ABRAM’S LAW: A rule stating that with given concrete materials and conditions of testing, the ratio of the amount of water to the amount of cement in the mixture design determines the strength of the concrete.

If only mixing concrete was as simple as knowing and applying Abram’s Law. Just calculate the water to cement ratio for the desired mix design – and “POOF” – you have perfect concrete to deliver to the jobsite, every time. What could go wrong?

As every producer knows, life is not that easy and batch computers sometimes need a little help to be sure all material variables are accounted for to ensure that the desired strength, flowability, workability, stability, and durability of concrete mixtures met the job specs.

Effects of Water Variability

Proper measurement of moisture content in concrete production is critical. For example, here are some individual variables affected by moisture content that affect the properties of the finished concrete product.

Aggregate Weight Control: A one-percent variation in moisture content in a dry aggregate by weight results in a change of 10kg of aggregate loaded into the mixer for every 1,000kg of dry aggregate weighed. The greater the variation in moisture, the greater the weight of aggregate used. The lower the grade of aggregate used, the greater the moisture percentage range that can be held by the aggregate, which increases the potential effect of moisture variation in the final product produced.

Water Volume Dispensed into the Mixer: Aggregate moisture variations require adjustments in the volume of water added to the mix design. For example: an aggregate density of 2400kg/m3 would require a water change of about 24 liters due to a one-percent change in aggregate moisture content.

Concrete Strength: Here we are back at Abrams Law. Variations in the water content by weight of the aggregates affects the water/cement ratio, thereby affecting the compressive and tensile strengths of the concrete.

Concrete Durability: Creep is related to the mix’s water/cement ratio and therefore water can be indirectly detrimental to durability. Autogenous shrinkage may also occur from varying water contents as it is caused by the internal consumption of water during hydration.

Flow Consistency: Obviously, the free water content in the mix directly affects the flowability of the concrete. But, there are also indirect effects. Changes in aggregate grading, as well as content ratio of fine/coarse material in the mix affects the flowability due to the water holding capacity of the different moisture contents.

Mix Stability: Maintaining the relative proportions as defined in the mix design is important to avoid bleeding, segregation and plastic settlement. Precise and accurate water control will help to control the occurrence of all these issues.

Problem 1: Aggregate Moisture Content Too Wet

Let’s face it – rain on your aggregate stockpiles is a game changer in mix designs. The pore spaces in aggregates can be full of water, yet the aggregate appears dry on the surface to the naked eye. This means that when placing this high moisture aggregate into your mix design without accounting for the additional water contained in the pore spaces of the aggregates, the water-to-cement ratio is higher, thus affecting the strength and durability of the finished product.

Problem 2: Aggregate Moisture Content Too Dry

Super dry aggregate piles act like a porous sponge when added into a wet concrete mixture. The air-filled pockets between the aggregates tend to take up water from the concrete paste mix, depleting the water available to allow the particles to slip past each other during mixing, pumping, pouring, placement, and finishing.

Solution to Problem 1 and Problem 2: Install a Moisture Probe Sensor

Providing accurate moisture content and absorption information to a modern batch computer allows the system to compensate for the water content missing or carried within the aggregate added to the mix. This level of quality control can be easily achieved with the addition of concrete moisture probe sensors or moisture meters to your concrete operation.

Digital microwave moisture measurement is recognized today as one of the most important methods for online measurement of bulk materials. Microwave moisture probe sensors have a range of sensors that utilize digital techniques for measuring the change in a microwave resonator frequency with changes of moisture in the material. This type of sensor has very significant benefits over existing analogue techniques. Possibly the most important factor is that this technique provides a greater accuracy of measurement over a far wider range of moisture contents, and is suitable for working with a far wider range of materials.

Depending on your plant configuration, the concrete moisture probe sensors or moisture meters can be installed into aggregate bins or hoppers, at material loadout points, in mixer walls, mixer floors, now available for mixer paddles. The moisture probe sensors can be used for many types of mixers from pan, ribbon, twin shaft and planetary mixer.

The first company to develop microwave moisture sensors specifically for use in the concrete industry and is recognized as the world leader in this technology is Hydronix. The unique Hydronix digital microwave moisture sensor not only has a digital measurement technique which provides a very precise level of accuracy but also a choice of measurement modes that enable the producer to select the mode best suited to their material.  Combined with Digital Signal Processing filters for improved performance and a variable frequency technique which is designed to operate in a bespoke range as opposed to an off-the-shelf frequency such as 433MHz or 2.4GHz, Hydronix sensors provide producers with the most accurate microwave moisture measurement available.

Choice of Measurement Modes

Hydronix sensors use different measurement modes to achieve the best signal response to different materials. In the concrete industry for example, different aggregates have their own distinct dielectric property behavior and will therefore respond differently to each of the underlying measurement modes. These response differences between the modes are also applicable to variables in the application itself. The best mode may now be selected for the material and the application to best account for the temperature range, precision required and bulk density changes over time.

Variable Frequency Technique

Hydronix sensor design starts and finishes with performance as the key design objective. The variable frequency technique is designed to operate in a bespoke range as opposed to an off-the-shelf frequency such as 433MHz or 2.4GHz. This ensures that the sensor is continually focused on detecting changes in the number of water molecules in the material with no interference from temperature changes and variations in the ionic content of the material (commonly salt content).

Benefits of Maintaining Optimum Control of Aggregate Moisture

Installing Hydronix digital microwave sensors into a new or existing system is simple and for minimal cost the benefits are extensive, often providing payback on the capital outlay in a few months. The benefits of maintaining optimum control of moisture may be briefly summarized as follows:

• Reduce costs by reducing the amount of spoiled material
• Save energy costs by only heating driers/burners to temperature required
• Save energy costs by reducing the amount of mixing time required
• Reduce additive costs by knowing the moisture levels of your raw materials
• Increase production levels
• Automation usually increases production rates
• Reduce the amount of time needed to rework
• Improve quality of outputs
• Easy adjustment to changes in moisture in materials to ensure consistency of final product

For more information on Hydronix digital microwave moisture probe sensors, moisture meters, and other moisture control products, call the experts at Gulf Atlantic: (800) 792-7427 or send them an inquiry.