Editing Concrete (section)

==Production==
[[File:Concrete plant in Mansfield, Ohio.jpg|thumb|upright|[[Concrete plant]] showing a [[concrete mixer]] being filled from ingredient silos]]
[[File:Concrete mixing plant, Birmingham, Alabama, view 2.jpg|thumb|upright|Concrete mixing plant in [[Birmingham, Alabama]], in 1936]]

Concrete production is the process of mixing together the various ingredients—water, aggregate, cement, and any additives—to produce concrete. Concrete production is time-sensitive. Once the ingredients are mixed, workers must put the concrete in place before it hardens. In modern usage, most concrete production takes place in a large type of industrial facility called a [[concrete plant]], or often a batch plant. The usual method of placement is casting in [[formwork]], which holds the mix in shape until it has set enough to hold its shape unaided.

Concrete plants come in two main types, ready-mix plants and central mix plants. A ready-mix plant blends all of the solid ingredients, while a central mix does the same but adds water. A central-mix plant offers more precise control of the concrete quality.  Central mix plants must be close to the work site where the concrete will be used, since hydration begins at the plant.

A concrete plant consists of large hoppers for storage of various ingredients like cement, storage for bulk ingredients like aggregate and water, mechanisms for the addition of various additives and amendments, machinery to accurately weigh, move, and mix some or all of those ingredients, and facilities to dispense the mixed concrete, often to a [[concrete mixer]] truck.

Modern concrete is usually prepared as a viscous fluid, so that it may be poured into forms.  The forms are containers that define the desired shape. Concrete [[formwork]] can be prepared in several ways, such as [[slip forming]] and [[steel plate construction]]. Alternatively, concrete can be mixed into dryer, non-fluid forms and used in factory settings to manufacture [[precast concrete]] products.

Interruption in pouring the concrete can cause the initially placed material to begin to set before the next batch is added on top. This creates a horizontal plane of weakness called a ''cold joint'' between the two batches.<ref>{{Cite web |title=Cold Joints |url=https://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=372 |website=www.concrete.org.uk|archive-url=https://web.archive.org/web/20160304074543/http://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=372 |archive-date=4 March 2016 |publisher= [[The Concrete Society]]|access-date= 30 December 2015}}</ref> Once the mix is where it should be, the curing process must be controlled to ensure that the concrete attains the desired attributes. During concrete preparation, various technical details may affect the quality and nature of the product.

===Design mix===
''Design mix'' ratios are decided by an engineer after analyzing the properties of the specific ingredients being used. Instead of using a 'nominal mix' of 1 part cement, 2 parts sand, and 4 parts aggregate, a civil engineer will custom-design a concrete mix to exactly meet the requirements of the site and conditions, setting material ratios and often designing an admixture package to fine-tune the properties or increase the performance envelope of the mix. Design-mix concrete can have very broad specifications that cannot be met with more basic nominal mixes, but the involvement of the engineer often increases the cost of the concrete mix.

Concrete mixes are primarily divided into nominal mix, standard mix and design mix.

Nominal mix ratios are given in volume of <math>\text{Cement : Sand : Aggregate}</math>. Nominal mixes are a simple, fast way of getting a basic idea of the properties of the finished concrete without having to perform testing in advance.

Various governing bodies (such as [[British Standards]]) define nominal mix ratios into a number of grades, usually ranging from lower [[compressive strength]] to higher compressive strength. The grades usually indicate the 28-day cure strength.<ref>{{cite web |url=http://www.civilology.com/grades-of-concrete/ |title=Grades of Concrete with Proportion (Mix Ratio)| date=26 March 2018}}</ref>

===Mixing===
{{See also|Volumetric concrete mixer|Concrete mixer}}

Thorough mixing is essential to produce uniform, high-quality concrete.

{{em|Separate paste mixing}} has shown that the mixing of cement and water into a paste before combining these materials with [[Construction aggregate|aggregates]] can increase the [[compressive strength]] of the resulting concrete.<ref>{{Cite web |title=Concrete International |url=https://www.concrete.org/publications/concreteinternational.aspx |access-date=2022-08-13 |archive-url=https://web.archive.org/web/20070928092034/http://www.concreteinternational.com/pages/featured_article.asp?ID=3491 |archive-date=28 September 2007|website=concrete.org|date=1 November 1989}}</ref> The paste is generally mixed in a {{em|high-speed}}, shear-type mixer at a [[Water-cement ratio|w/c]] (water to cement ratio) of 0.30 to 0.45 by mass. The cement paste premix may include admixtures such as accelerators or retarders, [[Plasticizer|superplasticizers]], [[pigment]]s, or [[silica fume]]. The premixed paste is then blended with aggregates and any remaining batch water and final mixing is completed in conventional concrete mixing equipment.<ref>{{cite web | title=ACI 304R-00: Guide for Measuring, Mixing, Transporting, and Placing Concrete (Reapproved 2009) | url=https://www.concrete.org/store/productdetail.aspx?ItemID=30400}}</ref>

Resonant acoustic mixing has also been found effective in producing ultra-high performance cementitious materials, as it produces a dense matrix with low porosity.<ref>{{Cite journal |last=Vandenberg |first=Aileen |last2=Wille |first2=Kay |date=2019-06-02 |title=The Effects of Resonant Acoustic Mixing on the Microstructure of UHPC |url=https://www.iastatedigitalpress.com/uhpc/article/id/9636/ |journal=International Interactive Symposium on Ultra-High Performance Concrete |volume=2 |issue=1 |doi=10.21838/uhpc.9636 |issn=0000-0000|doi-access=free }}</ref>

===Sample analysis—workability===
{{Main|Concrete slump test}}
[[File:Cannon Renewal Project - October 2016 (30662609012).jpg|thumb|Concrete floor of a [[parking garage]] being placed]]
[[File:Concreteathruz.jpg|thumb|Pouring and smoothing out concrete at Palisades Park in Washington, DC]]

Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (pouring, pumping, spreading, tamping, vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of [[hydration reaction|hydration]]) and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures increases concrete workability. Excessive water leads to increased bleeding or [[Segregation in concrete|segregation of aggregates]] (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. Changes in gradation can also affect workability of the concrete, although a wide range of gradation can be used for various applications.<ref>{{cite book |last1=Sarviel |first1=Ed |title=Construction Estimating Reference Data |date=1993 |publisher=Craftsman Book Company |isbn=978-0-934041-84-3 |url={{google books|plainurl=y|id=TopgKO4x_2kC}}|page=74 |language=en}}</ref><ref>{{cite journal |last1=Cook |first1=Marllon Daniel |last2=Ghaeezadah |first2=Ashkan |last3=Ley |first3=M. Tyler |title=Impacts of Coarse-Aggregate Gradation on the Workability of Slip-Formed Concrete |journal=Journal of Materials in Civil Engineering |date=1 February 2018 |volume=30 |issue=2 |doi=10.1061/(ASCE)MT.1943-5533.0002126 }}</ref> An undesirable gradation can mean using a large aggregate that is too large for the size of the formwork, or which has too few smaller aggregate grades to serve to fill the gaps between the larger grades, or using too little or too much sand for the same reason, or using too little water, or too much cement, or even using jagged crushed stone instead of smoother round aggregate such as pebbles. Any combination of these factors and others may result in a mix which is too harsh, i.e., which does not flow or spread out smoothly, is difficult to get into the formwork, and which is difficult to surface finish.<ref>{{cite web |url=https://www.concretenetwork.com/aggregate/ |title=Aggregate in Concrete – the Concrete Network |access-date=15 January 2017 |url-status=live |archive-url=https://web.archive.org/web/20170202232307/https://www.concretenetwork.com/aggregate/ |archive-date=2 February 2017  }}</ref>

Workability can be measured by the [[concrete slump test]], a simple measure of the plasticity of a fresh batch of concrete following the [[ASTM]] C 143 or EN 12350-2 test standards. Slump is normally measured by filling an "[[Duff Abrams|Abrams cone]]" with a sample from a fresh batch of concrete. The cone is placed with the wide end down onto a level, non-absorptive surface. It is then filled in three layers of equal volume, with each layer being tamped with a steel rod to consolidate the layer. When the cone is carefully lifted off, the enclosed material slumps a certain amount, owing to gravity. A relatively dry sample slumps very little, having a slump value of one or two inches (25 or 50&nbsp;mm) out of {{convert|1|ft|mm|spell=in}}. A relatively wet concrete sample may slump as much as eight inches. Workability can also be measured by the [[flow table test]].

Slump can be increased by addition of chemical admixtures such as plasticizer or [[superplasticizer]] without changing the [[water-cement ratio]].<ref>{{cite journal |last1=Ferrari |first1=L. |last2=Kaufmann |first2=J. |last3=Winnefeld |first3=F. |last4=Plank |first4=J. |title=Multi-method approach to study influence of superplasticizers on cement suspensions |journal=Cement and Concrete Research |date=October 2011 |volume=41 |issue=10 |pages=1058–1066 |doi=10.1016/j.cemconres.2011.06.010 }}</ref> Some other admixtures, especially air-entraining admixture, can increase the slump of a mix.

High-flow concrete, like [[self-consolidating concrete]], is tested by other flow-measuring methods. One of these methods includes placing the cone on the narrow end and observing how the mix flows through the cone while it is gradually lifted.

After mixing, concrete is a fluid and can be pumped to the location where needed.

===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.