Waterproofing Admixtures for Concrete – Requirement, Functions, Dosage and Mechanism

What is Waterproofing Admixture for Concrete?

Waterproofing admixture is a type of admixture which prevents the passage of water through hardened concrete under a pressure head. It is also called as water resisting admixture or permeability reducing admixture or damp proofing admixture.

Requirement of Waterproofing  Admixture for Concrete

Water is a vital part of concrete construction from the mixing of concrete to the curing of concrete. But once the concrete has gained its required strength, thereafter any moisture or water penetration in concrete can cause damage to the concrete. The damage can be due to corrosion of reinforcement or due to freeze thaw.

When a concrete work has finished and curing completed, the water content evaporates and leaves voids inside the concrete. Thus for a good durable concrete construction, low water/cement ratio is recommended so that porosity of concrete can be reduced. But then also, a concrete cannot be completely made waterproof even by using the minimum water/cement ratio.

Concrete is a porous material and water can penetrate through pores and micro-cracks of concrete due to capillary actions. Thus there is a need of an admixture which can control the porosity of concrete.

Functions of Waterproofing 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, or
• It may line the capillary pores with hydrophobic materials. This prevents the absorption of water in the pores due to capillary absorption.

Capillary absorption is the passage of water through pores in concrete due to presence of an externally applied hydraulic head.

These water resisting admixtures cannot significantly prevent the movement of water through cracks in concrete or through poorly compacted concrete which are the most common reasons for water leakage in concrete.

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.

American Concrete Institute (ACI) Committee 212 offers guidance on use of this admixture in its ACI 212.3 Rev10 titled “Report on Chemical Admixtures for Concrete “. Details about permeability reducing admixtures are covered in Chapter – 15 of the report.

Materials Required to Prepare Waterproofing Admixtures

The ACI 212.3 Rev.10 in its report states that following types of materials to be used in waterproofing admixtures:

• Hydrophobic or water-repellent chemicals such as oils, petroleum products which can block the entry of water into the pores by forming a layer along the pores in concrete. These materials do not fill the pores.
• Finely divided solid particles (powders)of inert and chemically active fillers to act as densifiers and restrict the movement of water through pores.
• Crystalline materials with hydrophilic nature which increases the density of calcium silicate hydrate and / or generate the pore-blocking deposits to resist water penetration.

The report does not limit the use of materials only to as mentioned above. Any material can be used as water-resisting admixture which can block the water penetration. These materials can be used as single or combination of two or more.

Dosage of Waterproofing Admixtures for Concrete:

Dosage of these admixtures vary depending on the type of admixture and the level of performance required. 2% of hydrophobic type admixture is commonly used, and may be 5% or more for the pore blocker types. These percentage is calculated based on weight or volume of concrete and not on cement.

Mechanism of Water Resisting Admixtures:

The mechanism of water resisting admixtures is to reduce the porosity of concrete. To reduce the porosity of concrete, the water content or the water cement ratio must be reduced. If water cement ratio is decreased below 0.45, the porosity of concrete reduced considerably.

If we further reduce the water/cement ratio, the workability of concrete decreases. This can be prevented by the use of water-reducing admixture that can ensure sufficient workability of concrete for full compaction and reduce shrinkage cracking.

If it is not possible to reduce water cement ratio, then there is another mechanism in which pore blocking admixtures are used. These are small in size roughly 0.1 microns and fine reactive or unreactive or insoluble polymer emulsions. During hydration process, they can settle in the pores of concrete and block them which helps the concrete to prevent further absorption of water.

The hydrophobic admixtures are soluble with the concrete but they react with calcium in fresh concrete and forms a layer which is adsorbed by the surfaces of capillaries. After the process of hydration, this layer prevents the penetration of water into the pores. But the resistance of hydrophobic admixture is limited and depends hydraulic head, the quality of the concrete and the effectiveness of the admixture.,

What is Screed in Concrete Construction? Properties and Composition of Screeds

What is Screed in Concrete Construction?

Screed in concrete construction is a flat board used to smoothen the concrete after it has been placed over a surface. Properties and composition of screed in construction is discussed.

Screed is a construction element laid in a range of thickness and its purpose is to bring the installation surface for the flooring to the design height and to provide a surface suitable for installing the specified flooring.

Screed in concrete construction can be defined as a flat board or a purpose that is made of an aluminum tool, which is used to smoothen the concrete after it has been placed over a surface.

Screeds are made from pre-blended mortar which is mixed with cementitious binders or anhydrite based binders. They are set as guides for straight edges which helps in bringing the surface of the floor of concrete to the desired elevation.

The screed must be sufficiently rigid in nature. This helps them to resist the stresses and the distortion that is caused during the spreading and leveling of the floor topping. The metal strips or the pipe spaced not more than 10 feet apart make effective screeds.

The following specifications are guaranteed by an effective screed:

1. Behave as substrate that is essential for installing the specified flooring

2. It will be laid on schedule

3. Will show durability under various service conditions

The service conditions specified may be external or internal for either civil, commercial or industrial flooring purpose etc.

Screeding can be defined as the process of cutting off the excess wet concrete to bring the top surface of a slab to the proper grade and smoothness.

Characteristics of Screed in Concrete Construction

Screed must possess certain technical and performance characteristics for it to be used for floor installation. The properties of screed are:

1. Sufficient Thickness

The thickness of the screed depends on the type of screed planned to install, the thickness of floor and its type, and the intensity of traffic that is estimated to fall over the floor.

2. Mechanical Resistance of Screed

To sustain the final load (service load) along with the contribution of the load from the type of floor, certain mechanical resistance must be gained by the screed selected.

Generally, it is recommended that the screed employed for a flooring for the domestic purpose must have a minimum strength of 20MPa. In case of industrial purpose, the minimum strength must be 30MPa.

3. Compactness of Screed

The screed must be compact and homogeneous on the whole surface and throughout the whole thickness. If the screed shows layers or areas that are crumbles or lower consistency, it is a sign of poor mechanical characteristics. This defect will result in the breakage or the detachment of the flooring.

4. Screed must be Properly Cured and Dimensionally Stable

Before the installation of the floor, it is essential to check that the screed is properly placed and cured. This proper curing will let to complete the shrinkages in it if any.

During the curing cycles of screed, they are prone to hygrometric shrinkage. This hygrometric shrinkage may be due to part of evaporation of the mixing water or the drying process. This shrinkage will result in the curling or the cracking of screeds.

If these cracks are formed just after the installation of the floor, then there is the possibility of floor detachment or damage.

5. Crack Free Screed

The cracks in screeds can be caused due to several factors. One of which is hygrometric shrinkage. Other reasons are due to the presence of large amount water in the mix. The use of aggregates which are very fine or mix with too much cement content also results in cracks.

It is advised that all the cracks must be monolithically sealed. This sealing is done with high precision by means of epoxy resins. These procedures are carried out before the installation of the floor.

To avoid hairline cracks in the surface of the screeds, it is recommended to make use of the anti-fracture membrane.

6. Clean

The surface of the screed must be cleaned perfectly. Any presence of dust, dirt, rubble, detached areas or any other material or substance on the surface of the screed must be removed before the installation of flooring.

Unclean surfaces will result in the prevention of adhesion between the floor and the screed.

7. Dry

The level of residual humidity must be checked in the screed surface. This value must conform with the maximum level for that type of floor covering and must be uniform throughout the whole thickness of the screed. This is especially carried out during the installation of flooring that is sensitive o humidity.

For anhydrite screeds, the level of residual humidity must be less than 0.5%. An electric or carbide hydrometer is used to measure the residual humidity in a screed.

8. Flatness

The flatness of the screed is checked by laying a straight edge at least 2m long in all the directions on the surface of the screed. The maximum acceptable tolerance with this straight edge is specified to be 2mm.

If the flatness is not within the tolerance, then the surface must be leveled off using a suitable product, before carrying out the floor installation

9. Smoothness

The suitability of grade of finish of the surface and the level of roughness mainly depends on the type of flooring that is chosen to be installed. For the reflective finishing, it is advised to apply a skimming product that is designed specifically for this purpose.

The Composition of Screed Concrete

The major composition of screeds showing good performance characteristics are mentioned below:

1. Admixtures for mixing it with water, liquid and powdered superplasticizers, cement, and suitable aggregates.
2. Special binders to mix with water and suitable aggregates.
3. Mixes of aggregates in a granulometric curve to make the screeds.
4. Special pre-blended mortars to mix with water.

Defects in Screeds

The most common defects that are seen in screed are mentioned below:

1. Cracks
2. Surface dust or bleeding
3. Crumbly surface
4. Fractures around the pipe work
5. Localized crumbly surface

Concrete Curing Time and Duration -Right Time to Cure Concrete

Concrete Curing Time – The Right Time to Cure Concrete

Curing of concrete is defined as the process of maintaining the moisture and temperature conditions of concrete for hydration reaction to normally so that concrete develops hardened properties over time. The main components which needs to be taken care are moisture, heat and time during curing process.

Why curing of concrete is required?

Curing of concrete is required for the following reasons:

• To prevent the concrete to dry out prematurely due to solar radiation and wind. This prevents plastic shrinkage of concrete.
• It helps to maintain the concrete temperature by allowing the hydration process. Hydration process requires water to carry on and releases heat.
• Curing helps the concrete to harden and bond with internal materials and reinforcement. This helps to prevent damage to bond between concrete and reinforcement due to vibration and impact.
• This helps development of impermeable, crack free and durable concrete.

Fig: Curing of Concrete Roof Slab by Ponding

What is the right time for curing of concrete?

The time to start curing of concrete depends on the evaporation rate of moisture from the concrete. The evaporation rate is influenced by wind, radiant energy from sunshine, concrete temperature, climatic conditions, relative humidity.

The evaporation of moisture is driven by the difference in vapor pressure on concrete surface and the in surrounding air. When the difference is high, evaporation rate is high.

ACI 308 – Guide to Curing Concrete suggests three phases of concrete curing. These phases are shown in figure 1.6 of ACI 308.

The right time of curing of concrete depends on:

Initial Curing – Bleeding of Concrete:

When the concrete is placed and compacted, bleeding of water occurs and rises through the surface of concrete due to settlement of concrete. The rate and duration of bleeding depends on many factors including concrete mix properties, depth or thickness of concrete, method of compaction of concrete etc.

These bleed water starts to evaporate from the surface. When all the bleeding water has disappeared from the surface, the drying of concrete starts, then initial curing of concrete is required to minimize the moisture loss and prevent plastic shrinkage cracks to concrete before and during finishing operations.

The initial curing of concrete can be done by techniques such as fogging or using the evaporation reducers, or by providing the sunshades and windscreens.

Intermediate curing:

Intermediate curing is done when the concrete surface finishing operations has been carried out before the final setting of concrete. This happens when the required surface texture of concrete member is achieved rapidly or when the setting of concrete is delayed.

Final Curing:

When the concrete is finished after the final setting of concrete, the final curing of concrete should be done. This helps to prevent surface drying of concrete because the loss of moisture from the concrete surface occurs immediately.

What is the duration of concrete curing?

Curing of concrete for longer duration increases the strength and durability of concrete structural member. The following figure explains how the compressive strength of concrete increases with time when it is cured for longer duration.

Fig: Effect of duration of water curing on strength of concrete

The curing duration of concrete depends on:

• The reason for curing i.e. to prevent plastic shrinkage, temperature control, strength and durability of concrete.
• The size of concrete structural member
• The type of concrete grade and rate of hardening of concrete
• The temperature and moisture conditions of surroundings
• The exposure conditions of the concrete surface during and after curing
• The requirement of curing duration as per specification of concrete

The American Concrete Institute (ACI) Committee 301 recommends a minimum curing period corresponding to concrete attaining 70 per cent of the specified compressive strength. The often specified 7 day curing commonly corresponds to approximately 70 per cent of the specified compressive strengths.

The Indian Standard IS 456 – 2000recommends that curing duration of concrete must be at least 7 days in case of ordinary Portland Cement, at least 10 days for concrete with mineral admixtures or blended cements are used. It also recommends that the curing duration should not be less than 10 days for concrete exposed to dry and hot weather conditions and 14 days for concrete with mineral admixtures or blended cement in hot and dry weather.

References:

• ACI 308, American Concrete Institute – Guide to Curing Concrete
• BS 8110 – British Standard for the Design and Construction of Reinforced and Prestressed Concrete Structures
• IS 456 – 2000 : Indian Standard Code of Practice for Plain and Reinforced Concrete