10.4
影响蠕变的因素
在普通重量骨料混凝土中，硬化水泥浆是蠕变的主要因素，而骨料比水泥浆更硬，对应力引起的变形更有弹性。骨料的刚度由其弹性模量决定，混凝土中骨料的体积越大，蠕变越小。
此外，水灰比至关重要，因为较低的水灰比会增加混凝土强度，从而减少蠕变。混凝土在加载时的强度与蠕变成反比;强度越高的混凝土蠕变越小。随着时间…
在混凝土混合物中，硬化水泥浆体是导致徐变的主要因素，而混合物中的骨料则会限制徐变。
弹性模量较高的集料刚度更大，能够减少徐变。因此，集料在混凝土中所占体积越大，徐变越小。
在设计限值范围内，蠕变随施加的应力呈线性增加。超过设计限值后，蠕变随施加的应力呈非线性增加。
因此，对于水泥浆含量相近的混凝土拌合物，水灰比较低的拌合物表现出更高的强度和更小的徐变。
此外，随着施加荷载时混凝土龄期的增加，由于混凝土随时间推移强度逐渐增长，其徐变会减小。
在加载前将混凝土在较高温度下养护，可提高其强度，进一步减少徐变。
采用早强水泥的早龄期加载构件，其徐变小于采用普通硅酸盐水泥的构件。
混凝土中增加持水细孔体积的外加剂会导致徐变增大。
最后，混凝土构件的尺寸和周围空气的相对湿度会影响徐变。相对湿度越高，徐变越小；在相对湿度恒定时，尺寸较大的构件表现出的徐变也较小。
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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.