Keywords

1 Introduction

The demand and utilization of rocks in various construction projects have continued to increase proportionally to infrastructural development. In the Abakaliki area of southeastern Nigeria (Fig. 240.1), construction aggregates are produced from crushing of pyroclastic rocks, commonly referred to as the ‘Abakaliki pyroclastics’. Due to their perceived hardness, arising primarily from fine-grained to porphyritic texture and igneous origin, the connection between the performance of the rocks in service and structural failure of projects in which they are used are of major concern. This research attempts to employ basic but standard laboratory tests to assess the usability of the Abakaliki pyroclastics as construction aggregates.

Fig. 1
figure 1

Geological map of the Abakaliki metropolis showing studied pyroclastic suites (Modified after Aghamelu and Okogbue 2013)

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1.1 Geology

The Abakaliki pyroclastic rocks consist of a sequence of tuff, mafic lavas, pyroclastic flows, agglomerates and amygdaloidal lavas of basaltic composition, alkaline in nature (Ofoegbu and Amajor 1987), which extruded the Albian Abakaliki Shale (see Fig. 240.1). The light grayish tuffs and unstratified lavas, however, form the dominant lithology (Aghamelu and Okogbue 2013).

2 Methodology

2.1 Sampling and Testing

A total of ten fresh representative samples from different pyroclastic rock bodies within the Abakaliki metropolis (Fig. 240.1) were collected and tested. The aggregate tests followed standards specified by British Standard Institution (BS 812 Parts 100 and 110 1990a, b), and American Society of Testing and Materials (ASTM C 127 1990; C 535 1988). The tests include petrographic modal analysis, water absorption, specific gravity (SG), Los Angeles abrasion (LAAV), aggregate crushing (ACV), aggregate impact value, ten percent fines (TFV) and flakiness index. Petrographic analysis was carried out in accordance with standard procedure given by ISRM (1978).

3 Results and Discussion

3.1 Modal Analysis

The results of the petrographic modal analyses are summarized in Table 240.1. The textures of the samples are between fine-grained and porphyritic. Presence of some weak materials such as chlorite in aggregates can render them deleterious. It is safe to assume that samples of the pyroclastic rocks with very significant contents of lithic fragment and glassy groundmass will be susceptible to faster alteration and material deterioration. Low quartz (≤4 %) in the Abakaliki pyroclastics may mean that they are ‘hydrophobic’ (i.e., higher affinity for bitumen than water), hence, would not be susceptible to stripping.

Table 240.1 Results of modal analyses on the samples of the Abakaliki pyroclastics
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3.2 Water Absorption (Wa)

The results of W a analysis are presented in Table 240.2. The amount of water aggregates can absorb tends to be an excellent indicator as to their strength, in other words, their weakness. According to ASTM C 127 (1990), W a of construction aggregate should be less than 2.5 %. The tested samples have low W a (all below 2.5 %) and may be an indication of very low porosity and degree of weathering. Feldspathic minerals and glassy materials in the rocks on weathering could increase secondary porosity, as well yield clay, which in turn increases their W a capacity.

Table 240.2 Results of geotechnical tests on the Abakaliki pyroclastic rocks
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3.3 Specific Gravity and Los Angeles Abrasion

The Abakaliki pyroclastics have mean SG of 2.64 (Table 240.2), and on the basis this could be judged fairly suitability as a construction material. Aggregates with high SG values, usually ≥2.65, are generally suitable in construction. The appreciable SG of the Abakaliki pyroclastics may have resulted from significant amount of pyroxene and iron oxides, which predominate the opaque minerals (Aghamelu and Okogbue, 2013). SG, however, is likely to drop, in service, as appreciable weak minerals (especially chlorite) are recorded and are prone to deterioration when exposed to weathering conditions. The samples Abakaliki Pyroclastics have their LAAV all above 15 % (see Table 240.2), despite being fine-grained to porphyritic, and not coarse-grained. According to Waltham (1994), fine-grained rocks usually yield crushed aggregates with low LAAV (below 15 %) while coarse-grained rocks yield higher values. Hence, ‘trap rocks’ (i.e., basalt and its likes) have their LAAVs within acceptable limits, generally less than 10 % (Waltham 1994), and are desired in most construction projects.

3.4 Aggregate Crushing and Ten Percent Fines

The results of ACV tests on the studied samples are presented in Table 240.2. Generally, fine-grained rocks yield aggregates with higher crushing resistance than coarse-grained rocks that yield aggregates with lower crushing resistance, thus, the lower the ACV the stronger the aggregate, in terms of its ability to resist crushing. The ACV of the studied samples compares fairly well that recorded on basalt (ACV of 14 %) by Waltham (1994); both rocks are classified generally as fine-grained and basic. Waltham (1994) had also noted that an aggregate with ACV below 5 % would usually serve well as a construction material, while that with ACV above 35 % would most likely perform poorly. The tested samples all have their TFV between 102 and 202 kN (see Table 240.2). Fine-grained variety, however, yielded TFVs that are slightly higher than that recorded on the porphyritic variety, indicating that texture variation could have resulted in TFV variation within the pyroclastic suite.

3.5 Flakiness Index (FI)

The results of FI tests on the studied samples are presented in Table 240.2, and revealed that the tested samples contained low to moderate percentage of flaky aggregates (FI 20–31 %). A flaky aggregate has low density under compaction and less strength than a cubical or an angular aggregate. Waltham (1994) has pointed out that aggregates with FI value of 20 % and below will serve well as a construction aggregate; for use as road stone FI should be below 3 %. Volcanic rocks commonly yield aggregates with moderate to high FI value.

4 Conclusions

  1. (1)

    Petrographic modal analyses indicate significant difference in the mineralogy and texture of the tested samples from the Abakaliki pyroclastic rocks. This is likely due to the variation in the magmatic and depositional processes that occurred during the emplacement of the volcanic suites. Presence of chlorite, shaley to muddy lithic fragment and glassy groundmass are recorded in all the tested samples. In most cases, samples with fine-grained texture and low glassy groundmass gave better engineering properties.

  2. (2)

    The pyroclastic rocks yielded aggregates with marginal performance in road projects. This is due to the fact that although the aggregates meet the ten percent fines value and water absorption specifications, they fail to meet a number of other requirements as a road stone, such as aggregate impact value, Los Angeles abrasion value and flakiness index (Table 240.3).

    Table 240.3 Comparison of the Abakaliki pyroclastic rocks with general road stone specification
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  3. (3)

    This study suggested that similarity in mode of formation, mineralogy and texture are not enough for common rating and predicting of the geotechnical behaviour of all rock types, Abakaliki pyroclastic rocks as an example.