Concrete, a composite material made from binding materials, aggregates, and water, is widely used in construction due to its durability and adaptability. Proper water quality is crucial in concrete production, as impurities can significantly impact both the strength and durability of the structure.
This article discusses the importance of water quality, acceptable sources, and the effects of different contaminants, along with guidelines for assessing the suitability of water for concrete production.
Role of Water in Concrete
Water is essential in concrete for hydration, ensuring the binding material (typically cement) forms a strong and durable matrix with aggregates. Given that cement production is energy-intensive, releasing about 900 kg of CO₂ per ton, optimizing the quality of water ensures sustainable and efficient construction.
Water Quality in Concrete
Water quality affects the concrete at two critical stages:
- Fresh State: Impure water can cause improper setting or segregation of materials.
- Hardened State: Over time, contaminants can cause structural defects such as reduced strength, cracking, or steel corrosion.
Thus, ensuring that the water used in mixing is free from harmful impurities is essential for achieving long-term durability and strength.
Potable Water
The general recommendation is to use water that is fit for drinking (potable water). Such water is free from harmful substances, with inorganic solids typically under 1000 ppm. When mixed with cement at a water-cement ratio (w/c) of 0.5, it contributes minimal solids (0.05%) and does not compromise the concrete’s strength.
However, in some cases, non-potable water may also be used if it does not contain harmful contaminants. Even water with a dark color or bad smell can be utilized, provided it passes chemical testing. Water with a pH between 6 and 9 is generally acceptable, though excessive chloride or alkali content must be monitored to avoid durability issues.
Acceptable Sources of Water
In addition to potable water, various sources of water can be used, either as primary or supplementary sources:
- Groundwater: Often used in areas with limited access to municipal water.
- Reclaimed Water: Treated water reused from domestic sources.
- Municipal Treated Water: Water from wastewater treatment plants.
These alternatives help conserve potable water while maintaining the concrete's performance, provided they undergo appropriate quality testing.
Guidelines for Sampling and Testing
Testing water used in concrete production is an integral part of quality control. A systematic testing process ensures that concrete meets performance expectations for strength and durability.
- Sample Collection: Collect samples carefully to avoid contamination that could affect test results.
- Storage and Timing: Store samples in a controlled environment and test them promptly to avoid chemical changes.
- Regular Testing: Conduct frequent tests, especially when using water sources with no prior performance records.
Determine the Suitability of Water
A simple yet effective method is to compare the setting time and strength of concrete using the water under consideration against those made with distilled water or a known suitable source.
- Tolerance Levels: Up to 10% deviation in strength and setting time is typically acceptable.
- When to Test: Testing is especially necessary if dissolved solids exceed 2000 ppm or if the water source contains unusual substances.
Effects of Contaminants in Mixing Water
Contaminants in water can lead to undesirable changes in concrete’s performance. Below are some common impurities and their effects:
1. Suspended Solids
- High levels of suspended solids introduce clay and silt into the mix, potentially weakening the concrete.
- Allowing water to settle before use helps remove suspended particles.
2. Acidity and Alkalinity
- Slightly acidic water (without organic acids) is usually harmless.
- Alkaline water should be tested as it can interfere with cement hydration and setting.
3. Algae Presence
- Algae in water can cause air entrainment in concrete, leading to reduced strength.
- Green or brown algae contamination should be addressed through proper testing and treatment.
4. Sea Water
- Sea water contains 3.5% salinity, with a mix of sodium chloride and magnesium salts.
- While sea water can accelerate early strength gain, it reduces long-term strength by about 15%.
- Use of sea water is discouraged for reinforced concrete due to the risk of steel corrosion.
5. Chloride Content
- High chloride levels cause persistent dampness and efflorescence on concrete surfaces.
- Chloride also promotes corrosion of embedded steel, weakening the structure over time.
6. Moisture Content in Aggregates
- Aggregates often contain surface moisture, contributing to the total water content in concrete.
- Fine aggregates may carry more than 10% surface moisture, affecting the water-cement ratio.
- Proper moisture control ensures that aggregates do not introduce excess water or impurities.
Impact of Lead in Mixing Water
An investigation conducted by Madhusudana Reddy et al. (2011) examined the effect of lead (Pb²⁺) in mixing water on concrete properties. Cement mortar specimens were cast using deionized water with varying lead concentrations (10 to 5000 mg/L).
- Findings:
- Higher lead concentrations led to a significant reduction in strength and a noticeable increase in setting time.
- However, at 2000 mg/L, only marginal changes in setting time and compressive strength were observed.
This study highlights the importance of monitoring heavy metal contamination in water used for concrete.
The quality of water used in concrete plays a critical role in ensuring the structure’s strength, durability, and serviceability. While potable water is ideal, non-potable water and alternative sources can also be used if they meet quality standards. Contaminants such as chlorides, algae, suspended solids, and heavy metals must be carefully monitored to prevent adverse effects on the concrete.
A rigorous sampling and testing program ensures that water used in concrete production is suitable, helping maintain long-term structural integrity. Proper control over water quality is essential, not only to optimize performance but also to reduce costs and contribute to sustainable construction practices.
