Although we seek out hard, dense, clean, sound, stable and non-reactive aggregates for use in precast concrete, high-quality coarse aggregate still contain miniscule pores. These pores are capable of holding measurable amounts of moisture, which can drastically affect a concrete mix. Fine aggregate also need to be carefully monitored since they are susceptible to bulking, which causes fine aggregate to hold water between the aggregate particles.
Aggregate moisture content fluctuates from day-to-day and can even change from hour-to-hour depending on how they are stored as well as exposure conditions like sun, wind, rain and humidity.
Weighing and batching wet aggregate into a mix without accounting for its unique moisture condition at the time of batching would result in adding an insufficient amount of aggregate to the mix while adding excess water. The opposite is true when weighing and batching dry aggregate without accounting for their moisture condition. Aggregate bins with high walls keeps aggregate types and sizes separated. There are three commonly used test methods for performing moisture content tests of normal-weight and lightweight aggregates.
When plants use moisture probes or automatic mixing water adjustment systems, plants must still conduct an aggregate moisture content test on each aggregate at least once per week to validate the moisture probe accuracy and performance.
First, ensure you have the necessary and properly functioning equipment and personal protective equipment readily available. Before collecting the sample, weigh the sample container. Turn the balance on and ensure it displays a measurement of zero. Then place the empty sample container on the scale and record its weight to the nearest 0.
Be sure to validate this weight measurement periodically, but not less than annually. Keep in mind the larger the sample size beyond the values required in Table 1, the longer it may take to conduct the test. Ensure the sample that is collected represents the entire stockpile of aggregate not only in size and gradation, but also in moisture content. The aggregate near the bottom edges of the coarse aggregate stockpile may be slightly larger in size than the general gradation of the entire stockpile, as bigger pieces have a tendency to roll to the bottom.
Also, keep in mind the moisture content of the aggregate within the stockpile will vary depending on depth in the stockpile and environmental exposure conditions.
Aggregate deep in the stockpile may be wetter since it is not exposed to the drying sun or wind. Conversely, aggregate on the surface of the stockpile may be wetter than those further down if there was a recent rain or if covered in morning dew.
Ensure the collected aggregate sample is protected from moisture loss or gain prior to performing the test. Turn the balance on, ensure it reads zero and then place the receptacle containing the aggregate sample on the scale.
Record the weight and the unit of measurement. This value is the combined weight of the sample container plus the aggregate sample. Next, subtract the weight of the sample container from the combined weight of the container plus the aggregate sample. Record the weight to the nearest 0. This value is the weight of the aggregate sample. If using a hot plate, place the sample container with the aggregate on the hot plate first and then turn it on.
If using an oven, place the sample container with the aggregate inside the oven first and then turn on the oven. Rapid heating of coarse aggregate can cause some particles to explode, which is both a safety concern and will result in the loss of aggregate particles and skew the testing results.
Allowing the heat source to gradually heat the aggregate can help prevent the aggregate from exploding. Gradually heat the aggregate sample using the selected method. If using a hot plate or burner, use the stirrer periodically during the heating process to help expedite moisture evaporation and to prevent localized overheating of the aggregate.
Carefully tap the stirrer on the side of sample container to remove any stuck-on aggregate and ensure it goes back into the rest of the sample for drying. If using a microwave or oven, stirring the sample during heating is optional.
There is no set duration for heating and drying the aggregate sample. The amount of time needed to achieve oven-dry aggregate will vary depending on the size of the sample, the aggregate gradation, the method of heating and the wetness of the representative aggregate sample. Carefully watch the sample during the heating and drying process and never leave the sample unattended. Carefully remove the sample container with the aggregate from the heat source using hot pads and place the sample container on the hot pad positioned on the scale.
Ensure the heat source is turned off. Record the value to the nearest 0. This value is the combined weight of the sample container plus the dried aggregate sample. Next, subtract the weight of the sample container from the combined weight of the sample container plus the dried aggregate sample.
This value is the weight of the dried aggregate sample. Using hot pads, carefully place the sample container back on the hot plate or burner, or back in the microwave or oven, and turn on the heat source. Continue to gradually heat the sample, taking care to consistently heat the sample. Therefore, by minimizing the amount of cement, the cost of concrete can be reduced.
However, excessively large aggregate tends to lower strength by reducing available bonding area. ASTM has limits for grading of concrete aggregates. A fineness modulus for fine aggregates should be 2. Two aggregates with the same fineness modulus can have different grading curves. A low fineness modulus requires more cement paste to maintain workability. Variations from mix design requirements for fineness modulus should not exceed 0.
However, there are solid restrictions on very fine particles to prevent increased water demand and volume instability. Gap Grading -- An aggregate where one or more of the intermediate-sized fractions is omitted. The resulting concrete is very stiff and has low workability.
An extreme case is no-fines concrete. This concrete is difficult to handle and compact; developing low strength and high permeability. Aggregate can contain water, both internal, based on porosity, and external, surface moisture. This gives aggregate the ability to absorb water. This will effectively reduce the amount of water available for hydration; or conversely, if the aggregate is very wet, add excess water to a cement mix.
Of these four states, SSD, saturated-surface-dry, is considered the best reference state. It is an equilibrium state, where the aggregate will not absorb or give water to the cement paste, simulates actual field conditions more closely, and used to determine bulk specific gravity. However, this moisture state is not easy to obtain. To determine the amount of water an aggregate will add or subtract from a cement paste, the following three quantities are used:.
The weight of water absorbed by the aggregate W abs is calculated from the weigh of the aggregate W agg in a concrete mix using effective absorption EA. It is used to calculate the additional water W add of the concrete mix. If the moisture content MC is positive, there is surface moisture. If the MC is negative, it has the potential for absorption.
Therefore, the total moisture associated with an aggregate is:. More water can be held in the interspace between particles than in coarse aggregates. This also leads to thicker films of water which in turn push the aggregate apart and increase the apparent volume. This is called bulking. Absolute specific gravity ASG considers the weight and volume of the solid part of the aggregate.
Unit weight UW or bulk density is the weight of a given volume of material. Basically, unit weight is measured by filling a container of known volume with a material and weighing it. The degree of moisture and compaction will affect the unit weight measurement.
Therefore, ASTM has set a standard oven-dry moisture content and a rodding method for compaction. Therefore, this is the most economical concrete aggregate since it will require the least amount of cement. Aggregates makeup the largest part of concrete mixes and are responsible for the durability of the mix.
Durability is a measure of how well concrete will handle freezing and thawing, wetting and drying, and physical wear. Chemical reactions also can contribute to problems with durability. Solid materials - particles passing a mesh sieve. These fine particles may increase water requirements and interfere with surface bonding between cement and coarse aggregates. Soluble substances - organic matter may interfere chemically with alkaline cement pastes affecting setting time.
Aggregates obtained from the sea should be thoroughly cleaned to avoid problems from salt contamination. Unsound particles -- Soft particles such as clay lumps, wood, and coal will cause pitting and scaling at the surface. Organic compounds can be released which interfere with setting and hardening. Weak material of low density which have low wear resistance should also be avoided.
It should be noted that tests on aggregates alone are not an effective means of predicting aggregate performance in the field.
Tests for aggregate properties for mix design are straightforward. However, tests for durability and performance have limitations. The use of waste materials as aggregate in concrete is gaining increased attention, especially in view of our escalating solid-waste problems. A wide variety of materials are being considered as aggregates: garbage, building rubble, industrial waste products, and mine tailings. All of these potential aggregates are evaluated on their 1 economy, 2 compatibility with other materials, and 3 concrete properties.
Successful utilization of waste material as aggregate depends on anticipating potential problems and ensuring that the properties of concrete will remain unchanged. Aggregates are classified by their specific gravities into three categories; 1 lightweight, 2 normal-weight, 3 heavy-weight; each with different applications.
Your comments and questions are more than welcome. Part 6. Shape and Texture Shape and texture affect workability of fresh concrete.
Size Gradation Grading or aggregate size distribution is a major characteristic in concrete mix design. Sieve Analysis -- determines the grading of an aggregate. Coarse aggregate is that retained on the 4 sieve and fine aggregate is that passing a 4 sieve. In a sieve analysis a series of sieve are used with smaller and smaller openings.
Coarse aggregates are analyzed with standard sieves and fine aggregates with half-sized sieves. Maximum Aggregate Size -- Smallest sieve in which the entire sample will pass through. Fineness Modulus -- a parameter for checking the uniformity of grading. Therefore, for fine and coarse aggregates respectively, the fineness modulus is: F.
Moisture Content Aggregate can contain water, both internal, based on porosity, and external, surface moisture. There are four moisture states: Oven-dry OD ; all moisture removed. Air-dry AD ; surface moisture removed, internal pores partially full Saturated-surface-dry SSD ; surface moisture removed, all internal pores full.
Wet ; pores full with surface film. Absorption and Surface Moisture To determine the amount of water an aggregate will add or subtract from a cement paste, the following three quantities are used: Absorption capacity AC -- maximum amount of water the aggregate will absorb. Specific Gravity A dimensionless ratio of density of the material in question to the density of water. Unit Weight Unit weight UW or bulk density is the weight of a given volume of material. Durability of Aggregates Aggregates makeup the largest part of concrete mixes and are responsible for the durability of the mix.
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