Quality Tip: How to Avoid Excessive & Uncompensated Moisture in a Concrete Batch Plant

The most important part of a concrete mix design is the water-to-cement ratio. The ratio is the amount of water to the quantity of cement, typically both measured in pounds. This ratio is important for creating enough cement paste to be able to bind all of the other materials together. The materials in a concrete batch include stone, sand, cementitious powder and water. More complex mixes could include non-cementitious powders, admixtures, and fibers.

When the materials are used wet – above saturated surface dry (SSD) weight – the water in the material is included in the weight of material. For example, if your material has 5% moisture above SSD weight and you batch 1,000 lbs. of this material, you will have 50 lbs. of extra water and you will have only 950 lbs. of material. This affects two parts of the batch. First, it will result in 6 gal of extra water. Second, it will reduce the yield. This wastes cement because more cement is added than needed to paste a smaller amount of material together.

Excessive moisture can result in:

• Overly wet mixes: Too much moisture can make the mix too fluid, reducing strength and leading to segregation of material.
• Inconsistent strength: Variations in aggregate moisture cause fluctuations in mix proportions, leading to unpredictable compressive strength.
• Extended setting times: Higher water content can delay curing and setting, impacting production schedules.
• Surface defects: Excess moisture can lead to problems like scaling, cracking and poor finish quality.

Understand Stockpile Moisture Variations
Aggregate moisture levels can vary significantly depending on where material is sourced from within the stockpile. Different moisture levels arise due to the natural settling of water in the pile, particularly in rainy or humid conditions. Trucks may pick up materials from different sections of the quarry’s stockpiles, which will also affect the moisture content in the batch.

The loading of the bins should be done as close to the start of batching as possible, as this timing will make for more consistent moisture readings from probe. The longer the materials sit in the bins the more water drains to the bottom of the aggregate.

To mitigate the risk of excess moisture:

• Equipment operators should avoid taking aggregates from the bottom of the pile, as this area tends to retain more moisture.
• Operators should keep the bucket level at least 18” to 24” from the bottom of the pile.
• Whenever possible, aggregates should be taken from higher points in the stockpile where moisture levels are lower and more consistent.
• Frequently stir up the pile to redistribute the moisture in the pile.
• In rainy weather or hot weather covering the pile will keep the moisture in the pile more consistent.

By ensuring proper loading and stockpile protection practices, concrete producers can significantly reduce variations in moisture content and enhance mix consistency.

Regularly Test for Yield Compensation
Regular testing of aggregate content and proper stockpile management are necessary to ensure a consistent and high-quality concrete mix.

The frequency of testing depends on whether the plant has moisture probes installed.

• For plants with moisture probes installed: Testing should be conducted at least once a week to ensure proper probe calibration.
• For plants without moisture probes: More frequent testing is necessary, with moisture checks performed in the morning, late morning and mid-afternoon before the shift ends.

This routine helps maintain an accurate water-to-cement ratio and your yield, reducing the risk of excess moisture that can compromise your concrete’s performance.

Incorporate Moisture Burn off Testing into Daily Procedures
To improve overall product quality, it is important to formally integrate burn off tests into your plant’s daily operating procedures which includes; accurate material moisture testing, taking samples, weighing and the drying of the samples. Standardizing this procedure will ensure all team members will remain vigilant about moisture fluctuations and take corrective actions when needed.

By adopting these proactive strategies, concrete producers can prevent excess moisture and correct yields leading to a more reliable and durable final product. A well-maintained quality control process not only enhances product integrity but also minimizes costly labor, rework and material waste. Staying ahead of moisture-related challenges will contribute to a more efficient and successful concrete batching operation.

For more information on how Mixer Systems batch plant controls can ensure proper compensation for moisture, contact us today.