Checking Quality of Cement

 How to check the Quality of Cement when it is received at site?

    Did you know that when concrete fails, one of the possible culprits is the quality of cement? Ensuring that the cement received at the construction site meets quality standards is crucial to avoid project delays or failures.

Cement Delivery Cement typically arrives at the site in bags or bulkers. Upon receipt, it's essential to carry out specific quality checks to verify its condition. Here's what to do: 1. **Check for the ISI Mark** Look for the ISI (Indian Standards Institute) mark on the cement bag. This indicates that the brand follows standard manufacturing practices to ensure product quality. If the ISI mark is present, third-party testing may not be mandatory unless specifically requested by the client. However, if there's any doubt or if a client insists, a third-party test should be conducted. 2. **Verify Manufacturing Details** Cement bags are usually marked with the week, month, and year of manufacturing. This information is crucial to determine the freshness of the cement. **When does Week 1 start?** Week 1 always begins on January 1st of the year. The day of the week on January 1st sets the weekly cycle for that year. For example, if January 1st falls on a Tuesday, every Tuesday will mark the start of a new week. 3. **Check Cement Age** Cement should ideally be consumed within three months of its manufacturing date. Beyond this period, the quality may deteriorate, especially if storage conditions are uncertain. It is often unclear how cement was stored by the supplier or trader, which can affect its performance. 4. **Source Cement Directly from the Manufacturer** To avoid the risks of compromised cement due to poor storage, always encourage procurement directly from the manufacturer. This helps ensure that the cement is fresh and has not been subjected to damage or alteration at the supplier’s facility. 5. **Manufacturing Week Printing** The manufacturing week is typically printed on the cement bags once they are loaded onto a conveyor, either for stacking or direct shipment in wagons. At the manufacturer's facility, the use of iron hooks is strictly avoided to prevent damage. **Printing Issues** The printing process is automated, and occasionally, bags may arrive with incorrect or missing manufacturing details. In such cases, notify the manufacturer immediately and request written confirmation before accepting the shipment. This will not only cover your liability but also prompt the manufacturer to rectify the issue, ensuring future shipments are correctly labeled.

I would like to point out that cement bags ordered directly from manufacturers will often have “Not for Sale” or “Not for Retail” printed on them. These bags are intended for direct use and not for retail distribution. Conversely, if a price is printed on the bag, it is meant for retail sale, and there is a risk of receiving old stock when purchasing through a retailer. Age of Cement Do not accept cement that is more than a month old. Ensure that this condition is clearly mentioned in the purchase order to avoid disputes later. Freshness is critical to cement performance, and the longer it sits, the more its quality deteriorates. Random Weight Checks It is essential to check the weight of cement bags randomly. A standard bag should weigh 50 kg, but improper handling can lead to discrepancies. This is crucial because using a bag that weighs less than 50 kg in concrete mixes can significantly alter the mix proportions, impacting the quality and strength of the concrete. Post-Dispatch Handling Issues In India, once cement is dispatched from the manufacturing unit, manufacturers often do not take responsibility for maintaining its quality. Cement is transported to storage yards across the country, primarily by train. Unfortunately, at unloading points, particularly at railway stations, local laborers—often using iron hooks—damage the cement bags. These unskilled laborers are not supervised by the manufacturer, and there is no quality oversight to prevent such damage. Manufacturers lack the authority to replace these local laborers at each station. Instead of attempting the impossible task of replacing labor, they should focus on improving bag design to minimize damage during handling. Industry-Wide Action As engineers, we should collectively pressure cement manufacturers to address the issue of improper handling at multiple stages of transportation. Writing to manufacturers to demand better handling practices and more durable packaging would help safeguard cement quality. The Weight Problem Due to Handling Improper handling—such as puncturing of bags at various locations—can lead to underfilled bags. When the bags are damaged, they may contain less than the standard 50 kg. This inconsistency can lead to concrete failures, as the bag’s weight is crucial for accurate mix design calculations. If someone uses a bag weighing less than 50 kg without realizing it, the chemistry and mathematics of the concrete mix are thrown off, leading to weaker or failed concrete. Manufacturers’ Response Manufacturers are aware of the potential losses due to improper handling. Rather than addressing the root cause, some manufacturers have taken the approach of slightly overfilling the bags to ensure that the net weight remains at 50 kg after transportation and handling losses.

After verifying the freshness of cement, we can further inspect its engineering properties to ensure optimal performance. Temperature of Cement in the Bag Sometimes, cement arrives hot. This does not necessarily indicate that hydration has started. Cement that is freshly manufactured and directly loaded for transport may reach temperatures as high as 50°C. In such cases, it is advisable to store the cement for 2 to 3 days before use, allowing it to cool down. This situation often arises when demand exceeds supply. Using hot cement can lead to a "false set," where the concrete becomes stiff prematurely. To restore plasticity, you may need to remix the concrete or increase the mixing time. Physical Properties of Cement Color The color of cement depends largely on the limestone used and any performance-enhancing additives, such as fly ash. A slightly off-white or lighter gray color does not indicate poor quality. For example, AAC cement, made from whitish limestone, has a fainter shade than typical gray cement. It's important to understand the variations in cement color—whether dark or light. Engineers should visit cement manufacturing plants to gain firsthand knowledge of the production process. Manufacturers often arrange such visits free of charge for their clients. Smoothness When you rub cement between your fingers, it should feel silky smooth due to its fine particle size. If it feels rough, the grinding process might not have been done properly. In such cases, it's important to check the fineness of the cement. Lumps in Bags - **Soft Lumps:** These are caused by the weight pressing down on the cement bags and will break apart easily when the bag is rolled. - **Hard Lumps:** These form due to moisture exposure or direct contact with water, such as rain entering the bags during transport. Cement containing hard lumps should be rejected as this indicates partial hydration has occurred. Contamination Test To check for contamination, take a handful of cement and throw it into water. If the cement floats for a short time before sinking, it is likely in good condition. If it sinks immediately, this may indicate that the cement has been tampered with or modified after dispatch. In such cases, cubes should be cast to test 1-day strength, and if it fails, the cement should be rejected. Consistency Consistency reflects the water demand of the cement. Higher water demand leads to a higher water-cement ratio, which can affect the strength and durability of the concrete. Generally, the standard consistency of cement ranges between 27% and 34%. Older cement may show a lower water demand due to partial hydration, while coarser cement (with less surface area) also tends to require less water. Testing should be conducted at 27°C ± 2°C with 65% ± 5% relative humidity, following IS 4031 (Part 4). If your lab conditions don’t meet these precise standards, you can still perform the consistency test, but make sure to compare your results with previous ones obtained under similar environmental conditions. Compressive Strength A good engineer always verifies the compressive strength of each received batch of cement. Testing the compressive strength ensures the quality of the cement and prevents failures during real-world applications. Since many site labs don’t meet ideal standards, results should be compared with previous tests conducted under the same conditions. It’s crucial to advocate for a properly equipped lab setup, as it can save significant costs by preventing large-scale failures, far outweighing the expense of setting up a standard lab.

Cement Received in Bulk (Bulkers) Loose cement transported in bulkers does not have manufacturing details printed on the cement itself. Therefore, it’s crucial to verify the batch details from the dispatch challan (delivery note). The challan should include important information such as seal numbers, which are printed on the seals fixed at the bulker openings after the cement is loaded. Seal Verification Before opening the bulker, ensure that the seal numbers on all openings match the numbers written on the dispatch challan. This is vital for confirming that the cement has not been tampered with during transit. After verifying the seals, proceed with the same inspection process that is used for cement bags. Preserving Samples from Each Lot It is essential to preserve a sample of the cement from each lot received. Store the sample in an airtight bag or container and label it with all relevant details, including the batch number, date of receipt, and any other information from the dispatch challan. Preserving these samples serves two critical purposes: 1. **Dispute Resolution:** If a future issue arises, such as concrete failure, having preserved samples allows for accurate testing to verify the quality of the cement at the time of receipt. 2. **Accountability:** In the event of failure, if the manufacturer does not accept responsibility, preserved samples provide evidence of the actual quality delivered, helping to blacklist non-compliant suppliers. Potential Causes of Concrete Failure Failures in concrete may occur for various reasons, including: - Improper storage conditions of cement at the site. - Errors in the batching process for concrete. - Mistakes during cube casting or other testing procedures. It’s important to note that while cement manufacturers employ advanced systems and automation, human error can still occur at the plant. This is why careful inspection and testing at the site are crucial. A Note to Engineers Don’t hesitate to hold cement manufacturers accountable if issues arise. Mistakes can happen even in automated plants, although they are rare. Careful testing and quality control are your best defense against failure. I hope this guidance helps you inspect and verify cement quality during your next delivery. If you have any questions or if I missed something, feel free to reach out. Ensuring thorough checks is key to maintaining the integrity of your project.