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Citation Information : Architecture, Civil Engineering, Environment. Volume 11, Issue 3, Pages 97-105, DOI: https://doi.org/10.21307/ACEE-2018-042
License : (BY-NC-ND-4.0)
Received Date : 10-September-2016 / Accepted: 06-March-2018 / Published Online: 04-April-2019
The paper concerns the effect of coarse aggregate type on the strength and strain properties of concrete under instantaneous loading. The studies deal with properties of the crushed aggregates and their effect on the basic mechanical properties and instant deformability of concretes in comparison to gravel concrete. The paper presents test results of mineral composition, crushing strength, absorption by weight, content of irregular grains and mineral dust as well as bulk and specific density of the 5 coarse aggregates: basalt, granite, dolomite, quartzite and natural aggregate (gravel) derived from the Polish rock beds. Strength and strain properties of concretes under compressive instantaneous loading were analyzed. The analysis showed that strength properties and modules of elasticity of tested concretes are proportional to crushing strength of aggregates. Based on the test results of strains it was concluded that the effect of coarse aggregate type on the modulus of elasticity and the limit strain is more significant than that of concrete strength.
Solid rocks with low porosity, low absorption and high strength are used as raw materials for the production of coarse crushed aggregates suitable for structural concretes. Due to the deficiency of aggregates of that type in Poland as well as not central location of their beds crushed aggregates made from some low porosity sedimentary rocks: limestone-dolomite carbonate rocks, sandstones and especially quartzite sandstone have been used since the sixties . Although natural coarse aggregate (gravel), the oldest in the history of concrete-related construction, is produced in Poland in largest quantities, crushed aggregates have been readily used for special concretes and high performance concretes as well as ordinary concretes of low strain, as for example for pre-stressed concretes. Whereas natural fine aggregate (quartz sand) is practically the only one which is applied in all types of concrete – only in few cases fine crushed aggregate is used.
As far as igneous rocks are concerned, concrete aggregate is most often produced from basalt and granite. Igneous rocks belong to the most desirable raw materials. Basalt is an alkaline extrusive volcanic rock which has a fine-grained texture and contains mainly plagioclases and pyroxenes, however, it does not include quartz at all or very little of it, which was found in the case of the tested aggregate (Fig 1) The total content of calcium and magnesium is usually more than 20.0%. Density of the Polish basalts ranges from 3.00 to 3.15 kg/dm3, porosity from 1.0% to 3.0%, absorption from 0.1% to 0.4%, and compressive strength from 120 MPa to 260 MPa . Basalts show the variability of characteristics within their mineral composition, but also within one bed. The best properties are those of rocks from volcanic vents, and the worse ones derive from coastal areas of lava covers, which are more susceptible to weathering. The occurrence of basaltic sunburn crackings is risky for concrete . Big advantages of basalt rocks are high strength and a very fine-grained texture, due to which the grain of the crushed aggregate is characterized by micro-roughness which ensures good adhesion of the hardened cement paste to the grains , .
Granite as a plutonic acid igneous rock containing quartz in the amount of up to even 60.0% by volume, alkali feldspars, plagioclases and biotite, is a valuable building material. However, it is found to be of quite variable properties dependent on the bed, and sometimes on the degree of weathering. Strength of Polish granites is from 100 MPa to 210 MPa, porosity from 1.5% to 3.0%, absorption from 0.2% to 0.5% . for the production of aggregates, granite is taken primarily from beds in the Strzegom-Sobotka Massif, but unfortunately the properties of this rock are pretty average, as compared to other Polish granites, for example, from the Strzelin Massif. And as demonstrated one can also have reservations towards the properties of concretes made from these aggregates , , .
Carbonate rocks from the limestone – dolomite group, which are a mixture of calcite and dolomite, are of great importance in aggregates used for concrete. While selecting rocks for the production of aggregate to be used for concrete the most important role is that of the porosity - it is highly desired to be as low as possible, and much less important is the amount of calcite and dolomite in the rock. Devonian limestone and dolomites are of the lowest porosity. In the best limestone, the absorption is less than 1.0%, and the strength higher than 100 MPa. Strength of the Devonian dolomite or calciferous Devonian dolomites is even higher, often exceeding 130 MPa, and sometimes even 200 MPa . Aggregates from carbonate rocks, despite their low hardness and usually lower strength than that of the igneous rocks and quartzite sandstones, have gained considerable technical appreciation as a dense interfacial transition zone is formed at carbonate aggregate grains [7, 10, 11, 12].
Among sedimentary rocks, there are also sandstones, which are used for the production of concrete aggregates. Quartzite sandstones have high physical properties and they are readily used for aggregate production. They consist of compressed quartz grains with crystallized quartz cement, the same as specific quartzites – which are metamorphic rocks of Precambrian origin . Compressive strength of Cambrian quartzite sandstones exceeds 200 MPa, and the Devonian 150 MPa, absorption is less than 0.6%. The SiO2 content is more than 95.0%, and sometimes even 99.0% . Thus, they are ultra-acidic rocks.
Petrographic composition of natural aggregates can be very complex. In gravels, (especially those from Northern Polish Lowland and Lower Silesia Region), there may be grains of both strong igneous rocks and of weak sedimentary rocks. The quality of concrete with gravel is fundamentally influenced by its petrographic and mineral composition. Significant importance for the physical properties of concrete has a content of weak grains, which are derived from weak and weathered rocks, such as for example porous sandstones or fossilized limestone. One should avoid gravels as well as any rock aggregates which contain minerals susceptible to adverse reactions with components of the cement paste. Aggregates containing reactive silica in forms such as opal, chalcedony, tridymite or strained quartz belong to the most susceptible to alkali-silica reaction that causes expansion of concrete . These minerals occur in chert, andesite, rhyolite tuff, dacite, porphyry, fossilized limestone and strained quartz in granitic rocks, as well as in many gravels [15, 16].
Crushed aggregates are obtained directly from rock material breaking, thus the properties of those aggregates are associated to a great extent with the mineral composition and the properties of individual rock. However, in the case of natural gravel aggregates, consisting of grains of different petrographic origin, and hence of the properties often very diverse and varying depending on the location of the bed, such a direct correlation does not exist.
Evaluation of the quality of aggregates was accurately recognized in the standard . Specific requirements were determined in the range of aggregate crushing strength. It is measured with aggregate crushing value (ACV) for any fraction individually . The crushing strength is reduced by irregular grains and therefore their content should be tested. Other necessary tests to be carried out concern the content of dust in aggregate. Covering the aggregate grain the dust decreases aggregate-cement paste bond strength and weakens the strength of concrete. Adhesion of the paste to aggregate is significantly affected by the texture of the grains [18, 19]. As emphasized with respect to aggregates made from basalt rocks.
The tests involved five coarse aggregates of two fractions 2-8 and 8-16 mm. There were 4 crushed aggregates: granite, basalt, dolomite, quartzite sandstone and natural gravel. Aggregates came from the following Polish rock beds:
granite – Graniczna quarry, Strzegom Massif – Sobótka;
basalt – Gracze quarry, valley of Nysa Kłodzka;
dolomite – Laskowa Góra quarry, Świętokrzyskie Mountains;
quartzite sandstone – Wiśniówka quarry, Świętokrzyskie Mountains
natural gravel – gravel plant Suwałki.
Basic properties of the rocks were shown in Table 1. In order to identify the mineral composition, the X-ray diffraction of aggregates was carried out. Aggregates were tested in terms of grain composition, crushing strength, absorption by weight, content of irregular grains and mineral dust, as well as bulk and specific density, on the basis of which the total porosity was determined.
In order to determine the effect of the coarse aggregate type on the properties of concrete, the 5 concretes made with those aggregates were tested. The same volume of coarse aggregate in each concrete was considered. The value of the w/c was 0.58. Portland cement CEM I 42.5 R was applied. The composition of concrete mixtures was given in Table 2.
In basalt (Fig 1) two basic minerals were found: plagioclase – in the form of anorthite (calcium aluminosilicate) and pyroxene – in the form of augite (magnesium and iron silicate), and a very small amount of olivine. There is no quartz in basalt.
In granite (Fig 2) the dominant mineral is quartz, and next to it two feldspars - orthoclase (potassium aluminosilicate) and albite (sodium aluminosilicate). Furthermore, there is biotite in the rock (basic potassium aluminosilicate, magnesium and manganese) which is the main component of mica.
On the basis of diffraction patterns (Figs 3 and 4), it was found that the composition of dolomite and quartzite sandstone is of mono-mineral character. The aggregate from Laskowa Góra contains practically dolomite only (CaMg(CO3)2). There are only trace elements of calcite and quartz. However, the aggregate made form quartzite sandstone contains quartz only (SiO2). Other minerals, by using X-ray diffraction were not found.
The most complex is the petrographic and mineral composition of natural gravel, which contains both grains derived from sedimentary rocks and igneous rocks (Fig 5). Gravel from Suwałki beds has got a high content of quartz occurring in sandstone grains and grains from igneous rocks. Orthoclase and albite and biotite indicate the presence of granitic rock grains. The gravel contains a large amount of various grains including calcite and dolomite.
It is very important that none of the tested aggregates contain clay minerals which make the quality of aggregate worse.
On the basis of the results of own tests (Table 3) all aggregates were classified as of the highest quality within their petrographic classes . The exception is granite, classified to the lowest brand of 20, due to the low crushing strength. Analyzing the results of absorption tests, the content of irregular grains and mineral dust exceeded the limit standard value only in the case of finer fractions of basalt (at nw), quartzite (at zp) and gravel (at nw). The specific density of aggregates is close to the values quoted from the references (differences do not exceed 2.0%) (Table 1). Porosity of aggregates (Table 3) is higher than that given in the references (Table 1). Except for the porosity of gravel, which amounted to 3.62%, the porosity of aggregates does not exceed 3.0%. There was no correlation found between the aggregate absorption and the porosity of aggregate grains, which is most probably associated with the fact that absorption is determined by open porosity and the distribution of pore sizes, however, total porosity was shown in Table 1. There was also no relationship found between the porosity and the crushing strength of aggregate, since it depends most probably on the mineral composition and intercrystalline bonds as in the case of rock strength.
A significant variation regarding compressive strength of concrete is connected with the crushing strength of aggregates (Fig 6). The lowest strength of concrete with granite aggregate is caused by its low crushing strength. The ACV of granite is as high as 15.0%. The strength of gravel concrete was one class lower than that of concretes with basalt, dolomite and quartzite sandstone. It is also related to crushing strength of gravel and the smoothness of its grains. Despite that, higher strength of gravel concrete than that of concrete with granite is associated with a significant amount of calcite and dolomite grains in its composition (Fig 5). A very low crushing value of basalt and quartzite aggregates of fraction 8–16 mm clearly explains the highest strength of concretes made with those aggregates. However, the strength of concrete with dolomite aggregate is of the same class as concretes with basalt and quartzite sandstone, despite a slightly higher crushing value. High strength of concrete with dolomite can be explained by good adhesion of the paste to this type of aggregates, due to low porosity of the aggregate-cement paste interfacial transition zone [21, 22, 23].
A clear effect of aggregate type on compressive strength and elasticity modulus of concrete was observed (Fig 7). The highest values of both properties were determined for concretes with basalt and dolomite aggregates. The properties of concrete with granite aggregate were the worst. In general, elastic modules were consistent with compressive strength of concrete, except for the concrete with quartzite aggregate, whose modulus was surprisingly low at considerable strength.
The concrete results were evaluated to identify the character and significance of compressive strength changes versus the aggregate crushing value. The elasticity modules of concrete with those aggregates were analysed with linear regression and correlation coefficients.
Figure 8 shows the regression straight line that characterizes the relationship between the compressive strengths of concrete and corresponding ACVs. The coefficient of determination, R2 = 0.841, indicates the degree to which the total compressive strength variability is dependent on the ACV. The coefficient of correlation between the ACV and the strength is - 0.9168, and the probability value p = 0.0284 < 0.05, thus the correlation is statistically significant.
The same statistical analyses were performed to evaluate the relationship between the elastic modulus of the concretes (dependent variable) and their compressive strength (independent variable). Figure 9 shows the regression line that confirms a rising trend of elastic modulus variation against the compressive strength of concretes. Fig 9 shows that the coefficient of determination R2 is 0.543 and the strength-modulus correlation coefficient is 0.737, with p = 0.156 > 0.05, thus, there is no evidence to reject the null hypothesis, the correlation is not statistically significant.
As the lack of relationship between elastic modules on compressive strength of the concretes might be due to the high deformability of concrete with quartzite aggregate (low elastic modulus) at high compressive strength (46.4 MPa), an additional analysis was performed that did not include the results of concrete containing quartzite aggregates (Fig 9).
The coefficient of determination was substantially higher, R2 = 0.920, and p = 0.0410 < 0.05 indicated a statistically significant correlation.
The analyses confirmed a significant effect of the ACV on the compressive strength of the concretes. The relationship between the modulus of elasticity and compressive strength was also confirmed but not for all aggregates. Although the test results (fcm, fctm) prove excellent strength parameters of quartzite aggregate (ACV), the deformability of concrete with this aggregate is very high (Ecm, εc1m, νcm), as shown clearly by the relationship between the modulus of elasticity of concrete and its compressive strength.
These findings support the thesis that the evaluation of suitability of aggregates should not be based only on their strength parameters or the strength of concrete containing the given aggregate. Broader studies, providing for the deformability of concrete, should be performed.
Figure 10 compares the relative quantities of uncrushed grains % (100 – ACV) under load (equivalent of crushing strength in test of ACV) of individual aggregates vs uncrushed gravel grains and relative strengths %fcm, %fctm and modulus %Ecm of individual concretes vs corresponding properties of gravel concrete. The properties associated with gravel aggregate and gravel concrete were taken as the reference (100%), similarly as in the standard modules of concretes with different aggregates . Aggregates have comparable crushing strengths, with the exception to granite. The ACV of this aggregate qualified it to the lowest class. The properties of dolomite and basalt aggregate of and concretes with them were the best. In addition to concrete with granite, the strength of concretes with crushed aggregates was higher from 10.0% to nearly 20.0% compared to concrete with gravel. Only modulus of concretes with basalt and dolomite is higher than that of concrete with gravel.
Figure 11 shows brittleness, that is the ratio of tensile strength to compressive strength fctm/fcm of the tested concretes. For all types of aggregates, it was found that the brittleness of concretes is higher than standard values (for a given concrete class). It is noted that the material is more brittle when the value of the relationship fctm/fcm is lower. Given the fact that concretes with quartzite and basalt are of the same class, and concrete with gravel is of a lower class, then the most brittle concrete is that with gravel and in the second place that with basalt. Concrete with dolomite is of the lowest brittleness, and then that with granite.
In the whole range of uniaxial instantaneous compressive loading (Fig 12) there occurred nearly the same strains of concretes with basalt, dolomite aggregate and natural gravel for the corresponding effort levels. Strains of concretes with granite aggregate and quartzite sandstone were even up to 25.0% higher. The curves in Figure 12 clearly show the limit strains εc1. They range from 2.10‰ for concrete with dolomite aggregate, 2.12‰ with natural gravel, 2.22‰ with basalt aggregate, 2.48‰ with granite aggregate and even up to 2.65‰ with quartzite aggregate. Limit strains of concrete with granite and quartzite obtained in tests are higher by about 15.0% from the standard value (Table 4). This underestimation of the strain can be significant in the non-linear analysis of concrete structure, when the strain values εc1 are applied . Quite surprising is a very low limit strain (2.12‰) of concrete with natural gravel containing oval and smooth grains, and a very high strain (2.65‰) of concrete with quartzite sandstone containing crushed and irregular grains.
Based on the relationship of the effort versus strain (Fig 12) it is difficult to evaluate the effect of strength and type of aggregate on the elasticity modulus of concretes and the limit strain. Only by analyzing the relationship of stress strain (Fig 13) one can simultaneously assess the effect of concrete strength and aggregate type on the strain properties of concrete.
Large angles of experimental stress strain curves equivalent with high elasticity modules of concretes with dolomite and basalt aggregates, as well as concrete with natural gravel, are consistent with high elasticity modules and low limit strains of these concretes. Despite of high strength, concrete with quartzite sandstone aggregate has a low elasticity modulus and a very high limit strain. Strain properties of concrete with granite aggregate are even worse. In addition to low strength (two classes lower than other concretes with crushed aggregates) one can clearly see in the figure that inclination angle of the σc-εc curve is much smaller, this proves a very low elasticity modulus of concrete with granite aggregate.
The low value of the elasticity modulus of concretes with quartzite sandstone and granite (Table 4, Fig 13) and their high brittleness suggest that displacements (deflections, curvature columns) and the crack widths in structural elements made from these concretes will be larger than those of concrete with dolomite, basalt or gravel.
Compressive strengths of concretes with all the aggregates (except dolomite) are proportional to crushing strength of aggregates (inversely proportional to the aggregate crushing value).
Concretes with dolomite aggregate, and then basalt and quartzite had the highest tensile strength, which is primarily related to aggregate-paste bond strength and crushing strength.
Concretes with dolomite and basalt aggregate reached the highest elasticity modulus. The modulus of elasticity is dependent on aggregate-paste bond strength, affecting initiation of microcracks in interfacial transition zone.
The best properties of concretes with basalt and dolomite aggregates are associated with aggregate-paste bond strength due to fine texture of basalt and tighter microstructure of cement paste-dolomite aggregate interfacial transition zone.
The test results of strains suggest that the effect of aggregate type on the modulus of elasticity and the limit strain is more significant than strength of concrete.
This study was supported by the Kielce University of Technology and Lublin University of Technology, within the research project “The influence of technological and operational factors on the technical properties of building materials and durability of structures”. This work was financially supported by Ministry of Science and Higher Education, within the statutory research number S/14/2015.