The quest to simplify concrete production using locally available materials, construction equipments, labor force and practices is on the increase on a daily bases and has led to the need for a variety of methods of producing concrete. This encourages the researchers in the construction industry to attempt to enhance the current concrete technology practices and develop new ones. The construction industry has therefore witnessed a lot of modifications in the production of concrete around the world for some time now.

Japanese researchers invented self-compacting concrete in 1980 which is a kind of concrete that preserves its homogeneity and flows under its particular weight in its state of plastic during filling any form even surrounding the congested reinforcement of the framework, attaining compaction without vibrating mechanically [1]. Other researchers investigated a high-strength concrete using coarse aggregates and crushed lime stone of 10, 15 and 20 mm sizes [2]. It was also shown that with the application of coarse recycled concrete aggregate, as the component materials in the concrete mixtures, it is possible to produce structural concrete that can be satisfactory and even of high quality, which primarily depends on the characteristics of crushed demolished concrete [3].

In another research which investigated the mechanical properties of concrete containing recycled concrete aggregate (RCA) and ceramic waste as coarse aggregate replacement it was found that the concrete containing 35% RCA and 35% ceramic waste showed the best properties compared with the normal or conventional concrete [4]. Other researchers recounted that, there are various ways by which concrete has been modified for various purposes without compromising its strength, such as coarse aggregate combination and partial replacement of either fine or coarse aggregate. They further advised that concrete production can be modified provided the minimum required strength is reached and serves the intended purpose [5].

Some investigated the mixture of glass powder and silica fume in concrete as a partial cement replacement, to study its effect upon concrete strength. The mix proportion of 1:2:4 was selected for all the concrete samples with water to binder ratio of 0.55. For comparison, a control sample of concrete was prepared without mixture of glass powder and silica fume to compare it with the various samples containing different percentages of mixture of glass powder and silica fume as a partial replacement of cement in concrete. Results showed that the usage of mixture of glass powder and silica fume in concrete as a partial replacement of cement increases the concrete strength [6].

The safety, strength and structural integrity of concrete structures depend largely on the quality of concrete used for their construction and one of the most important quality parameters is the compressive strength of the concrete [7]. Concrete grade/strength class C20/25 is the minimum concrete grade/strength class recommended for the construction of reinforced concrete load-bearing members (beams, slabs and columns) of building under mild exposure condition [8]. Class 20/25 concrete is a concrete with a minimum cylinder crushing strength of 20 MPa or a minimum cube crushing strength of 25 MPa.

In making concrete, it is important to use the correct concrete mixing ratios. The inappropriate mix ratios or incorrect proportioning of concrete ingredients, leads to production of concrete that do not meet the designed target strengths. Research affirms that concrete mix for normal concrete is conventionally batched in ratios depending on the targeted design strength. Concrete mix ratios are the proportions of concrete components namely cement, sand, aggregates and water decided based on the type of construction and mix designs [9].

Some researchers undertook a prediction of fresh and hardened properties of normal concrete via choice of aggregate sizes, concrete mix-ratios and Cement and arrived at a series of conclusions. First they concluded that the different mix ratios 1:2:4, 1:2:3 and 1:3:6 should be adopted for different purposes. Secondly they found that, the 1:2:4 mix ratio is a very suitable mix ratio for normal concrete while the 1:2:3 mix ratio (very high in cement content and therefore more expensive) should be used for constructions where extra compressive strength may be required as it has a higher strength than the 1:2:4 mix ratio. Thirdly, the 1:3:6 mix ratio has the lowest compressive strength of the three and is not suitable for the casting of major structural elements such as beams and columns. It could be used for blinding purposes and for structural elements of less importance. Fourth they found that the concrete produced with the mixed-size aggregate had the highest compressive strength. This may be due to the fact that all pore spaces between the aggregates were completely filled with the smaller aggregates and cement. This confirms the preference for mixed aggregate sizes to single aggregates in practice [10].

From the literature above, it can be observed that different mix ratios have been adopted for different purposes. But little information is available on rationalized concrete mix ratio despite the fact that its usage is on the increase on most building sites in Cameroon. The compressive strength of concrete produced with the rationalized mix ratio is still not investigated and documented. The main objective of this study is therefore to investigate if the concrete produced with equal volumes of sand and gravel mix ratio can be adopted as conventional mix ratio in terms of minimum required compression strength of concrete for buildings. The specific objective of this work was to compare the conventional mix ratio of 350 kg of cement: 400 liters of sand: 800 liters of gravel for a cubic meter and the rationalized batch of 350 kg of cement: 600 liters of sand: 600 liters of 5/15 gravel, 15/25 gravel and a combination of 5/15 + 15/25 gravel.

Physical characteristics of cement: the physical characteristics of cement studied as presented in Table 1 are specific gravity, specific surface area, consistency and apparent (bulk density).

The specific gravity shows that it is within the range of 3.1 to 3.16. This shows that the cement was properly ground into fine powder during its production and that the pores of cement are not filled with moisture which can affect the mix and bonding. The specific surface area of cement is greater than 2250 cm²/g, it can be deduced that the cement has a good fineness which offers a greater surface area for hydration and hence faster the development of strength. The consistency obtained is within the range of 25% to 35%. This shows that for every 1000 g of cement, 290 g of water will be the minimum required to make the cement paste. Apparent (bulk) density is within the range of 0.9 to 1.2 t/m³.

Properties of sand and gravel

Table 2 shows the result of sieve analysis for sand and gravel that was carry out on the following sieve size: 25 mm, 20 mm, 15 mm, 12 mm, 10 mm, 5.00 mm, 2.5 mm, 1.25 mm, 0.63 mm, 0.315 mm, 0.16 mm, 0.08 mm and the bottom bowl as per AFNOR [12] to determine the distribution of sand and gravel by sieving them per sieve. The results of sieve analysis of percentage passing for each aggregate type are presented here below in Table 2.

The table above shows that the sand is continuous graded and the grading is within zone II envelope of the British standard showing that the Wum road sand is well graded for the concrete work. Aggregates have to be graded so the whole mass of concrete acts as a relatively solid, homogeneous, dense combination with the smallest particles acting as inert filler for the voids that exist between the larger particles [13]. The Specific gravity for Wum road sand obtained is within the range of 2.65 to 2.67 and that for the various gravel sizes are also within the range of 2.5 to 3. Therefore the pores in the Wum road sand and also

in the various gravel sizes are within the acceptable range. The apparent density of both the sand and the gravel are minimum 1.6 and 1.5 respectively. Therefore the volume that the graded aggregate will occupy in concrete, including the solid aggregate particles and the voids between them will produce good concrete at constant moisture content for each mix ratio. The Sand Equivalent (SE) is within the range of above 80% as specified in the French norms AFNOR [14], therefore it is very clean sand. The fineness modulus (FM) obtained is within the FM range of 2.2 to 2.8 therefore, the Wum road sand is suitable for construction works. Table 2 also shows the result of sieve analysis for gravel 5/15, gravel 15/25 and gravel 5/15 + 15/25 that was carried out on as per AFNOR [12]. All the percentages passing shown in table for the various gravel sizes considered for this study are continuous. This proofs that their distributions are good for production of dense concrete.

Quantities of Materials used for the experiments

The quantity of cement for every test was 50 kg meanwhile the quantity of

water was 25 liters for every test. The batching of aggregates for the conventional mix ratio consisted of 60 liters of sand plus 120 liters of gravel in all the single size gravel cases meanwhile in the case of combination of two gravel sizes, 60 liters of gravel from the 5/15 size and also 60 liters from the 15/25 size were used. For the rationalized mix ratio, the batching 90 liters of sand and 90 liters of gravel in all single size gravel meanwhile in the case of combinations of the two gravel sizes, 45 liters of gravel from the 5/15 size and also 45 liters from the 15/25 size were used. The average slump test for the all conventional mix was 6.4 cm and for the rationalized mix it was averagely 5.1 cm. This shows that the rationalized mix ratio is less workable and requires more water as compared to the conventional mix ratio at the same water cement ratio. However, as the amount of water in a concrete mix increases, the strength decreases.

Compressive tests results of concrete

Case of 5/15 gravel

From the result in Figure 1 for gravel size 5/15, the average compressive strength of concrete for seven days of curing for concrete made of 350 kg of cement: 400 liters of sand: 800 liters of gravel mix ratio is 13.5 N/mm² and concrete made of 350 kg of cement: 600 liters of sand: 600 liters of gravel mix ratio is 12.78 N/mm², we can deduce that, the concrete strengths in both mix ratios met

the minimum required standard for seven days of curing which is 60% - 65% of the 20 N/mm² cylinder strength considered in this study. The average compressive strength of concrete for 14 days of curing for concrete made of 350 kg of cement: 400 liters of sand: 800 liters of gravel mix ratio is 18.2 N/mm² and concrete made of 350 kg of cement: 600 liters of sand: 600 liters of gravel mix ratio is 17 N/mm², we can deduce that, the concrete strengths in both mix ratios met the minimum required standard for 14 days of curing which is 90% of 20 N/mm² cylinder strength. Still on Figure 1, the results of the average compressive strength of concrete for 28 days of curing for concrete made of 350 kg of cement: 400 liters of sand: 800 liters of gravel mix ratio is 19.2 N/mm² and concrete made of 350 kg of cement: 600 liters of sand: 600 liters of gravel mix ratio is 18.7 N/mm², we can deduce that, the concrete strengths in both mix ratios did not meet the minimum required standard for 28 days of curing which should be 99% of 20 N/mm² cylinder strength.

Case of 15/25 gravel

From the result in Figure 2 for gravel 15/25, the average compressive strength of concrete for seven days of curing for concrete made of 350 kg of cement: 400 liters of sand: 800 liters of gravel mix ratio is 14.1 N/mm² and concrete made of 350 kg of cement: 600 liters of sand: 600 liters of gravel mix ratio is 13.5 N/mm², we can deduce that, the concrete strengths in both mix ratios met the minimum required standard for seven days of curing which is 60% - 65% of 20 N/mm² cylinder strength. The average compressive strength of concrete for 14 days of curing for concrete made of 350 kg of cement: 400 liters of sand: 800 liters of gravel mix ratio is 18.6 N/mm² and concrete made of 350 kg of cement: 600 liters of sand: 600 liters of gravel mix ratio is 17.5 N/mm², we can deduce that, the concrete strengths in both mix ratios met the minimum required standard for 14 days of curing which is 90% of 20 N/mm² cylinder strength. Again, from the result in Figure 2 of the average compressive strength of concrete for 28 days of curing for concrete made of 350 kg of cement: 400 liters of sand: 800 liters of gravel mix ratio is 20.8 N/mm² and concrete made of 350 kg of cement:

600 liters of sand: 600 liters of gravel mix ratio is 19 N/mm², we can deduce that, the concrete strength in the conventional mix ratio met the minimum required standard for 28 days of curing which should be 99% of 20 N/mm² cylinder strength but the rationalized mix ratio did not.

Case of mix with 5/15 + 15/25 gravel

From the result on Figure 3 for the combination of gravel 5/15 + 15/25, the average compressive strength of concrete for seven days of curing for concrete made of 350 kg of cement: 400 liters of sand: 800 liters of gravel mix ratio is 14.6 N/mm² and concrete made of 350 kg of cement: 600 liters of sand: 600 liters of gravel mix ratio is 13.8 N/mm², we can deduce that, the concrete strengths in both mix ratios met the minimum required standard for seven days of curing which is 60% - 65% of 20 N/mm² cylinder strength. The average compressive strength of concrete for 14 days of curing for concrete made of 350 kg of cement: 400 liters of sand: 800 liters of gravel mix ratio is 19 N/mm² and concrete made of 350 kg of cement: 600 liters of sand: 600 liters of gravel mix ratio is 18 N/mm², we can deduce that, the concrete strengths in both mix ratios met the minimum required standard for seven days of curing which is 90% of 20 N/mm² cylinder strength. Still on Figure 3, from the result of the average compressive strength of concrete for 28 days of curing for concrete made of 350 kg of cement: 400 liters of sand: 800 liters of gravel mix ratio is 21.9 N/mm² and concrete made of 350 kg of cement: 600 liters of sand: 600 liters of gravel mix ratio is 20 N/mm², we can deduce that, the concrete strengths in both mix ratios met the minimum required standard for 28 days of curing which is 20 N/mm² for structural concrete.

From the study, it is observed that average compressive tests result for both the conventional and the rationalized mix ratios met the minimum compressive strength at 7 days and 14 days in one case meanwhile in some cases, both did not meet the minimum required standard for 28 days of curing which is minimum

20 N/mm² for structural concrete for building projects. Using the 5/15 gravel size alone, resulted in strengths below 99% of the 20 N/mm² cylinder strength for both the conventional and the rationalized mix ratios hence leading to the conclusion that 5/15 gravel results in lower strength concrete and should not be used for rationalized concrete mix ratio except it has to be combined with larger size gravel.

When gravel size 15/25 was used alone, the cylinder strength was 20.8 N/mm² for the conventional mix ratio which is a good result for compressive strength. The strength of the rationalized mix ratio did not meet the minimum required standard for 28 days of curing which should be 99% of 20 N/mm² cylinder strength. This result also leads to the conclusion that 15/25 gravel results in lower strength concrete and should not be used for rationalized concrete mix ratio except if it has to be combined with other size aggregates.

When a combination of gravel sizes 5/15 + 15/25 was used, the result of the average compressive strength of concrete for 28 days of curing for both the conventional mix and the equal volumes of sand and gravel mix ratio met the minimum required standard for 28 days of curing. Based on this, this study arrives at the conclusion that the equal volumes of sand and gravel mix ratio of 350 kg/m³ of cement: 600 liters of sand: 600 liters of gravel mix ratio can be adopted as a conventional mix design using a combination of 5/15 + 15/25 gravel. Therefore, in determining the compressive strength of concrete it is important to consider the mix ratio of concrete.

However, from the slump test result, the rationalized mix ratio is less workable and requires more water as compared to the conventional mix ratio at the same water cement ratio. The high fine aggregates in the rationalized mix ratio caused an increase in surface area of particles within the mixture, thereby increasing the demand for the amount of water required to coat the surfaces which will result in segregation and bleeding in the fresh state and also increased drying shrinkage and cracking in the hardened state. Further study is required in the area of mixing water in the equal volumes of sand and gravel mix ratio.

We sincerely appreciate Bambui engineering Bamenda for letting us use their laboratory. We equally thank Dr. Egr. Mbuh Moses, head of civil engineering department at the University of Bamenda who directed us to this laboratory.

The authors declare no conflicts of interest regarding the publication of this paper.

Che, P.B., Emmanuel, Y.B. and Billong, N. (2022) Equal Volumes of Sand and Gravel Concrete Mix Ratios in Cameroon and Its Effect on Concrete Compressive Strength. World Journal of Engineering and Technology, 10, 539-549. https://doi.org/10.4236/wjet.2022.103034