Abstract

The qualities of construction materials and the influence of insurgency on the availability of local coarse aggregates for production of concrete in Maiduguri were assessed in this study. Ten block production industries were selected from major wards in Maiduguri metropolis, while the brands of cement sold in Maiduguri including Dangote 3X (i.e. Obajana) and Ashaka, of grades 42.5N and 32.5N respectively were selected with two different sizes of blocks. Four sizes of reinforcement bars; 10 mm, 12 mm, 16 mm and 20 mm; were randomly selected. Local aggregates and water were used during the investigations. Microsoft Excel was used to analyse and interpret the data. The results show that, the dry compressive strength of the sandcrete blocks samples for the 150 mm blocks were 0.016 to 0.999 N/mm² which is less than between 0.03 to 98.44 % of the recommended minimum of 1.0N/mm² at age 7 days; similarly, the average compressive strength of the sampled 225 mm blocks were also 0.253 to 0.634 N/mm² which is less than between 57.73 to 83.14 % of the recommended minimum of 1.5N/mm² at the age of 7 days. The highest value of 0.634 N/mm² was less than the recommended value of 1.5N/mm² for load bearing walls. The diameters of all the steel reinforcements were 1.42 to 3.09% less than the specifications; characteristic strengths of 10 mm steel reinforcement proved to be 506.7N/mm² which is greater than 460N/mm² the recommended value. A factor of safety of 1.115 was recommended instead of 1.15. The insurgency in the north eastern part has negatively impacted the quality and availability of construction material as well as the local coarse aggregates in the building industry.

1. Introduction

Various parts of the world have been rocked by insurgencies such as the FARC in Colombia, Afghanistan, Somalia, Mali and Nigeria among others. Several impacts of these insurgencies have been studied by authors in various fields ^{1, 2, 3, 4, 5, 6}. The insurgency in the North Eastern Nigeria started in 2002 but it became dramatically more destructive between 2010 and 2016. During this period, Borno state was the most affected and at the peak of the crisis, Maiduguri the state capital, located on latitude 11.85° North and longitude 13.08° East, received the greatest influx of the refugees. Construction activities continued in the state capital even though it was with a lot of difficulties, mainly due to the fact that majority of sites of building materials such as local coarse aggregates referred to as Bama gravel, named after the town located 75 km from Maiduguri, the state capital, could no longer be safely accessed by the aggregates vendors since travelling outside Maiduguri most especially beyond Konduga along the southern flag of the metropolis was risky, if not impossible. Other gravels are also found in large quantities around Madube, Kawuri and Konduga ⁷.

Concrete is a composite material composed of aggregate bonded together with fluid cement which hardens over time. Most use of the term "concrete" refers to Portland cement concrete or to concretes made with other hydraulic cements, such as ciment fondu ^{8, 9}.

Aggregates are essential materials in civil engineering construction processes; their inclusion in concrete and asphalt mixes has always made their production to be more economical ¹⁰. Aggregate characteristics comprise of shape, texture, and grading. They together influence workability, finish ability, pump ability, segregation, physical, mechanical properties of fresh and hardened concrete. Construction and durability problems have been reported due to poor mix proportioning and variation on grading ^{11, 12}. It is therefore not surprising that the quality of these materials is of considerable importance and hence it is useful to establish the quality of aggregate from time to time to ensure quality control and be able to specify materials correctly and ensure materials will perform accordingly ¹³.

Except for water, aggregate is the cheapest component of concrete. On the other hand, cement and reinforcements are the most expensive components; consequently, they are responsible for about 85 percent of the total cost of materials for reinforced concrete production. Therefore, it becomes reasonable and expedient that quality research assessment be made on the locally available aggregate materials because of its quantity, availability and nearness to site of use to verify the quality and suitability for medium grade concrete production ^{8, 14}.

According to Ahamadu ¹⁵ and ASTM C 125 ¹⁶ and ACI 116 ¹⁷, the maximum size of coarse aggregate used in concrete has a bearing on the economy and strength of concrete. Usually more water and cement are required for small-size aggregates than for large sizes, due to an increase in total aggregate surface area otherwise referred to as the “aggregate specific surface”, for a given water-cement ratio. The amount of cement required decreases as the maximum size of coarse aggregate increases. Furthermore, aggregates of different maximum sizes may give slightly different concrete strengths for the same water-cement ratio. In some instances, at the same water-cement ratio, concrete with a smaller maximum-size aggregate could have higher compressive strength. This is especially true for high-strength concrete. The optimum maximum size of coarse aggregate for higher strength depends on factors such as relative strength of the cement paste, cement aggregate bond, and strength of the aggregate particles ^{16, 17}.

Previous studies on this fluviolalustine gravel ¹⁸ indicate that Bama gravel has an average size of 10 mm interspersed with intermediate sizes ranging from 6-2mm. Its origin is linked to the Bama-ridge that began at the Cameroon plains, passing the northern tip of the Mandara Mountains in Nigeria and extending some 300 km northwards towards Bama and Maiduguri. Bama gravel is thus a natural product of weathering on the granitic rocks of Mubi and Gwoza, and transported to the riverbed by run-off because of falling rains and other agents of transportation. As a result of the scarcity and high cost of crushed stones (chippings) in most parts of North-East (Borno,Yobe, Adamawa and Taraba states), Bama gravel has long been used as one of the aggregates for concrete making.

Previous studies carried out shows that, large percentage of the aggregate is less than 10mm in diameter. Onundi et al., ⁷ have successfully categorised the Bama gravel into three specific groups A, B and C as a function of their particle size distributions for concrete production. Since, minimizing the aggregates voids content should be one of the objectives of optimization of concrete mixtures; mixture proportioning methods should encourage concrete optimization or aggregate optimization.

This paper attempts to identify and categorise the Bama gravel; determine the qualities of construction materials and the influence of insurgency on the availability of local coarse aggregates in Maiduguri. The objectives of the research are to provide insight and solutions into some of the critical areas that poses toothing problems to young engineers and students when important decisions must be taken to achieve a successful project delivery.

2. Materials and Methods

Ten block production industries were selected from major wards in Maiduguri metropolis, while the brands of cement sold in Maiduguri including Dangote 3X (i.e. Obajana) and Ashaka, of grades 42.5N and 32.5N respectively were selected as shown in Table 1, with two (2) different sizes of blocks. Four sizes of reinforcement bars (i.e. 10 mm, 12 mm, 16 mm and 20 mm) were randomly selected from sites in Maiduguri. Local aggregates (i.e. coarse and fine grades) and water were used during the investigations. Microsoft Excel was used to analyse and interpret the data.

a. BS8110 ¹⁹, Clause 2.4.2.1 discussed “Characteristic strengths of materials” and recommended that:

The characteristic strength of concrete f_cu, is given by

(1)

where,

f_m = mean strength,

S = standard deviation (8.0 N/mm² for samples less than 40 and 4.0 N/mm² for samples more than 40),

whereas, the target mean strength f_m

(2)

The characteristic strength f_cu (i.e. 95% success or confidence limit is assumed).

The average or mean strength for any design only ensures 50% success and 50% failure confidence limit and is considered too risky for a reliable structural design.

Still quoting many of the relevant international normative adaptations on this subject matter, regardless of the governing body, the information provided in most global and local standards is quite detailed and intended to help the user understand the following basic testing requirements ^{20, 21}.

a. equipment required

b. associated terminology and symbols

c. specimen preparation

d. testing procedures or methods

e. calculations or results to be determined.

For rebar tensile testing, it is helpful to break down the tensile test into the separate stages of the test. This applies regardless of which test standard is being followed ^{19, 20}.

The five basic regions are:

a. pretest

b. preload

c. elastic region

d. yielding

e. plastic region.

AC133-(2010) ²⁰ and ASTM A706/A706M-14 ²¹ give the elastic region or straight line portion of the test as seen on the Stress-Strain plot.

AC133-(2010) ²⁰ and ASTM A706/A706M-14 ²¹ similarly, once yielding begins, many rebar grades exhibit a defined yield point that is seen as an abrupt bend in the Stress-Strain test curve.

There are two definitions for the factor of safety:

a. One as a ratio of absolute strength (structural capacity) to actual applied load, measure of the of a particular design(i.e. reliability index).This is a variable measure of degree of reliability or structural capacity and is usually considered at the ultimate or collapse limit state.

b. Factor of Safety(FoS) is a constant value imposed by , , , or to which a structure must conform or exceed. This is a constant value of factor of safety which is usually considered at the serviceability or elastic limit state.

(3)

3. Results and Discussion

The Bama gravels possess almost the same value for most of its physical parameters such as specific gravity (SG), silt content (SC), aggregates crushing value (A.C.V) and aggregates impact value (A.I.V). The values of each of these parameters are within the limits as shown in Table 2.

The elastic region or straight line portion of the test as seen on the Stress-Strain plot in Figure 1a can often exhibit some non-linear behaviour initially due to further straightening of the rebar specimen. In using an extensometer, this may show up as slightly negative strain at the beginning of the test and is generally considered normal for rebar and should not be seen as critical error in the experimental process.

Once yielding begins, many rebar grades exhibit a defined yield point that is seen as an abrupt bend in the Stress- Strain test curve in Figure 1a. It is then followed by a period of specimen elongation with little or no increase in force. Because of this, servo-controlled systems must be controlled using crosshead or actuator displacement feedback in order to maintain a constant rate of travel throughout yielding. It is very important to note that using stress control during yielding will cause the test to accelerate excessively, which is in direct violation of the standards. This can also cause the yield point (upper yield) to be masked or smoothened and cause yield strength results to be higher than expected. Likewise, strain control from an extensometer can also become erratic during yielding and, is therefore, not recommended when testing rebar. Figure 1a, Figure 1b, Figure 1c & Figure 1d show the wrong and correct results interpretations for the determination of the yield stresses of tested steel materials which serves as guides for practising engineers, researchers and students of Structural and Mechanical Engineering.

Table 3a, Table 3b & Table 3c are the results of research conducted on the steel diameters Ø, characteristic strength f_y, partial materials safety factor ϒ_m and % strain e, they show that the characteristic parameters of the reinforcements used in Maiduguri are not complying with the recommended values by the BS 8110 (1997) ¹⁹.

From equation (3),

From the above results, since the steel characteristic values are below the standard specifications, the recommended factor of safety for Nigerian National Annex from the normative conditions for Euro code is:

Table 4 shows the laboratory test results of the Portland cements in Maiduguri with the following interpretations and implications.

Ÿ Fineness of cement increases the rate of evolution of heat.

Ÿ Finer cement offers a great surface area for hydration and hence faster the development of strength.

Ÿ Increase in fineness of cement also increases the drying shrinkage of concrete and hence creates cracks in structures.

Ÿ Excessive fineness requirement increases cost of grinding.

Ÿ Excessive fine cement requires more water for hydration, resulting reduced strength and durability.

Ÿ Fineness of cement affects properties like gypsum requirement, workability of fresh concrete and long term construction time.

Ÿ Coarse cement particles settle down in concrete to causes bleeding.

Ÿ Volume expansion (i.e. Soundness) in cement mortar in concrete is caused by the presence of unburnt lime (CaO), dead burnt MgO and also CaSO₄.

Ÿ By Le-chatelier method we can only find out presence of unburnt lime (CaO); Presence of unburnt lime may develop cracks in the cement because of increase in volume.

Ÿ Free lime (CaO) and magnesia (MgO) are known to react with water very slowly and increase in volume considerably, which result in cracking, distortion and disintegration of mortar and concrete.

Consistency limit of Portland cement indicates the quantity of mixing water to form a paste of workable material matrix or cement paste, the viscosity of which will be such that the Vicat's plunger penetrates up to a point 5 to 7 mm from the bottom of the Vicat's mould.

These tests were conducted in accordance with the recommendations of BS EN 196-3 ²². The setting time of Ordinary Portland Cements (OPC), as specified by the standards requires that an initial set time of not less than 60 minutes, and a final set or hardening time of not more than 600 minutes. These values are to serve as important guides to prevent cement from setting too quickly as to affect the intended strength and long time delay before hardening or gaining enough strength for the shuttering to be removed to facilitate early completion of projects.

Costs of production were mostly affected by industry’s policies which were influenced by demand, supply and other economic factors leading to the variations in prices. Raheem et al., ²⁴ (2012) estimated that the cost of a piece of 225 mm sandcrete hollow blocks of mix ratio of 1:9 would be ₦195. It can thus be deduced according to Sule ²⁵ (2016) that blocks are produced to meet market demand as against required quality, because, if the cost is higher to improve the sandcrete block quality, most customers are so poor that patronage would be poor and that could lead to the closure of the block industry. This survey also noted through the producers that, they (the producers) know the appropriate mix proportion that could improve the general production of good quality blocks but if the cost of sale is high they lose most customers that would not care about quality but cost of unit block. The cost which influences the sale-ability by the industry is mostly a driving factor for increased patronage and quality of blocks being produced in Maiduguri.

All the ten sampled industries had unskilled labour but very experience in block production since, experience teaches best; they have the capability to produce higher quality blocks. Unfortunately, lack of skilled staff and the paramount aim of maximizing profit by the vendors have also prevented quality control in the industry. Most of the particles of the fine aggregates used for the production of the local sandcrete hollow blocks were classified as Zone 2 as presented in Figure 2, except for industry X (Dalori) which is coarser fell in Zone 3.

The dry compressive strength analyses of the sandcrete blocks samples are presented in Table 5a & Table 5b), the average compressive strength of the sampled 150mm blocks ranged from 0.016 to 0.999 N/mm² which is less than between 0.03 to 98.44 % of the recommended minimum value of 1.0N/mm² at age 7days. Similarly, the average compressive strength of the sampled 225mm blocks ranged from 0.253 to 0.634 N/mm² which is also less than between 57.73 to 83.14 % of the recommended minimum value of 1.5N/mm² at the age of 7 days of the allowable value by Nigerian Industrial Standard. This deviation shows that many of the sandcrete blocks cannot be used for loading blocks; since, the highest value of 0.634 N/mm² is less than the recommended standard value of 1.5N/mm² for load bearing walls. These confirm the reason we have many buildings with series of cracks and various deformations on our buildings along Damboa roads where the soil is of soft clay.

The test results of the local aggregate are shown in Figures (3a, b, c & d). For aggregates to be considered as coarse, they must be higher than or retained on a 5mm diameter sieve. Therefore,

1. C_A is the best aggregate category (i.e. Madube 1 and 2) =76-80% retained on 5mm sieve, they can no longer be accessed due to insurgency

2. Figure 3b & Figure 3d, C_D (i.e. Bama Bakingada, Chabbal 1 & 2) = 2% retained on 5mm sieve cannot be recognised as coarse aggregates but the locals use them because they are cheaper.

3. Figure 3c, C_C is family of aggregates that can no longer be accessed.

4. The local aggregates currently sustaining Maiduguri is C_B shown in Figure 3b & Figure 3d respectively. C_B = 25-55% retained on 5mm diameter sieve.

a. Produces medium grades concrete (i.e. 20-25N/mm²).

b. Appropriate sizes of crushed stones must be combined with these categorise of locally available coarse aggregates for higher grades of concrete that are more than 25 N/mm² to be successfully produced.

c. The most appropriate mix ratio in Borno since there is peculiar local coarse aggregate which may warrant having a mix ratio that are in four part (e.g.1:1:1:4 or 1:1½:1½:3 and other most appropriate after trial mix proportions) instead of the conventional mixes that are three as reported by Ahamadu ¹⁵.

There were thirteen (13) different types of naturally occurring aggregates available now on the Maiduguri market, among which six (6) are obtained from Konduga, four (4) from Maiduguri and only three (3) are from Bama. According to gravel vendors the three (3) materials from Bama are an old stock because no one goes to Bama area to fetch the materials because of the security challenges caused by the insecurity; therefore the prefix of Bama gravel should be replaced with Konduga and Maiduguri gravels. Furthermore:

1. Using the BS 882 ²⁶ the materials are mainly fine aggregates as more 95 percent of the sample passed through sieve 4.75 mm (i.e. approximately 5 mm), this is further proven by the fact that more than 70 % of the particle size distribution curves of these samples fall into the Zone 1 standard envelop of fine aggregate. Although the remaining materials did not satisfy the BS standard as none of the particle size distribution curve fall into the standard zone of coarse aggregate. Based on USCS only two materials from Konduga Tsallaken Ruwa and Maiduguri are classified as well graded materials, two materials From Chabbal 1 Chabbal 2 and Bama Bakingada are classified as Gravel-Sand-Silt materials while the remaining 8 materials are classified as poorly Graded materials.

2. There were no significant changes in the categorization done by the previous scholars, although the groups remain the same as it were A, B and C with the materials maintaining their previous group. The only changes are the behaviour of the pattern of the particle size distribution curves produced because of the difference in the type of graph used. Madube 1 and Madube 2 were classified as well graded materials having their average coefficient of uniformity (CU) and coefficient of concavity (CC) as 4.17 and 1.06 as against the previous USCS classification which classified Madube 1 and Madube 2 as poorly graded having an average CU and CC as 3.4 and 1.12 respectively.

3. When all the newly identified materials were combined with the old ones as they may still be available in the market when the security improves; then four groups of gravels were majorly identified in Maiduguri market (i.e. Groups C_A, C_B, C_C, C_D) but C_A is currently not available in the market because of insurgency and security related problems and C_D is proved to be technically fine aggregate. Finally, we have Groups C_B and C_C as currently locally used aggregates in Maiduguri markets for construction purposes by the locals for production of medium grade concrete. It must be emphasised that these group of aggregates must be combined with crushed stones of various sizes for us to produce higher grade of concrete.

4. The most appropriate mix ratio in Borno, since there is a peculiar local coarse aggregate which may warrant having a mix ratio that are in four part (e.g.1:1:1:4 or 1:1½:1½:3 and other most appropriate after trial mix proportions) instead of the conventional mixes that are three.

The summaries of results of tested steel reinforcements selected in Maiduguri have shown that:

1. the diameters, Ø of all the steel reinforcements are less than the specifications by between 1.42 to 3.09%;

2. the characteristic strengths, f_y of diameter 10mm steel reinforcement proved to be 506.7>460N/mm² than recommended value, whereas, all other diameters of 12,16,and 20mm steel reinforcements proved to be between 302.7 to 430<460N/mm² which is the recommended value;

3. similarly, the partial material safety factors, ϒ_m of diameter 10mm of the steel reinforcement 1.157>1.05 of recommended value, but, all other diameters of 12,16,and 20mm steel reinforcements proved to be between 0.682 to 0.938<1.05 which is the recommended value; and

4. the relative elongation for all the steel reinforcements proved to be between 2.19 to 11.79<14% which is the recommended value.

Therefore, it is only 10mm steel reinforcement that successfully passed the laboratory test, but all other category of steels failed and they were considered not satisfactory. They would under-perform and endanger the life and property of the public when used for construction purposes. While the Portland cement in Maiduguri are satisfactory in terms of the tests conducted for fineness, consistency, initial and final Setting time and Soundness; these deviations experienced from the results of the sandcrete blocks show that many of the 225 mm sandcrete blocks cannot be used as loading blocks; since, the highest value of 0.634 N/mm² is less than the recommended standard value of 1.5N/mm² for load bearing walls. This survey also noted through the producers that, the sandcrete block producers know the appropriate mix proportion that could improve good quality blocks but if the cost of sale is high, they lose most customers that would not care about quality but cost of unit block. Although, all the samples of Ashaka and Obajana Dangote cements tested were satisfactory with respect to fineness, consistency, initial and final setting, and soundness respectively. These confirm the reason why there are many buildings with series of cracks and various deformations on buildings along Damboa road where the soil is of soft clay. Similar tests must be conducted in the country; since if this trend is replicated in many cities in Nigeria, it can be a major contributory factor towards the incessant collapse of buildings in many parts of Nigeria.

4. Conclusion

The following conclusions were draw at the end of this research work:

a). The recommended factor of safety for Nigerian National Annex from the normative conditions for Euro code is: 1.115.

b). The Bama gravels values for most of its physical parameters such as specific gravity, silt content, aggregates crushing value and aggregates impact value; are within the limits of the BS 882.

c). The insurgency in the North Eastern part of Nigeria has negatively impacted the quality and availability of construction material as well as the local coarse aggregates in the building industry.

Acknowledgements

The authors wish to thank the TETFUND for funding this research work and the local block production industries and the staff of Ramat Polytechnic Maiduguri, for their support and cooperation.

References