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Curing, Concrete Cosmetics, Protective Systems for Durable Precast Construction

ABSTRACT: Construction without of the use of Construction Chemicals is not possible today. Precast elements addresses many of the shortcomings with respect to quality of the concrete and it is imperative that Construction Chemicals be a vital com-ponent of improving precast production to promote sustainable construction.

Depending on the units produced, and their eventual use, a great many demands are made on pre-cast concrete: early and ultimate strengths, durability and aesthetics being the most im-portant. A particular challenge in the precast concrete industry is the production of consistently high quality units. In this re-spect a flawless surface finish and well-formed edges are of prime importance. In addition the specifications, with regard to strength and durability, must be met.

Apart from offering High Performance Superplasticizers for Concrete, Construction Chemicals today offer a wide variety of solutions for the Precast Industry to protect the elements and enhance the durability of the elements cast. Solutions include:

1. Latest Generation Curing Compounds that can provide pro-tection to segments against weather, heal micro-cracking on surfaces and aid adhesion of segments

2. New Generation Concrete Cosmetics that enhance the fin-ish of the segment surface and provide protection against carbonation and weathering

3. Impregnation Systems / Coating Systemsas protective coating on segments to prevent water ingress and carbonation. This article reviews the underlying mechanisms of these

additives and the latest applications / joint studies undertaken at MC-Bauchemie and other industry partners incorporating these materials for use in Precast Construction.

Over the years pre-cast concrete construction has evolved as an alternative to in-situ concrete to speed up construction times, and achieve better quality in terms of finish and durabili-ty. Precast Concrete Units offer this advantage by virtue of being factory finished. A production process ensures excellent finish, dimensional tolerances, required strength, adequate quality controls and consistency.

From the precast manufacturer’s perspective, the require-ments above makes it imperative that units produced are of con-sistently high quality. For a manufacturer, targets on optical fin-ish, sharp unbroken edges, compressive and flexural strengths and durability must be met. At the same time, the primary aim for every manufacturer is the achievement of high early strength

to allow early stripping times / shorter cycle times to ensure financial viability.

Often despite the strict quality controls and standard of care, the elements are subject to damage from various quarters due to handling, incorrect curing, etc.

Figures 1 to 5 show some of these damages.

Sunny SurlakerHead Admixtures DivisionMC Bauchemie India Pvt. Ltd.

Figure 1: Honeycombs in Precast Element due to incomplete compaction

Figure 2: Damaged Edges in Precast Element due to handling

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Construction chemicals can provide solutions to mitigate these damages and protect the elements from further wear in service. Some of the solutions that construction chemicals provide are:

1. Latest Generation Curing Compounds to provide protection to segments against weather, heal micro-cracking on sur-faces and aid adhesion of segments

2. New Generation Concrete Cosmetics for restoring damaged

surfaces of the segments to enhance the surface finish and provide protection against carbonation and weathering

3. Impregnation Systems / protective coatings as protection to prevent water ingress and carbonation in the segments

These materials are discussed in the segments below

Latest Generation Curing Compounds for Precast Segments

One basic constraint in precast segments is the curing of the concrete elements once it has been cast and the formwork re-moved. Curing does take place and is well intended to be done, but due to several reasons it cannot be completed optimally when done with WATER.

Figure 6 shows general storage of precast elements. When stored outdoors, due to rising temperatures and wind, the con-crete elements dry up so fast that it becomes very difficult to maintain anefficient curing cover with water or wet burlap. This incomplete curing problem is amplified on mega sites. All of us are well aware of problems incomplete curing causes; viz.

a. Lowering of compressive and flexural strengthsb. Sanding and dusting of surfaces and lowering abrasion re-

sistancec. Higher permeabilityd. Cracking due to plastic shrinkage, drying shrinkage and

thermal cracking.e. Increased rate of carbonationf. Lower weathering and frost resistanceg. Higher ingress of chlorides and atmospheric chemicals

Figure 3: Damaged Edges in Precast Element due to incomplete compaction

Figure 4: Damage around lifting ring in Precast Element

Figure 5: Larger Picture Shows Cracking due to incomplete curing, inset shows cracking due to improper loading during lifting and handling

Figure 6: Storage of Precast Elements Outdoors

Hence there has to be some consideration given to the cur-ing of concrete elements. In such cases it becomes very im-portant for us to understand the need of the curing so that the elements attain the required strength. Also curing with water has certain disadvantages to it like late beginning, no quality control and excessive supervision needed, causes wetting and drying cycles for concrete, allows entry of water in concrete, which may create problems later on. Of course in regions of northwest India, where rain is scarce, water curing also puts pressure on water resources. Therefore, in such cases, using a curing compound, offers best possible advantages.

Considering Curing Compounds, the following key proper-ties are essential:

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1. Should be Membrane Forming to stop water loss from the surface

2. Early Beginning3. One Time application: from QC Perspective4. Should prevent entry of H2O in concrete5. Should reduce concrete temperature6. Should be solar reflective

Types of Curing Compounds most often used and most suitable for concrete works:

1. Wax Based2. Acrylic Resin Based3. Resin Based

These types of curing compounds all have curing efficiency between 80-90%. They are all membrane forming compounds. Some other types of curing compounds may be commercial-ly available but may not be of membrane forming type. When we discuss curing efficiency, it is generally the comparison of rate of strength gain of compound cured concrete vs. rate of strength gain in concrete placed underwater for 28 days. Since submerged curing is never possible on site, curing compounds provide the next safest alternative.

The next selection criteria for selection of curing com-pounds should be for bonding of subsequent layers. Certain wax based or resin based curing compounds are based on materials that degrade over time in the presence of UV. If the curing compound does not degrade, it needs to be removed by mechanical, thermal or chemical means before the next layer of treatment (grouts, segmental bonding adhesives, etc.) can be applied. In such cases, use of an acrylic based curing compound will provide the best possible solution, especially in precast construction. Some wax or resin based compounds are more suited for large expanses of exposed concretes, such as roads, parking areas, decks, etc. which needs the curing compounds to degrade over the curing period, under action of UV rays.

The fundamental difference between Acrylic Curing com-pounds and other materials is that they are based on select-ed Acrylic Polymer Resins emulsified in water. Other curing compounds are based generally on waxes emulsified in water or solvents or on aromatic hydrocarbon resins. These curing compounds need complete degradation or manual / mechan-ical removal of the wax or resins (these act like bond breaking materials between concrete and subsequent layers of glue or coatings) before subsequent layers of treatment. If any part of the curing compounds remains within surface pores of the con-crete, it may affect bonding of subsequent layers. Here Acrylic curing compounds display an advantage, that due to its inher-ent formulation, it does not affect bond of subsequent layers. This property provides a failsafe against incomplete bonding and subsequent bond failure during construction. In addition, it heals any microcracks formed in concrete, thereby providing an excellent visual finish for concrete elements and improves its overall durability.

Properties of Acrylic Based Curing Compounds most suit-ed to Precast Concrete are:

- Early Beginning- One Time application: Excellent from QC perspective- No entry of water in concrete

- Reduction of concrete temperature- No wetting and drying problem- Heals / Seals Microcracks in Concrete- Does not affect bond of subsequent layers like bonding ad-

hesives or coatings- Seals Concrete Safely- UV Stable- Solar reflectivity- No need for removal from concrete surface as it does not

affect bonding of subsequent layers

These Properties make it an excellent option for use in Building Projects for curing precast concrete segments.

Concrete Cosmetics

Precast Concrete is cast under controlled conditions and is subjected to handling at early strengths. Due to this, the ele-ments are often prone to surface damages such as blowholes, fine cracks, chipped edges, uneven surfaces and so on. These damages do not significantly hamper the durability of the ele-ment, but the visual damage often makes the elements unac-ceptable to the end users.

To address this concern, the concept of concrete cosmet-ics was introduced. Concrete cosmetics are very fine polymer modified cement mortars, that can be drawn into very thin lay-ers and add durability to the applied surfaces. As the name sug-gests, these mortars are not intended as structural mortars, but are rather fairing coats that can be used to repair minor damages to the elements. Different applications of this material are shown in Figure 7.

The advantages as opposed to using conventional cement for such repairs are:

1. These mortars are non-shrink2. These materials are modified with suitable polymers to

improve adhesion and properties in flexure to ensure ade-quate bond to the substrate

3. These materials can be drawn into very thin layers without cracking

4. These materials are resistant to water and carbonation5. They provide a smooth finish similar to fair faced concrete6. Colors can be matched by mixing selected quantities of a

white and grey mortars7. Easy to mix and apply8. Rapid development of mechanical properties

Figure 7: Different applications of Concrete Cosmetics

Areas of application for Concrete Cosmetics

- For porous or rough concrete the entire surface can be coated with Cosmetic repair mortar

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- To repair precast elements with shrinkage cracks- To level off broken corner and edges on pre-cast elements- To repair uneven surfaces in precast concrete elements- Finishing coat for concrete in isolated areas and whole sur-

faces as part of a concrete repair system- Smoothening and restoring of concrete surfaces. Surface

profile and character of fair-faced concrete can be retained.- Closing of pores and blowholes (scratch coat)

Hydrophobic Impregnations / Coatings to Protect Elements

After production, precast elements are often stored for ex-cessive periods in the stockyard, until it is time to dispatch. In this time, the elements are subject to inclement weather, rain, water, harmful gases and carbonation effects. To prevent ag-ing of the elements and protect it, the entire concrete surface must be provided with a final seal coating. This is mandatory to maintain status quo of corrosion. Such surface treatments perform several duties at the one time. Firstly, all the concrete is protected from further stress due to aggressive pollutants in the air and from progressive carbonation.

These coating systems must have to a high CO2 resistance if they are to be effective in protecting against carbonation and, on the other hand, they must not have a negative effect on the element’s water vapor diffusion rate.

To achieve this, as an additional, protective measure, par-ticularly in the case of porous substrates susceptible to abra-sion, rough handling, a hydrophobic impregnation agent can be applied up to saturation point to the surfaces first treated by a fine finish. This impregnation will ensure that there will be no water infiltration or carbonation, leading to more extensive damage if hairline cracks form or if the surface is subjected to mechanical damage. This helps to drastically improve the du-rability of the elements.

These sealers / impregnations are based on advanced na-no-technology, which help these treatments to penetrate the concrete deeply, react with the substrate and provide durabili-ty. When cured, these impregnations allow water vapor trans-mission through the concrete, while preventing liquid water that could contain dissolved chlorides or acids, from passing through the concrete. At the same time, they are UV resistant, so that there is no degradation of the impregnation into the con-crete or its protective quotient. Figure 8 shows the mechanism of action for these materials.

Also where possible, it would help to use an elastic, elasto-meric, anti-carbonation, crack-bridging, UV resistant coating to protect precast elements. These coatings work on the concept of providing equivalent cover, by which, the coatings (microns thick) provide the concrete with cover equivalent to centimeters of concrete. Figures 9 and 10 show this concept. These impreg-nations and coating systems are applicable to protect the entire

range of precast elements that can be produced. These vastly improve the durability of precast elements and help prevent corrosion damage prior to erection.

Conclusions

Admixtures apart, the construction chemicals segment has a lot to offer for precast construction to enhance durability of structures. The use of these materials should become more popular in interest of sustainability and durability of the concrete produced. These factors apart, these materials aid the precast manufacturers to reduce rate of rejection, improve quality of the segments and cut down on processing steps, which all translate to long term financial benefits for the producer. w

Figure 8: Working Mechanisms for Hydrophobic Impregnations in Concrete

Figure 9

Figure 10

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