Admixtures For Water Proofing In concrete

Submitted By:Sanjipta Behera

Civil EngineeringRoll no-13011521Sem-7th , Sec-B2

Contents1. Introduction

2. Damages to structure

3. Permeability Reducing Admixtures(PRAs)

4. Functions of Water Proofing Admixtures

5. Types of materials used in PRAs

6. Reduction of permeability using PRAs

7. Comparison of reduction of permeability

8. When to use PRAs

9. Dosage of PRAs

10. Application procedure

11. Quality assurance

12. Successful projects

13. Conclusion

Introduction Permeability is one of the main factors that can affect the

durability of the structure.

Permeable structures can result for various reasons such as low cement content, poor compaction too much water during concrete mixing etc.

The passage of water through cracks ,joints, voids, and inter connected capillaries in the mass concrete makes the concrete pervious, ultimately leads to poor protection of structures, increases the rate of alkali aggregate reaction and also accelerates the rate of corrosion in the steel.

Introduction (cont.) The impervious property of concrete can be improved by addition

of admixture in the green concrete or by surface treatments and the phenomenon is generally known as waterproofing.

The products used in both the stages of concrete are known as construction chemicals or admixtures and waterproofing is one of the aspects in construction aid.

These products are required for modifying the properties of concrete or mortar to make them more suitable for the work at hand or for the economy for such purposes are saving energy

Damages to Structure

Permeability Reducing Admixtures(PRAs)

So to counteract these problems there is a need of an admixture which can control the porosity of concrete. These admixtures are known as permeability-reducing admixtures (PRAs) .

There are two broad classifications depending on concrete exposure

1.concrete exposed to non hydrostatic conditions (PRAN)

2.concrete exposed to hydrostatic conditions (PRAH)

Functions of Water Proofing Admixtures A permeability reducing concrete admixture or waterproofing admixtures function in following ways: It reduces the size of capillary pores, their numbers and continuity

inside the concrete structure. It blocks the capillary pores of concrete It may line the capillary pores with hydrophobic materials. This

prevents the absorption of water in the pores due to capillary absorption.

Besides reducing permeability, some PRAs impart other beneficial characteristics, such as reduced drying shrinkage, reduced chloride-ion penetration, improved freeze/thaw resistance.

Types of materials used in PRAsAccording to American Concrete Institute (ACI) Committee 212 the materials used to produce PRAs vary, but they generally fall into three categories.

1.The largest category consists of hydrophobic, or water-repellent, chemicals derived from soaps or fatty acids, vegetable oils, and petroleum. These materials form a water-repellent layer along pores in the

concrete, but the pores themselves remain open.

2.The second category is finely divided solids—either inert or chemically active fillers such as talc, clay, siliceous powders, hydrocarbon resins, and coal-tar pitches. These materials densify the concrete and physically limit the passage

of water through the pores. Some experts also consider supplementary cementitious materials (SCMs) to be in this category.

Types of materials used in PRAs(cont.)

3.The third category consists of crystalline products—proprietary active chemicals in a carrier of cement and sand. These are hydrophilic materials that increase the density of

calcium silicate hydrate or generate crystalline deposits that block concrete pores to resist water penetration.

The various types of materials can be used alone or in combination to give different levels of performance.

Reduction of permeability using PRAs According to the ACI report, concrete produced with

hydrophobic chemical admixtures could theoretically resist some hydrostatic pressure. It reduces the permeability upto 49%.

The hydrophilic crystalline admixtures provide the concrete with

the greatest resistance to infiltration of water under hydrostatic pressure. It reduces permeability upto 70%.

The colloidal silica is very less effective in reduction of permeability. It reduces permeability upto 19%.

Comparison of reduction of permeabilityGiven below the table reproduced from ACI 212.3, summarizes results from a series of permeability tests performed on concrete mixes with three different types of PRAs

Admixture type Coefficient of permeability of reference concrete

Coefficient of permeability of test concrete

Percent reduction in permeability

Crystalline 4.29× 1.28× 70

Colloidal silica 1.98× 1.61× 19

Hydrophobic pore blocker

2.23× 1.14× 49

When to use PRAsThe value of a PRA depends entirely on the environment the concrete will be exposed to and the importance of keeping water from passing through. For interior columns, beams, and floor slabs in a high-rise,

permeability isn’t a big issue. For structures that will be exposed to moisture, salt or salt water,

wicking, or water under hydrostatic pressure, using a PRA can help prevent problems such as water migration, leaks, freeze/thaw damage, corrosion, carbonation, and efflorescence.

When to use PRAs(cont.) PRANs are often used in architectural concrete, precast panels,

and concrete brick, block, and pavers to repel rain and minimize dampness. Reducing permeability can help minimize efflorescence and make it easier to keep walls clean.

PRAHs are needed for more extreme and continuous exposures, such as below-grade structures, tunnels and subways, water tanks and pools, bridges, and dams. Manufacturers of the crystalline PRAHs say the products can eliminate the need for membrane waterproofing systems and epoxy-coated reinforcement, thereby reducing the cost of waterproofing.

Dosage of PRAs.Like other admixtures, PRAs are typically specified by the architect or engineer and added to the concrete at the ready-mix plant.

Dosage of these admixtures vary depending on the type of admixture and the level of performance required. Hydrophobic type- 2% of weight of concrete. Colloidal silica- 5% by weight of concrete. Crystalline PRAH-2% by weight of cementious material.

Application procedure:1.The part of the structure where water proofing is to be done is determined. So more importance is given to that part which is more exposed to water.

2.Water cement ratio is fixed properly as more water or less cement is vulnerable to permeability. Cement is provided as per the ratio.

3.Fine and coarse aggregates are added as per the mixed design.

4.Admixure of specified dosage is added.

5.Water is added as per the mixed design.

6.Mixing is done with the help of machine.

7.After mixing the concrete is applied at the desired location.

Quality assurance:1. Waterproofing admixture to be certified by the International Code

Council (ICC) as a Chemical Admixture Used in Concrete (AC198).

2. Waterproofing admixture must be certified to NSF/ANSI Standard 61.

3. Waterproofing admixture shall hold a valid agreement certificate from the British Board of Agreement.

Successful projects1. Waterproofing admixture was used in the renovation and

expansion of the Mark Jefferson Science Complex at Eastern Michigan University in Ypsilanti.

2. South Cobb Tunnel Lift Station near Atlanta.

3. Gardens by the Bay, Marina Bay, Singapore.

4. National Road Bikeway Tunnel near St. Clairsville.

5. Singapore international airport terminal 3.

DrawbacksBesides having so many benefits PRAs has some drawbacks

1. It will not form membrane on cracks larger than half a millimeter.

2. Joints, penetrations, and transitions will need attention as well. In some instances, such as basements of single family homes, a contractor may still need to provide an adequate drainage system to avoid water buildup through the life of the structure.

Conclusion

There are many products in the market now a day which are available to protect the concrete structure from damage due to water penetration. For example, coatings, sealers, membranes etc. are used to prevent water penetration. But all these require huge amount of money and time. These process can be simplified by the use of water resisting admixture which prevent the ingress of water through concrete by making it waterproof.

For projects and applications that need waterproof concrete, the use of PRAs is worth considering. Contractors need only follow sound placement and finishing practices to install it successfully, and owners may be able to cover the costs by saving the labor and materials required for other waterproofing methods.

References

1. http://www.kryton.com/products/krystol-internal-membrane-kim/attachment/concrete-waterproof_admixture_csi_kim/

2. http://www.concreteconstruction.net/how-to/materials/waterproofing-admixtures-for-concrete_o

3. www.penetron.com/doc/pdf/PRANvsPRAH_ThePenetronDifference.

4. http://theconstructor.org/concrete/waterproofing-admixtures-concrete/11110/

Questions