Measurement of Properties
2.2 Basic and Index Properties
2.2.1 Intact Rock GeneralGeneral
Testing is normally performed in the laboratory on a specimen of fresh to slightly weath-ered rock free of defects.
Basic properties include volume–weight relationships, hardness (for excavation resist-ance), and durability and reactivity (for aggregate quality).
Index tests include the uniaxial compression test (see Section 2.4.3), the point load index test (see Section 2.4.3), and sonic velocities, that are correlated with field sonic velocities to provide a measure of rock quality (see Section 2.5.3).
Volume–Weight Relationships
Include specific gravity, density, and porosity as defined and described in Table 2.6.
Hardness General
Hardness is the ability of a material to resist scratching or abrasion. Correlations can be made between rock hardness, density, uniaxial compressive strength, and sonic veloci-ties, and between hardness and the rate of advance for tunneling machines and other
TABLE 2.6
Volume–Weight Relationships for Intact Rock Specimens
Property Symbol Definition Expression Units
Specific gravity Gs The ratio of the unit weight of a pure Gsγm/γw
(absolute) mineral substance to the unit weight of water at 4°C. γw 1g/cm3or 62.4 pcf
Specific gravity Gs The specific gravity obtained from a mixture Gsγm/γw
(apparent) of minerals composing a rock specimen
Density ρor γ Weight W per unit volume V of material ρ W/V t/m3
Bulk density ρ Density of rock specimen from field ρ W/V t/m3
(also g/cm3, pcf)
Porosity n Ratio of pore or void volume Vvto total n Vv/Vs % volume Vt.
In terms of density and the apparent n 1(ρ/Gs) % (metric) specific gravity
Notes: Specific Gravities: Most rock-forming minerals range from 2.65 to 2.8, although heavier minerals such as hornblende, augite, or hematite vary from 3 to 5 and higher.
Porosity: Depends largely on rock origin. Slowly cooling igneous magma results in relatively nonporous rock, whereas rapid cooling associated with escaping gases yields a porous mass. Sedimentary rocks depend on amount of cementing materials present and on size, grading, and packing of particles.
Density: Densities of fresh, intact rock do not vary greatly unless they contain significant amounts of the heavier minerals.
Porosity and density: Typical value ranges are given in Table 2.12.
Significance: Permeability of intact rock often related to porosity, although normally the characteristics of the in situ rock govern rock-mass permeability. There are strong correlations between density, porosity, and strength.
excavation methods. The predominant mineral in the rock specimen and the degree of weathering decomposition are controlling factors.
Measurement Criteria
The following criteria are used to establish hardness values:
1. Moh’s system of relative hardness for various minerals.
2. Field tests for engineering classification.
3. “Total” hardness concept of Deere (1970) based on laboratory tests and devel-oped as an aid in the design of tunnel boring machines (TBMs). Ranges in total hardness of common rock types are given in Figure 2.1.
4. Testing methods for total hardness (Tarkoy, 1975):
FIGURE 2.1
Range of “total” hardness for common rock types. Data are not all inclusive, but represent the range for rocks tested in the Rock Mechanics Laboratory, University of Illinois, over recent years. HR Schmidt hardness; HA
abrasion test hardness. (From Tarkoy, P. J., Proceedings of the 15th Symposium on Rock Mechanics, Custer State Park, South Dakota, ASCE, New York, 1975, pp. 415–447. With permission.) (a) Inset: Schmidt hammer.
Total hardness HTis defined as
HT HR√HAg1/2 (2.1)
where HRis the Schmidt hardness and HAthe abrasion test hardness.
● Schmidt rebound hardness test: An L-type concrete test hammer (Figure 2.1a), with a spring in tension, impels a known mass onto a plunger held against the speci-men (energy 0.54 ft lb. or 0.075 m kg). The amount of energy reflected from the rock–hammer interface is measured by the amount that the hammer mass is caused to rebound (ASTM C805).
● Shore (C-2) sclerescope is also used to measure rebound hardness. The rebound height of a small diamond-tipped weight falling vertically down a glass tube is measured and compared with the manufacturer’s calibration.
● Abrasion hardness test is performed on a thin disk specimen which is rotated a specific number of times against an abrading wheel, and the weight loss recorded.
Durability General
Durability is the ability of a material to resist degradation by mechanical or chemical agents. It is the factor controlling the suitability of rock material used as aggregate for road-way base course, or in asphalt or concrete. The predominant mineral in the specimen, the microfabric (fractures or fissures), and the decomposition degree are controlling factors.
Test Methods
Los Angeles abrasion test (ASTM C535-03 and C131-03): specimen particles of a specified size are placed in a rotating steel drum with 12 steel balls (1 7/8 in. in diameter). After rotation for a specific period, the aggregate particles are weighed and the weight loss compared with the original weight to arrive at the LA abrasion value. The maximum acceptable weight loss is usually about 40% for bituminous pavements and 50% for concrete.
British crushing test: specimen particles of a specified size are placed in a 4-in.-diameter steel mold and subjected to crushing under a specified static force applied hydraulically.
The weight loss during testing is compared with the original weight to arrive at the British crushing value. Examples of acceptable value ranges, which may vary with rock type and specifying agency, are as follows: particle size (maximum weight loss), 3/4–1 in. (32%), 1/2–3/4 in. (30%), 3/8–1/2 in. (28%); and 1/8–3/16 in. (26%).
Slake durability test (ASTM D4644): determines the weight loss after alternate cycles of wetting and drying shale specimens. High values for weight loss indicate that the shale is susceptible to degradation in the field when exposed to weathering processes.
Reactivity: Cement–Aggregate Description
Crushed rock is used as aggregate to manufacture concrete. A reaction between soluble sil-ica in the aggregate and the alkali hydroxides derived from portland cement can produce abnormal expansion and cracking of mortar and concrete, often with severely detrimental effects to pavements, foundations, and concrete dams. There is often a time delay of about 2 to 3 years after construction, depending upon the aggregate type used.
The Reaction
Alkali–aggregate reaction can occur between hardened paste of cements containing more than 0.6% soda equivalent and any aggregate containing reactive silica. The soda equiva-lent is calculated as the sum of the actual Na2O content and 0.658 times the K2O content of the clinker (NCE, 1980). The alkaline hydroxides in the hardened cement paste attack the silica to form an unlimited-swelling gel that draws in any free water by osmosis and expands, disrupting the concrete matrix. Expanding solid products of the alkali–silica reaction help to burst the concrete, resulting in characteristic map cracking on the surface.
In severe cases, the cracks reach significant widths.
Susceptible Rock Silicates
Reactive silica occurs as opal or chalcedony in certain cherts and siliceous limestones and as acid and intermediate volcanic glass, cristobolite, and tridymite in volcanic rocks such as rhyolite, dacites, and andesites, including the tuffs. Synthetic glasses and silica gel are also reactive. All of these substances are highly siliceous materials that are thermodynam-ically metastable at ordinary temperatures and can also exist in sand and gravel deposits.
Additional descriptions are given in Krynine (1957).
Reaction Control
Reaction can be controlled (Mather, 1956) by:
1. Limiting the alkali content of the cement to less than 0.6% soda equivalent. Even if the aggregate is reactive, expansion and cracking should not result.
2. Avoiding reactive aggregate.
3. Replacing part of the cement with a very finely ground reactive material (a poz-zolan) so that the first reaction will be between the alkalis and the pozzolan, which will use up the alkalis, spreading the reaction and reaction products throughout the concrete.
Tests to Determine Reactivity Tests include:
● The mortar-bar expansion test (ASTM C227-03) made from the proposed aggre-gate and cement materials.
● Quick chemical test on the aggregates (ASTM C289-01).
● Petrographic examination of aggregates to identify the substances (ASTM C295).
2.2.2 Rock Masses