Mechanical Behavior of Basaltic Rocks from Serra Geral Formation Used as Road Material in Santa Catarina State, Brazil

This work presents physical, petrographic and mechanical characteristics of basalt aggregates from Serra Geral Formation in the State of Santa Catarina, Brazil, used as layers of simple graded crushed stone in pavement. The simple graded crushed stone chosen for the study was obtained from a quarry in the municipality of São José do Cerrito/SC, located in the mountainous region of Santa Catarina. The aggregates of this quarry were used to pave a segment of highway BR-282/SC from 2002 to 2006. This study presents the results of thin section analysis, which identifies the mineralogical composition of the material and clarifies aspects associated with the rock texture. In addition, resilient modulus tests are conducted, followed by numerical evaluation of the pavement’s structural behavior, field evaluation of elastic displacement with Benkelman beam testing, and comparison of the results. The entire highway segment was monitored throughout its implementation and was determined to be in good condition. The material used offers good performance as a pavement layer. Moreover, the methodology used for the design presents good correlation with the field tests.


Introduction
The Serra Geral Formation, covering much of the southern region of Brazil and extending through parts of São Paulo, Mato Grosso do Sul and Minas Gerais states, includes the known world's largest occurrence of Earth fissure basaltic magmatism.This formation caps the Paraná sedimentary basin and is approximately 1,000 m thick in some sections.Owing to its immensity, this formation is an important element of study for Brazilian geotechnical engineers.
Accurate characterization of these basalts is important for the implementation of major geotechnical works because changes in this material can often pose engineering problems.
Characterization of the aggregate used in road paving is accomplished through physical and mechanical tests.Although not common for this purpose, thin section analysis can greatly contribute to understand the test results.The Serra Geral Formation is a source for asphalt aggregates used in layers of dry-bound macadam sub-base and simple graded crushed stone base in addition to asphalt mixtures for road paving in that region of Brazil.
Most of the Serra Geral Formation is characterized by a simple volcanic sequence represented by a wide predominance of basalt (97% in volume) and diabase dikes of approximately 133,000 years old.These rocks are dark gray to black, have very fine to medium granulation and are hypo crystalline, massive or vesicular.To a lesser extent, this formation presents a sequence of acidic rocks represented by rhyolites and rhyodacites with intermediate volcanic rocks including andesites and porphyritictrachyandesites (Schneider et al., 1974).Figure 1 shows a geological map of Brazil, in which the hatched area to the south represents the basaltic lava flows of the Serra Geral Formation.

Petrologic Characteristics
The texture of a rock corresponds to the relationship among its components and is defined by the shape, arrangement and distribution of minerals in its composition (Bigarella et al., 1985).Texture depends essentially on the relative size of mineral grains; thus, rocks formed by macrocrystals of distinct crystalline structure, such as granites, tend to disaggregate more easily under the action of the crushing process than fine-grained basalts.A good description of rock texture is linked to the degree of crystallization, the absolute size of the grains, the crystal shapes and the mutual relationship among these features.This fact highlights the importance of thin section analysis of rocks selected for crushing.
Basalt is an effusive rock of the gabbro family, the silica content of which varies from 45% to 55%.Its most common components are plagioclase, pyroxene, olivine and magnetite.It is a melanocratic rock that is dark, dense, finely granulated and holocrystalline or hypocrystalline.
A set of samples was collected from the Brazilian Army's quarry located at road mark km 254 of the BR-282/SC in the county of São José do Cerrito/SC.Thin section slides were created for the petrographic analyses presented herein.The rock is classified as basalt; complete petrographic analysis was reported by (Guimarães, 2007).
From a macroscopic perspective, this is a gray finegrained rock of massive structure.From a microscopic perspective, this material has a subophitic holocrystalline texture with euhedralcrystals of pyroxene in an intergranular array composed of parallel and subparallel laths of labradorite and interstitial granules of pyroxene and opaque minerals.
Its essential mineral is labradorite plagioclase occurring in parallel and subparallel well-developed laths reach-  ing 0.7 mm.These crystals are sometimes zoned, and most are twinned according to the albite law.Figures 2, 3, 4, and 5 show photomicrographs of thin section analysis including images without polarized light (NL) and with polarized light (PL).An additional set of samples was collected in a slope along the highway BR-282/SC section between Lages/SC and São José do Cerrito/SC, approximately 8 km from the aforementioned quarry.These samples have a more greenish color than the previously described samples and were subsequently classified as diabase, originating very likely from an intrusion in a basaltic rock pre-existing in place.The presence of diabase dikes is quite common in alkaline rocks of the Serra Geral Formation in the Paraná sedimentary basin.
From the macroscopic perspective, this rock is greenish gray colorless with massive structure and fine to me-dium granulation.From the microscopic perspective, it presents an intergranular holocrystalline texture in which heuhedral crystals of pyroxene occur in an intergranular array composed of parallel and subparallel laths and granules of labradorite and interstitial euhedral opaque crystals of pyroxene.In addition, many minerals of interstitial alteration are present as products of saussuritization of the interstitial plagioclase and uralitization of pyroxene with masses of interstitial micropegmatite.
Its essential minerals are labradorite plagioclase occurring in parallel and subparallel well-developed laths reaching 0.7 mm.These crystals are sometimes zoned, and most are twinned according to the albite law.Some, however, are completely changed through saussuritization into a yellowish green mass with chlorite and calcite occupying interstitial spaces.Pyroxene, augite and pigeonite occur in Soils and Rocks, São Paulo, 39(2): 203-210, May-August, 2016.205 Mechanical Behavior of Basaltic Rocks from Serra Geral Formation Used as Road Material in Santa Catarina State, Brazil    anhedral and euhedralgranular crystals occupying the gaps between plagioclase laths.The opaque and sparse accessory minerals included in the mafics and between plagioclase laths are possibly ilmenite and magnetite.The secondary minerals form masses of micropegmatite with an intergrowth of vermicular quartz commonly found at the borders of plagioclase.Some deuteric calcite occurs as a secondary product of plagioclase.The products of uralitization and saussuritization include chlorophaite.The percentage of pyroxene is low.Figures 6 and 7 show photomicrographs of this material.

Materials
Graded crushed basalt samples selected for study of their mechanical behavior were collected from the quarry of the 10th Construction Engineering Battalion of the Brazilian Army, in São José do Cerrito/SC.This material was used to pave a segment of highway BR-282/SC.

Subgrade
The materials constituting the subgrade of the road, or the final layer of earthmoving, originated from slopes of excavation along the road and are composed of residual soils of basalts with high levels of change.The alteration is predominantly clay and is expansive in some cases.Table 1 presents some geotechnical characteristics of these materials; compaction energy equivalent to the standard proctor test was used for analysis.Guimarães et al. (2004) showed that the subgrade soils of this highway present a low resilient modulus and have a significant negative influence on the fatigue life of the asphalt coating.This fact emphasizes the importance of using high-quality materials in the base and sub-base layers of pavement.

Simple graded crushed stone base
The crushed stone produced in the quarry of São José do Cerrito/SC is classified as grade A according to the National Department of Transport Infrastructure (DNIT) standard, as shown in Fig. 8.

Methodology
In addition to the microscopic characterization of the material through petrographic thin section slides, resilient modulus tests were conducted, followed by numerical evaluation of the pavement's structural behavior, field evaluation of its elastic displacement with Benkelman beam testing, and comparison of the results.
Preparation of aggregates and soils was carried out at the geothecnical laboratory of the Federal University of Rio de Janeiro (COPPE/UFRJ).
The study of highway BR-282/SC pavement included soils previously dried in an oven at 80 °C.In the case of fine soils, the clods were broken; in the case of soils with boulders, the clods were not broken.The hygroscopic moisture content was determined, and the samples were sealed in plastic bags.
The samples were then separated into samples of approximately 4 kg and were homogenized by adding water to achieve the optimum water content previously calculated.Each sample was then packaged in a plastic bag.The bags were then tightly closed and kept in a moist chamber for at least 12 h before the resilient modulus test was conducted.
On the day of the resilient modulus test, each sample was removed from the moist chamber and immediately subjected to mechanical compression in a tripartite cylinder 10 cm in diameter and 20 cm in height with compactation energy equivalent to the standard Proctor test for subgrade soil or modified Proctor test for crushed stone base.
The resilient modulus tests were conducted according to the COPPE/UFRJ method, as reported by Motta and Me-dina (2005).The instrumentation details of this method were reported by Guimarães et al. (2004).

Results -The Case for BR-282/SC
The pavement structure implemented along the segment of highway BR-282/SC analyzed in this study is presented in Fig. 9. Its resilient characteristics as presented below were employed for the achievement of structural responses of the pavement under loads.

Asphalt mix
The asphalt coating is composed of hot mix asphalt classified as grade C of the DNIT standard for implementation of well graded stabilized bases with a bitumen content of 6.3% (CAP 50/70).The value for the resilient modulus was 5,495 MPa and the tensile strength determined through split (Brazilian) test was 1.15 MPa. Figure 10 shows the fatigue behavior of such hot mix asphalt obtained through dynamic-repeated split tests over cylindrical samples.
It was determined that the values K1 and N for expression of the fatigue life of the coating are higher than   those presented by Motta and Pinto (1995) (K1 = 4,200 and N = 3.47) similar to asphalts mixtures, indicating that this material is of satisfactory quality.

Graded crushed basalt base
The resilient modulus tests with the graded crushed stone from BR-282/SC were performed with three different levels of compression moisture, as shown in Fig. 11.Good classification in the linear model was not obtained, as indicated by the R 2 value very close to zero in all cases.This fact indicates that the value of the resilient modulus can be considered approximately as constant in this case.
By adopting the model compound, as shown in Fig. 11, the equation in this case is obtained as follows: where MR: resilient modulus; s 3 : confinement stress; and s d : deviatoric (axial) stress.

Dry-bound Macadam sub-base
In the case of the dry macadam sub-base, determination of the resilient modulus was not possible from conventional laboratory tests.Therefore, in this study, the modulus was obtained from backcalculation of deflection basins obtained on the dry bound macadam layer in the field.In this case, an approximate value of 200 MPa was adopted.

Subgrade soil
The material forming the subgrade of Highway BR-282/SC proved to be highly variable, as shown in Table 1.The values of the resilient modulus of soil from deposit 3 are shown in Fig. 12, considering three distinct cases of moisture content.In this deposit, the resilient modulus values for compaction moisture above the optimal levels were slightly lower than those in other soils.In addition, the proposed model shows good classification.

Field tests
During the implementation phase of each pavement layer, Benkelman beam deflection tests were conducted to evaluate the elastic deformability level of the layers.In addition, the homogeneity level of the surface was evaluated.Surveys were conducted for the coating layer in two phases.The first was performed immediately after the implementation of the coating, and the second was conducted after several months of road operation.For the survey immediately following implementation of the asphaltic mix, referred to as coating 1, the average values obtained were 45.7 x 10 -2 mm at the surface central line and 58.6 x 10 -2 mm at the right edge.The surface after four months of road operation, referred to as coating 2, had average deflection values of 37.3 x 10 -2 mm at the surface central line and 40.8 x 10 -2 mm at the right edge. Figure 13 shows the deflection values of the asphalt layer in the first phase.The results show significant dispersion, which was reduced in the second field measurement.
Figure 14 shows average deflection values for each layer of the pavement, considering the edges and the axle.It was noted that the deflection values significantly decreased with the implementation of subsequent layers.
Numerical simulation of the structural behavior was performed by using version 2 of Finite Element Analysis of Pavement Structures (FEPAVE2) software.
The total pavement deflection calculated by the software was 32 x 10 -2 mm, whereas the average field values after four months of operation remained between 37.3 and 40.8 x 10 -2 mm; there is therefore good correlation.

Conclusions
The soils of the subgrade of the road in which pavement has been studied were obtained from excavation slopes located along the road.These predominantly clay soils are residual soils of basalt showing a high degree of change and 5.7% expansion.Determined by compression of the testing materials, the resilient modulus of the subgrade proved to be significantly influenced by the moisture con-  tent.Very low values are related to higher humidity, although these values were within the limit of two percentage points acceptable by DNIT standards.
The simple graded crushed stone used as pavement base layer was classified as DNIT granulometric grade A, material granulometrically stabilized, and presented resilient modulus values almost constant and over 300 MPa.This result is considered very good for this type of paving material.
The sub-base resilient modulus result, obtained by backcalculation from deflection basins, using the Benkelman beam, was 200 MPa, indicating a dry-bound macadam layer with good bracing.
The deflection tests conducted with the Benkelman beam showed high levels of dispersion; however, the average values indicated a downward trend.In the case of road axle, the following levels were measured: subgrade, 94.4; sub-base, 69.3; base, 55.7; asphalt coating at first measure, 45.7; and asphalt coating at second measure, 37.3.This final value of final deflection measurement in the axle proved to be compatible with that obtained through numerical simulation of structural behavior, 32.0.
In general, it can be stated that the basalt aggregate from Serra Geral Formation, used in this analysis, presented satisfactory characteristics for use in paving and that the pavement presented very good structural behavior.

Figure 1 -
Figure 1 -Geological map of Brazil obtained by Mineral Resources Research Company (CPRM, 2015).

Figure 2 -
Figure 2 -Photomicrograph showing subophitic texture of basalt sample Pyroxene crystals occur in an array with labradorite plagioclase in form of parallel laths.The polarized light (PL) field width is 1.5 mm.

Figure 3 -
Figure 3 -Same rock section undernonpolarized light (NL) with afield width of 1.5 mm.

Figure 4 -
Figure 4 -Photomicrograph showing the subophitic texture of the basalt sample.Labradorite plagioclase crystals occur in laths; pyroxene occurs in grains between the plagioclase (with interference colors blue and yellow) and many euhedralopaques.The polarized light (PL) field width is 0.6 mm.

Figure 5 -
Figure 5 -Same rock section in which opaques appear in nonpolarized light (NL).The field width is 1.5 mm.
Macadam sub-base Dry-bound macadam is a paving technique widely used in the southern region of Brazil and is usually associated with a subsequent layer of graded crushed stone.State road agencies such as the Highway Department of Rio Grande do Sul (DAER) and the Infrastructure Department of Santa Catarina (DER; currently DEINFRA) have estab-lished proper technical specifications for this service, such as DAER ESP 07/91 and DEINFRA-SC-ES-P-03/15.Coarse aggregate is limited from 2 inches to 4 inches, and the filling material particle size must be below 3/8 inch.

Figure 6 -
Figure 6 -Photomicrograph showing the intergranular holocrystalline texture of diabase.Labradorite plagioclase crystals occur in laths; pyroxene twinning (taking half of the field) occurs in a few granules among the plagioclase.The polarized light (PL) field width is 1.5 mm.

Figure 7 -
Figure 7 -Same rock section in which opaque appear in nonpolarized light (NL).The field width is 1.5 mm.

Figure 9 -
Figure 9 -Profile of the pavement segment of Highway BR-282/SC analyzed in this study.

Figure 10 -
Figure 10 -Fatigue life of the asphalt mix used in the coating of Highway BR-282/SC.

Figure 8 -
Figure 8 -National Department of Transport Infrastructure (DNIT) grain size distribution grade A for graded crushed stone produced in the quarry of the 10th Construction Engineering Battalion of the Brazilian Army.This material was used to pave Highway BR-282/SC.

Figure 11 -
Figure 11 -Variation of the resilient modulus of graded crushed stone from BR-282/SC for moisture content.The tests were conducted with a cylinder 10 cm in diameter and 20 cm in height with energy equivalent to the modified Proctor test.

Figure 12 -
Figure 12 -Resilient modulus variation in moisture content for the subgrade of Highway BR-282/SC.

Figure 14 -
Figure 14 -Variation of average deflection per pavement layer.

Figure 13 -
Figure 13 -Deflection survey of the asphalt layer immediately after its implementation: stakes 760 to 840.

Table 1 -
Geotechnical characteristics of subgrade soil of Highway BR-282/SC.