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Preventing concrete failures by proper curing method :Construction materials

BY: Guest | Category: Others | Post Date: 2009-09-26
 



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Concrete failures are associated to several reasons from design mixtures, mixing process, component properties and so many more. Improper curing is considered one of those eminent reasons why we face such concrete failures in slabs and more evident to pavements which we can easily notice because of cracks visible to the naked eye. Concrete is a construction material considered to be an artificial conglomerate stone composed of cement, aggregate, water, and chemical admixtures. The word concrete comes from the Latin word "concretus", which means "hardened" or "hard". As the cement and water mixes, a chemical reaction called hydration occurs and normally the concrete changes from plastic to solid state in about two hours. In this stage the concrete continuous to gain strength as it cures.

Concrete strength is considered an important property which is affected by factors such as porosity, aggregates, cement and curing. In curing, if the internal relative humidity drops below 80%, hydration and strength gain will stop. The rate of strength gain is directly related to the amount of moist curing. Thus, curing should be taken essential care by the engineer in charge in the site. In all applications, extensive care needs to be taken properly in the curing of concrete to achieve best strength and hardness. Curing happens after the concrete has been placed. In order that the cement will gain its maximum strength, it requires a moist and controlled environment. The cement hardens as time passes and gaining in strength by passing of days.


Concrete mix designers or engineers needs to specify the required compressive strength of concrete, which is normally given as the 28 day compressive strength. That is why, concrete care should be taken for a period of twenty eight days, although since it is a long wait to determine if desired strengths are going to be obtained, most engineers predict the concrete strength in a three-day and seven-day analysis. In this period, a maximum care to curing is being done.

This article will discuss in laymen's terms, as possible, how curing process should be done and what necessary precautions should be made. Curing is the maintenance of a satisfactory moisture content and temperature in concrete for a suitable period of time immediately after following placing and finishing so that desired properties may be obtained. It is time, temperature and moisture that affect curing. Initial curing period is considered as the time between when concrete mixture is placed and when final curing operations can be initiated. While, the final curing period is the time between applications of final curing procedures and the termination of deliberate curing activity. Both periods of curing have their objectives in which the previous is needed to prevent excessive loss of mixing water from the plastic concrete, which can lead to plastic shrinkage cracking, while the latter is to insure that necessary water is either retained or added and that temperature is maintained within a range that the hydration of the cementitious materials can progress sufficiently for development of necessary physical properties and that temperature is controlled sufficiently to avoid damaging thermal gradients. Both stages should be given enough attention in order to avoid future failures.

The next delicate step is to know what curing method is appropriate to apply. We will just go through with the general aspect of some of the methods being practiced at present. During the initial curing stage, wind breaks and water misting devices are commonly used although there are impracticalities using this method in large pavement construction because of the large amount of materials and labor that is required. In some practices, long canopy were utilized that could be towed behind the paving operation to protect the concrete from the rain, sun and wind. In the market, evaporation reducers or commonly named as evaporation retarders are one of the popular products used in the engineering practice as of date to protect the concrete during the initial period of curing. During the final curing period, water-retention methods or water-added methods are usually utilized. Water-retention method is used to retain the mixing water in the concrete for the period of time necessary for curing to be completed while water-added methods provide water in excess of the mixing water.

Adding of water should be done to at least minimize or eliminate the effect of internal desiccation that leads to the development of shrinkage cracks. During the curing process, water is lost due to hydration. That is why, careful analysis of the methods to be used should be analyzed and properly done. Based on literatures, concrete mixtures achieve adequate strength even if all the cement does not hydrate, however it is important that additional water gets into the near-surface zone of the concrete for durability purposes. With the present concrete technology, it is noted base on practicality and economical reasons that water-retention method in the form of curing compounds is the most appropriate.

It is important to control evaporation of bleed water during the initial curing period to avoid shrinkage and this can be done by either the use of water in the form of mist and evaporation reducers. Applying water should follow specification guidance. The US Federal Highway and transportation suggested that at an application rate of 0.2 kilograms per meters squared (kg/m2) and an evaporation rate of 1 kilogram per meter squared per hour (kg/m2/h), water would need to be applied every 12 minutes (min) to avoid loss of mixing water. Another method used by concrete experts are evaporation reducer products which are potentially quite valuable for helping protect concrete pavements from excessive water under conditions of high evaporation rates. Guidance for usage is available for every product. In the final stage of curing, water-retentive method (waterproof sheeting and curing compounds) and water-added methods (water, wet burlap, wet soil, and wet cotton mats) are commonly used.

Concrete curing should start after concrete is placed, finishing is complete, and the surface sheen has disappeared. In different cases, there are different curing definitions. In the case of conventional concrete, the initial curing period would start immediately after placing, and then would end, and the final curing period would start at approximately the time of initial setting. This would normally be 2-4 hours, although it could extend for several more hours if admixtures cause retardation. In the case of slip-form paving, the initial curing period would start immediately after the paving machine passes, and the transition point to the final curing period is unclear. ACI, AASHTO and the US Department of Transportation had provided guidance on the curing duration based on specific purposes.

It is important to develop a method that would detect deficiencies before damage starts to build up. One possible approach would be to monitor the surface sheen until the end of the initial curing period. Loss of sheen during this period is an indication that evaporation is exceeding bleeding, potentially conditioning plastic shrinkage cracking. Observation of laboratory specimens indicates that in the absence of surface evaporation, a surface sheen is present until approximate time of initial setting even if there is essentially no detectable bleeding.

Another common engineering practice is direct visual inspection when pigmented curing compounds are used. The engineer should know that uneven appearance of color or whiteness will indicate problems in curing compound application.

In order to verify the strength of concrete after curing, a number of test methods such as measuring strength and by measuring surface or near-surface physical properties are applicable. Other methods include in place curing of test cylinders, ultrasonic pulse velocity, rebound hammer, pull out, and penetration resistance.

It is of great importance to know the four properties of concrete mixtures and concrete materials that affect curing practice: (1) type of cement, (2) presence of pozzolan or slag, (3) water-cement ratio and, (4) rate of strength gain. It should be noted that proper testing and proper design of water-cement ratio should always be utilized to achieve the desired strength with out any failure after the curing process. It is recommended to read the ACI and AASHTO standards for more complete and detailed explanation on concrete curing.

In conclusion, proper curing method of concrete will lead to increase in its strength and lower permeability, and definitely avoid cracking caused by surface drying prematurely. Extensive care must also be taken to avoid freezing, or overheating due to the exothermic setting of cement. This will also avoid scaling, reduced strength and abrasion resistance and cracking. It is also important that during the first week up to 10 days the concrete should not be permitted to freeze or dry out.

Acknowledgments:

US Department of Transportation
Federal Highway and transportation
04/09/07
http://www.fhwa.dot.gov/pavement/pccp/pubs/05038/chapt2.cfm

Concrete Properties
http://www.geocities.com/concretesite/tmch3.htm

Wikipedia, the free encyclopedia
www.wikipedia.com

The British Standards Institution (BSI)
European Committee for Standardization
AASHTO
American Concrete Institute (ACI)

WRITTEN BY:
FLORANTE C. POSO, JR., MCE, PhD Lecturer, Salalah College of Technology Sultanate of Oman Member: Philippine Institute of Civil Engineers

Article Source: http://www.saching.com



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