How concrete gains strength and how to accelerate hardening

Recommendation points

• Curing chemistry
• Strength of concrete
• Seasonal specifics
• Acceleration of setting and strength development

After the completion of monolithic work, a rather long stage of exposure and reinforced concrete strength gain begins. We will tell you what kind of care concrete needs during hardening, how to accelerate it and what physical and chemical phenomena accompany this process..

Curing chemistry

The construction of concrete structures that fully meet the design characteristics is a real art that cannot be comprehended without understanding the complex and continuous sequence of transformations taking place in the structure of the material. The prototypes of building binders, vaguely reminiscent of modern cement, appeared in the 3rd or 2nd millennium BC. However, the composition and ratio of the components of such mixtures were selected exclusively experimentally until the end of the 18th century, when the so-called “Roman cement” was patented. This was the first milestone in the scientific approach to the development of structural concrete..

The chemical nature of the hardening of modern cement is very complex, it includes a long chain of processes flowing into each other, during which first the simplest chemical bonds are formed, and then increasingly strong physical bonds, leading to the formation of a monolithic stone-like material. It makes no sense to consider these processes in detail for a person inexperienced in chemistry as a science, it is much more useful to assess the external signs of such phenomena and their practical meaning.

In modern construction, a predominantly Portlandic cement mixture is used, consisting of fired clay, gypsum and limestone, and from the point of view of chemistry, from oxides of calcium, silicon, aluminum and iron. Primary raw materials undergo heat treatment and fine grinding, after which the components are mixed in a precisely defined proportion. The main purpose of processing in the production process is to destroy the natural chemical and physical bonds of substances, which are subsequently restored in the presence of water. Cement, unlike untreated clay and lime, hardens due not to drying, but hydration, therefore, wetting it after final curing does not lead to softening and an increase in viscosity.

Strength of concrete

Unlike atmospheric binders, which quickly harden in air, cement hardens almost the entire service life of concrete structures. This is due to the fact that substances remain in the thickness of the frozen product that have not had time to react with water. In fact, in the production of a concrete mixture, water is added to it in an amount that is obviously insufficient for the reaction of all the particles of the mineral binder. This is due to the fact that the increased water content in concrete leads to its delamination, significant shrinkage during hardening and the appearance of internal stresses.

Nevertheless, the remnants of minerals continue to react, because concrete has a non-zero moisture content in its thickness. Because of this, its hardening does not occur instantly, but over a long time. Of the entire hardening period, the most intense period can be distinguished, which for concrete on Portland cement is 28-30 days. If during this time the concrete product is in appropriate conditions, it assumes 100% of the design strength. At the same time, in just 6-8 days of hardening, the strength of concrete reaches 60-70% of the brand, and a third of the calculated strength of the product acquires already on 2-3 days..

Seasonal specifics

The hardening of mixtures on a cement binder is accompanied by two processes – a slight increase in volume and heat release. Because of this, the course of curing reactions can differ significantly depending on external conditions..

First you need to deal with the increase in volume. This process has certain practical benefits: it facilitates easier separation of the formwork and pre-stretches the reinforcement, increasing the quality of adhesion and allowing the steel to perceive the tensile load almost immediately after its occurrence, bypassing the stage of elastic deformation. Negative consequences of expansion arise in situations where concrete is constrained by the form, for example, when pouring concrete screeds, dowels in prefabricated monolithic structures and the production of products in rigid permanent formwork. In such cases, a compressible shell device is required to compensate for linear expansion..

The release of heat can have both positive and negative effects. First you need to understand that the heating of the hardening concrete mass is most pronounced in the first 50 hours after the preparation of the mixture. The intensity of heating increases in proportion to the dimensions of the product, because it is more difficult to remove heat from the concrete. You also need to take into account that concrete with a high cement content will heat up more than low-quality.

At low air temperatures, the ability of concrete to heat up during hardening makes it relatively easy to maintain normal temperature conditions. Despite the fact that under normal conditions the minimum temperature mark for concrete work is +5 ° C, products can be poured into a permanent formwork made of expanded polystyrene even in frost down to -3 ° C: the own heat release will maintain the required temperature. Even ordinary concrete structures can be protected with insulating materials to maintain the desired temperature regime or to equip greenhouses in which a positive temperature is simply maintained. It is important to note that after the concrete has set 50-60% of its strength, frost does not have a destructive effect, for the reason that most of the water has already managed to react. However, the hardening rate drops to almost zero, which must be taken into account when determining the exposure time.

In hot weather, the natural heating of the concrete mix has a negative effect. Water evaporates from the surface too quickly, besides, heating provokes linear expansion, accompanied by the opening of cracks, which is unacceptable during the hardening of concrete. Therefore, massive products exposed to the sun must be constantly moistened and cooled with running water at least in the first 7–10 days after pouring. The rest of the curing time, the concrete can remain under the cover of the polyethylene film.

Acceleration of setting and strength development

Depending on the brand, the concrete takes 20-30 hours to finally take shape, after which it can be poured abundantly with water to make the curing process more intense. High temperature also promotes accelerated hardening, but only on condition that the heating is uniform throughout the entire thickness of the cast product. So, at the factories of concrete goods, hardening is accelerated by pouring steam over the product at a temperature of 70–80 ° C, but one must remember that heating above 90 ° C is destructive for hardening concrete.

The maximum speed of strength development can be ensured by the correct water-cement ratio of the prepared mixture, established by GOST 30515 2013. You can also speed up the process by adding various additives: calcium chloride, sodium sulfate and chloride, sodium carbonate (soda). But it must be remembered that the use of setting accelerators is limited by their limiting content, as well as by the type of concrete structure, the brand of concrete and reinforcement, and the type of cement used. GOST 30459–96 can provide more clarity on this issue..

In conclusion, it should be noted that in civil engineering, the need to accelerate the hardening of concrete is extremely rare. Concrete acquires most of the brand strength quickly enough, therefore, in the case of pouring floors or reinforced belts, construction operations can be continued already 7-10 days after the completion of monolithic works. If we are talking about the foundation, then it makes almost no sense to accelerate the hardening: the base of the building must undergo shrinkage within a year so that the support soil layer has time to stabilize and a possible distortion can be eliminated with a corrective layer or during the construction of the box.