This paper is based on an experiment of establishing the consistency, density, and compressive strength of concrete. The experiment involves casting of 12 sets of concrete cubes to with three different water to cement ratios. Therefore, four sets of cubes are prepared using one W/C ratio. The essence of making all the four pieces from a common ratio is to boost the confidence of the results. Compaction of the concrete cubes produced is carried out to determine the density of the casting used. The product is also subjected to a compressive load which is basically a destructive test. The load obtained at the fracture point of the cube is used to calculate the compressive strength of the concrete. The results obtained from the experiment showed some discrepancy from the theoretically proven concepts and this could be attributed to the errors discussed in the report. This paper is sectioned into an introduction, experimental procedure, results and analysis, and discussion of the results. Each section provides the desired coherence for the paper.
Introduction
Concrete is a composite material that is made by a mixture of aggregate, cement paste, and water. The greatest bulk of concrete is made of the aggregate. Aggregate is divided into fine and coarse aggregate. The fine aggregate is usually sand while ballast is usually the ballast. Crushed stone and natural gravel may also serve the purpose (Rouquerol 2013, p.12). In a typical application, the course aggregate normally carries the bigger portion of the mixe design. The size distribution directly affects the amount of cement that will be used. The smaller the size, the smaller the amount of binding material needed and vice versa. The recent developments have also seen the introduction of artificial aggregate such as bottom ash. Water has used an ingredient to bind together all the material make up. It converts the cement powder into a binding paste that to unite together all the ingredients. The strength of the resultant concrete is influenced by different factors such as the ratio of the aggregates and the size of the coarse aggregate. Concrete may be used alone as plain concrete cement for blinding where strength is not an issue of concern (Rouquerol 2013, p.12). It can also be used along with reinforcement bars as steel to achieve an appropriate strength. The strength of a concrete can be determined by different parameters such as compressive strength, flexural, and splitting strength (Rouquerol 2013, p.12). This report will look into these factors by analysing the results of the concrete experiment. The objective of this experiment was to use different methods to evaluate the workability of freshly prepared concrete and to establish their consistency to the BS EN 1230 standards.
Apparatus
To achieve the objective of this experiment different apparatus were used. These include form box, mixing rod, compacting factor apparatus, drier, measuring cylinder, and meter rule.
Materials
1. 10kg of Ordinary Portland cement
2. Coarse aggregate i.e. 30kg of limestone
3. Fine aggregate, that is 15kg of sand
4. water
Procedure
Using the above-listed apparatus and materials, the experiment was carried out to achieve the stated objective. Each of the material listed above except water was divided into two equal portions. Twelve sets of concrete were prepared which were grouped into four classes. Every four set was cast using a different cement-water ratio. The water-cement ratios used are 0.35:1, 0.55:1, and 0.70:1. Therefore, four sets of concrete were prepared from each of the W/C. The following set of steps was used to conduct the experiment.
i. Using the drier, the ingredients were dried for approximately two minutes.
ii. The water was then added and the mixture was stirred for about 2 minutes to make a homogeneous casting.
iii. The concrete was then poured the form box an let to settle.
iv. To test the wet density of the concrete, water was poured into the concrete and compaction was carried out until no more change in volume was registered.
v. The original mass and the new mass of the concrete was measured.
vi. Using the measuring cylinder, the volume of water displaced as a result of the additional concrete was recorded.
vii. The same process was used for all the water-cement ratios until all the 12 sets of blocks were produced.
Results
The results collected in this experiment were tabulated as shown in the tables below:
Table 1: Results collected for the W/C experiment
Concrete Workability Results Sheet
Mix (W/C)
0.35
0.55
0.70
Slump Test (mm)
0
53
collapse
VEBE (Seconds)
32
8.38
1
Flow (mm/mm)
0/0
332/318
530/470
Compaction (kg)
17.42
20.32
21.54
21.30
21.92
21.94
Displacement (ml)
795
815
820
Sample Density Calculation
Density (r) =
This calculation will use the data for W/C of 0.35 as shown in the table above.
r =
= 4,880.50 kg/m3
The rest of the densities can be computed with the aid of Ms excel. A tabular representation of these calculations is given below with the density calculations been highlighted on the table as shown below:
Table 2: Summarised calculations for density
Mix (W/C)
0.35
0.55
0.70
Compaction (kg)
17.42
20.32
21.54
21.30
21.92
21.94
Displacement (ml)
795.00
815.00
820.00
Density (kg/m3)
4880.50
1963.19
487.80
Based on table 2 above, a graph of water-cement ratio against the density of the concrete can be plotted to clearly illustrate the effect of compaction in the density of the concrete formed. The diagram below represents this graph:
Figure 1: Effect of compaction on the density of concrete
Table 3: Data collected from three cubes with different W/C ratio
W/C
Cube
Weight (kgs)
Length
Breadth
Width
Maximum Load
0.35
1
8.34
150
150
150
1304
2
8.41
150
150
150
1318
3
8.35
150
150
150
1340
4
8.40
150
150
150
1213
0.55
1
8.22
150
150
150
994
2
8.12
150
150
150
992
3
8.06
150
150
150
1030
4
8.29
150
150
150
986
0.70
1
8.08
150
150
150
655
2
7.76
150
150
150
730
3
7.80
150
150
150
710
4
7.84
150
150
150
723
Sample Compressive strength Calculation
To get the compressive strength for any given W/C, it is important to calculate an average maximum load associated with each ratio. The average maximum load for 0.35 W/C can be computed as:
Average Load (W/C = 0.35) =
=1293.75kN
Therefore,
Compressive Strength, Fc =
The cross-sectional area of the cube used is:
A = (150x 150) x 106 m2
Fc
=
= 57.50 (N/mm2)
The rest of compressive strengths have been computed using excel and the solutions presented in the table below:
Table 4: Summarised calculations for compressive strength
W/C
Maximum Load
Average Load
Compressive strength (N/mm2)
0.35
1304
1293.75
57.50E+6
1318
1340
1213
0.55
994
1000.50
44.47E+6
992
1030
986
0.70
655
704.50
31.31E+6
730
710
723
Using the results tabulated in table 4 shown above, a relationship between water-cement ratio and the compressive strength can be established. For a better illustration, these results are presented in the graph shown below. A discussion of the shape of the graph will be done at a later stage of this paper.
Figure 2: Graph of Compressive strength against W/C ratio
Discussion
The relationship between Compaction and Density
Compaction refers to the rearrangement of particles of a material by application of mechanical techniques. Compaction increases the shear strength, bearing capacity, and boosts the dryness level of a material (Singh and Majumdar 2018). Compaction is applied in engineering to achieve these properties for the parent material.
For our case at hand, the relationship between concrete compaction and its density can be seen clearly from figure 1. The compaction force decreased gradually from approximately 4kg to about 0.3kg. When the compaction force of 4kgs was applied, concrete produced had the highest density. Conversely, the least force applied resulted in the least density of concrete as well. Therefore, the compaction force is directly proportional to the density of the concrete. The higher the force the higher the density of the product. Compaction of concrete results in evacuation of the air from the concrete structure. The voids and spaces existing between the concrete particles are reduced or eliminated by applying a compaction force. As a result, a continuous mass of concrete is produced by application of an external pressure. the resultant product is a denser mass than the uncompacted concrete.
The relationship between Density and Strength
The strength of concrete is affected by different parameters. One of these factors is density. The density of a material has some direct effects on the strength of the material since it affects the distribution of particles (Singh and Majumdar 2018). A dense material means that the air cavities do not exist in the material. Similarly, the dense concrete has a surety of a homogeneous structure which makes it have a high ability to resist any deformation. In fact, from table 2 and table 4, the first cube is found to have a compressive strength of 57.5N/mm2 which corresponds to the highest density of 4880.50kg/m3.Therefore, the strength of concrete is directly proportional to its density.
The relationship between Water-Cement Ratio and Strength of the Concrete
The water-cement ratio has a direct impact on the strength of the concrete. When the water to cement ratio is high in a concrete mix, a gel-like the product will be formed when hydration takes place. In this case, water tends to take up a significant volume of the space. The outer product of hydration is formed as a result of this process. When the product dries up, the large air spaces and voids are left out (Singh and Majumdar 2018). As previously discussed, large air spaces result in low strength concrete. Similarly, even after drying up, it is possible that a certain amount of water will be trapped in the structure of the concrete. This automatically compromises on the strength integrity of the structure. On the other hand, when the water-cement ratio is low, almost all the water used in the mixture will be used up in the reaction process. As a result, a very little amount of water will be left out to evaporate which result in a limited number of air spaces and voids in the concrete structure (Singh and Majumdar 2018). Therefore, low water to cement ratio will lead to the formation of a strong product. In our experiment, the effect of water to cement ratio can be seen from the figure number 2. The compressive strength of the concrete reduces drastically as the water proportion in the mixture increases. The experiment is therefore in tandem with the theoretically proven concepts.
Comment on the Accuracy of the Results
The density of conventional concrete is 2320kg/m3 whereas; wet concrete has a density range of 648–808 kg/m3. In our experiment, the calculated densities were distributed as 4880.580kg/m3, 1963.1980kg/m3, and 487.80kg/m3. The values are not badly off and the discrepancies from the theoretical values can be attributed to factors such as:
1. Inaccuracy in taking the measurements of the concrete weights.
2. The inconsistency of the concrete blocks due to defects such as honeycombs.
3. Poorly finished form box which gives wrong dimensions.
Order of Strength
Table 5: Compressive Strengths
W/C
Compressive Strength
0.35
57.5MPa
0.55
44.46MPa
0.7
31.31MPa
The results obtained from different water-cement ratio depicted varying strengths as shown in table 5 above. The combination with the least volume of water had the highest strength while the one with the highest proportion showed the least compressive strength.
References
Rouquerol, J, 2013. Adsorption by powders and porous solids: principles, methodology and applications. Academic Press.
Singh, P.R., Shah, N.D. and Majumdar, P.K., 2018. Effect of Density and Porosity on the Durability of Flyash blended Concrete.
