Can concrete bend over time?

As concrete cures, it shrinks, which can lead to cracking. And as it reacts with water, concrete does something else. But it's not a wonder material. And as it reacts with water, concrete does something else: it creeps or deforms progressively over time.

This has been known for decades, and is included in all concrete-related calculations used in construction projects, so it's not news. But what really causes it to drag on remains a mystery. As concrete became critically important for everyday life, its limitations also became increasingly evident. Concrete is a material that is strong in compression but weak in tension (when pulled, bent or stretched).

It is essentially a brittle material. The amount of stress, weight, or load a material can withstand determines its strength. Under tensile load, concrete responds with a sudden loss of load capacity when a crack propagates from a pre-existing failure, usually at the weak interface between aggregates and cement binder. Although concrete is generally not designed to withstand tensile loads, tensile stresses exist when a concrete member undergoes bending, shearing (or sliding), or restrained deformation (such as shrinkage as it dries).

There are useful techniques to make concrete usable in large structural elements. In addition to steel reinforcement, stretched cable “tendons” can be added to cast slabs to balance tensile forces with compressive forces. If the tendons are stretched before casting, the process is called prestressing; if they are stretched later, it is called post-tensioning. Concrete is plastic until it reaches more than 90% of its final cure strength.

Concrete is brittle and subject to something called brittle fracture. Imagine holding a long piece of chalk with both hands and trying to bend it. No matter how much pressure you put on, the chalk won't bend, but it will eventually split in two. Now, imagine that the piece of chalk is a bridge.

Well, you can see the potential problem of fragility fracture there. The deformation of a structure due to a sustained load is known as concrete creep. Concrete can alter its shape if subjected to prolonged stress or stress. This deformation most often occurs in the direction of the applied force.

Examples of this are the compression of a concrete column or the bending of a beam. Concrete does not always fail or break down as a result of creep. When a load is applied to concrete, instantaneous elastic deformation occurs, which converts to creep strain if the load is sustained. The elastic deformation regime ends at a strain value of approximately 0.01 percent, similar to the deformation of a normal concrete in the event of catastrophic failure.

Concrete does not possess such mechanisms, so cracks result in an unstable fracture and a rapid loss of bearing capacity. In recent years, it has become common practice to incorporate industrial by-products such as fly ash from coal-based power plants or slag (agglomerations of mineral impurities) from steel production into greener concrete mixes. Unlike fully developed cracks in concrete or common fiber reinforced concrete, these microcracks continue to carry an increasing amount of load until composite deformation capability is achieved. On the subject of warping and warping, standard practices for slab design, vapor barrier installation and concrete placement have remained remarkably consistent ever since.

In this case, the question asked by a group of American scientists was “What causes concrete to crawl? , and published their answer in the latest issue of The Journal of Chemical Physics (free). In addition to the safety concerns of concrete infrastructure exposed to major hazards, there are also economic, social and environmental implications associated with the deterioration of infrastructure under normal service conditions. When concrete is replaced with an engineered cementitious compound (ECC), the initial flex crack extends into a band of multiple microcracks (bottom). The time-dependent component of stress that arises as a result of stress is also known as concrete creep.

The current practice of repeatedly repairing concrete structures due to surface cracks and chips (where parts break) is decidedly unsustainable. A current review of this 1976 study on concrete cracking would indicate that researchers observed plastic shrinkage cracking and shrinkage cracking as a result of short-term deformation when their test slabs cracked, even though their study did not specifically evaluate the latter phenomenon. Fiber-reinforced concrete, which contains a network of short strands usually made of steel, glass, or polymer, can significantly improve fracture strength, the amount of energy (per unit area) that dissipates at the crack tip as the crack extends. Concrete is not known for its ability to bend, but as this experimental example shows, special formulations of the material can be both flexible and strong.

Twenty-eight years after the 1976 study on concrete cracking, the 2004 edition of ACI 302 had undone, corrected or improved almost all of the findings of the original study. Because these compounds hold the concrete together, this stress was found to produce a kind of internal flow of C-S-H along the nanometer grains of the concrete, causing creep. . .