Editing Concrete (section)

===Curing===
[[File:Curing-concrete.jpg|thumb|A concrete slab being kept hydrated during water curing by submersion (ponding)]]

====Maintaining optimal conditions for cement hydration====
Concrete must be kept moist during curing in order to achieve optimal strength and [[Reinforced concrete structures durability|durability]].<ref>"Curing Concrete" Peter C. Taylor CRC Press 2013. {{ISBN|978-0-415-77952-4}}. eBook {{ISBN|978-0-203-86613-9}}</ref> During curing [[hydrate|hydration]] occurs, allowing calcium-silicate hydrate (C-S-H) to form. Over 90% of a mix's final strength is typically reached within four weeks, with the remaining 10% achieved over years or even decades.<ref>{{cite web | title=Concrete Testing | url=http://technology.calumet.purdue.edu/cnt/rbennet/concrete%20lab.htm | access-date=10 November 2008 | archive-url=https://web.archive.org/web/20081024193802/http://technology.calumet.purdue.edu/cnt/rbennet/concrete%20lab.htm | archive-date=24 October 2008 | df=dmy-all }}</ref> The conversion of [[calcium hydroxide]] in the concrete into [[calcium carbonate]] from absorption of [[carbon dioxide|CO<sub>2</sub>]] over several decades further strengthens the concrete and makes it more resistant to damage. This [[carbonation]] reaction, however, lowers the pH of the cement pore solution and can corrode the reinforcement bars.

Hydration and hardening of concrete during the first three days is critical. Abnormally fast drying and shrinkage due to factors such as evaporation from wind during placement may lead to increased tensile stresses at a time when it has not yet gained sufficient strength, resulting in greater shrinkage cracking. The early strength of the concrete can be increased if it is kept damp during the curing process. Minimizing stress prior to curing minimizes cracking. High-early-strength concrete is designed to hydrate faster, often by increased use of cement that increases shrinkage and cracking. The strength of concrete changes (increases) for up to three years. It depends on cross-section dimension of elements and conditions of structure exploitation.<ref name="Veretennykov Yugov Dolmatov et al 2008"/> Addition of short-cut polymer fibers can improve (reduce) shrinkage-induced stresses during curing and increase early and ultimate compression strength.<ref>{{Cite web|url=http://www.minifibers.com/documents/ADMIXUS-Admixtures-for-Cementitious-Applications.pdf|archive-url=https://web.archive.org/web/20161017073633/http://www.minifibers.com/documents/ADMIXUS-Admixtures-for-Cementitious-Applications.pdf|title="Admixtures for Cementitious Applications."|archive-date=17 October 2016}}</ref>

Properly curing concrete leads to increased strength and lower permeability and avoids cracking where the surface dries out prematurely. Care must also be taken to avoid freezing or overheating due to the [[exothermic]] setting of cement. Improper curing can cause [[Spalling#Spalling in mechanical weathering|spalling]], reduced strength, poor [[abrasion (mechanical)|abrasion]] resistance and [[fracture|cracking]].

====Curing techniques avoiding water loss by evaporation====
During the curing period, concrete is ideally maintained at controlled temperature and humidity. To ensure full hydration during curing, concrete slabs are often sprayed with "curing compounds" that create a water-retaining film over the concrete. Typical films are made of wax or related hydrophobic compounds. After the concrete is sufficiently cured, the film is allowed to abrade from the concrete through normal use.<ref>{{cite web |url=http://www.daytonsuperior.com/docs/default-source/tech-data-sheets/section-05---curing-compounds.pdf?sfvrsn=3 |title=Home |access-date=12 November 2015 |url-status=live |archive-url=https://web.archive.org/web/20151208184425/http://www.daytonsuperior.com/docs/default-source/tech-data-sheets/section-05---curing-compounds.pdf?sfvrsn=3 |archive-date=8 December 2015  }}</ref>

Traditional conditions for curing involve spraying or ponding the concrete surface with water. The adjacent picture shows one of many ways to achieve this, ponding—submerging setting concrete in water and wrapping in plastic to prevent dehydration. Additional common curing methods include wet burlap and plastic sheeting covering the fresh concrete.

For higher-strength applications, [[accelerated curing]] techniques may be applied to the concrete. A common technique involves heating the poured concrete with steam, which serves to both keep it damp and raise the temperature so that the hydration process proceeds more quickly and more thoroughly.