23rd May 2024 -

Moisture is one of the biggest threats to the structural integrity of buildings made from concrete blocks. When water penetrates a concrete block wall, it can lead to cracking, spalling, efflorescence and even reinforce corrosion.  

Therefore, effective moisture management should be a priority for protecting concrete block structures from deterioration. For this feature, we explore various moisture management solutions for you to introduce during construction as well as maintenance techniques.

Are concrete blocks waterproof?

Concrete blocks are not fully waterproof because of their composition and manufacturing processes. While concrete has relatively low porosity, most concrete blocks contain microscopic pores and capillaries to allow moisture penetration and absorption. 

There are various factors which create potential pathways for water intrusion. These include mortar mix for concrete blocks, the presence of hollow cores or voids, concrete block density, porous mortar joints and any cracks or gaps. 

While concrete blocks lack waterproofing properties, their moisture resistance can be substantially improved through installation techniques. 

What are the different types of concrete blocks?

Concrete block types offer varying levels of moisture resistance depending on their composition and manufacturing process. Below, we outline the different options and their moisture resistant capabilities. 

1. Standard concrete blocks

Standard blocks are the most basic type made from a mixture of cement, aggregate and water. Generally, standard concrete blocks have moderate moisture resistance. They are susceptible to water penetration over time, especially if not properly sealed or maintained.

2. Dense concrete blocks

Overall, these blocks have a low porosity. This is because they are manufactured with a very low water-to-cement ratio and often use smaller aggregate sizes. Their compact matrix gives high density concrete blocks exceptional resistance to moisture penetration and absorption.

3. Interlocking concrete blocks

These dry-stacked blocks interlock together with tongue-and-groove joints rather than using mortar. Their moisture resistance depends heavily on using high-quality, dense concrete in manufacturing to limit porosity. 

4. Hollow concrete blocks

Also known as concrete breeze blocks, they are similar to standard blocks but with hollow cores or cavities. The voids make a hollow concrete block lighter and more insulative than solid concrete blocks. However, the cores can allow moisture penetration if not properly filled or insulated.

5. Autoclaved aerated concrete (AAC) blocks

Made from a mixture of cement, lime, aluminium powder and silica sand, aerated concrete blocks are cured under pressurised steam. This produces a highly porous but lightweight and moisture resistant material. Their closed cellular structure gives aerated blocks excellent thermal insulation and moisture control properties.

6. Concrete masonry units (CMUs)

Often referred to as hollow blocks, CMUs have a characteristic two-hollow core design. This makes them lighter than solid blocks while retaining sufficient strength. The voids of CMUs can be filled with insulation or grout to improve thermal and moisture performance further.

7. Decorative concrete blocks

These speciality blocks come in various shapes, textures and colours to enhance aesthetics. Decorative concrete blocks are made with water repellent admixtures or given protective surface coatings to maintain their appearance in damp environments.

Does concrete block thickness affect moisture resistance?

The thickness of concrete blocks does impact their overall resistance to moisture penetration and potential for water damage. There are several key factors related to block thickness that impact moisture vulnerability. 

Path length

Thicker blocks simply provide a longer path that moisture must travel to fully penetrate through the unit. Increased path length makes it more difficult for moisture to reach the cores of thicker blocks compared to thinner ones.

Capillary absorption

Moisture is drawn into concrete through capillary pores and air voids via absorption and capillary action. With thicker blocks, there is a more solid material that this capillary network must bypass, providing more resistance to moisture.

Bulk water resistance

Under rain or flood conditions, thicker blocks are better able to resist bulk water ingress and saturation. The increased mass and reduced surface area to volume ratio make it harder for moisture to saturate thicker units.

Freeze-thaw durability

In cold climates, thicker blocks have greater resistance to freeze-thaw damage caused by absorbed moisture freezing and expanding. The increased thickness provides more protective cover over the interior portions of the block.

For example, 8-inch thick blocks generally provide better moisture protection than 4-inch units. However, high-density 4-inch blocks can potentially outperform low-quality 8-inch blocks in damp environments. 

Proper moisture protection detailing is still critical for thicker blocks if they are made from porous concrete. 

How to prevent moisture in concrete block buildings

Preventing moisture when you lay concrete blocks requires a layered approach with several proven techniques depending on the block type. 

Standard/hollow concrete blocks

  • Apply a water repellent sealer to the exterior block surfaces after construction. It helps repel bulk water from the porous concrete matrix of standard blocks which lack integral moisture protection.
  • Use mortar admixtures like liquid polymers to reduce mortar porosity and water absorption. This is beneficial since mortar joints are a common moisture entry point for hollow blocks.
  • Fill all hollow cores with grout or insulation to eliminate moisture pathways. This is crucial for hollow block walls to prevent moisture from absorbing through interconnected cavities.
  • Install drainage planes like rigid insulation behind veneers to allow moisture to drain from behind hollow block veneer walls.
  • Add sufficient weep holes (outlets for water to drain away from structures) for moisture to escape.
  • Ensure proper overhangs and flashing to prevent bulk water shedding issues. This is important as standard blocks lack inherent moisture resistance.

Dense concrete blocks

  • The concrete block density and low-permeability nature provide excellent moisture resistance.
  • Application of water repellent sealers is still recommended, especially for high-exposure areas.
  • Use a quality mortar and ensure full head/bed joints to prevent moisture from absorbing through mortar.
  • Coating of exterior surfaces with elastomeric paint or stucco improves water-shedding ability.

Interlocking concrete blocks

  • Use high-quality concrete for interlocking blocks with tongue-and-groove joints as the blocks lack mortar for added moisture resistance.
  • A water repellent admixture added during manufacturing enhances moisture protection from the start.
  • Proper alignment and tight interlocking are essential to prevent moisture penetration through the dry-stacked joints.
  • A quality sealant can be applied over the assembled wall to create a continuous moisture barrier.

AAC blocks

  • Coat exterior faces with elastomeric paints or water repellent sealers to prevent moisture absorption through the porous block faces.
  • Use specially formulated AAC-approved mortars and grouts because regular masonry mortars can damage the AAC material due to moisture migration.
  • Detail for complete moisture drainage and ventilation behind exterior walls. This is crucial as the low density means moisture will not easily re-evaporate.

CMUs/decorative blocks

  • Fully grout cores in reinforced and load-bearing applications stabilise walls to eliminate moisture through cores.
  • Fill un-reinforced voids with perlite/vermiculite insulation to stop moisture transfer through open cores.
  • Apply weatherproof coatings or water-repellent admixtures if used externally.

General maintenance techniques

  • Proper site grading and drainage divert water away from foundations and stop moisture accumulation at the footing or foundation.
  • Install dampproofing membranes or plastic vapour retarders under slabs to stop moisture migration from the soil into the building.
  • Maintain consistent through-wall flashing and weather-resistant barriers to create discontinuous moisture barriers between wall sections.
  • Caulk all joints, cracks and penetrations for continuous moisture barriers by sealing any gaps where water could potentially enter.
  • Limit moisture exposure during construction until protection is complete to allow proper curing before exposure to prevent moisture entrapment.

Can you repair concrete block moisture damage?

Yes. Concrete blocks can be repaired if water damage does occur. However, the extent of reparability depends on the severity of the issue. Several repair techniques can be implemented.

1. Surface repairs

It is possible to remove efflorescence (a powdery salt residue) by dry brushing or applying an efflorescence remover acid wash. When moisture enters and freezes, surface scales known as spalling or scaling can form. 

These may be patched with a repair mortar or cementitious coating. Hairline cracks that allow moisture entry points can be repaired with a high-quality concrete crack filler or sealant.

2. Masonry repairs

If moisture has eroded the mortar, repointing (removing old mortar and pointing with new) may be needed for deteriorated mortar joints. For blocks damaged beyond repair, selective demolition and replacement with new units integrated into the existing wall are sometimes required.

3. Coatings/sealers

Crystalline waterproofing coatings use chemicals that absorb into the concrete and grow crystalline formations to seal moisture pathways. Flexible, rubberised liquid-applied elastomeric coatings create a waterproof barrier over concrete masonry walls.

4. Pressure grouting

If hollow block cores have moisture-related defects, they can potentially be rehabilitated by pressure-grouting techniques to densify and displace moisture.

5. Drainage enhancements

For exterior walls, installing drainage planes like insulation or membranes provides better moisture escape instead of trapping it. Adding weep holes or vents creates pathways for incidental water to properly drain out of wall systems.

Top concrete blocks and bricks with quick UK delivery

At Building Materials Nationwide, we offer an extensive selection of concrete blocks to meet all your needs. From standard to hollow and dense concrete blocks, our selection is suited for you to build various concrete block projects. We pride ourselves on carrying high-quality concrete blocks from trusted manufacturers. 

For trade customers seeking superior building materials, opening a trade account with Building Materials Nationwide provides excellent benefits. Enjoy access to our comprehensive product range at competitive trade prices and personalised account management. Sign up today or contact the team for a chat.  

    * These fields are mandatory