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The process of curing concrete, handling it in different climatic conditions and the various diseases that can arise in concrete structures are the key points of this paper.
The word concrete is derived from a latin word “concretus”which means compact or condensed. Concrete is a thick composite artificial stone ‘like material which is made by mixing cement (generally Portland cement) and various other aggregates, such as sand, pebbles, gravel, shale, etc., with water and allowing the mixture to harden by hydration. Since the ancient times it had been used in constructing structures and today also it is used more than any other man ‘made construction material in the world.
Depending upon the reqirement of the structure,the proportions of the main ingredients discussed below are varied to avail different types of concrete. By doing so the finished product can be tailored to its application with varying strength, density, or chemical and thermal resistance properties.
Portland cement is a basic ingredient of concrete. It consists of a mixture of oxides of calcium, silicon and aluminium. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay, and grinding this product (called clinker) with a source of sulfate (most commonly gypsum).
When water is added in a cementitious material, cement paste is formed through the process of hydration. The cement paste glues the aggregate together, fills voids within it, and allows it to flow more smoothly. Water should be added in the mix very carefully as less water in the cement paste will yield a stronger, more durable concrete; more water will give an free-flowing concrete with a higher slump. Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure.
In the process of hydration many different reactions occur at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles, and other components of the concrete, to form a solid compact mass.
The presence of aggregate greatly increases the robustness of concrete above that of cement.Following are the different items which could be used as aggregates:
Sand, natural gravel and crushed stone
Recycled aggregates (from construction, demolition and excavation waste) and manufactured aggregates like air-cooled blast furnace slag and bottom ash
Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative “exposed aggregate” finish.
CURING OF CONCRETE
The term curing concrete signifies the process of stopping freshly poured concrete from drying out too quickly. Unless the concrete mass is not cured, or if it is left out to dry out on its own, it will not develop the full bond between all of its ingredients. In order to form a water cement paste, water is added in Portland cement after which chemical reactions take place in the mixture turns the paste into a bonding agent. This reaction is known as hydration and it produces a stone-like compact substance’the hardened cement paste. Both the rate and degree of hydration, and the resulting strength of the final concrete, depend on the curing process that follows placing and consolidating the plastic concrete. As long as the mixture contains water and the temperature conditions are favorable, the process of Hydration continues ,though at a decreasing rate . The strength of concrete is dependent on crystal growth within the concrete matrix through the process of hydration If water is not adequate, the crystals cannot grow, due to which the concrete is unable to attain the desired strength. The presence of water through proper curing of the concrete facilitates the crystal growth that encloses the gravel and sand mix, causing interweaving with each other.
.The time-span of the process of Curing begins from the time of consolidation and ends at the point where the concrete reaches its design strength. The period can vary from a few days to a month or longer. For most structural use, the curing period for cast-in-place concrete is usually 3 days to 2 weeks. During this period, the concrete mass should be kept moist and as near 73’F as practical. Bridge decks and other slabs exposed to weather and chemical attack usually require longer curing periods. The time span of the curing period also depends on the factors such as
‘ type of cement used
‘ mix proportions
‘ required strength
‘ size and shape of the concrete mass
‘ weather conditions and temperature
‘ future exposure conditions
SIGNIFICANCE OF CURING
A proper and effective curing process helps in improving various properties of concrete such as
‘ freeze and thaw resistance
‘ wear resistance
‘ volume stability.
‘ Serviceability and apperance
The compressive strength of concrete that has been properly cured is 75 to 95 per cent more than the concrete that has not been cured. Figure 1 illustrates this by comparing the compressive strength of concrete(at 180 days) for which the surface has been either kept moist for the entire duration of 180 days or is kept moist for various periods of time and allowed to dry out or is allowed to dry out from the time it was first made.It is quite clearly depicted that the compressive strength of the concrete is highest when it is fully cured for the entire period of 180 days.
Fig 1:Moist curing effect on compressive strength of concrete source:www.tpub.com
The permeability and absorptivity of concrete mix depends upon the porosity of the mix ie whether the pores and capillaries are discrete or interconnected. The porosity of the mix is reduced overtime through proper and effective curing of concrete thereby increasing the durability of concrete. Also, proper curing of concrete mix will reduce crazing, dusting and scaling of the slab thereby ensuring better serviceability and appearance.
STAGES IN CURING PROCESS
After the concrete mix have been placed, it is imperative to keep it moist and maintain specified concrete temperatures.The process of curing starts immediately after the placement of concrete and it goes through two stages :
Initial curing stage: time of placement to initial set.
This stage would include all the deliberate action taken between placement and final finishing of concrete. Approximate conditions during the initial curing period should be forecast prior to construction. Under this stage,the rate of strength gain of the concrete is minimal.
The various aspects to be dealt with construction ‘initial curing period as shown in diagram below are :
verifying evaporation conditions: concrete temperatures, wind velocity, air temperature, and relative humidity are required to be taken into account during placement. These elements are used to make nomograph to calculate the evaporation rate and to determine whether critical drying rates exist. Using the concrete placing temperature, the time of initial setting can be estimated. The time of initial setting is the optimal time for application of final curing.
Major items requiring attention during construction-initial curing period. Source : www.fhwa.dot.govt
Onsite adjustments for Excessive drying: In order to reduce evaporation rates of bleed water the following two adjustments could be made-
reducing concrete placing temperatures- Cooling aggregate stockpiles, cooling mixing water, or using ice for mixing water are very effective ways of reducing concrete temperatures.
use of evaporation reducers. Evaporation reducers are water emulsions of film-forming compounds. The film-forming compound is the active ingredient that slows down evaporation of water. There is also a benefit from the water fraction of the evaporation reducers, in that it compensates to a small degree for losses of mixing water to evaporation.Evaporation reducers may need to be applied several times, depending on the conditions.
Concrete that are liable to quick drying is required to be covered with wet gunny bag or wet hessian cloth properly squeezed, so that the water does not drip and at the same time, does not allow the concrete to dry.
This condition should be maintained for 24 hours or at least till the final setting time of cement at which duration the concrete will have assumed the final volume.
The Final curing stage: The final curing period is defined as the time interval between application of curing procedures and the end of deliberate curing. Final curing methods fall into four categories:
Water curing : Water curing can be done in various ways like immersion,ponding, spraying or fogging and wet covering.This is by far the best method of curing as it satisfies all the requirements of curing, namely, promotion of hydration, elimination of shrinkage and absorption of the heat of hydration.
Curing a house slab by flooding. Source: http://www.builderbill-diy-help.com/curing-concrete.html
The precast concrete items are normally immersed in curing tanks for a certain duration. Pavement slabs, roof slab etc. are covered under water by making small ponds. Vertical retaining wall or plastered surfaces or concrete columns etc. are cured by spraying water. In some cases, wet coverings such as wet gunny bags, hessian cloth, jute matting, straw etc., are wrapped to vertical surface for keeping the concrete wet. For horizontal surfaces saw dust, earth or sand are used as wet covering to keep the concrete in wet condition for a longer time so that the concrete is not unduly dried to prevent hydration. The diagram below summarises the major criteria for using water-added curing methods.
Major features of curing with added water.
Membrane curing : this curing method is mostly used when there is less availability of water in the region where curing is done. Under this method, concrete could be covered with membrane which will effectively seal off the evaporation of water from concrete. A continuous seal over the concrete surface is maintained by means of a firm impervious film to prevent moisture in concrete from escaping by evaporation. Some of the materials, which can be used for this purpose, are bituminous compounds, polyethylene
PLASTIC SHEETING source:www.builderbill-diy-help.com
or polyester film, waterproof paper, rubber compounds etc. When waterproofing paper or polyethylene film are used as membrane, care must be taken to see that these are not punctured anywhere and also see whether adequate lapping is given at the junction and this lap is effectively sealed.
Application of heat : When concrete is subjected to higher temperature it accelerates the hydration process resulting in faster development of strength. Concrete cannot be subjected to dry heat to accelerate the hydration process as the presence of moisture is also an essential requisite. Therefore, subjecting the concrete to higher temperature and maintaining the required wetness can be achieved by subjecting the concrete to steam curing.
The exposure of concrete to higher temperature can be done by Steam curing at ordinary pressure, Steam curing at high pressure ,Curing by Infra-red radiation ,Electrical curing.
Traditional steam curing of concrete pipes (www.construction-int.com)
Miscellaneous : Calcium chloride is used either as a surface coating or as an admixture. It has been used satisfactorily as a curing medium. Both these methods are based on the fact that calcium chloride being a salt shows affinity for moisture. The salt not only absorbs moisture from atmosphere but also retains it at the surface. This moisture held at the surface prevents the mixing water from evaporation and thereby keeps the concrete wet for a long time to promote hydration. Formwork prevents escaping of moisture from the concrete, particularly, in the case of beams and columns.
Keeping the form work intact and sealing the joint with wax or any other sealing compound prevents the evaporation of moisture from the concrete. This procedure of promoting hydration can be considered as one of the miscellaneous methods of curing.
At the end of the curing process majority of cement gets hydrated.There is slow rate of compressive strength gain of around 50-100 psi/hours and little exothermic heat generation.
In the entire process of curing if one of the curing procedures is used initially, it may be replaced by one of the other procedures after the concrete is 1 day old, provided that
the concrete surface is not permitted to become dry at any time.
After the termination of the curing process the adequacy of a curing program could be verified.Although strength is the primary variable around which curing specifications are based several other approaches can also be used like
Surface Water Absorption: the amount of water a dry concrete specimen absorbs in the first minute or so after contact with liquid water is directly related to the quality of the curing of the near-surface zone of the concrete. This method has direct applicability to verifying curing. The method is reasonably applied to cores, which can be dried to a constant low moisture content before testing.
Rebound Hammer: The rebound hammer method basically measures the modulus of elasticity of the nearsurface concrete. This may actually recommend the method for use in evaluating the curing of concrete pavements, where near-surface effects are considered most important. The test method is suitable for in-place measurements. Laboratory work has shown that rebound numbers of uncured concrete exposed to modestly severe drying are reduced by about 50 percent at 7 days relative to well-cured concrete.
Strength of Cores: The strength of concrete is strongly affected by inadequate curing, and, in theory, could be detected by measuring strength of cores. the effects of poor curing are only strongly apparent in the properties of the top 50 mm of concrete, and sometimes even less.
Ultrasonic Pulse Velocity: The ultrasonic pulse velocity (UPV) method is an indirect measure of the modulus of elasticity of concrete. The modulus of elasticity of concrete tends to increase with increasing hydration (or quality of curing) of the cement paste fraction of the concrete. UPV testing can be set up in a number of configurations, each of which tends to focus on slightly different features of the concrete. A simple pulse velocity taken through a piece of concrete, which is the traditional way of using UPV to investigate concrete properties, gives information on the average quality of the concrete.
Abrasion Resistance: The degree of curing has been shown in numerous research publications to be strongly reflected in the abrasion resistance of the cement-paste fraction of concrete. This truth is easily verified qualitatively using an electrically powered wire brush and observing the ease with which the near-surface mortar can be removed from a small spot of concrete. Poorly cured concrete is easily abraded away, while well-cured concrete is quite difficult to abrade away with such equipment.
DISEASES OF CONCRETE
Deterioration of concrete buildings is becoming a cause of concern now a days. Often the structures located along open water, lakes, rivers and oceans are extremely vulnerable to attack from the harsh environment i.e. salt, moisture, humidity, carbon dioxide, etc. cracks, spalls and rust stains are some of the visual symptoms of the deterioration of concrete.
Crack is the most common diseases of concrete structures and it occurs as a result of material discontinuity. Thermal and shrinkage cracks can be associated with high cement content and high strength concrete. Usually cracks occur in the concrete structures due to various reasons such as excess water in the mix ,rapid drying of the concrete, improper strength concrete poured on the job, lack of control joints, if Concrete is poured on frozen ground, alkali-silica reactionetc.
In order to prevent cracks in concrete structures, following preventive measures could be taken:
‘ Estimate in advance the quantity of water to be mixed to get the required concrete mix. Adding too much water in the mix is one of the main causes of cracked concrete
‘ Do not pour concrete on the frozen ground .A compacted subgrade could be used to pour concrete upon to prevent cracking
‘ Cut control joints into the concrete so that the slab will be able to shrink and expand with temperature changes. Control joints should be cut the same depth of the slab and spaced a maximum of three times the thickness of the concrete.
‘ The slab must retain enough moisture so that the drying and shrinking happens as slowly as possible in the days and weeks after pouring. Curing helps the concrete to retain moisture in the concrete so that it can continue to gain strength to resist cracking.
CRACKS ON CONCRETE SURFACE
When the concrete is critically saturated i.e. approximately 91% of its pores are filled with water then deterioration of concrete from freeze thaw actions may occur. When water freezes to ice it occupies 9% more volume than that of water.In case there is no space in concrete for this expanded volume , a kind of distress is caused which will result in loss of concrete surface.
A surface active agent should be added to the concrete mixture in order to prevent frost in a concrete structure. This creates a large number of closely spaced, small air bubbles in the hardened concrete which would act as expansion chambers to relieve the pressure build-up. It is to be noted that concrete with high water content and high water to cement ratio is less frost resistant than concrete with lower water content.
ABRASION AND CAVITATION
Hydraulic Concrete structures are affected by Abrasion-erosion due to the action of debris rolling and grinding against surface. The sources of the debris include construction trash left in a structure, riprap brought back into a basin by eddy currents because of poor hydraulic design or asymmetrical discharge, and riprap or other debris thrown into a basin by the public. Mechanical abrasion is usually characterized by long shallow grooves in the concrete surface and spalling along monolith joints. general abrasion and cavitation erosion results in coarse aggregate exposed concrete surface, concrete uneven surface, resulting in holes.
In order to prevent the structure to suffer from abrasion the concrete should include the maximum amount of the hardest coarse aggregate that is available and the lowest practical w/c. In addition to this,high-range water-reducing admixtures (HRWRA) and condensed silica fume have been used to develop high compressive strength concrete 97 MPa (14,000 psi) , at this strength the concrete mix assumes a greater role in resisting abrasion-erosion damage. Also vacuum-treated concrete, polymer concrete, polymer-impregnated concrete, and polymer portland cement concrete could also be used to increase the abrasion resistance. In existing structures, balanced flows should be maintained into basins by using all gates to avoid discharge conditions where eddy action is prevalent.
Carbonation is a chemical reaction between atmospheric carbon dioxide and hydrated cement compounds which causes a reduction in the alkalinity of the concrete. The permeability and moisture content of the concrete directly affects the rate of carbonation .
“Pop-corn” like calcite crystals present in carbonated paste.
In the process of carbonation the calcium bearing phases present in the concrete mass are attacked by carbon dioxide of the air and converted to calcium carbonate.In this process the alkalinity of the concrete is lowered from an initial pH of around 13.5 to around 8.5 over some years.
One method of testing a structure for carbonation is to drill a fresh hole in the surface and then treat the cut surface with phenolphthalein indicator solution. This solution will turn [pink] when in contact with alkaline concrete, making it possible to see the depth of carbonation.
In order to combat the process of carbonation aquron products from Allcrete industries could be used which helps in the following ways
‘ seals out moisture to a depth of 100mm or more
‘ reduces oxygen permeability by a pore-blocking process
‘ brings up the alkalinity of the concrete
‘ purging and binding chlorides in the colloidal silicate hydrogel formed in the pores and capillaries of the concrete
It is to be noted that when Concrete is treated within 24 hours of casting ,it will be protected for life against carbonation problems and aged concrete, once treated, will become immune to further deterioration.
Corrosion of steel reinforcement in concrete structures is well known to be “Concrete Cancer”, which happens to be a significant area of concern in the area of infrastructure and building across the world. Corrosive species can enter the concrete mix if the ingredients of the concrete mix such as water, aggregates, additives are contaminated.When such corrosive species reacts with the chemical compounds under the porous nature of concrete,the problem of corrosion occurs.
Corrosion damage to the reinforcing steel results in the build-up of voluminous corrosion products, generating internal stresses and subsequent cracking and spalling of the concrete as shown schematically in the diagram below:
Corrosion mechanism Concrete Cancer
The most frightening thing is that the corrosion can be going on under the surface and will not be noticed until the concrete starts visibly disintegrating. The outcome of such corrosion is that concrete reduces its strength and also the steel re-enforcing within the concrete can rust and the pressure this creates can cause the concrete to crack and crumble. Buildings in coastal areas are especially at risk.
The preventive measures to avoid such kind of concrete cancer are listed below:
Epoxy coating: The concrete structures that are exposed to deicing salt may benifit from use of epoxy-coated, hot dip galvanised or stainless steel rebar. Epoxy coated rebar can easily be identified by the light green colour of its epoxy coating.
Applying Sealants: After the process of curing, penetrating sealants must be applied. Sealants include paint, plastic foams, films and aluminum foil, felts or fabric mats sealed with tar, and layers of bentonite clay, sometimes used to seal roadbeds.
corrosion inhibitors: calcium nitrite can also be added to the water mix before pouring concrete. Generally, 1’2 wt. % of calcium nitrate with respect to cement weight is needed to prevent corrosion of the rebars.
Extreme weather conditions such as high ambient and concrete temperature, low relative humidity or a mere 40′ F or less average daily air temperature for more than 3 consecutive days, tend to impair the quality of freshly mixed or hardened concrete thereby giving detrimental results. Therefore, it becomes imperative to handle concrete with caution both in extreme hot and cold weather.
EFFECTS OF HOT WEATHER AND PROPER HANDLING
‘ On the compressive strength: When the temperature of concrete is high cement hydrates at a much faster rate, it sucks up water and grows crystals around the aggregate particles but don’t have time to grow strong . Although, the early strength will be higher but 28-day strength suffers. If the concrete is about 18′ hotter than normal (for example, 88′ instead of 70’), the ultimate compressive strength of the mix will be about 10% lower.
‘ On the colors of integrally colored concrete: under hot weather conditions , slump decreases rapidly as the cement sets up and and more mixing water is needed. In integrally colored concrete,this can lead to variations in water content which can result in significant differences in concrete color between adjacent pours.
‘ Surface drying: high concrete temperature and hot dry wind across the concrete can cause more drying and surface shrinkage.
‘ Thermal differentials: sometimes the hot weather condition makes one part of the concrete mass warmer than another part. If this differential is greater than about 20’F then concrete gets cracks.
‘ Difficulty in maintaining air content can be a problem in warm concrete. Mixing is more likely to drive air out of the concrete making the level difficult to control.
Dealing with hot weather concreting
Plastic shrinkage is a particular problem in hot weather concreting. As a general rule, each 10o F increase in ambient temperature reduces slump about 1″. A switch from ASTM C494 Type A to Type D water reducing and set retarding mix may be part of an effective plan for hot-weather concreting.
Aggregates forms a major part in a concrete mix so it’s temperature has the greatest effect on concrete temperature. Shading of aggregate piles is ideal, although not always possible. Using cool water is another way to get cool concrete. Ready mix producers in hot climates use chilled water or ice to lower the concrete temperature.
Retarding admixtures can also contribute towards controlling concrete in warm weather. When the concrete is hot, the setting time is very quick .At that time Retarder can be added at the plant or on the job site . Retarders delays the setting time but they also give the concrete more time to dry out, so curing is critical. Retarders come as straight retarders or as water-reducing and retarding admixtures. Mid-range water reducers can increase the air content of the concrete. For concrete that is to be stamped, consider using step retardation–adding retarder to the mix after half of the batch or after one-third and two-thirds have been placed. For textured concrete one of the strongest things to do in hot weather is step retardation.
In order to avoid slump loss superplasticizer (high-range water reducer) could be used. These admixtures can increase slump without affecting the concrete’s final strength or appearance.
Before placing the concrete on a subgrade wet down everything, subgrade and forms, with cool water so moisture isn’t absorbed from the concrete, which can lead to cracking.
In order to prevent the evaporation of surface water use a monomolecular film or evaporation retarder as soon as the concrete is down and bull floated. These materials evaporate after a couple of hours. Monomolecular film will prevent plastic shrinkage cracking and surface crusting.
In hot and dry weather curing needs to start as soon as finishing operations are completed. A white pigmented curing compound could be used with plain gray concretein order reflect sunshine. Also the white curing blanket could be used for the same purpose.
A white pigmented curing compound on plain gray concrete white curing blankets keep the concrete cooler by reflecting can help reflect some heat from the sun the sun
In a hot climate, stain the concrete in the coolest part of the day as staining concrete relies on a chemical reaction that happens faster in hot weather.If concrete is stained in 95′ to 100′ weather ,it can ruin the entire job.
Staining is best done early in the day when the concrete is cooler
For an overlay installation it is advisable that the concrete surface temperature should be between 50′ and 80′.Installing an overlay during the hottest part of the day amidst direct sunlight could adversely affect the quality of the result.
EFFECTS OF COLD WEATHER AND PROPER HANDLING
During cold weather until the concrete gains compressive strength of 500psi , it is under threat of either getting freezed up which in turn could break up the matrix or the concrete mix sets at a very slow pace. Below 40’F the hydration reaction basically stops and the concrete doesn’t gain any further strength. when the ground is cold, the concrete in contact with it will be cold and will set more slowly. This can lead to the problem of crusting, with the top part of the concrete set and the bottom still soft.
During the process of shifting the concrete mix from the ready mix plant to the job site there will be some heat loss. While placing concrete in cold it is advisable to remove all snow and ice from that area. Also any standing water should also be removed that could get mixed into the concrete. In case of a frozen land, it is advisable to thaw it using hydronic heat pipes and blankets (such as those from Ground Heaters), or electric blankets (check out Power Blanket). It is also suggested to Warm up anything that will come in contact with the concrete, including forms and any embedments, to at least 32’F. Covering everything with tarps the day before the pour, will keep it dry and warm enough.also blankets could be used for the same purpose as well. Place triple layers of insulating blankets at corners and edges that could freeze. Wrap any protruding rebars. Make sure the blankets won’t blow off during the night.
Ground heaters Inc Portland cement association
While placing decorative concrete in cold weather it is suggested to use a dial pocket thermometer or an infrared thermometer to test the concrete temperature as it is needed to be kept above 50’F for the concrete to keep gaining strength. Also on exterior concrete, customers should be reminded to keep deicing chemicals off the surface during the first winter. Deicers can lead to spalling of newer concrete.
During the process of finishing concrete in cold weather, it is advisable to let all the bleed water to evaporate first otherwise water-cement ratio would increase surface of concrete would get weak. Bleed water is basically the concrete particles settling (like mud in a stirred up pond) and squeezing out all the extra water. Vacuums can also be used for this purpose. It should also be noted that sealing concrete in cold weather conditions is very risky.
Power blanket Layfield Group
In order to keep the slab warm hydronic heating pipes or electric heating blankets could be used. These are laid on top of the slab and insulated. Also, in case it is too cold to even place the concrete, then the only option left is to enclose the work and heat the air. In case of an enclosure,it is required to consider the potential problem of carbonation. With unvented heaters (salamanders), or even with gas-powered equipment, the carbon dioxide levels can increase. This carbon reacts with the concrete, creating a chalky carbonated layer at the surface which is unacceptable. To fix this problem use heaters that exhaust to the outside of an enclosure or building and just blow in warm air. Maintain the concrete temperature above 40′ degree
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