Seminar Report on Concrete in different weathering Condition2015-16

ABSTRACTConcrete is a composite material composed mainly of water, aggregate, and cement. Often, additives and reinforcements (such as rebar) are included in the mixture to achieve the desired physical properties of the finished material. When these ingredients are mixed together, they form a fluid mass that is easily molded into shape. Over time, the cement forms a hard matrix which binds the rest of the ingredients together into a durable stone-like material with many uses.

INTRODUCTIONConcrete is a composite material composed mainly of water, aggregate, and cement. Often, additives and reinforcements (such as rebar) are included in the mixture to achieve the desired physical properties of the finished material. When these ingredients are mixed together, they form a fluid mass that is easily molded into shape. Over time, the cement forms a hard matrix which binds the rest of the ingredients together into a durable stone-like material with many uses.[2]Famous concrete structures include the Hoover Dam, the Panama Canal and the Roman Pantheon. The earliest large-scale users of concrete technology were the ancient Romans, and concrete was widely used in the Roman Empire. The Colosseum in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world's largest unreinforced concrete dome.[3]After the Roman Empire collapsed, use of concrete became rare until the technology was re-pioneered in the mid-18th century. Today, concrete is the most widely used man-made material (measured by tonnage).

COLD WEATHER CONCRETEConcrete placed during cold weather will develop sufficient strength and durability to satisfy intended service requirements only if it is properly produced, placed and protected.ACI 306 Cold Weather Concreting defines cold weather concreting as a period when for more than three (3) consecutive days, the following conditions exist: The average daily air temperature is less than 5C (40F) and, The air temperature is not greater than 10C (50F) for more than one-half of any 24 hour period.Even though not defined as cold weather, protection during Spring and Fall is required during the first 24 hours to avoid freezing.What Happens When Concrete Freezes? Pore water in concrete starts to freeze around -1C (30F) As some water freezes the ion concentration in the unfrozen water goes up, furtherdepressing the freezing point. At around -3 to -4C (25 to 27F), enough of the pore water will freeze so that hydration will completely stop, and depending on the extent of hydration, and thus the strength of theconcrete, the forces generated by the expansion of ice (ice occupies ~9% more volume than water) may be detrimental to the long term integrity of the concrete.Objectives of Cold Weather ConcretingThe objectives of cold weather concreting are to: Prevent damage to concrete due to freezing at early ages Assure that concrete develops the required strength for the safe removal of forms Maintain curing conditions that foster normal strength development without usingexcessive heat Limit rapid temperature changes in the concrete to prevent thermal cracking Provide protection consistent with the intended serviceability of the structureFor every 10C (18F) reduction in concrete temperature, the times of setting of the concrete double, thus increasing the amount of time that the concrete is vulnerable to damage due to freezing. It should be noted that warm concrete placed on cold sub-grade will lose heat and its temperature will drop. It is important to understand that having the concrete reach the specified 28-day strength is irrelevant if the structure is damaged by inadequate curing and protection. Concrete that is protected from freezing until it has attained a compressive strength of at least 3.45 Mpa (500 psi) will not be damaged by exposure to a single freezing cycle. Concrete that is protected and properly cured will mature to its potential strength despite subsequent exposure to cold weather.Except in heated, protective enclosures, little or no external supply of moisture is required for curing during cold weather.Recommended Practices and Basic PrinciplesPlanningPrior to the pour, clearly define the cold weather concreting methods that will be used. A pre-placement meeting with the contractor, specifier, producer, laboratory and other interested parties is highly recommended.Curing and ProtectionWhere a specified concrete strength must be attained in a few days or weeks, protection at temperatures above 10C (50F) is required.Temperature RecordsTemperature of the concrete determines the effectiveness of protection, regardless of air temperature. Maintaining temperature records of concrete in place is essential.Heated EnclosuresMust be strong enough to be windproof and weatherproof. Combustion heaters must be vented to the outside to prevent carbonation.Exposure to Freezing and ThawingConcrete should be properly air entrained if it will be saturated and exposed to freezing and thawing cycles during construction.

SlumpAll else being equal, lower slump and/or lower water/cement ratio mixes are particularlydesirable in cold weather for flatwork. This reduces bleeding and decreases setting time.Truck Travel TimeThe distance from the plant to the point of placement can have a severe effect on thetemperature of concrete. Hot WaterWhile hot water improves setting time of cold weather concrete, after the first few batches of concrete hot water heaters may not be able to maintain hot water temperature. Later in the pour, concrete may be cooler than at the beginning of the pour.Temperatures for Placement and Protection in Concrete WeatherACI 306 provides recommended temperatures for placement and protection of concrete.

Additional Accelerator Toward the End of the PourAdding more accelerating admixture at the end of the pour will offset the effects of cooler mixing water and provide a more consistent set. Another reason to add more accelerating admixture as the pour progresses is to speed up the setting of the latter mixes and have all the concrete in the pour reach set at about the same time in order to facilitate the finishing operation.Acceleration of Concrete Hydration in Cold WeatherThe reduction of setting time and the acceleration of strength gain often result in substantial savings due to shorter protection periods, faster form reuse, earlier removal of shores, and less labor in finishing flatwork. Setting time is more important in flatwork finishing Early strength gain is more important for early form removalAcceleration may be encouraged by using: Type III Portland cement 20% additional Type I or II cement to provide Type III response Set-accelerating admixtures such as PolarSet, DCI, Daraccel, Gilco or Lubricon NCA Calcium chloride is the most cost effective accelerator available, but it causes corrosion of embedded metals in the presence of oxygen and moisture. This is why limits exist on the use of chlorides in concrete (see Grace technical bulletin TB-0105).It is important to verify that non-chloride accelerating admixtures are also noncorrosive. Some accelerating admixtures which are labeled as non-chloride may still contain materials which cause the products to be corrosive to embedded metals. Non-chloride, noncorrosive accelerators are more expensive up-front, but when life-cycle costs and regulations limiting chlorides are considered, they are the most cost effective products. Accelerators have been introduced successfully into concrete both before and after the addition of cement to the mix, but it may be best policy to add the accelerator to the mix after the cement has been wetted. On rare occasions, when accelerators are added to the mix prior to the batching of under-sulfated cements, there may be adverse reactions with the tricalcium aluminate (C3A) in the cement which may result in retardation. Therefore, we recommend that if the accelerator is to be added up-front, before the cement, it should be tested with the intended cement at the intended use temperature, prior to placement. Different mixes and materials will exhibit different setting times. It should not be assumed that two different Portland cements will set at similar rates. If pozzolans are to be used in theconcrete, they should also be included in trial mixes prior to placement. If used correctly, products such as PolarSet can be used to add 2C to 6C (5F to 10F) tothe temperature range within which concrete can be used (possibly more). At high enough dosages, concrete can be brought down to temperatures of -7C (20F) (depending on materials and mix design) without hydration stopping and without any degradation of long-term strength. In addition, in more massive structures, the heat of hydration can help protect the structure from freezing.HOT WEATHER CONCRETE Things to Consider about Hot Weather ConcretingWhen the temperature of freshly mixed concrete approaches approximately 77 degrees Fahrenheit adverse site conditions can impact the quality of concrete. Ambient temperatures above 90 degrees Fahrenheit and the lack of a protected environment for concrete placement and finishing (enclosed building) can contribute to difficulty in producing quality concrete.

The use of liquid nitrogen is one option to reduce concretes temperature during hot-weather concreting.The precautions required to ensure a quality end product will vary depending on the actual conditions during concrete placement and the specific application for which the concrete will be used. In general, if the temperature at the time of concrete placement will exceed 77 degrees Fahrenheit a plan should be developed to negate the effects of high temperatures. The precautions may include some or all of the following: 1. Moisten subgrade, steel reinforcement, and form work prior to concrete placement.2. Erect temporary wind breaks to limit wind velocities and sunshades to reduce concrete surface temperatures.3. Cool aggregates and mixing water added to the concrete mixture to reduce its initial temperature. The effect of hot cement on concrete temperature is only minimal.4. Use a concrete consistency that allows rapid placement and consolidation.5. Protect the concrete surface during placement with plastic sheeting or evaporation retarders to maintain the initial moisture in the concrete mixture.6. Provide sufficient labor to minimize the time required to place and finish the concrete, as hot weather conditions substantially shorted the times to initial and final set.7. Consider fogging the area above the concrete placement to raise the relative humidity and satisfy moisture demand of the ambient air.8. Provide appropriate curing methods as soon as possible after the concrete finishing processes have been completed.9. In extreme conditions consider adjusting the time of concrete placement to take advantage of cooler temperatures, such as early morning or night time placement.With proper planning and execution concrete can be successfully placed and finished to produce high quality durable concrete at temperatures of 95 degrees Fahrenheit or more.Setting Time

The effect of high ambient temperatures and high temperature concrete component materials have on the setting time of concrete mixtures is a topic of concern due to the reduced time in which concrete must be placed, consolidated and finished; increased potential for plastic shrinkage cracking, thermal cracking and cold joints; potential strength reduction due to high water demand and high curing temperatures; difficulty in controlling air content; and increased urgency for applying appropriate curing method at an early age. As a general rule of thumb an increase of 20 degrees Fahrenheit will reduce the setting time of a concrete mixture by as much as 50 percent. As an example, a concrete mixture that reaches final set in three hours at 60 degrees Fahrenheit may reach final set in as little 1 hours at 80 degrees Fahrenheit. As the concrete temperature increases the setting time is further reduced. The actual temperature of the concrete mixture as delivered is effected by the temperature of the materials used in the mixture, the cementitious content of the mixture, the temperature of the equipment used to batch and transport the concrete, and the ambient temperature and conditions at the project site. Concrete applications may be considered hot weather concrete at temperatures ranging from 77 to 95 degrees Fahrenheit depending on the specific application. Precautions should be planned in advance to counter the effects of high temperature well in advance of execution to counter these effects.

Precautions may include use of materials with a good performance history in high temperature conditions, cool concrete materials or concrete mixture, provide concrete consistency and placement equipment and crew for rapid placement, reduce time of transport, schedule placement to limit exposure to atmospheric conditions (night time placement or more favorable weather), plan to limit rapid moisture loss (sun screens, wind screens, misting, or fogging), and consider the use of an evaporation retarder. Schedule a preconstruction meeting including all of the participants to discuss the plan to control the effects specific to the project and expected conditions.

DURABILITY OF CONCRETEConcrete durability has been defined by the American Concrete Institute as its resistance to weathering action, chemical attack, abrasion and other degradation processes.Durability is the ability to last a long time without significant deterioration. A durable material helps the environment by conserving resources and reducing wastes and the environmental impacts of repair and replacement. Construction and demolition waste contribute to solid waste going to landfills. The production of new building materials depletes natural resources and can produce air and water pollution The design service life of most buildings is often 30 years, although buildings often last 50 to 100 years or longer. Most concrete and masonry buildings are demolished due to obsolescence rather than deterioration. A concrete shell can be left in place if a building use or function changes or when a building interior is renovated. Concrete, as a structural material and as the building exterior skin, has the ability to withstand natures normal deteriorating mechanisms as well as natural disasters. Durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and properties desired. For example, concrete exposed to tidal seawater will have different requirements than an indoor concrete floor. Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of concrete. Seawater Exposure: Concrete has been used in seawater exposures for decades with excellent performance. However, special care in mix design and material selection is necessary for these severe environments. A structure exposed to seawater or seawater spray is most vulnerable in the tidal or splash zone where there are repeated cycles of wetting and drying and/or freezing and thawing. Sulfates and chlorides in seawater require the use of low permeability concrete to minimize steel corrosion and sulfate attack. A cement resistant to sulfate exposure is helpful. Proper concrete cover over reinforcing steel must be provided, and the water-cementitious ratio should not exceed 0.40. Chloride Resistance and Steel Corrosion: Chloride present in plain concrete that does not contain steel is generally not a durability concern. Concrete protects embedded steel from corrosion through its highly alkaline nature. The high pH environment in concrete (usually greater than 12.5) causes a passive and noncorroding protective oxide film to form on steel. However, the presence of chloride ions from deicers or seawater can destroy or penetrate the film. Once the chloride corrosion threshold is reached, an electric cell is formed along the steel or between steel bars and the electrochemical process of carrions begins. The resistance of concrete to chloride is good; however, for severe environments such as bridge decks, it can be increase by using a low water-cementitious ratio (about 0.40), at least seven days of moist curing, and supplementary cementitious materials such as silica fume, to reduce permeability. Increasing the concrete cover over the steel also helps slow down the migration of chlorides. Other methods of reducing steel corrosion include the use of corrosion inhibiting admixtures, epoxy-coated reinforcing steel, surface treatments, concrete overlays, and cathodic protection. Resistance to Alkali-Silica Reaction (ASR): ASR is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures, and mixing water. The reactivity is potentially harmful only when it produces significant expansion. Indications of the presence of alkali-aggregate reactivity may be a network of cracks, closed or spalling joints, or movement of portions of a structure. ASR can be controlled through proper aggregate selection and/or the use of supplementary cementitious materials (such as fly ash or slag cement) or blended cements proven by testing to control the reaction. Abrasion Resistance: Concrete is resistant to the abrasive affects of ordinary weather. Examples of severe abrasion and erosion are particles in rapidly moving water, floating ice, or areas where steel studs are allowed on tires. Abrasion resistance is directly related to the strength of the concrete. For areas with severe abrasion, studies show that concrete with compressive strengths of 12,000 to 19,000 psi work well. Concrete crack? Concrete, like most materials, will shrink slightly when it dries out. Common shrinkage is about 1/16th of an inch in a 10-foot length of concrete. The reason contractors place joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint, where concrete cracks due to shrinkage are expected to occur. Control or construction joints are also placed in concrete walls and other structures. Concrete surfaces spallConcrete spalling (or flaking) can be prevented. It occurs due to one or more of the following reasons. 1.) In cold climates subjected to freezing and thawing, concrete surfaces have the potential to spall if the concrete is not air-entrained. 2.)Too much water in the concrete mix will produce a weaker, more permeable and less durable concrete. The water-cementitious ratio should be as low as possible (0.45 or less).3.) Concrete finishing operations should not begin until the water sheen on the surface is gone and the excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because finishing operations have begun too soon, the concrete on the surface will have too high of a water content and this surface will be weaker and less durable. Inedible: Vermin and insects cannot destroy concrete because it is inedible. Some softer materials are inedible but still provide pathways for insects. Due to its hardness, vermin and insects will not bore through concrete. Gaps in exterior insulation to expose the concrete can provide access for termite inspectors.Moderate to Severe Exposure Conditions for Concrete: The following are important exposure conditions and deterioration mechanisms in concrete. Concrete can withstand these effects when properly designed. The Specifiers Guide for Durable Concrete is intended to provide sufficient information to allow the practitioner to select materials and mix design parameters to achieve durable concrete in a variety of environments. Resistance to Freezing and Thawing: The most potentially destructive weathering factor is freezing and thawing while the concrete is wet, particularly in the presence of deicing chemicals. Deterioration is caused by the freezing of water and subsequent expansion in the paste, the aggregate particles, or both. With the addition of an air entrainment admixture, concrete is highly resistant to freezing and thawing. During freezing, the water displaced by ice formation in the paste is accommodated so that it is not disruptive; the microscopic air bubbles in the paste provide chambers for the water to enter and thus relieve the hydraullic pressure generated. Concrete with a low water-cementitious ratio (0.40 or lower) is more durable than concrete with a high water-cementitious ratio (0.50 or higher). Air-entrained concrete with a low water-cementitious ratio and an air content of 5 to 8% will withstand a great number of cycles of freezing and thawing without distress.Chemical Resistance: Concrete is resistant to most natural environments and many chemicals. Concrete is virtually the only material used for the construction of wastewater transportation and treatment facilities because of its ability to resist corrosion caused by the highly aggressive contaminants in the wastewater stream as well as the chemicals added to treat these waste products. However concrete is sometimes exposed to substances that can attack and cause deterioration. Concrete in chemical manufacturing and storage facilities is specially prone to chemical attack. The effect of sulfates and chlorides is discussed below. Acids attack concrete by dissolving the cement paste and calcareous aggregates. In addition to using concrete with a low permeability, surface treatments can be used to keep aggressive substances from coming in contact with concrete. Effects of Substances on Concrete and Guide to Protective Treatments discusses the effects of hundreds of chemicals on concrete and provides a list of treatments to help control chemical attack. Resistance to Sulfate Attack: Excessive amounts of sulfates in soil or water can attack and destroy a concrete that is not properly designed. Sulfates (for example calcium sulfate, sodium sulfate, and magnesium sulfate) can attack concrete by reacting with hydrated compounds in the hardened cement paste. These reactions can induce sufficient pressure to cause disintegration of the concrete. Like natural rock such as limestone, porous concrete (generally with a high water-cementitious ratio) is susceptible to weathering caused by salt crystallization. Examples of salts known to cause weathering of concrete include sodium carbonate and sodium sulfate. Sulfate attack and salt crystallization are more severe at locations where the concrete is exposed to wetting and drying cycles, than continuously wet cycles. For the best defense against external sulfate attack, design concrete with a low water to cementitious material ratio (around 0.40) and use cements specially formulated for sulfate environmentsSulfate attack in concrete and mortarSulfate attack can be external or internal.External: due to penetration of sulfates in solution, in groundwater for example, into the concrete from outside.Internal: due to a soluble source being incorporated into the concrete at the time of mixing, gypsum in the aggregate, for example. External sulfate attackThis is the more common type and typically occurs where water containing dissolved sulfate penetrates the concrete. A fairly well-defined reaction front can often be seen in polished sections; ahead of the front the concrete is normal, or near normal. Behind the reaction front, the composition and microstructure of the concrete will have changed. These changes may vary in type or severity but commonly include: Settlement and BleedingConcrete is said to be in a plastic state before it begins to set. The aggregate is dispersed by the cement paste and the particles in the paste are dispersed in the water. After placing, there is a period of settlement when the particles come closer together; most of this settlement usually occurs within an hour or so of placement. Total volume change may, in extreme cases, amount to 1 per cent or more, but it is not of great significance because the concrete is in a plastic or semiplastic state and no appreciable stresses can result from these changes. During settlement, water often appears at the surface, having exuded from the plastic mass. This phenomenon is called bleeding.Accumulation of water at the top of a mass of concrete is often undesirable; for example, when concrete is placed continuously in a deep form, the upper part can gain progressively more water as the filling of the form progresses, leading to relatively poor quality at the top. On the other hand, the accumulation of some water at the surface is not always undesirable because surface water is required to prevent plastic shrinkage and to lubricate the tools used for finishing the surface. Again, an excess of surface water may lead to a thin layer of slurry on the finished surface and a weak susceptible layer on the surface of the concrete. Care must be taken that finishing does not begin before the bleeding period is over.Settlement may give rise to structural flaws. A layer of water may be left under horizontal reinforcing bars so that half the area of contact between the steel and concrete is lost. This problem can be eliminated by proper vibration or revibration of the plastic concrete, care being taken not to touch reinforcing. It must not be overlooked, however, that settlement and bleeding do result in a reduction of water content. If not offset by one of the undesirable features discussed, the effect is beneficial to strength, permeability and volume stability.

FACTORS AFFECTING DURABILITY OF CONCRETEDurability of Concrete depends upon the following factors:Cement contentMix must be designed to ensure cohesion and prevent segregation and bleeding. If cement is reduced, then at fixed w/c ratio the workability will be reduced leading to inadequate compaction. However, if water is added to improve workability, water / cement ratio increases and resulting in highly permeable material.CompactionThe concrete as a whole contain voids can be caused by inadequate compaction. Usually it is being governed by the compaction equipments used, type of formworks, and density of the steelworkCuringIt is very important to permit proper strength development aid moisture retention and to ensure hydration process occur completely

CoverThickness of concrete cover must follow the limits set in codesPermeabilityIt is considered the most important factor for durability. It can be noticed that higher permeability is usually caused by higher porosity .Therefore, a proper curing, sufficient cement, proper compaction and suitable concrete cover could provide a low permeability concreteTypes of Durability of Concrete There are many types but the major Concrete Durability types are:1. Physical durability2. Chemical durabilityPhysical Durability Physical durability is against the following actions1. Freezing and thawing action2. Percolation / Permeability of water3. Temperature stresses i.e. high heat of hydrationChemical Durability Chemical durability is against the following actions1. Alkali Aggregate Reaction2. Sulphate Attack3. Chloride Ingress4. Delay Ettringite Formation5. Corrosion of reinforcementCauses for the Lack of Durability in Concrete 1. External Causes:a. Extreme Weathering Conditionsb. Extreme Temperaturec. Extreme Humidityd. Abrasione. Electrolytic Actionf. Attack by a natural or industrial liquids or gases2. Internal Causesa) Physical Volume change due to difference in thermal properties of aggregates and cement paste Frost Actionb) Chemical Alkali Aggregate Reactions i. Alkali Silica Reactionii. Alkali Silicate Reactioniii. Alkali Carbonate Reaction Corrosion of Steel

CONCLUSIONPrecautions may include use of materials with a good performance history in high temperature conditions, cool concrete materials or concrete mixture, provide concrete consistency and placement equipment and crew for rapid placement, reduce time of transport, schedule placement to limit exposure to atmospheric conditions (night time placement or more favorable weather), plan to limit rapid moisture loss (sun screens, wind screens, misting, or fogging), and consider the use of an evaporation retarder. Schedule a preconstruction meeting including all of the participants to discuss the plan to control the effects specific to the project and expected conditions.

REFERENCE http://www.cement.org/ http://www.cipremier.com http://www.cement.org/for-concrete-books-learning/concrete-technology/concrete-construction/hot-weather-concreting

Dept. of Civil Engg. 21 GPTC Muttom