Accelerated curing

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Accelerated curing is any method by which high early age strength is achieved in concrete. These techniques are especially useful in the prefabrication industry, wherein high early age strength enables the removal of the formwork within 24 hours, thereby reducing the cycle time, resulting in cost-saving benefits. [1] The most commonly adopted curing techniques are steam curing at atmospheric pressure, warm water curing, boiling water curing and autoclaving.

Contents

A typical curing cycle involves a preheating stage, known as the "delay period" ranging from 2 to 5 hours; heating at the rate of 22 °C/hour or 44 °C/hour until a maximum temperature of 50−82 °C has been achieved; then maintaining at the maximum temperature, and finally the cooling period. The whole cycle should preferably not exceed 18 hours. [2] [3]

Mechanism

At heightened temperatures, the hydration process moves more rapidly and the formation of the Calcium Silicate Hydrate crystals is more rapid. The formation of the gel and colloid is more rapid and the rate of diffusion of the gel is also higher. However, the reaction being more rapid leaves lesser time for the hydration products to arrange suitably, hence the later age strength or the final compressive strength attained is lower in comparison to normally cured concrete. This has been termed as the crossover effect. [4]

The optimum temperature has been found to be between 65 and 70 °C, beyond which the losses in later age strength have been found to be considerably higher. [3]

Delay period

Accelerated curing techniques invariably involve high temperatures. This may induce thermal stresses in the concrete. Further, the water in the pores starts to exert pressure at higher temperatures. The combined effect of the pore pressure and thermal stresses causes a tensile stress within the body of the concrete. If the accelerated curing process is begun immediately after the concrete has been poured, then the concrete will not be able to withstand the tensile stresses as it requires time to gain some strength. Moreover, these microcracks formed may then lead to the delayed formation of ettringite, which is formed by the transformation of metastable monosulfate. Delayed ettringite formation (DEF) induces expansion in the concrete thereby weakening it. DEF is promoted by the formation of the cracks which enables the easy entry of water. Therefore, a delay period is allowed to elapse before the commencement of the curing process to allow the concrete to gain a certain minimum tensile strength. The setting time of the concrete is an important criterion to determine the delay period. Generally, the delay period is equal to the initial setting time which has been found to give satisfactory results. Lesser delay periods result in compressive strength losses. [1]

Excessive temperatures

Excessive temperatures cause a drop in the Compressive strength due to the "crossover" effect. Higher temperatures would reduce the cycle time and therefore improve the economy of the manufacturing process, however, the compressive strength obtained would also be lower. Therefore, it is a trade-off between cost saving benefits and the loss in compressive strength. Depending on the type of project and economic considerations, either the cycle time is designed to suit the concrete mix or vice versa. [3]

Role of pozzolanic material

Pozzolona increases the later age strength of concrete as it reacts with calcium hydroxide and turns it into calcium-silicate-hydrates (C-S-H). However Portland pozzolona cements have higher activation energy and therefore, their rate of hydration is lower as compared to ordinary Portland cement (OPC). This results in lower early age strength as compared to OPC. Accelerated curing techniques radically help to increase the rate of strength gain. Halit et al. [5] showed that steam curing improved the 1 day compressive strength values of high volume fly ash concrete mixtures (40%, 50% and 60% fly ash by replacement) from 10MPa to about 20MPa which is sufficient to enable the removal of formwork and greatly aids the precast concrete industry.

Related Research Articles

Concrete Composite construction material

Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement that hardens (cures) over time. In the past, limebased cement binders, such as lime putty, were often used but sometimes with other hydraulic cements, such as a calcium aluminate cement or with Portland cement to form Portland cement concrete. Many other non-cementitious types of concrete exist with other methods of binding aggregate together, including asphalt concrete with a bitumen binder, which is frequently used for road surfaces, and polymer concretes that use polymers as a binder.

Cement Hydraulic binder used in the composition of mortar and concrete

A cement is a binder, a substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Concrete is the most widely used material in existence and is behind only water as the planet's most-consumed resource.

Portland cement

Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th century by Joseph Aspdin, and usually originates from limestone. It is a fine powder, produced by heating limestone and clay minerals in a kiln to form clinker, grinding the clinker, and adding 2 to 3 percent of gypsum. Several types of Portland cement are available. The most common, called ordinary Portland cement (OPC), is grey, but white Portland cement is also available. Its name is derived from its resemblance to Portland stone which was quarried on the Isle of Portland in Dorset, England. It was named by Joseph Aspdin who obtained a patent for it in 1824. However, his son William Aspdin is regarded as the inventor of "modern" Portland cement due to his developments in the 1840s.

Reinforced concrete Concrete with rebar

Reinforced concrete (RC), also called reinforced cement concrete (RCC), is a composite material in which concrete's relatively low tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength or ductility. The reinforcement is usually, though not necessarily, steel reinforcing bars (rebar) and is usually embedded passively in the concrete before the concrete sets.

Pozzolana or pozzuolana, also known as pozzolanic ash, is a natural siliceous or siliceous-aluminous material which reacts with calcium hydroxide in the presence of water at room temperature. In this reaction insoluble calcium silicate hydrate and calcium aluminate hydrate compounds are formed possessing cementitious properties. The designation pozzolana is derived from one of the primary deposits of volcanic ash used by the Romans in Italy, at Pozzuoli. The modern definition of pozzolana encompasses any volcanic material, predominantly composed of fine volcanic glass, that is used as a pozzolan. Note the difference with the term pozzolan, which exerts no bearing on the specific origin of the material, as opposed to pozzolana, which can only be used for pozzolans of volcanic origin, primarily composed of volcanic glass.

Lime mortar

Lime mortar is composed of lime and an aggregate such as sand, mixed with water. The Ancient Egyptians were the first to use lime mortars, which they used to plaster the pyramids at Giza. In addition, the Egyptians also incorporated various limes into their religious temples as well as their homes. Indian traditional structures built with lime mortar, which are more than 4,000 years old like Mohenjo-daro is still a heritage monument of Indus valley civilization in Pakistan. It is one of the oldest known types of mortar also used in ancient Rome and Greece, when it largely replaced the clay and gypsum mortars common to ancient Egyptian construction.

Metakaolin is the anhydrous calcined form of the clay mineral kaolinite. Minerals that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than cement particles, but not as fine as silica fume.

Ground-granulated blast-furnace slag is obtained by quenching molten iron slag from a blast furnace in water or steam, to produce a glassy, granular product that is then dried and ground into a fine powder. Ground-granulated blast furnace slag is highly cementitious and high in CSH which is a strength enhancing compound which improves the strength, durability and appearance of the concrete.

Pozzolan Siliceous volcanic ashes commonly used as supplementary cementitious material

Pozzolans are a broad class of siliceous or siliceous and aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties. The quantification of the capacity of a pozzolan to react with calcium hydroxide and water is given by measuring its pozzolanic activity. Pozzolana are naturally occurring pozzolans of volcanic origin.

Ettringite Calcium sulfo-aluminate

Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2(SO4)3(OH)12·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration. It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.

Tricalcium aluminate Ca3Al2O6, often formulated as 3CaO·Al2O3 to highlight the proportions of the oxides from which it is made, is the most basic of the calcium aluminates. It does not occur in nature, but is an important mineral phase in Portland cement.

Alkali–silica reaction Deleterious expansive chemical reaction affecting reactive siliceous aggregates and damaging concrete

The alkali–silica reaction (ASR), more commonly known as "concrete cancer", is a deleterious swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive amorphous silica found in many common aggregates, given sufficient moisture.

Types of concrete

Concrete is produced in a variety of compositions, finishes and performance characteristics to meet a wide range of needs.

Concrete degradation Damages inflicted to concrete by various harmful physical phenomena and chemical or biological processes

Concrete degradation may have various causes. Concrete can be damaged by fire, aggregate expansion, sea water effects, bacterial corrosion, calcium leaching, physical damage and chemical damage. This process adversely affects concrete exposed to these damaging stimuli.

Concrete has relatively high compressive strength, but significantly lower tensile strength. The compressive strength is typically controlled with the ratio of water to cement when forming the concrete, and tensile strength is increased by additives, typically steel, to create reinforced concrete. In other words we can say concrete is made up of sand ,ballast, cement and water

The environmental impact of concrete, its manufacture and applications, are complex. Some effects are harmful; others welcome. Many depend on circumstances. A major component of concrete is cement, which has its own environmental and social impacts and contributes largely to those of concrete.

The pozzolanic activity is a measure for the degree of reaction over time or the reaction rate between a pozzolan and Ca2+ or calcium hydroxide (Ca(OH)2) in the presence of water. The rate of the pozzolanic reaction is dependent on the intrinsic characteristics of the pozzolan such as the specific surface area, the chemical composition and the active phase content.

Energetically modified cement Class of cements, mechanically processed to transform reactivity

Energetically modified cements (EMCs) are a class of cements made from pozzolans, silica sand, blast furnace slag, or Portland cement. The term "energetically modified" arises by virtue of the mechanochemistry process applied to the raw material, more accurately classified as "high energy ball milling" (HEBM). This causes, amongst others, a thermodynamic transformation in the material to increase its chemical reactivity. For EMCs, the HEBM process used is a unique form of specialised vibratory milling discovered in Sweden and applied only to cementitious materials, here called "EMC Activation".

Sulfate attack in cement, mortar and concrete.

Gyrolite Rare phyllosilicate mineral crystallizing in spherules

Gyrolite, NaCa16(Si23Al)O60(OH)8·14H2O, is a rare silicate mineral (basic sodium calcium silicate hydrate: N-C-S-H, in cement chemist notation) belonging to the class of phyllosilicates. Gyrolite is also often associated with zeolites. It is most commonly found as spherical or radial formations in hydrothermally altered basalt and basaltic tuffs. These formations can be glassy, dull or fibrous in appearance.

References

  1. 1 2 Erdem, T. (2003). "Setting time: an important criterion to determine the length of the delay period before steam curing of concrete". Cement and Concrete Research. 33 (5): 741–050. doi:10.1016/S0008-8846(02)01058-X.
  2. ↑ ACI 517.2 R-87, Accelerated Curing of Concrete at Atmospheric Pressure-State of the Art, ACI Manual of Concrete 1992, Revised.
  3. 1 2 3 Turkel, S.; Alabas, V. (2005). "The effect of excessive steam curing on Portland composite cement concrete". Cement and Concrete Research. 35 (2): 405–411. doi:10.1016/j.cemconres.2004.07.038.
  4. ↑ Paya, J.; Monzo, J.; Perismora, E.; Borrachero, M.; Tercero, R.; Pinillos, C. (1995). "Early-strength development of portland cement mortars containing air classified fly ashes". Cement and Concrete Research. 25 (2): 449–456. doi:10.1016/0008-8846(95)00031-3.
  5. ↑ Yazici, H.; Aydin, S.; Yigiter, H.; Baradan, B. (2005). "Effect of steam curing on class C high-volume fly ash concrete mixtures". Cement and Concrete Research. 35 (6): 1122–1127. doi:10.1016/j.cemconres.2004.08.011.
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