fundamental to controlling creep through concrete mix design and material selection.
Q2: How do aggregates influence creep behavior in concrete?
Aggregates restrain creep because they are stiffer than hardened cement paste. Their modulus of elasticity determines their stiffness, and greater aggregate volume in the concrete reduces overall creep. The cement paste, not the aggregates, is the primary contributor to creep deformation in normal-weight aggregate concrete.
Q3: What is the relationship between applied stress and creep within design limits?
Within design limits, creep increases linearly with applied stress. However, when stress exceeds design limits, creep increases non-linearly and accelerates due to microcracking in the concrete material. This distinction is critical for predicting creep behavior under different loading conditions and stress ranges.
Q4: How does the age of concrete at loading affect creep?
Creep decreases as the age of load application increases because concrete gains strength over time. Applying loads at later stages diminishes creep significantly. Additionally, curing at elevated temperatures before loading enhances strength and further reduces creep development in structural members.
Q5: Why does relative humidity influence creep in concrete members?
Higher relative humidity reduces creep in concrete. Admixtures that increase fine pore volume holding moisture lead to increased creep. The concrete member's size, expressed as volume-to-surface ratio, also affects creep; larger members exhibit less creep at constant relative humidity conditions.
Q6: How does cement type affect creep development?
Early-loaded members with high-early-strength cement exhibit less creep than those with ordinary Portland cement. High-early-strength cement allows concrete to gain strength faster before loading, reducing subsequent creep significantly. Cement type selection is therefore important for controlling creep in structural applications and design.
Q7: What role does concrete strength play in determining creep?
Concrete strength is inversely related to creep; stronger concrete exhibits significantly less creep. Strength can be enhanced through lower water-to-cement ratios, elevated temperature curing, and delayed loading. All these factors work together to minimize stress-induced deformation and creep in concrete members.