Part 6. Concrete Aggregates

Aggregates generally occupy 70 to 80% of the volume of concrete and therefore have a significant effect on its properties. Strength of concrete and mix design are independent of the composition of aggregate, but durability may be affected. Aggregates are classified based on specific gravity as heavyweight, normal-weight, and lightweight. Normal weight aggregates make-up 90% of concrete used in the United States.

Shape and Texture

Shape and texture affect workability of fresh concrete. The ideal aggregate would be spherical and smooth allowing good mixing and decreasing interaction between particles. Natural sands are close to this shape. However, crushed stone is much more angular and requires more paste to coat the increased surface area. Long, flat aggregate should be avoided due to increased interaction with other particles and the tendency toward segregation during handling.

Shape and texture of coarse aggregates affects the strength of the concrete mix. Increased surface area provides more opportunity for bonding and increases strength. However, excessive surface area in an aggregate can lead to internal stress concentrations and potential bond failure.

Size Gradation

Grading or aggregate size distribution is a major characteristic in concrete mix design. Cement is the most expensive material in concrete. Therefore, by minimizing the amount of cement, the cost of concrete can be reduced.

• 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. The maximum nominal size is the smallest sieve in which at least 95%, by weight, of the sample will pass. Maximum size should not be larger than 1/5 the minimum dimension of a structural member, 1/3 the thickness of a slab, or 3/4 the clearance between reinforcing rods and forms. These restrictions limit maximum aggregate size to 1 1/2 inches, except in mass applications.

Higher maximum aggregate size lowers paste requirements, increases strength and reduces w/c ratios. However, excessively large aggregate tends to lower strength by reducing available bonding area. ASTM has limits for grading of concrete aggregates.

• Fineness Modulus -- a parameter for checking the uniformity of grading. Generally calculated for fine aggregates but also for coarse aggregates assuming 100% is retained on #8 - #100 sieves. Therefore, for fine and coarse aggregates respectively, the fineness modulus is:

F.M. = (Cumulative percent retained on half-sized sieves)/100

F.M. = (Cumulative percent retained on standard sieves including #4 + 500 )/ 100

A fineness modulus for fine aggregates should be 2.3 - 3.1. 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 0.2 (ASTM standards). ASTM allows for an increase in fine aggregates (% passing #50 and #100) if smoother surface finishing is required. 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. Advantages of gap grading are more economical concrete, use of less cement, and lower w/c ratios. 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.

Moisture Content

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.

There are four moisture states:

1. Oven-dry (OD); all moisture removed.
2. Air-dry (AD); surface moisture removed, internal pores partially full
3. Saturated-surface-dry (SSD); surface moisture removed, all internal pores full.
4. Wet; pores full with surface film.

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.

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:

1. Absorption capacity (AC) -- maximum amount of water the aggregate will absorb. The range for most normal-weight aggregates is 1 - 2%.

2. Effective Absorption (EA) -- amount of water required to bring an aggregate from the AD state to the SSD state.

The weight of water absorbed by the aggregate Wabs is calculated from the weigh of the aggregate Wagg in a concrete mix using effective absorption (EA).

3. Surface Moisture (SM) -- amount of water in excess of SSD

It is used to calculate the additional water W_add of the concrete mix

The moisture content (MC) of aggregate is given by:

 

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:

Stockpiled fine aggregate is often in a wet state with a surface moisture of 0 to 5%. 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.

Specific Gravity

A dimensionless ratio of density of the material in question to the density of water.

SG = [density of solid] / [density of water]

Absolute specific gravity (ASG) considers the weight and volume of the solid part of the aggregate. Whereas, bulk specific gravity (BSG) is a measure of the weight/volume of solids and pores of a material.

ASG > BSGSSD > BSGOD

However, since the porosity of most rocks used in concrete is 1 to 2%, the values of all specific gravities are approximately the same; in the range of 2.5 to 2.8.

Unit Weight

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. The maximum unit weight of a blend of two aggregates is about 40% fine aggregate by weight. Therefore, this is the most economical concrete aggregate since it will require the least amount of cement.

Durability of Aggregates

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.

• Soundness -- rocks that undergo volume changes due to wetting and drying are rare. However, aggregate is susceptible to volume change during freezing and thawing cycles. Freezing can cause internal stresses to build up as water inside the aggregate freezes and expands. A critical size can be calculated below which freeze-thaw stress is not a problem; however, for most rock it is greater than normal sizes.
  
• Wear Resistance -- a good aggregate will be hard, dense, strong, and free of porous material. The abrasion resistance of aggregate can be tested by the Los Angeles abrasion test; however, this test does not match well with concrete wear in the field.
  
• Alkali-Aggregate Reaction -- An expansive reaction between some reactive forms of silica with the aggregate and alkalis in the cement paste. The result is overall cracking in the structure, manifesting itself in map or pattern cracking at the surface. This reaction can be controlled most easily by using low-alkali cements. However, due to changes in manufacturing, low-alkali cements may not be feasible. A better approach is to avoid aggregate with the potential or proven record of reactivity. A low w/c ratio is very impermeable and will slow down the reaction but not stop it. No adverse reactions will occur without external water.
  
• Other Alkali-Silica Reactions -- sand-gravels found in river systems of Kansas and Nebraska are highly reactive and cause map cracking. Replacement of 30% of the aggregate with crushed limestone is effective in reducing the damage. Basically, it results in the separation of flat clay minerals causing very slow expansion.
  
• Alkali-Carbonate Reactions -- an expansive reaction involving clayey carbonate rock. Reaction can be controlled by using low-alkali cements or blending aggregate with other less reactive material. ASTM has set standards for deleterious substances in aggregates, which depend on application. This can be divided into two categories:
  

  • Impurities

    • Solid materials - particles passing a 200-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.

Evaluation of Aggregates

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.

Physical Tests

• Abrasion Resistance
  
  • Abrasion resistance -- The Los Angeles test for abrasion involves ball milling an aggregate sample for a given time and measuring how the sample particles are reduced in size.
    
  • Scratch hardness test -- assumes a relationship between hardness and abrasion.  Neither of this tests are an accurate or reliable measure of the concrete hardness. An indication would be to test the concrete itself.
    
• Frost Resistance
  
  • Soundness test -- This test is a simulation of ice formation in an aggregate sample. The sample is saturated with a solution of sodium or magnesium salt and dried in an oven. The salt crystals which form in the pores simulate ice. Correlation between this test and field tests are not good. Again, a better approach is testing aggregate in concrete.

Chemical Tests

• Alkali-silica Reaction -- A rapid reliable test for alkali-aggregate reactivity has not yet been developed. Most acceptable tests require long curing times of about 6 months. In this test, the aggregate is ground into a fine sand and used to make a variety of mortar bars. The mortars are stored in hot, moist conditions to accelerate the reaction. Expansion of the sample is measured and compared to ASTM specifications.
  
• Aggregate Beneficiation -- If an aggregate does not pass the ASTM tests, an engineer may choose to try to upgrade the material. Beneficiation may be useful in areas where aggregate is scarce. There are several possible ways of treatment:
  
  
  • Crushing -- Soft, porous rock may be removed by crushing.
  • Heavy-media separation -- Lightweight particles may be separated by floating them to the top of a liquid.
  • Reverse water flow or air flow -- used to remove lightweight particles like wood.
  • Hydraulic jigging -- Stratification of aggregate in a vertical pulsation of water. Lightweight particles separate to the top.
  • Elastic fractionation -- Aggregate is dropped on an incline steel plate. Hard particles bounce higher off the plate than do softer particles. Appropriate placement of collection bins can provide good separation.
  • Washing and scrubbing -- Removes fine surface particles.

Waste Materials as Aggregate

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.

Special Aggregates

Aggregates are classified by their specific gravities into three categories; 1) lightweight, 2) normal-weight, 3) heavy-weight; each with different applications.

• Lightweight Aggregates -- A general characteristic of lightweight aggregate is high internal porosity. Most of these materials are synthetic, however, some natural materials can be treated to provide low specific gravity. Clays, shale, or slates will bloat at high temperatures resulting in an expansion in volume. Other synthetic materials are produced using pyroprocessing techniques, such as volcanic glass, slags, or waste glass. Lightweight aggregates have high absorption capacity associated with their high porosity. However, some materials have a coating resulting from the fusion process and water cannot penetrate. This coating can be damaged during handling resulting in an abrupt increase in absorption.
  
  
• Heavyweight Aggregates -- A material with a high specific gravity. These types of materials are mostly used for radiation shielding and application where a high mass-to-volume ratio is required.
  
  
• Abrasion and Skid-Resistant Aggregates -- Hard, dense aggregates used in heavy-industry applications where high resistance to abrasion is required. The strength of the cement paste and the cement-aggregate bond are more important than the aggregate hardness.
  
  
• Marginal Aggregates -- Use of this type of aggregate will require more care and thought in design, and generally more cost. In considering marginal aggregates, there are four areas of interest: 1) concrete properties, 2) weaknesses of aggregate, 3) beneficiation, and 4) use of protective measures.
  
  

This website was originally developed by Charles Camp for his CIVL 1101 class.
This site is maintained by the Department of Civil Engineering at the University of Memphis.
Your comments and questions are more than welcome.