Abstract: Dwellers along the coastline area of Lagos state, Nigeria are exposed to abundant supply of the natural resources of Atlantic ocean and Lagoon water and may not have the privilege of having potable drinking water at their disposal for producing concrete. This study evaluates the characteristics of concrete produced with Atlantic ocean and Lagoon water. Concrete produced with tap water (regarded as potable drinking water) serves as the control experiment. Compressive strength, workability and density were used to evaluate the characteristics of concrete specimens. All the concrete samples have true slump with Lagoon water concrete having low workability and both tap and Atlantic ocean water concrete having medium workability. The concrete specimens produced with the three types of water fall into the category of normal weight concrete as their densities lie within the range of 2200-2600 kg m^-3 specified. The 28th day compressive strength of concrete specimens produced with Atlantic ocean, tap and Lagoon water are 25.0 and 33.5 N mm^-2, 17.9 and 28.6 N mm^-2 as well as 15.1 and 19.4 N mm^-2 for mix ratios 1:3:6 and 1:2:4, respectively. It was concluded that tap water should be used in mixing concrete where strength is of major concern, Atlantic ocean water could be used where early strength is required and Lagoon water should only be used for general concrete works where strength is of less importance such as in mass concrete, floor screed and mortar.

INTRODUCTION

Lagos state of Nigeria consists of a large Lagoon and an archipelago of large islands. The coastline is bound by the Atlantic ocean which is the 2nd largest ocean in the world. Thus, Lagos state is blessed with two great natural sources of water Lagoon water and Atlantic ocean water. Dwellers along the coastline area of Lagos state are exposed to the abundant supply of these natural resources and may not have the privilege of having potable drinking water at their disposal for producing concrete.

This research investigates the characteristics of concrete produced with these available sources of water for construction works along the coastal line area of Lagos state with a view to ascertaining their adequacy or otherwise.

Concrete for many years has been one of the major materials used in the construction of buildings and other engineering structures. It is a construction material composed mainly of three constituents namely; cement, fine and coarse aggregates and water. Cement is the chemically active constituent but its reactivity is only brought into effect on mixing with water. Thus, water is an important ingredient of concrete as it actively participates in the chemical reaction with cement (Shetty, 2001). Water used in concrete in addition to reacting with cement and thus causing it to set and harden also facilitates mixing, placing and compaction of the fresh concrete. According to Barry (1980), water for concrete should be reasonably free from such impurities as suspended solids, organic matter and dissolved salts which may adversely affect the properties of concrete.

There is the popular saying that only water fit for drinking is acceptable as being satisfactory for mixing concrete. However in Shetty (2001)’s argument, some waters containing a small amount of sugar would be suitable for drinking but not for mixing concrete. A simple way of determining the suitability of such water is to compare the setting time of cement and the strength of the mortar cubes using the water in question with the corresponding results obtained using known good water or distilled water.

A tolerance of about 10% is usually permitted to allow for chance variations in strength (BS 3148) (BSI, 1980). Table 1 shows the tolerance of concentrations of some impurities in mixing water.

Table 1:	Tolerable concentrations of some impurities in mixing water
Shetty (2001)

The objectives of this study are to:

MATERIALS AND METHODS

Burham portland cement was used in the production of the concrete and it was obtained from a local dealer at Costain Orile Iganmu in Lagos state. The fine aggregate is sharp sand with sizes not >5 mm diameter and it was obtained from Tin Can Island, Lagos. The coarse aggregate is granite from crushed quarried rocks. The size of the granite is not >19 mm and it was bought from local granite seller at Costain, Orile Iganmu in Lagos state. Three types of water were used in mixing the concrete namely; Atlantic ocean water which was obtained from Lagos Bar beach, Lagoon water which was obtained from Lagoon front at University of Lagos, Akoka and tap water which was obtained from Civil Engineering Concrete Laboratory, University of Lagos, Akoka, Lagos state.

Sieve analysis of aggregates: The sieve analysis of the fine and coarse aggregates used were carried out in accordance with the procedure in BS 1377-2 (BSI, 1990).

Water analysis: Water analysis was carried out on the three samples of water used to determine their physical and chemical properties.

Table 2:	Format for identifying concrete specimen

The physical properties determined include appearance, colour, odour, pH at 20°C, turbidity and conductivity. The chemical properties determined include acidity, alkalinity, total hardness, chloride, sulphite, sulphate, nitrite, ammonia and silica. The water analysis was performed in Chemistry Laboratory in University of Lagos, Akoka.

Specimen preparation: Concrete specimens were produced using Lagoon and Atlantic ocean water for mixing and curing purposes. Another set of concrete specimens with potable drinking water as the matrix serves as the control experiment. A format was used to identify the specimens as shown in Table 2. Specimen preparation for Compressive strength test was performed using 150 mm cube steel moulds. The mix proportions of cement:sand:granite used are 1:2:4 and 1:3:6 with 0.5 and 0.6 water-to-cement ratios, respectively. After casting, the specimens were stored in the curing room at 27±5°C with 90% relative humidity for 24 h and then demoulded and placed under water until the testing ages of 7, 14, 21 and 28 days. The compressive strength was determined with Compressive strength testing machine (Clock house model) with maximum capacity of 3000 KN. The strength value was the average of three specimens. Slump test was carried out to check the workability of concrete. The test was carried out in accordance with the requirements of BS 1881:Part 102 (BSI, 1983).

RESULTS AND DISCUSSION

Sieve analysis: The result of the sieve analysis for both fine and coarse aggregates is shown in Fig. 1. The uniformity coefficient (Cu) of the sand is 2.40 which is <4 as suggested by Emesiobi (2000). This indicates a uniform soil.

Also, the Coefficient of curvature (Cc) is 0.82 which is between 0.5 and 2.0 as suggested by Shetty (2001), therefore this indicates a well graded soil. Similarly, the uniformity coefficient of the granite is 2.14 while the coefficient of curvature is 1.15 indicating that it is uniform and well graded too. Thus, the fine and coarse aggregates used are quite adequate for producing good concrete.

Water analysis: The results of the physical and chemical characteristics of the three water samples used are shown in Table 3 and 4, respectively. Table 3 showed that tap water has a clear appearance, odourless and a pH of 6.4. For Lagoon water, it showed a clear sample, green in colour and the odour is pungent indicating the presence of organic matters and a pH of 6.2 which is acidic. The Atlantic ocean water was clear in appearance, odourless and a pH of 8.0 indicating alkalinity. Only tap water nearly satisfied the WHO and NIS requirements for drinking water (NAFDAC, 2001). The Lagoon water was acidic while the Atlantic ocean water was alkaline. Table 4 showed that tap water has chlorine content of 70 ppm, total hardness of 20 ppm CaCO₃ and sulphate of 0.04 ppm. Lagoon water has chlorine content of 150 ppm, total hardness of 104 ppm CaCO₃ and sulphate of 0.17 ppm while Atlantic ocean water has chlorine content of 15250 ppm, total hardness of 550 ppm CaCO₃ and sulphate of 1.15 ppm. Only Atlantic ocean water failed to meet the tolerable concentrations in mixing water as specified in Table 1.

Fig. 1:	Grading curves for sand and granite used

Table 3:	Physical characteristics of water samples

Table 4:	Chemical characteristics of water samples
ND = Not Detected

Table 5:	Slump of concrete specimens

Workability: The result of the Slump test indicating the workability of the concrete specimens is shown in Table 5. All the concrete samples have true slump with Lagoon water concrete having low workability (slump between 15-30 mm) and both tap water and Atlantic ocean concrete having medium workability (slump between 35-75 mm) as specified by Neville (2000). The low workability of the Lagoon water could be attributed to the presence of organic matter in it.

Density: The results of the densities of the three concrete specimens are shown in Fig. 2-4. Figure 2 showed that the density of concrete made with tap water (the control) ranges from 2421-2459 and 2459-2637 kg m^-3 for 1:3:6 and 1:2:4 mix ratios, respectively as the curing age increases from 7-28 days.

As expected, the density increases as the curing age increases. This is in line with previous studies of Shetty (2001) and Raheem et al. (2008). Figure 3 shows the density of concrete made with Atlantic ocean water followed a similar pattern with values ranging from 2370-2519 and 2489-2569 kg m^-3 for 1:3:6 and 1:2:4 mix ratios, respectively during the same period. However, concrete made with tap water are denser than those with Atlantic ocean water.

Figure 4 showd that the density of concrete made with Lagoon water does not follow a regular pattern with values actually decreasing at 28 days. This could be due to the presence of organic matter in the mixing water. The concrete specimens produced with the three types of water fall into the category of normal weight concrete as their densities lie within the range of 2200-2600 kg m^-3 specified by Neville (2000).

Fig. 2:	Density of concrete specimens produced with tap water

Fig. 3:	Density of concrete specimens produced with Altantic ocean water

Fig. 4:	Density of concrete specimens produced with Lagoon water

Compressive strength: The results of the compressive strength of the three concrete specimens at curing ages of 3, 7, 21 and 28 days are shown in Fig. 5-7. The result of concrete made with tap water as shown in Fig. 5 that the compressive strength gradually increases from 12.7-17.9 and 19.3-28.6 N mm^-2 for 1:3:6 and 1:2:4 mix ratios, respectively as the curing age rises from 7-28 days. This is in line with previous findings on concrete made with potable drinking water (Raheem and Aderounmu, 2001; Adesanya and Raheem, 2009). For concrete made with Atlantic ocean water, the compressive strength as shown in Fig. 6 followed a similar pattern to that of tap water but with higher values. The strength increases from 23.3-25.0 and 28.0-33.5 N mm^-2 for 1:3:6 and 1:2:4 mix ratios, respectively.

The increase in strength is more pronounced at early ages as 46 and 32% increases were recorded for 1:3:6 and 1:2:4 mix ratios, respectively at 7th day. This increase in strength may be attributed to the high alkalinity and high total hardness of the mixing water indicating a large quantity of calcium. The high chlorine content may be responsible for the early strength of concrete specimens.

Fig. 5:	Compessive strength of concrete specimens produced with tap water

Fig. 6:	Compessive strength of concrete specimens produced with Atlantic ocean water

However, the long term strength of the concrete may be adversely affected as claimed by Neville (2000). Figure 7 showed that the compressive strength of 1:3:6 and 1:2:4 concrete specimens produced with Lagoon water were 11.7, 12.2, 16.0 and 15.1 N mm^-2 as well as 15.6, 19.9, 21.5 and 19.4 N mm^-2 for ages 7, 14, 21 and 28 days, respectively. The strengths are generally lower than those obtained for tap water and Atlantic ocean water concrete. A decrease in strength was also observed between 21 and 28 days for both mix ratios. The reduction in compressive strength of the specimens may be attributed to the acidic nature of the mixing water as well as the presence of organic substance as depicted by its green colour and pungent odour.

The effect of the three types of water on the compressive strength of concrete with mix ratios 1:3:6 and 1:2:4 are shown in Fig. 8 and 9, respectively. As can be observed from Fig. 8 and 9, concrete specimens produced with Atlantic ocean water have highest values followed by those produced with TAP water while the ones with Lagoon water recorded the lowest values. The NIS 439 (SON, 2000) requirement for minimum compressive strength of 26 N mm^-2 at 28 days was satisfied by only 1:2:4 concrete specimens produced with tap water and Atlantic ocean water.

Fig. 7:	Compessive strength of concrete specimens produced with Lagoon water

Fig. 8:	Effect of mixing water of the compressive strenght of concrete (mix ratio 1:3:6)

Fig. 9:	Effect of mixing water of the compressive strenght of concrete (mix ratio 1:2:4)

The very high compressive strength of concrete specimens produced with Atlantic ocean water notwithstanding, the high chloride content in it could be detrimental to its use where reinforced steel bars are required. It would also be unsuitable in places where appearance is of great importance due to staining. Lagoon water concrete specimens showed a decrease in the compressive strength on the 28 day. This is an indication that it should not be used on construction sites for concrete works requiring as this will lead to failure of the structure.

The implication of these findings is that only tap water is recommended for use in mixing concrete where strength is of major concern. However since, all the specimens meet the minimum strength of 6 N mm^-2 after 28 days of curing recommended by BS 5224 (BSI, 1976) all the three types of water could be used for general concrete works where strength is of less importance such as in mass concrete, floor screed and mortar.

CONCLUSION

From the results of the various tests performed, different conclusions can be drawn; only tap water nearly satisfied the WHO and NIS requirements for drinking water. The Lagoon water was acidic while the Atlantic ocean water was alkaline. All the concrete samples have true slump with Lagoon water concrete having low workability and both tap water and Atlantic ocean concrete having medium workability. Based on their densities, concrete specimens produced with the three types of water fall into the category of normal weight concrete. Concrete specimens produced with Atlantic ocean water have highest compressive strength values followed by those produced with tap water while the ones with Lagoon water recorded the lowest values.

Tap water is recommended for use in mixing concrete where strength is of major concern. Atlantic ocean water could be used where early strength is required and Lagoon water should be used for general concrete works where strength is of less importance such as in mass concrete, floor screed and mortar.