Why Should Swimming Pools Be Filled and Drained Slowly? 25 mm (1 Inch) per Hour

One of the most common recommendations encountered during the commissioning of swimming pools is that the pool should be filled and drained at a rate of approximately 25 mm (1 inch) per hour. This guidance is frequently included in project specifications, waterproofing system recommendations, and pool construction manuals.

 

Why Should Swimming Pools Be Filled Slowly?

Although there is no mandatory EN requirement, gradual filling is considered good engineering practice for several important reasons.

1. Gradual Application of Hydrostatic Pressure

As the water level rises, hydrostatic pressure increases continuously.

If the pool is filled too rapidly, this pressure is applied suddenly to:

·        Reinforced concrete

·        Cementitious levelling mortar

·        Waterproofing membrane

·        Tile adhesive

·        Ceramic or glass mosaic tiles

Gradual filling allows the structure and finishes to accommodate these loads progressively.

 2. Reducing Differential Moisture Movement

Concrete and cement-based materials absorb water when submerged.

Rapid filling can create steep moisture gradients between saturated and unsaturated areas, resulting in:

·        Differential expansion

·        Internal stresses

·        Localized cracking

A controlled filling rate allows moisture to penetrate more uniformly throughout the structure.

3. Protecting Waterproofing Membranes

Liquid-applied waterproofing membranes are designed to withstand hydrostatic pressure after full curing.

Rapid filling may:

·        Overstress weak or improperly cured areas

·        Expose application defects

·        Increase the likelihood of localized debonding

Gradual filling provides a more controlled loading condition during the initial commissioning of the pool.

4. Protecting Tile Installation Systems

Swimming pool tile systems depend on several bonded layers working together:

·        Levelling mortar

·        Waterproofing membrane

·        Tile adhesive

·        Tile finish

·        Grout

If the pool is filled too quickly, hydrostatic pressure increases rapidly.

Where defects exist—such as:

·        Poor adhesive coverage

·        Inadequate tile beating

·        Air voids beneath the tiles

·        Weak substrate preparation

Rapid water loading may accelerate tile debonding or reveal latent installation defects.

5. Allowing Continuous Inspection

One of the greatest advantages of slow filling is that engineers can monitor the pool throughout the process.

During filling, inspections can identify:

·        Water leakage

·        Crack development

·        Movement joint performance

·        Waterproofing integrity

·        Tile system behaviour

If problems are detected early, filling can be stopped before the pool reaches full capacity.

 

Why Should Swimming Pools Be Drained Slowly?

Draining a swimming pool can be even more critical than filling.

When water is removed rapidly:

·        The internal hydrostatic pressure decreases suddenly.

·        External groundwater pressure may exceed the internal pressure.

·        Differential stresses develop within the concrete shell.

·        Waterproofing systems experience stress reversal.

·        Bonded tile systems may become vulnerable if underlying defects exist.

In extreme cases, excessive groundwater pressure can even lead to uplift (flotation) of the pool structure if adequate hydrostatic relief measures are not in place.

For these reasons, controlled draining is equally important.

 

Where Does the 25 mm (1 Inch) per Hour Recommendation Come From?

The recommendation is based primarily on practical engineering experience and manufacturer guidance.

It is commonly referenced in:

·        Waterproofing manufacturers' installation manuals

·        Tile installation guidelines

·        Structural engineering recommendations

·        Pool commissioning procedures

·        Project-specific technical specifications

The exact filling or draining rate may vary depending on:

·        Pool dimensions

·        Water depth

·        Structural design

·        Groundwater conditions

·        Waterproofing system

·        Tile installation system

·        Engineer's recommendations

Some projects permit faster filling, while others specify much slower rates for large or critical structures.

 

Is the 25 mm per Hour Rule Mandatory? - No.

• EN 15288-1 – Swimming Pools for Public Use – Safety Requirements for Design
• EN 15288-2 – Swimming Pools for Public Use – Safety Requirements for Operation
• EN 16582 – Domestic Swimming Pools

These standards address the design, safety, operation, and performance of swimming pools. However, they do not specify a mandatory filling or draining rate.

The commonly quoted 25 mm (1 inch) per hour filling or draining rate is not a mandatory requirement under European Standards.

Instead, it should be regarded as good engineering practice intended to:

·        Minimize hydrostatic shock.

·        Reduce differential structural stresses.

·        Protect waterproofing membranes.

·        Safeguard bonded tile systems.

·        Allow progressive inspection during commissioning.

Ultimately, the filling and draining rate should be determined by the project specification, structural engineer, and system manufacturers, taking into account the specific characteristics of the swimming pool.

 

Conclusion

While European Standards do not mandate a filling or draining rate of 25 mm (1 inch) per hour, the recommendation remains a widely accepted engineering practice. Slow and controlled filling and draining help reduce structural stresses, protect waterproofing and tile systems, and provide valuable opportunities for inspection during commissioning.

Every swimming pool is unique. Therefore, the appropriate filling and draining procedure should always be established based on engineering judgement, manufacturer recommendations, and project-specific requirements, rather than relying solely on a generalized rule.

 

Can a Cement Mortar Be Used for Levelling Swimming Pool Walls and Floors?

When constructing a swimming pool, one of the common questions is whether a cementitious mortar can be used as a levelling layer beneath the waterproofing membrane and tile installation system. The answer is yes, provided the mortar is specifically designed for the application and demonstrates the necessary performance characteristics through standardized testing.

It is important to understand that a levelling mortar is not intended to act as the waterproofing layer. Its primary function is to provide a sound, smooth, dimensionally stable, and durable substrate for the subsequent waterproofing and tile installation system. Therefore, acceptance of a cementitious mortar should be based on its engineering performance rather than solely on its compressive strength.

What Performance Is Required Cement Mortar Be Used for Levelling Swimming Pool Walls and Floors?

A levelling mortar intended for swimming pool applications should possess the following properties:

1. High compressive strength to withstand service loads. ASTM C109/C109M or EN 12190 - : 25 MPa at 28 days
2. Excellent bond strength to the concrete substrate. ASTM D4541 or EN 1542 - 1.0 MPa
3. Low water absorption and permeability. ASTM C642 - ≤ 6–8% by mass.
4. Good dimensional stability with minimal shrinkage. Drying Shrinkage - ASTM C157 - ≤ 0.10% (1000 microstrain).
5. Adequate flexural and tensile strength to resist cracking. ASTM C348) – evaluates crack resistance.
6. Durability under continuous water immersion. An immersion period of 28 days - No softening - No cracking - No delamination - No reduction in bond strength - No surface deterioration after prolonged immersion
7. Compatibility with the specified waterproofing membrane and tile adhesive. Pull-off strength as a system - BS EN 14891 - 0.5 MPa
8. Application thickness: As screed- min 15 - 50 mm, up to 100 with mesh. As plaster - min 8 - 15mm, up to 30mm in 2 pass.
9. Bonding to substrate - with bonding agent - min 1 MPa pull-off strength

Pool System:

A properly designed swimming pool does not rely on the levelling mortar for waterproofing. Instead, the complete system typically consists of:

1. Reinforced concrete (RCC) shell.
2. Cementitious levelling mortar or screed.
3. Waterproofing membrane.
4. Tile adhesive.
5. Ceramic, porcelain, or glass mosaic tiles.
6. Tile joint Grout.

Each layer performs a specific function, and the overall durability of the swimming pool depends on the compatibility and performance of the entire system.

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SWRI’s (Southwest Research Institute) Sealant Validation Program - ASTM C920

This standard is used for elastomeric sealants—the flexible materials used in joints in construction, like window seals, concrete joints, or expansion joints. The main things they check are adhesion, cohesion, peel strength, and hardness under movement.

Tests and procedure:

1. Cyclic Movement Test (Adhesion & Cohesion)
Purpose: To check if the sealant stays stuck to the surface (adhesion) and doesn’t break apart internally (cohesion) when the joint expands and contracts repeatedly.

Procedure: Prepare a standard test joint (often two metal or concrete blocks with a gap filled with sealant). Cure the sealant fully (let it harden as per manufacturer instructions). Move the joint back and forth repeatedly, simulating expansion and contraction of the building materials.
Inspect for cracks, pulling away from the substrate, or internal failure. Pass criteria: Sealant should remain adhered and intact without major cracks or failures.

2. Adhesion-in-Peel Test
Purpose: To measure how strongly the sealant sticks to the surface when it is peeled off.
Procedure: Apply the sealant to a standard substrate and allow it to cure. Cut a narrow strip and peel it off at a standard angle (usually 90°).
Measure the force needed to peel the sealant from the surface. Pass criteria: Force required should meet the minimum values specified in ASTM C920.

3. Indentation Hardness Test
Purpose: To check how hard or soft the sealant is, which affects flexibility and durability.
Procedure: Cure the sealant sample fully. Press a standard indenter (like a small metal tip) into the sealant with a defined force. 
Measure how deep the indenter goes into the sealant. Pass criteria: Depth of indentation must fall within the acceptable range (too soft or too hard can fail performance).

This test/program just makes sure the sealant:
1. Sticks well,
2. Doesn’t fall apart,
3. Can handle movement, and
4. Has the right flexibility.

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Basalt Stone in Construction — Material Characteristics, Applications and Installation Requirements

Basalt is a natural volcanic igneous rock produced by the rapid cooling of molten lava at the earth’s surface. Due to this fast cooling process, the mineral crystals remain extremely fine and compact, creating a very dense and durable stone. The material typically appears in dark grey to black tones with a uniform texture and is widely used in architectural and civil construction works. Basalt can be considered an engineering-grade natural stone, comparable to granite in strength but often exhibiting higher density and superior resistance to environmental exposure.

 

2. Physical and Mechanical Properties of Basalt stone

Property	Typical Range
Density	2.8 – 3.1 g/cm³
Water Absorption	Approx. 0.2 – 1%
Compressive Strength	150 – 300 MPa
Abrasion Resistance	Very High
Slip Resistance	High (especially flamed finish)
Weather Resistance	Excellent

The most critical property affecting installation is the very low water absorption of basalt.

 

3. Reasons for Use in Construction of Basalt stone

a. High Mechanical Strength
Basalt is capable of withstanding heavy traffic loads and impact stresses. It is therefore commonly used in public infrastructure and commercial developments.

b. Weather Resistance
The stone performs well under severe environmental conditions. It resists ultraviolet radiation, temperature variation, and moisture exposure. This is particularly important in hot climates where surface temperatures may exceed 70°C.

c. Slip Resistance
Surface treatments such as flamed or bush-hammered finishes provide high slip resistance, making basalt suitable for pedestrian and wet areas.

d. Chemical Resistance
Basalt has good resistance to salts, mild acids, and cleaning chemicals. It performs significantly better than limestone or marble in outdoor and coastal environments.

e. Low Maintenance
The material does not require polishing and has low staining potential. Routine cleaning is generally sufficient, reducing lifecycle maintenance costs.

 

4. Typical Applications of Basalt stone

Location	Application
External Areas	Walkways, plazas, pavements
Landscaping	Garden paths and stepping stones
Building Exterior	Façade cladding
Wet Areas	Pool decks and coping
Infrastructure	Kerbstones and cobbles
Interior Areas	Feature walls and lobby flooring

 

5. Installation Considerations for the installation of Basalt stone

Basalt is a dense and low-porosity material. Unlike porous stones, it does not absorb moisture from cement mortar. As a result, traditional mortar bedding or basic tile adhesives cannot develop sufficient bonding strength.

Improper installation commonly results in:

• Hollow sounding tiles
• Debonding
• Edge lifting
• Tile displacement
• Failure in exterior terrace areas

 

6. Installation Adhesive Requirements for basalt stone installation?

For successful installation, basalt must be installed using a high-performance deformable cementitious tile adhesive.

Minimum Classification:
EN 12004 – C2TES1

Recommended for Exterior Use:
EN 12004 – C2TES2 (highly deformable)

The adhesive must be capable of accommodating thermal movement between the concrete substrate and the stone finish.

 

7. Why Epoxy or Polyurethane Adhesives Are Unsuitable for Basalt stone installation?

Although epoxy and polyurethane adhesives provide high bond strength, they are not suitable for basalt installation.

Key limitations:

Rigidity
These materials form rigid bonds and cannot absorb differential thermal expansion between stone and concrete. This leads to stress transfer and eventual debonding or cracking.

Moisture Trapping
Epoxy and polyurethane adhesives are non-breathable. Moisture vapor within concrete becomes trapped beneath the stone, causing blistering, tenting, and efflorescence.

Thermal Movement
In hot climates, stone surfaces experience high temperatures. Flexible cementitious adhesives accommodate this movement, whereas rigid adhesives do not.

Tile adhesive is intended to act as a controlled stress-relief layer rather than a structural bonding compound.

 

8. Recommended Installation Practice

Successful basalt installation requires the following:

• Double bonding (adhesive applied to substrate and stone back)
• 100% contact coverage
• Appropriate notched trowel (approx. 10–12 mm)
• Movement joints at regular intervals (approx. 3–4 m externally)
• Proper substrate preparation
• Waterproofing below exterior stone installations

The following practices should be avoided: 

• Spot fixing
• Direct cement mortar bedding without bonding slurry
• Ready-mixed paste adhesives

Basalt is one of the most durable natural stones used in construction due to its strength, weather resistance, slip resistance, and low maintenance characteristics. It is highly suitable for exterior and heavy-traffic environments.

However, its low absorption and high density require proper installation methods. The long-term performance of basalt flooring or cladding depends primarily on selecting a deformable cementitious tile adhesive and following correct installation procedures.

Properly installed basalt systems provide long service life and reliable performance, while failures typically result from incorrect bonding systems rather than material deficiencies.

Tile Joint Grout Consumption Calculator

Tile Length (mm) Tile Width (mm) Tile Thickness (mm) Joint Width (mm) Total Area (Sqm) Select Grout Type 1500 Sanded Grout (10 Kg) 1600 Unsanded Grout (10 Kg) PERMACOLOR Grout (10 Kg) PERMACOLOR Grout FS (5 Kg) SPECTRALOCK Pro Premium Grout (5.2 Kg) SPECTRALOCK Pro Premium Grout FS (4.95 Kg) Latapoxy 2000 industrial epoxy grout (10.8 Kg) LATAPOXY SP 100 (5.385 Kg) STELLAR Grout (5 Kg) Wide Joint Flowable Grout (20 Kg)

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Does Absorption – Porosity of the Surface Affect Bonding?

In construction and finishing works, surface porosity plays a critical role in determining how well a product bonds to a substrate. Whether you are applying tile adhesives, self-leveling compounds, waterproofing membranes, or coatings, the absorption characteristics of the surface directly influence performance and long-term durability.

What is Porosity? 

Porosity refers to the proportion of open pores or voids within a material. These pores can be:

• Capillary pores – formed during hydration or solidification of cementitious materials.
• Air voids – trapped during mixing.

Effect of Porosity on Bonding

Highly porous substrates: Examples: old concrete, screeds, plaster, AAC blocks.

• Absorb mixing water from cementitious products too quickly.
• Adhesive does not get enough water for proper hydration → weak bond.

Low-porosity or non-porous substrates: Examples: glazed tiles, dense stone, metals.

• Provide little to no mechanical key for bonding.
• Adhesive sits on the surface without anchoring → debonding risk.

Moderate porosity substrates: Examples: cement boards, fair-faced concrete.

• Provide a good balance for mechanical interlock and hydration.
• Often ideal for strong adhesion.

How Absorption / Porosity Affects Bonding

1. Sealing the surface – Reduces excessive suction on highly porous substrates so the product does not lose water too quickly.
2. Mechanical bond improvement – Primers create a tacky or micro-rough layer for better grip.
3. Chemical bonding – Some primers contain resins or reactive agents to bond chemically with both the substrate and the applied product.
4. Uniform surface condition – Primers minimize dust, laitance, or micro-particles that block adhesion.

High vs. Low Porosity in Practice

• High Porosity → High Water Absorption, Product’s mixing water is drawn into the substrate → weak hydration → poor bonding.
• Low Porosity → Very Low Water Absorption, Adhesives can “float” without proper grip unless primed or roughened.

Example:

• Concrete screed with 8% absorption will pull water from cement-based adhesive in minutes.
• Glazed tile with <0.5% absorption won’t allow adhesive penetration without surface roughening or a primer.

Porosity, Strength, and Durability

In cement-based systems (concrete, mortar, screed):

• Cement + Water → Hydration → Binder formation.
• Higher strength requires lower porosity.
• Lower porosity generally means higher durability.

The hydration process links strength, porosity, and durability — improving one influences the others.

Standards for Measuring Porosity / Water Absorption

1. Cement, Concrete & Mortar

• ASTM C642 – Density, absorption, and voids in hardened concrete.
• BS 1881-122 – Water absorption in hardened concrete.
• IS 3085 – Permeability of cement mortar and concrete.

2. Ceramic Tiles & Fired Clay

• ASTM C373 – Porosity, water absorption, bulk density of ceramic tiles.
• ISO 10545-3 / EN ISO 10545-3 – Porosity of ceramic tiles.
• ASTM C20 – Porosity and density of refractory brick.

3. Natural Stone

• ASTM C97 – Absorption and bulk specific gravity of dimension stone.
• EN 1936 – Density and porosity of natural stone.
• IS 1124 – Porosity of building stones.

4. Refractories

• ASTM C830 – Porosity and density of refractory shapes by vacuum.
• ASTM C20 – Boiling water method for refractories.

5. Plastics & Polymers

• ASTM D570 – Water absorption of plastics.
• ISO 62 – Water absorption of plastics.

Simple Site-Level Porosity Test (Qualitative) as per ASTM F3191

1. Clean the surface.
2. Place a drop of water with a pipette.
3. Wait 1 minute.
4. Wipe the droplet.
5. Observe:
• Absorbs quickly → High porosity.
• Remains rounded → Low porosity.
• Moderate absorption → Ideal bonding potential.

Recommendations

• High porosity → Use primers to seal the substrate and reduce suction.
• Low porosity → Use bonding primers to create a mechanical/chemical key.
• Always ensure the surface is clean, sound, and free of contaminants.

Absorption and porosity directly affect adhesion. Understanding your substrate and applying the correct primer or preparation method ensures a strong, durable, and reliable bond — preventing costly failures in tile, stone, or coating installations.

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SASO ISO 13007 - Tile adhseive

Full form is SASO = Saudi Standards, Metrology and Quality Organization

ISO = International Organisation for Standardisation

13007 series standard specifies the performance requirements and test methods for adhesives and grouts used in the installation of ceramic tiles. The standard categorizes adhesives into different classes based on their performance characteristics. 

Below is a table summarizing the minimum requirements for each class of adhesive as outlined in SASO ISO 13007:

Adhesive Class	Open Time	Slip Resistance	Shear Adhesion Strength	Tensile Adhesion Strength	Transverse Deformation
C1	≥ 20 minutes	No slip	≥ 0.5 MPa	≥ 0.5 MPa	Not required
C2	≥ 30 minutes	No slip	≥ 1.0 MPa	≥ 1.0 MPa	Not required
C2S1	≥ 30 minutes	≤ 0.5 mm slip	≥ 1.0 MPa	≥ 1.0 MPa	≤ 0.5 mm
C2S2	≥ 30 minutes	≤ 0.5 mm slip	≥ 1.0 MPa	≥ 1.0 MPa	0.5 mm
D1	≥ 20 minutes	No slip	≥ 0.5 MPa	≥ 0.5 MPa	Not required
D2	≥ 30 minutes	No slip	≥ 1.0 MPa	≥ 1.0 MPa	Not required
D2S1	≥ 30 minutes	≤ 0.5 mm slip	≥ 1.0 MPa	≥ 1.0 MPa	≤ 0.5 mm
D2S2	≥ 30 minutes	≤ 0.5 mm slip	≥ 1.0 MPa	≥ 1.0 MPa	0.5 mm

Notes:

Open Time: The duration during which the adhesive remains usable after application.

Slip Resistance: The maximum allowable slip of the tile after placement.

Shear Adhesion Strength: The adhesive's resistance to shear forces.

Tensile Adhesion Strength: The adhesive's resistance to tensile forces. 

Transverse Deformation: The flexibility of the adhesive, indicating its ability to accommodate slight movements in the substrate.

C1

Normal cementitious adhesive

Basic performance (e.g., tensile adhesion strength ≥ 0.5 MPa)

C2

Improved cementitious adhesive

Higher performance (e.g., tensile adhesion strength ≥ 1.0 MPa)

D1

Normal dispersion adhesive

Basic performance

D2

Improved dispersion adhesive

Enhanced properties like water resistance, adhesion, etc.

R1

Normal reaction resin adhesive

Basic two-component adhesive

R2

Improved reaction resin adhesive

Better chemical and mechanical resistance

F

Fast-setting

Adhesive sets faster than standard time

T

Slip resistance

Adhesive prevents tile from slipping vertically

E

Extended open time

Adhesive maintains bonding strength for longer after application

S1

Deformable

Adhesive can flex ≥ 2.5 mm and < 5 mm (for moderate substrate movement)

S2

Highly deformable

Adhesive can flex ≥ 5 mm (used in areas with high movement or stress)

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