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Dry and wet laboratory tests and thermal fatigue of rocks
Engineering Geology, 9(1975) 249--265 ©Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands
DRY AND WET LABORATORY TESTS AND THERMAL FATIGUE OF ROCKS L. AIRES-BARROS, R. C. GRACA and A. VELEZ
Laboratory of Mineralogy and Petrology, Instituto Superior T~cnico, Lisbon (Portugal) (Received December 19, 1974; revision accepted May 15, 1975) ABSTRACT Aires-Barros, L., Gra~a, R. C. and Velez, A., 1975. Dry and wet laboratory tests and thermal fatigue of rocks. Eng. Geol., 9: 249--265. Investigation of the weathering and weatherability of three types of igneous rocks (granite, nepheline syenite and lamprophyric-minette dyke) was carried out by means of laboratory tests. For this purpose an experimental device was developed to obtain hot conditions alternating with room temperature and also with wet conditions, by immersion in two types of liquids (distilled water and a solution simulating "seawater"). Beyond qualitative effects emphasized in the text and in a photographic documentary, some quantitative determinations were made (weight loss and chemical decay of main cations of rock-minerals), which enable the authors to present an attempt at an alterability index for classification of the rocks. The research-program outlined on quantitative determinations is suitable for application in problems of engineering geology such as in determining suitability of rocks for use as breakwater stone or riprap for dams or in the stone deterioration measurements of the deterioration of stone used for buildings and ornamental facing stones. INTRODUCTION
Literature references refer to many papers discussing the effects of thermal variations on rocks. Nevertheless, papers dealing with laboratory work are not abundant. Griggs (1936) is the only author to mention laboratory research on the fatigue of rocks caused b y insolation effects. Griggs studied some rock types in an experiment where heating was accomplished by radiation from an electric heater and cooling b y a stream of dry air. The temperature range of the experiment was approximately l l 0 ° C . The conclusions from Griggs' experiments on several rock types (granite, syenite, felsite and sandstone) and with different textures (granite) are that "no change in the rock surface could be discerned" (op. cit. p.790). It must be emphasized that these tests were followed carefully b y means of micropetrographic observations and photographic methods in order to discover any slight change in the texture or composition of the minerals. In his final chapter, Griggs presents a plan to investigate the effect of humid conditions. Only an incomplete description is available on the
250 remarkable effects of a preliminary experiment where cooling was accomplished by a fine spray of water. Our research follows the main lines of Griggs' experiment, but with some differences. These are as follows: Firstly the rock specimens were cooled to room temperature without using any dry air current. Natural cooling was allowed in order to investigate the effects of air humidity and to facilitate comparison of the results obtained with those of Griggs' experiments. Secondly, we experimented with immersion in two liquid media (e.g. distilled water and saline solutions simulating seawater). Thirdly, we intended to quantify the results obtained in order to try classifying the rocks by means of a weathering index with possible geotechnical applications. RESEARCH PROGRAM We studied the behaviour of endogenetic rocks under the following experimental conditions: (a) insolation (alternate heating and cooling regimes); (b) alternate insolation and fluid submersion regimes; (c) alternate insolation and fluid submersion with salt crystallization effect. The main purpose was to investigate the fatigue caused by the effects of thermal changes on rocks with and without the action of several types of wetting. Disc-type rock specimens (42 mm diameter and 8 mm thick) with a polished face were subjected to the above tests. The study began with micropetrographic analysis (by reflected light) of the rock discs, which were also photographed for purposes of documentation. All stages of transformation during the tests were registered in the same way. The chemical transformations, mainly the cations that pass into solutions, were determined by atomic absorption spectrochemistry. Weight losses of the samples after each laboratory test were also calculated. For the experiments, specially developed laboratory apparatus -"climatizing apparatus" -- promoted quick effects and enabled severe heat (70°C) to alternate with either dry or wet conditions, both at r o o m temperature (20°C). The immersion step may be conducted in distilled water or in a salt solution simulating, for instance, the sea environment. The time selected for one cycle of the apparatus was 30 minutes. Thus one "apparatus year" corresponded to 182.5 laboratory hours. An apparatus cycle included a period of high temperature for 20 min and a period at room temperature (10 min), either dry or in the immersion medium. Each rock type studied was submitted to a total period of "ten laboratory years", i.e., 80 days of laboratory work. During this lapse of time there were several steps when the experiment was stopped for detailed study of the rock specimens and chemical analysis of the immersion liquids.
251
These steps occurred at the end of the "first year of laboratory work", at the end of the "third y e a r " and at the end of the test, i.e., at "ten years of work". In short, the presented research program was accomplished when each lot of rock specimens of a determined rock type underwent the following types of investigation: (1) Micropetrographic study of the rock specimens illustrated by microphotographic observation of the main aspects {e.g. texture, microfissures). (2) Tests covering 1, 3 and 10 "laboratory y e a r s " corresponding to 8 + 16 + 56 = 80 days of laboratory work. After each test a microphotographic observation of the same aspects referred to above also was obtained. By this method, it was intended to follow the transformations appearing in the specimens. (3) Determination of the weight losses and the leaching of cations carried from samples to the immersion solutions. These determinations were made after each of the three periods referred to. EXPERIMENTAL PROCEDURE
In order to conduct the tests, it was necessary to develop a device with the following parts and possibilities (see Figs.1 and 2):
6 L .
23-
.
.
STIRRER ROLLER SYST
Fig.1. General diagram of the climate-simulating system.
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Fig.2. Diagram of the electrical and mechanical
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TEMPORIZER ROLLIER SYSTEM ( UP & DOWN ELEVATOR ) MICRO- SWITCH CIRCUIT- BJtEAK(R ALARM-LAMP INF'RA RED LAMP SAMPLE STIRRER CYCLOMETER
connections.
(1) A heat source activated by a thermostat which controlled the time necessary to obtain the desired temperature. An infra-red lamp (250W) was employed which traversed the work table in front of the sample. The movement of the heat source thus permitted that different temperatures could be chosen to occur on the specimen. (2) A " t e f l o n " rock-sample that held the rock disc perpendicular to the infra-red rays. The sample holder was controlled by a roller that immersed it periodically in the selected solution. This solution was automatically stirred during sample immersion. (3) A system of two thermostats that controiled the heat source, the movement of the sample holder, the solution stirrers and a cyclometer. A system of control of the apparatus provided alarm-lamps, a thermometer and a hygrometer. After the construction of the laboratory system, the possibility of the association of more groups of stirrers, rollers and infra-red lamps was considered, in order to accelerate the research program. Thus, at present, the apparatus carries out three tests simultaneously. The size of the rock specimens was chosen in order to obtain a homogeneous distribution of the temperature on them. Thus, the rock discs have a diameter that permits the hot rays from the source to be centered with the axis of the
253 specimen and t h u s to a f f e c t it h o m o g e n e o u s l y . T h e thickness o f the discs was c h o s e n such t h a t t h e t e m p e r a t u r e o f the faces o p p o s i t e t o the h e a t source should n o t be m o r e t h a n 10% l o w e r t h a n the t e m p e r a t u r e s o n the faces e x p o s e d to t h e rays. RESULTS T h r e e d i f f e r e n t r o c k t y p e s were studied. One was a n e p h e l i n e syenite which is an igneous rock, sodic-alkali-silicates-rich (nepheline, albite-oligoclase and aegirine). It is a coarse-grained r o c k o f pink colour. A n o t h e r is a p s e u d o p o r p h y r i t i c granite, rich in alkali-feldspar and q u a r t z and w i t h a b u n d a n t b i o t i t e flakes. The t h i r d r o c k studied was a l a m p r o p h y r i c d y k e rich in alkali-feldspar and in biotite flakes ( m i n e t t e ) occurring a m o n g t h e r o c k s o f the Precambrian granite-gneissic b a s e m e n t o f t h e Cabora-Bassa region (Mozambique). In this area a large d a m is u n d e r c o n s t r u c t i o n and t h e l a m p r o p h y r i c d y k e s in t h e f o u n d a t i o n s are c o n s i d e r e d significant because o f their response to weathering. Granite and n e p h e l i n e syenite are t w o r o c k t y p e s occurring in Portugal, the first being c o m m o n in t h e n o r t h and central regions o f the c o u n t r y , the s e c o n d occurring in t h e south, as a b a t h o l i t h . T h e results (weight loss and chemical leaching) are p r e s e n t e d in Tables I and II. F r o m figures o f Table I and taking into a c c o u n t the chemical analysis o f the t h r e e s o u n d rocks studied, Table III was prepared. Here, percentages o f m o b i l i z e d chemical elements are calculated against t h e o x i d e s actually present in s o u n d rocks. TABLE I Weight loss Sample
Nephelinesyenite SN2 *1
SN3 .2
Granite SN4 .3
G1 *1
Lamprophyre G2 ,2
L1.1
L2 .2
Initial weight P0 (g)
33.1065 34.0243 32.9548 34.2675 31.6866 26.0158 27.8987
1 lab. year weight P1 (g)
33.0770 34.0010 32.9085 34.2632 31.6826 26.0007 27.8685
3 lab. year weight P3 (g)
33.0768 33.9867 32.8500 34.2591 31.6815 25.9899 27.7845
7 lab. year weight P7 (g) 33.0762 33.5630 32.5838 34.2576 31.6766 25.8857 27.2348 ,1 Laboratory assay with heating and dry cooling, alternating regime. ,2 Laboratory assay w i t h insolation and moisture, alternating action. ,3 Laboratory assay with insolation and moisture, alternating action with salt crystallization effect.
254 T A B L E II C h e m i c a l loss (% X 10 -3 ) Rock type
Duration of test (lab. years)
K2 O
Na20
CaO
MgO
1 3 10
0.1 2.1 2.8
1 3 10 1 3 10
Fe203
Total
7.0 3.0 26.0
0.5 3.2 6.0
0.4 0.4 0.4
0.5
8.0 9.2 35.2
0.3 5.6 9.1
3.7 38.0 50.0
1.6 2.6 11.0
0.5 3.4 5.9
22.0 28.0 32.0
7.1 5.9 23.0
3.9 6.1 15.0
4.6 9.3 8.6
2.1
37.6 51.7 78.6
1.4 2.0 17.0
* * *
8.2 11.0 23.0
0.4 1.0 1.6
1.7 3.6 7.0
11.7 17.6 48.6
Total
(a) fluid submersion in distilled water Nepheline syenite (SN3)
G r a n i t e (G2)
Lamprophyre (L2)
0.3 0.3
6.1 49.9 76.3
(b) fluid submersion in salt solution (35 g/l NaCl) Nepheline syenite (SN4)
1 3 10
* N o t d e t e r m i n e d regarding t h e t y p e o f assay.
T A B L E III C h e m i c a l loss calculated against t h e oxides p r e s e n t in s o u n d rocks Rock type
D u r a t i o n of test (lab. years)
K20
Na20
CaO
MgO
Fe203
Nepheline prior t o t e s t i n g syenite (SN3) 1 3 10
6.60 0.001 0.003 0.075
8.31 0.084 0.120 0.433
2.40 0.021 0.154 0.404
1.18 0.034 0.068 0.102
1.61
prior t o testing 1 3 10
6.43 0.005 0.092 0.233
2.42 0.153 1.723 3.789
1.46 0.110 0.288 1.041
0.96 0.052 0.406 0.021
prior to testing 1 3 10
7.16 0.307 0.698 1.145
1.22 0.582 1.066 2.951
6.73 0.058 1.149 0.371
10.18 0.45 0.136 0.221
(a) fluid submersion in distilled water (%)
Granite (G2)
Lamprophyre (L2)
0.031 0.031
0.040 0.086 0.261
0.34 0.088 0.176
0.052 0.482 1.140
3.41 0.062 0.062
0.131 0.310 0.584
255 TABLE III (continued) Rock type
Duration of test (lab. years)
K20
Na20
CaO
MgO
Fe203
Total
0.021 0.951 0.309
* * *
0.342 0.800 1.758
0.034 0.119 0.254
0.106 0.329 0.764
0.503 1.299 3.085
(b) fluid submersion in salt solution (35 g/l NaCl) Nepheline syenite (SN4)
1 3 10
* Not determined regarding the type o f assay.
250 -
/
/ ./ /
/' /
AP:
Pg-PI PO
ill00 % (i,1,],10)
NEPHELIN[ SY[NITE ....
eNANIT[
.......
LANPNOPHYN[
DY - DRY ,~
I
'
•
WI - WET SOLUTION
tO0
lie
/,Y/:
D1
o ~----1
................. I ~TIM[
_w,,o, 10
( LAB, YEAIIS ) ~
Fig.3. Relationship between weight loss (AP) and duration of test (time).
256 40
TEST WrTH
DESTILLED WATER
N E P H [ L I N [ SYENITE (SN$) GRANITE ( e 2 ) LAMPROPHYRE ( L 2 )
_+ / /
//
20
/ ©
t
J
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2
..'
/I
,'
/ /
/
" ."
....-
~
~,~o.
. .
I )"
,"
,;
r
,"
, J' ,'r
........
TIME
K~
( LAB
YEARS )
Fig.4. Relationship between Na20 and K20 leaching and duration of test.
Figs.3 and 4 are derived from data contained in Tables I and II. These data, microscopic and photographic observation (Figs.5-8) obtained on specimens tested, enable the presentation of some general conclusions: Weight loss in wet conditions is very drastic in comparison with those obtained in dry tests. Special attention must be given to rocks with fine and irregular microfissuring (such as the granite tested), since insolation fatigue may be sufficient to promote weight loss similar to that obtained in wet conditions In these irregular microfissured rocks, thermal fatigue may promote microspalling, i.e., the production of fine debris. All surfaces in the dry test show evident traces of transformations (Fig.5A and B). These transformations consist either of the appearance of new microfissures and the enlargement of the old ones, or the changing of clear polished surfaces of minerals into pitted and cloudy surfaces. It must be emphasized that Griggs' experiments were carried out with cooling by a dry air current. Another fact is that Griggs presents photographs with only 4 and 9 times magnification, which we believe to be too low to see mineralogical and textural changes. The photographs of the present investigations are enlarged 50 times.
257
It may be concluded that in the dry test the humidity of the air (7085% in winter and 40-63% in summer in the laboratory) is sufficient to promote the transformations referred to, including significant weight loss. Granite is the rock in which the weight loss is the lowest as compared to the other rocks studied; in that rock-type the weight losses in dry and wet experiments are of the same magnitude. This may be explained by great microfissuring of rock (mainly quartz) which leads to microspalling in both tests. The greater values of weight loss obtained in nepheline syenite must be related to the large quantity of secondary calcite which fills voids and microfissures. Concerning the lamprophyre, the photographs show a generalized pitting of biotite flakes and feldspar surfaces, giving rise to debris. The chemical decay of Na20, CaO and MgO was greater in granite than in the other two rocks. This fact is explained by greater permeability and porosity (5,5 mdy and 1,1%) of granite against the other rocks tested (5,0 mdy and 1,3% for nepheline syenite, and 0,18 mdy and 0,8% for lamprophyre). For K:O, lamprophyre shows the greatest chemical decay. This is due to the leaching through a rich biotite (and also alkali-feldspar) bearing rock. Differences between granite and nepheline syenite must be emphasized. In weight loss, syenite shows values greater than granite. For chemical leaching it is the contrary. The explanation may be due to the greater permeability of granite that permits a deeper penetration of water than in nepheline syenite. Fine mocrofissuring of granite promotes a deep leaching of this rock but it is not enough to cause a great weight loss. For nepheline syenite, the presence of a generalized system of fine cracks filled by calcite and a general impregnation of calcitic pitting are the causes for a greater weight loss by thermal fatigue. The experiments with salt solution show that weight loss is greater than for the other tests on a similar rock type only after four laboratory years have elapsed. After that time of laboratory work, salt deposition and crystallization in voids and cracks compensates significantly for the effective weight loss. It is not convenient to wash the specimens very drastically for removal of salt, because rock particles must then also be removed. The final weight losses are lower than the corresponding values for tests with distilled water. Perhaps this test should not be longer than five laboratory years in order to avoid internal salt deposition in voids and cracks of the specimens (Fig.8). Leaching of all chemical elements analysed is greater in this test than in that with distilled water (Fig.9). CONCLUSIONS
As an attempt to define an alterability classification for the rocks studied, the following formula may be used: K -- g m i n 4- g j gmin = ( l + g j ) K inln .
258
Fig.5. Lamprophyre dyke. A, prior to testing; B, after "10 years" of dry laboratory testing.
259
Fig.6. Granite. A, prior to testing; B, after "10 years" of wet laboratory testing.
1
261
Fig.7. Nepheline syenite. A, prior to testing; B,C,D, after "1, 3, and 10 years" of wet laboratory testing, respectively.
I i
263
Fig.8. Nepheline syenite. A, prior to testing; B, after "1 year" of wet laboratory testing in salt solution; C, idem but washed for salt removal; D, after "10 years" of wet laboratory testing in salt solution.
264 TEST
WITH
DISTILLED
WATER
- -
NEPHELINE SYENITE ( S k i s )
....
GRANITE
........
LAMPROPHYRE ( L 2 )
/I
/
/
--TIME
(02)
OhO,
/
/ J ~
O
csO . . .....
( LAB.
YEARS)~
20 TEST
15
WITH
N I CI SOLUTION
NEPHELINE SYENITE
I0
fib 0
--
TIME ( L A B .
YEARS)--
Fig.9. Relationship b e t w e e n leaching and duration o f test either in distilled water or with NaCl solution.
where: K = an alterability index; K m i n = a factor regarding mineralogical influence; it can be considered that Kmi,= AP (weight loss); gj = a factor regarding textural, permeability and porosity influence; it can be considered that gj = total chemical leaching. From results presented in Tables I and III, Table IV can be derived. Taking into a c c o u n t the figures of this latter table, the values for the weathering i n d e x K can be obtained:
265 TABLE IV Values of mineralogical and textural factors Rock type
Dry test Ap = Kmin 4.6
gJ 0
a P = Kmin
g/'
Granite
3.2
1.14
Nepheline syenite
9.0
0
136.0
0.26
Lamprophyre
50.0
0
238.0
0.58
dry ){KG=4. ~ ( = q6
test ~K~S== 50
Wet test
K6=7 IgsN= 171 test~K L = 376 wet
Thus the test rocks can be classified according to their alterability index as follows: KL> KSN> g 6
The experiments presented have been carried o u t on three rock-types whose relative alterability was not difficult to establish a priori. Nevertheless, the results obtained enable one to predict, e.g., the weatherability of a lot of crystalline limestones to be used as building facing stone, or to quantify the stone deterioration of a rock mass where tunnelling works have been made, or even in determining suitability of breakwater stone or riprap for dams. Some experiments for these purposes have already been made. These results will be published in future when some other correlations (e.g. with microhardness values and with microreflectance measurements) have been established. ACKNOWLEDGEMENTS
The authors acknowledge the support given to this work by the European Research Office of the U.S. Army Research & Development Group. REFERENCES Aires-Barros, L., 1970. Note pr61iminaire sur un indice d'alt6rabilit~. Proc. Int. Congr. Int. Ass. Eng. Geol., 1st, Paris, 1: 573-577. Griggs, D. T., 1936. The factor of fatigue in rock exfoliation. J. Geol., 44: 783--796.
DRY AND WET LABORATORY TESTS AND THERMAL FATIGUE OF ROCKS L. AIRES-BARROS, R. C. GRACA and A. VELEZ
Laboratory of Mineralogy and Petrology, Instituto Superior T~cnico, Lisbon (Portugal) (Received December 19, 1974; revision accepted May 15, 1975) ABSTRACT Aires-Barros, L., Gra~a, R. C. and Velez, A., 1975. Dry and wet laboratory tests and thermal fatigue of rocks. Eng. Geol., 9: 249--265. Investigation of the weathering and weatherability of three types of igneous rocks (granite, nepheline syenite and lamprophyric-minette dyke) was carried out by means of laboratory tests. For this purpose an experimental device was developed to obtain hot conditions alternating with room temperature and also with wet conditions, by immersion in two types of liquids (distilled water and a solution simulating "seawater"). Beyond qualitative effects emphasized in the text and in a photographic documentary, some quantitative determinations were made (weight loss and chemical decay of main cations of rock-minerals), which enable the authors to present an attempt at an alterability index for classification of the rocks. The research-program outlined on quantitative determinations is suitable for application in problems of engineering geology such as in determining suitability of rocks for use as breakwater stone or riprap for dams or in the stone deterioration measurements of the deterioration of stone used for buildings and ornamental facing stones. INTRODUCTION
Literature references refer to many papers discussing the effects of thermal variations on rocks. Nevertheless, papers dealing with laboratory work are not abundant. Griggs (1936) is the only author to mention laboratory research on the fatigue of rocks caused b y insolation effects. Griggs studied some rock types in an experiment where heating was accomplished by radiation from an electric heater and cooling b y a stream of dry air. The temperature range of the experiment was approximately l l 0 ° C . The conclusions from Griggs' experiments on several rock types (granite, syenite, felsite and sandstone) and with different textures (granite) are that "no change in the rock surface could be discerned" (op. cit. p.790). It must be emphasized that these tests were followed carefully b y means of micropetrographic observations and photographic methods in order to discover any slight change in the texture or composition of the minerals. In his final chapter, Griggs presents a plan to investigate the effect of humid conditions. Only an incomplete description is available on the
250 remarkable effects of a preliminary experiment where cooling was accomplished by a fine spray of water. Our research follows the main lines of Griggs' experiment, but with some differences. These are as follows: Firstly the rock specimens were cooled to room temperature without using any dry air current. Natural cooling was allowed in order to investigate the effects of air humidity and to facilitate comparison of the results obtained with those of Griggs' experiments. Secondly, we experimented with immersion in two liquid media (e.g. distilled water and saline solutions simulating seawater). Thirdly, we intended to quantify the results obtained in order to try classifying the rocks by means of a weathering index with possible geotechnical applications. RESEARCH PROGRAM We studied the behaviour of endogenetic rocks under the following experimental conditions: (a) insolation (alternate heating and cooling regimes); (b) alternate insolation and fluid submersion regimes; (c) alternate insolation and fluid submersion with salt crystallization effect. The main purpose was to investigate the fatigue caused by the effects of thermal changes on rocks with and without the action of several types of wetting. Disc-type rock specimens (42 mm diameter and 8 mm thick) with a polished face were subjected to the above tests. The study began with micropetrographic analysis (by reflected light) of the rock discs, which were also photographed for purposes of documentation. All stages of transformation during the tests were registered in the same way. The chemical transformations, mainly the cations that pass into solutions, were determined by atomic absorption spectrochemistry. Weight losses of the samples after each laboratory test were also calculated. For the experiments, specially developed laboratory apparatus -"climatizing apparatus" -- promoted quick effects and enabled severe heat (70°C) to alternate with either dry or wet conditions, both at r o o m temperature (20°C). The immersion step may be conducted in distilled water or in a salt solution simulating, for instance, the sea environment. The time selected for one cycle of the apparatus was 30 minutes. Thus one "apparatus year" corresponded to 182.5 laboratory hours. An apparatus cycle included a period of high temperature for 20 min and a period at room temperature (10 min), either dry or in the immersion medium. Each rock type studied was submitted to a total period of "ten laboratory years", i.e., 80 days of laboratory work. During this lapse of time there were several steps when the experiment was stopped for detailed study of the rock specimens and chemical analysis of the immersion liquids.
251
These steps occurred at the end of the "first year of laboratory work", at the end of the "third y e a r " and at the end of the test, i.e., at "ten years of work". In short, the presented research program was accomplished when each lot of rock specimens of a determined rock type underwent the following types of investigation: (1) Micropetrographic study of the rock specimens illustrated by microphotographic observation of the main aspects {e.g. texture, microfissures). (2) Tests covering 1, 3 and 10 "laboratory y e a r s " corresponding to 8 + 16 + 56 = 80 days of laboratory work. After each test a microphotographic observation of the same aspects referred to above also was obtained. By this method, it was intended to follow the transformations appearing in the specimens. (3) Determination of the weight losses and the leaching of cations carried from samples to the immersion solutions. These determinations were made after each of the three periods referred to. EXPERIMENTAL PROCEDURE
In order to conduct the tests, it was necessary to develop a device with the following parts and possibilities (see Figs.1 and 2):
6 L .
23-
.
.
STIRRER ROLLER SYST
Fig.1. General diagram of the climate-simulating system.
.
.
.
.
.
.
.
I .
.
.
.
.
.
.
.
.
.
.
i
252
. . . . . . . . . . . . . . . . . . . . . . . . . . .
" l ~ ' , ~, ~ - l
I]IIIIIIIII[IIIIIIIII r
.............................................
1 i
2 , 'i , ~rk I . .. ..... .. . . . . .
i I
j
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1
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~ .............
~
i
I
,
n
,,
,,,
I- . . . . . . . . . . . . . . . . . . .
i
1
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J
i
,
_ _
,, I
~
_ 01~
I I i I I
J | J
Fig.2. Diagram of the electrical and mechanical
_ _
~7
I
12 3 _ 4. 5_ 6 _ 7 6g-
TEMPORIZER ROLLIER SYSTEM ( UP & DOWN ELEVATOR ) MICRO- SWITCH CIRCUIT- BJtEAK(R ALARM-LAMP INF'RA RED LAMP SAMPLE STIRRER CYCLOMETER
connections.
(1) A heat source activated by a thermostat which controlled the time necessary to obtain the desired temperature. An infra-red lamp (250W) was employed which traversed the work table in front of the sample. The movement of the heat source thus permitted that different temperatures could be chosen to occur on the specimen. (2) A " t e f l o n " rock-sample that held the rock disc perpendicular to the infra-red rays. The sample holder was controlled by a roller that immersed it periodically in the selected solution. This solution was automatically stirred during sample immersion. (3) A system of two thermostats that controiled the heat source, the movement of the sample holder, the solution stirrers and a cyclometer. A system of control of the apparatus provided alarm-lamps, a thermometer and a hygrometer. After the construction of the laboratory system, the possibility of the association of more groups of stirrers, rollers and infra-red lamps was considered, in order to accelerate the research program. Thus, at present, the apparatus carries out three tests simultaneously. The size of the rock specimens was chosen in order to obtain a homogeneous distribution of the temperature on them. Thus, the rock discs have a diameter that permits the hot rays from the source to be centered with the axis of the
253 specimen and t h u s to a f f e c t it h o m o g e n e o u s l y . T h e thickness o f the discs was c h o s e n such t h a t t h e t e m p e r a t u r e o f the faces o p p o s i t e t o the h e a t source should n o t be m o r e t h a n 10% l o w e r t h a n the t e m p e r a t u r e s o n the faces e x p o s e d to t h e rays. RESULTS T h r e e d i f f e r e n t r o c k t y p e s were studied. One was a n e p h e l i n e syenite which is an igneous rock, sodic-alkali-silicates-rich (nepheline, albite-oligoclase and aegirine). It is a coarse-grained r o c k o f pink colour. A n o t h e r is a p s e u d o p o r p h y r i t i c granite, rich in alkali-feldspar and q u a r t z and w i t h a b u n d a n t b i o t i t e flakes. The t h i r d r o c k studied was a l a m p r o p h y r i c d y k e rich in alkali-feldspar and in biotite flakes ( m i n e t t e ) occurring a m o n g t h e r o c k s o f the Precambrian granite-gneissic b a s e m e n t o f t h e Cabora-Bassa region (Mozambique). In this area a large d a m is u n d e r c o n s t r u c t i o n and t h e l a m p r o p h y r i c d y k e s in t h e f o u n d a t i o n s are c o n s i d e r e d significant because o f their response to weathering. Granite and n e p h e l i n e syenite are t w o r o c k t y p e s occurring in Portugal, the first being c o m m o n in t h e n o r t h and central regions o f the c o u n t r y , the s e c o n d occurring in t h e south, as a b a t h o l i t h . T h e results (weight loss and chemical leaching) are p r e s e n t e d in Tables I and II. F r o m figures o f Table I and taking into a c c o u n t the chemical analysis o f the t h r e e s o u n d rocks studied, Table III was prepared. Here, percentages o f m o b i l i z e d chemical elements are calculated against t h e o x i d e s actually present in s o u n d rocks. TABLE I Weight loss Sample
Nephelinesyenite SN2 *1
SN3 .2
Granite SN4 .3
G1 *1
Lamprophyre G2 ,2
L1.1
L2 .2
Initial weight P0 (g)
33.1065 34.0243 32.9548 34.2675 31.6866 26.0158 27.8987
1 lab. year weight P1 (g)
33.0770 34.0010 32.9085 34.2632 31.6826 26.0007 27.8685
3 lab. year weight P3 (g)
33.0768 33.9867 32.8500 34.2591 31.6815 25.9899 27.7845
7 lab. year weight P7 (g) 33.0762 33.5630 32.5838 34.2576 31.6766 25.8857 27.2348 ,1 Laboratory assay with heating and dry cooling, alternating regime. ,2 Laboratory assay w i t h insolation and moisture, alternating action. ,3 Laboratory assay with insolation and moisture, alternating action with salt crystallization effect.
254 T A B L E II C h e m i c a l loss (% X 10 -3 ) Rock type
Duration of test (lab. years)
K2 O
Na20
CaO
MgO
1 3 10
0.1 2.1 2.8
1 3 10 1 3 10
Fe203
Total
7.0 3.0 26.0
0.5 3.2 6.0
0.4 0.4 0.4
0.5
8.0 9.2 35.2
0.3 5.6 9.1
3.7 38.0 50.0
1.6 2.6 11.0
0.5 3.4 5.9
22.0 28.0 32.0
7.1 5.9 23.0
3.9 6.1 15.0
4.6 9.3 8.6
2.1
37.6 51.7 78.6
1.4 2.0 17.0
* * *
8.2 11.0 23.0
0.4 1.0 1.6
1.7 3.6 7.0
11.7 17.6 48.6
Total
(a) fluid submersion in distilled water Nepheline syenite (SN3)
G r a n i t e (G2)
Lamprophyre (L2)
0.3 0.3
6.1 49.9 76.3
(b) fluid submersion in salt solution (35 g/l NaCl) Nepheline syenite (SN4)
1 3 10
* N o t d e t e r m i n e d regarding t h e t y p e o f assay.
T A B L E III C h e m i c a l loss calculated against t h e oxides p r e s e n t in s o u n d rocks Rock type
D u r a t i o n of test (lab. years)
K20
Na20
CaO
MgO
Fe203
Nepheline prior t o t e s t i n g syenite (SN3) 1 3 10
6.60 0.001 0.003 0.075
8.31 0.084 0.120 0.433
2.40 0.021 0.154 0.404
1.18 0.034 0.068 0.102
1.61
prior t o testing 1 3 10
6.43 0.005 0.092 0.233
2.42 0.153 1.723 3.789
1.46 0.110 0.288 1.041
0.96 0.052 0.406 0.021
prior to testing 1 3 10
7.16 0.307 0.698 1.145
1.22 0.582 1.066 2.951
6.73 0.058 1.149 0.371
10.18 0.45 0.136 0.221
(a) fluid submersion in distilled water (%)
Granite (G2)
Lamprophyre (L2)
0.031 0.031
0.040 0.086 0.261
0.34 0.088 0.176
0.052 0.482 1.140
3.41 0.062 0.062
0.131 0.310 0.584
255 TABLE III (continued) Rock type
Duration of test (lab. years)
K20
Na20
CaO
MgO
Fe203
Total
0.021 0.951 0.309
* * *
0.342 0.800 1.758
0.034 0.119 0.254
0.106 0.329 0.764
0.503 1.299 3.085
(b) fluid submersion in salt solution (35 g/l NaCl) Nepheline syenite (SN4)
1 3 10
* Not determined regarding the type o f assay.
250 -
/
/ ./ /
/' /
AP:
Pg-PI PO
ill00 % (i,1,],10)
NEPHELIN[ SY[NITE ....
eNANIT[
.......
LANPNOPHYN[
DY - DRY ,~
I
'
•
WI - WET SOLUTION
tO0
lie
/,Y/:
D1
o ~----1
................. I ~TIM[
_w,,o, 10
( LAB, YEAIIS ) ~
Fig.3. Relationship between weight loss (AP) and duration of test (time).
256 40
TEST WrTH
DESTILLED WATER
N E P H [ L I N [ SYENITE (SN$) GRANITE ( e 2 ) LAMPROPHYRE ( L 2 )
_+ / /
//
20
/ ©
t
J
/
2
..'
/I
,'
/ /
/
" ."
....-
~
~,~o.
. .
I )"
,"
,;
r
,"
, J' ,'r
........
TIME
K~
( LAB
YEARS )
Fig.4. Relationship between Na20 and K20 leaching and duration of test.
Figs.3 and 4 are derived from data contained in Tables I and II. These data, microscopic and photographic observation (Figs.5-8) obtained on specimens tested, enable the presentation of some general conclusions: Weight loss in wet conditions is very drastic in comparison with those obtained in dry tests. Special attention must be given to rocks with fine and irregular microfissuring (such as the granite tested), since insolation fatigue may be sufficient to promote weight loss similar to that obtained in wet conditions In these irregular microfissured rocks, thermal fatigue may promote microspalling, i.e., the production of fine debris. All surfaces in the dry test show evident traces of transformations (Fig.5A and B). These transformations consist either of the appearance of new microfissures and the enlargement of the old ones, or the changing of clear polished surfaces of minerals into pitted and cloudy surfaces. It must be emphasized that Griggs' experiments were carried out with cooling by a dry air current. Another fact is that Griggs presents photographs with only 4 and 9 times magnification, which we believe to be too low to see mineralogical and textural changes. The photographs of the present investigations are enlarged 50 times.
257
It may be concluded that in the dry test the humidity of the air (7085% in winter and 40-63% in summer in the laboratory) is sufficient to promote the transformations referred to, including significant weight loss. Granite is the rock in which the weight loss is the lowest as compared to the other rocks studied; in that rock-type the weight losses in dry and wet experiments are of the same magnitude. This may be explained by great microfissuring of rock (mainly quartz) which leads to microspalling in both tests. The greater values of weight loss obtained in nepheline syenite must be related to the large quantity of secondary calcite which fills voids and microfissures. Concerning the lamprophyre, the photographs show a generalized pitting of biotite flakes and feldspar surfaces, giving rise to debris. The chemical decay of Na20, CaO and MgO was greater in granite than in the other two rocks. This fact is explained by greater permeability and porosity (5,5 mdy and 1,1%) of granite against the other rocks tested (5,0 mdy and 1,3% for nepheline syenite, and 0,18 mdy and 0,8% for lamprophyre). For K:O, lamprophyre shows the greatest chemical decay. This is due to the leaching through a rich biotite (and also alkali-feldspar) bearing rock. Differences between granite and nepheline syenite must be emphasized. In weight loss, syenite shows values greater than granite. For chemical leaching it is the contrary. The explanation may be due to the greater permeability of granite that permits a deeper penetration of water than in nepheline syenite. Fine mocrofissuring of granite promotes a deep leaching of this rock but it is not enough to cause a great weight loss. For nepheline syenite, the presence of a generalized system of fine cracks filled by calcite and a general impregnation of calcitic pitting are the causes for a greater weight loss by thermal fatigue. The experiments with salt solution show that weight loss is greater than for the other tests on a similar rock type only after four laboratory years have elapsed. After that time of laboratory work, salt deposition and crystallization in voids and cracks compensates significantly for the effective weight loss. It is not convenient to wash the specimens very drastically for removal of salt, because rock particles must then also be removed. The final weight losses are lower than the corresponding values for tests with distilled water. Perhaps this test should not be longer than five laboratory years in order to avoid internal salt deposition in voids and cracks of the specimens (Fig.8). Leaching of all chemical elements analysed is greater in this test than in that with distilled water (Fig.9). CONCLUSIONS
As an attempt to define an alterability classification for the rocks studied, the following formula may be used: K -- g m i n 4- g j gmin = ( l + g j ) K inln .
258
Fig.5. Lamprophyre dyke. A, prior to testing; B, after "10 years" of dry laboratory testing.
259
Fig.6. Granite. A, prior to testing; B, after "10 years" of wet laboratory testing.
1
261
Fig.7. Nepheline syenite. A, prior to testing; B,C,D, after "1, 3, and 10 years" of wet laboratory testing, respectively.
I i
263
Fig.8. Nepheline syenite. A, prior to testing; B, after "1 year" of wet laboratory testing in salt solution; C, idem but washed for salt removal; D, after "10 years" of wet laboratory testing in salt solution.
264 TEST
WITH
DISTILLED
WATER
- -
NEPHELINE SYENITE ( S k i s )
....
GRANITE
........
LAMPROPHYRE ( L 2 )
/I
/
/
--TIME
(02)
OhO,
/
/ J ~
O
csO . . .....
( LAB.
YEARS)~
20 TEST
15
WITH
N I CI SOLUTION
NEPHELINE SYENITE
I0
fib 0
--
TIME ( L A B .
YEARS)--
Fig.9. Relationship b e t w e e n leaching and duration o f test either in distilled water or with NaCl solution.
where: K = an alterability index; K m i n = a factor regarding mineralogical influence; it can be considered that Kmi,= AP (weight loss); gj = a factor regarding textural, permeability and porosity influence; it can be considered that gj = total chemical leaching. From results presented in Tables I and III, Table IV can be derived. Taking into a c c o u n t the figures of this latter table, the values for the weathering i n d e x K can be obtained:
265 TABLE IV Values of mineralogical and textural factors Rock type
Dry test Ap = Kmin 4.6
gJ 0
a P = Kmin
g/'
Granite
3.2
1.14
Nepheline syenite
9.0
0
136.0
0.26
Lamprophyre
50.0
0
238.0
0.58
dry ){KG=4. ~ ( = q6
test ~K~S== 50
Wet test
K6=7 IgsN= 171 test~K L = 376 wet
Thus the test rocks can be classified according to their alterability index as follows: KL> KSN> g 6
The experiments presented have been carried o u t on three rock-types whose relative alterability was not difficult to establish a priori. Nevertheless, the results obtained enable one to predict, e.g., the weatherability of a lot of crystalline limestones to be used as building facing stone, or to quantify the stone deterioration of a rock mass where tunnelling works have been made, or even in determining suitability of breakwater stone or riprap for dams. Some experiments for these purposes have already been made. These results will be published in future when some other correlations (e.g. with microhardness values and with microreflectance measurements) have been established. ACKNOWLEDGEMENTS
The authors acknowledge the support given to this work by the European Research Office of the U.S. Army Research & Development Group. REFERENCES Aires-Barros, L., 1970. Note pr61iminaire sur un indice d'alt6rabilit~. Proc. Int. Congr. Int. Ass. Eng. Geol., 1st, Paris, 1: 573-577. Griggs, D. T., 1936. The factor of fatigue in rock exfoliation. J. Geol., 44: 783--796.