Concrete is one of the most widely used building materials in the world due to its versatility, durability, and relatively low cost. However, regular concrete is porous and allows water to pass through. This can lead to issues like dampness, leaks, and corrosion of reinforcing steel. High strength concrete has emerged as an alternative to improve concrete’s resistance to water penetration.
What is high strength concrete?
High strength concrete (HSC) refers to concrete with a high compressive strength, typically greater than 8,000 psi (55 MPa). It differs from regular concrete in its mixture proportions and manufacturing process:
- Lower water-to-cement (w/c) ratio – This improves strength and reduces porosity
- Lower aggregate content – This leaves more space for the cement paste to bind everything together
- Use of mineral and chemical admixtures – These enhance concrete properties like workability, strength, and impermeability
- Improved curing conditions – Proper moisture and temperature control accelerates strength gain
The optimized mixture proportions result in a dense, tightly-packed internal structure with low permeability. This prevents water from readily passing through the concrete.
Is high strength concrete waterproof?
High strength concrete is highly impermeable but not completely waterproof. The performance depends on several factors:
Water-Cement Ratio
The lower the w/c ratio, the denser and less permeable the hardened concrete becomes. Typical HSC has w/c ratios between 0.25 to 0.45, while conventional concrete is 0.50 to 0.60. At very low w/c, the compact microstructure significantly reduces capillary pathways for water to flow through.
Ingredients and Mix Proportions
Optimizing the mix design is key to improving impermeability. Finer cement particles and pozzolanic materials like fly ash or silica fume fill microscopic pores and voids. Lower aggregate content leaves more space for water-resistant cement paste. Using admixtures like superplasticizers allows lower water content without compromising workability.
Placement and Compaction
Proper concrete placement ensures there are minimal air voids. Careful compaction eliminates gaps around reinforcement and formwork. This minimizes pathways for moisture ingress.
Curing
Adequate curing is vital for strength gain and hydration of the cement paste. Extended moist curing improves microscopic pore structure. Proper curing results in a denser, tighter concrete with resistance to water entry.
How waterproof is high strength concrete?
There are two main physical properties used to quantify concrete’s water-resistance:
Water Absorption
This measures the amount of water absorbed under standard conditions, expressed as a percent of the dry mass. Typical values for high strength concrete range from 3-7%, compared to 5-12% for normal concrete. Lower absorption indicates higher water-resistance.
Permeability
This measures the rate at which water flows through saturated concrete under pressure. It is expressed in units of millimeters per second. High strength concrete generally has very low permeability, less than 10 x 10-12 mm/sec. Normal concrete is around 50-100 x 10-12 mm/sec.
| Property | High Strength Concrete | Normal Concrete |
|---|---|---|
| Water Absorption | 3-7% | 5-12% |
| Permeability | <10 x 10-12 mm/sec | 50-100 x 10-12 mm/sec |
While not completely impervious, high strength concrete provides significantly improved resistance and durability against water entry compared to conventional concrete. However, it is not a fully waterproof material.
What applications use high strength concrete for its water resistance?
The low permeability of high strength concrete makes it well-suited for use in wet environments and water retaining structures:
Liquid Retaining Structures
HSC is used in the construction of structures like water tanks, sewage treatment plants, swimming pools, and dams. Its low absorption minimizes leakage of contained liquid.
Foundations
HSC provides resistance against moisture ingress and mitigates issues like dampness and mold growth in basements and underground structures.
Bridges
For bridges over water bodies, HSC protects against moisture penetration that could corrode reinforcing steel and damage the structure.
Marine Structures
In docks, ports, and offshore platforms, HSC maintains integrity against constant exposure to seawater.
Water and Wastewater Tanks
Storage tanks for potable water and wastewater utilize HSC to prevent contamination from stored liquid permeating through concrete walls.
What are the limitations of HSC water resistance?
Despite its dense microstructure, high strength concrete still has limitations in water exposure:
Extreme Hydrostatic Pressure
When subject to high levels of prolonged hydrostatic pressure from one side, HSC may still allow water permeation over time in a process called osmosis. Additional external tanking or membranes may be required.
Presence of Cracks
Any cracks or openings wider than 0.2mm can allow water ingress through HSC. Good structural design and control of early-age cracking is important.
Chemical Attack
While impermeable to pure water, HSC is still prone to chemical attack from pollutants, chloride ions, and acidic groundwater, which can degrade and erode the concrete over time.
Repeated Freeze-Thaw Cycles
In cold climates, freezing water inside porous concrete can cause damage from expansive pressures. HSC resists but does not completely prevent this.
Carbonation
Over the long-term, atmospheric CO2 penetrating the concrete reacts with cement paste and reduces alkalinity. This slowly negates the passivating effect and risks corrosion of rebar.
How can high strength concrete be made more waterproof?
There are additional methods that can enhance high strength concrete’s liquid resistance for critical applications:
Use of Waterproofing Admixtures
Special admixtures like crystalline waterproofing chemicals, added during mixing, fill pores and react with concrete to form a water-resistant gel.
Epoxy-Coated Rebar
Coating the steel reinforcement with epoxy provides an extra protective barrier against moisture reaching the metal surface.
Impregnation Sealers
Penetrating sealers are applied to the hardened concrete surface. They chemically react to block pores and form an impermeable layer inside the concrete.
External Membranes
Sheet or liquid membranes applied externally on the concrete surface provide an added moisture barrier.
Drainage Systems
Installation of drainage pipes or layers in the concrete structure allows collected water to be diverted away.
Key Takeaways
- High strength concrete has very low permeability and absorption but is not completely waterproof.
- A dense microstructure and optimized mix design provide improved resistance compared to normal concrete.
- HSC is well-suited to liquid retaining and wet environment applications.
- Cracking, hydrostatic pressure, chemical attack can still allow water ingress over time.
- Additional treatments like sealers, membranes, admixtures can enhance water resistance.
Conclusion
While not impenetrable, high strength concrete possesses very low permeability and absorption properties that make it suitable for use in immersed and water-exposed environments. With careful design, placement, and curing, HSC provides vastly improved water resistance compared to conventional concrete. However, limitations remain when subject to high hydrostatic pressures, cracking, freeze-thaw exposure, and chemical attack over prolonged periods. Advanced mix designs, surface treatments, coatings, and drainage provisions can further enhance the water-resisting performance of high strength concrete.