Creep in concrete is the gradual, time-dependent deformation that occurs when concrete is under sustained load. Even after the initial elastic compression that happens the moment a load is applied, concrete continues to slowly deform over months and years. This ongoing strain can eventually reach one to three times the original elastic deformation, making it one of the most important long-term behaviors that engineers must account for in structural design.
How Creep Works at a Microscopic Level
Concrete is not a simple solid. The cement paste that binds everything together contains a gel-like material (calcium silicate hydrate, often abbreviated C-S-H) riddled with tiny pores filled with water. When a sustained load presses down on concrete, three things happen inside this microstructure over time.
First, water trapped in nanoscale pores slowly migrates under pressure, moving from areas of high stress to areas of lower stress. Second, the layered sheets of C-S-H gel slide against one another, rearranging gradually under load. Third, some of the bonds within the gel break and reform in new positions. Together, these mechanisms allow the concrete to deform without cracking, almost like an extremely slow flow. The process is fastest in the first few months after loading and tapers off over several years, though it never fully stops.
Basic Creep vs. Drying Creep
Engineers distinguish between two types of creep based on whether moisture is escaping the concrete.
Basic creep occurs even when no moisture leaves the concrete. If you sealed a loaded concrete specimen on all sides so no water could evaporate, it would still deform over time due to the internal gel rearrangement and pore water movement described above. Basic creep is driven primarily by the material’s own microstructure under stress.
Drying creep is the additional deformation that occurs when concrete is both loaded and allowed to dry. It’s sometimes called the Pickett effect. When water evaporates from the pores, it triggers extra deformation of the C-S-H gel and causes pore water to flow more aggressively. Drying creep results from four overlapping mechanisms: free shrinkage (which would happen without load), basic creep, stress-induced shrinkage (where the load accelerates moisture loss), and microcracking near the drying surface. In real structures exposed to air, both types happen simultaneously.
What Influences How Much a Concrete Creeps
Water-to-Cement Ratio
A higher water-to-cement ratio produces a more porous cement paste, and more porous paste creeps more. Research using microindentation testing has confirmed that both creep and creep recovery increase as the water-to-cement ratio rises. Interestingly, stiffer pastes (lower water-to-cement ratios) recover a larger proportion of their total creep when the load is removed, meaning more of their deformation is reversible. With higher water-to-cement ratios, a greater share of the deformation becomes permanent.
Aggregate Type and Volume
Aggregates (the gravel and sand in concrete) do not creep significantly on their own. They act as internal restraints, resisting the deformation of the cement paste around them. Stiffer aggregates with a higher elastic modulus reduce overall creep. Conversely, using weaker or more porous aggregates increases it. Recycled aggregate concretes, for example, show creep coefficients roughly 20 to 35% higher than conventional mixes made with natural stone, largely because recycled aggregates contain residual cement paste that is more compressible.
Concrete Strength
Higher-strength concrete generally creeps less. The denser cement matrix in high-strength mixes, often achieved through supplementary materials like silica fume, leaves fewer pores and less mobile water. High-strength lightweight concrete has shown creep resistance comparable to normal-weight high-strength concrete of similar compressive strength, demonstrating that it’s the density and quality of the cement matrix, not just the weight of the mix, that governs creep behavior.
Curing Age at Loading
Concrete loaded at a young age creeps more than concrete loaded after extended curing. As cement continues to hydrate over weeks and months, the microstructure becomes denser and stiffer. Creep decreases with increasing curing age because there are fewer unhydrated pores and the gel structure is more developed by the time load is applied.
How Humidity and Temperature Affect Creep
Ambient humidity has a direct effect on the total magnitude of creep. Lower humidity increases the rate of water evaporation from the concrete, which amplifies drying creep. Research comparing concrete beams in different environments found that specimens stored at lower humidity consistently showed higher total creep values. However, short-term fluctuations in humidity do not cause noticeable swings in creep rate on a day-to-day basis.
Temperature, on the other hand, drives short-term fluctuations in creep deformation. When temperatures rise, the viscosity of pore water drops and molecular movement within the C-S-H gel speeds up, both of which accelerate creep. Comparative studies between laboratory-controlled conditions and natural outdoor exposure have confirmed that temperature changes are the primary source of short-term variations in creep rate, while humidity determines the overall magnitude over the long term.
Why Creep Matters in Structures
Creep is not just a laboratory curiosity. It has real consequences for how structures perform over decades.
In prestressed concrete, the steel tendons are stretched and anchored against the concrete to keep it in compression. As the concrete creeps under this sustained compressive force, it shortens, and the tendons lose some of their initial tension. Combined with elastic shortening and shrinkage, total prestress losses typically fall in the range of 23 to 25% of the initial stress, with creep accounting for a substantial portion of that loss. Engineers design for this by initially overstressing the tendons so the final effective force, after all losses, is still adequate.
In tall buildings and long-span bridges, differential creep between columns or between different parts of a structure can cause uneven shortening. If one column carries more sustained load than its neighbor, it will creep more and shorten more over time. This can crack finishes, misalign mechanical systems, and redistribute forces in ways that were not part of the original design.
Creep also causes long-term deflection in beams and slabs. A concrete floor that meets deflection limits on day one may sag noticeably over several years as creep adds to the initial elastic deflection. Building codes require designers to estimate long-term deflections by applying a creep multiplier to the immediate deflection, typically doubling or tripling it depending on the duration of loading and when the load was first applied.
How Creep Is Estimated in Practice
Because creep develops over years, engineers rely on prediction models rather than waiting to measure it. The most widely used models take into account the concrete’s compressive strength, the age at which it was loaded, the relative humidity of the environment, and the size of the structural member (since thicker members dry more slowly and therefore experience less drying creep relative to their volume).
The result is usually expressed as a creep coefficient: the ratio of long-term creep strain to the initial elastic strain. A creep coefficient of 2.0, for instance, means the concrete will eventually deform an additional two times its initial elastic compression. For typical structural concrete loaded at 28 days in moderate humidity, creep coefficients generally fall between 1.5 and 3.0, with most of the deformation occurring in the first two to five years.
Reducing creep in practice comes down to the same factors that improve concrete quality overall: using a low water-to-cement ratio, choosing stiff and dense aggregates, allowing adequate curing before applying sustained loads, and controlling the drying environment during early age. In critical structures like nuclear containment vessels or long-span post-tensioned bridges, these measures are tightly specified because even modest excess creep can compromise performance over a service life measured in decades.