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Aggregates

            There are three main parts to a concrete mixture; Portland cement concrete, water and aggregates. The main function that aggregates carry out is to take up space. The more space filled by these aggregates makes the amount of Portland cement in a particular job much less and makes the project as a whole a great deal cheaper.

            This does not mean that we can use a mixture with a small amount of Portland cement and a great deal of rock. Like all things done in the engineering field, there is set of codes that must be followed. For all aggregates used in this particular type of job, it is necessary that all aggregates must not contain any substance that is deleteriously reactive with the alkalies in the cement so as not to cause excessive amounts of expansion. The following specs come from AC 150/5370-10A P-501. A link to this site can be found in the source list.

Fine Aggregates

            All fine aggregates shall conform to the requirements of ASTM C 33. The gradation tests shall conform to the ASTM C 136 in which some exceptions may apply. The fines should fall into the following table after the tests have been completed to the above standards.

 

TABLE 1.  GRADATION FOR FINE AGGREGATE

(ASTM C 33)

 

 

Sieve Designation (Square Openings)

Percentage by Weight

Passing Sieves

3/8 in. (9.5 mm)

100

 

No. 4 (4.75 mm)

95‑100

 

No. 8 (2.36 mm)

80‑100

 

No. 16 (1.18 mm)

50‑85

 

No. 30 (600 micro‑m)

25‑60

 

No. 50 (300 micro‑m)

10‑30

 

No. 100 (150 micro‑m)

2‑10

 

         

 

Course Aggregates

            All course aggregates shall conform to the ASTM C 33 and all gradation tests should be in compliance with ASTM C 136. It is also important to note that if the nominal maximum size of the aggregate is more than one inch in diameter, the aggregates should be given in two separate size groups. The gradation should conform to the following table once the testing is complete.

GRADATION FOR COARSE AGGREGATE

 

 

Percentage by Weight Passing Sieves

 

Sieve Designations

(square openings)

From 2" to No. 4

(50.8 mm ‑ 4.75 mm)

From 1‑1/2" to No. 4

(38.1 mm ‑ 4.75 mm)

From 1" to No. 4

(25.0 mm‑4.75 mm)

#3

#57

#4

#67

#57

in.

Mm

2"‑1"

1"‑No.4

1‑1/2"‑3/4"

3/4"‑No.4

1"‑No.4

2‑1/2

63

100

---

---

---

---

2

50.8

90-100

---

100

---

---

1‑1/2

38.1

35-70

100

90-100

---

100

1

25.0

0-15

95-100

20-55

100

95-100

3/4

19.0

---

---

0-15

90-100

---

1/2

12.5

0-5

25-60

---

---

25-60

3/8

9.5

---

---

0-5

20-55

---

No. 4

4.75

---

0-10

---

0-10

0-10

No. 8

2.36

---

0-5

---

0-5

0-5

·         * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

 

 

 

TABLE 2.  GRADATION FOR COARSE AGGREGATE

ASTM C 33

 

 

Percentage by Weight Passing Sieves

 

Sieve Designations

(square openings)

 

 

 

 

*

 

*

in.

mm

 

2‑1/2

63

*

*

 

2

50.8

*

*

 

1‑1/2

38.1

*

*

 

1

25.0

*

*

 

¾

19.0

*

*

 

1/2½

12.5

*

*

 

3/8

9.5

*

*

 

No. 4

4.75

*

*

 

No. 8

2.36

*

*

 

 

It is extremely important that the only acceptable aggregates for this particular type of application be: crushed stone, crushed or uncrushed gravel, air-cooled blast furnace slag, crushed or recycled concrete pavement or any combination thereof. Dust and any other coatings should be washed off of the aggregate before use.

The following is a list of Cementious Materials and the standards with which they must conform.

Fly Ash                        ASTM C 618 Class C, F, N

Blast Furnace Slag        ASTM C 989, grade 100 or 120

Fly ash can be used from any sources that are pre-qualified by other agencies, provided it meets the loss of ignition requirement of this specification and is accompanied by a certification and test data.

Water

            The water used in mixing or curing of aggregates should be clean of any contaminates such as oil, salt, acid, alkali, sugar, vegetable, or any other substance that will injure the finished surface, and must also conform to the requirements of AASHTO T 26. It should be noted that any potable quality water may be used without testing.

Cover Material for Curing

            Below are listed the types of curing covers and the requirements they must meet.

Liquid forming–membrane compounds must be in compliance with ASTM C 309, type 2, class B.

White Polyethylene film must be in compliance with ASTM C 171.

White burlap-polyethylene sheeting must be in compliance with ASTM C 171.

Waterproof paper shall conform to the requirements of ASTM C 171.

Some tests were performed in which pieces of aggregate, shaped to a 1-inch square area, were imbedded in cement bricks and tested for bond strength in tension. The results that follow are from the text “ Investigating Concrete, selected works of Bryant and Katharine Mather.”

 

           Surface texture

             of aggregate

                                                Bond strength in tension

28 days in      water

28 days in water, then 28 days in air

28 days in water, then 28 days cycles of wetting and drying

Rough, porous

350

260

235

Rough

240

275

230

Rough

215

300

245

Fairly rough

250

185

160

Smooth

120

45

                                                   -----

Smooth, non-choidal

285

170

45

Very smooth

195

40

25

 

This exhibits the effect of the aggregate shape in the concrete mixture. This type of information should be taken into account before aggregate is selected.