Aggregates contribute to 60-80% of a concrete mix, with water and cement being the other major constituents. Therefore considerable testing and consideration should be placed on aggregate type and size to ensure an adequate concrete mix.
Two tests have been performed to establish the properties of the aggregates: 1. Compacted bulk density test
2. Sieve analysis
II Testing For Compacted Bulk Density
* To determine the specific bulk densities of a range of course, fine and mixed aggregates. * To determine the void content of the samples.
* From this we can determine the optimum fine to coarse aggregate mix, given the bulk density and minimum void content. Equipment
* Electronic Balance
* Metal Cylinder (AS Compliant)
* Metal Scoop
* Tamping Steel Rod
1. Determine the weight and volume of the cylinder (for lab experiment written on side). 2. For the mixed aggregates ensure they are well mixed. Now fill the cylinder with the aggregate to 1/3 the height of the cylinder using the scoop. 3. Compact the aggregate with the tamping rod giving 25 strokes without touching the bottom of the cylinder then stomp the cylinder on the ground 3 times. 4. Repeat steps 2 and 3 another two times so that the cylinder is full and has now being compacted 3 times. 5. Remove the surface aggregate to the level of the cylinder rim by rolling the tamping rod over the rim of the cylinder, working from the centre out. 6. Weigh the final compacted mass of the cylinder comprising the compacted aggregate.
II.I Results – Data & Calculations
Two formulas where used to calculate the data in the following table.
Db = (w2 – w1) / Vb
Where Db = Bulk Density (kg/m3)
w2 = Cylinder + Aggregate Weight (kg)
w1 = Cylinder Wt. (kg)
Vb = Aggregate bulk volume (m3)
Void Content = 1 - (Db/ Dp) x 100%
Where Dp = Particle Density (kg/m3)
= 2600 kg/m3
Please note that if fine aggregate content = 20% then coarse aggregate content = 80%, so that the total = 100%.
TABLE 1: COMPACTED BULK DENSITY TEST RESULTS
| Fine agg. Content
| Agg. Type
| Cylinder Wt
| Cylinder + agg Wt.
| Bulk Density
| Void Content
| Volume (m3)
II.II Results – Graphs
The following two graphs show the fine aggregate content versus compacted bulk density, and void content. The optimum values are highlighted by the red lines.
From the graph, we see that the optimum fine aggregate content is 60%. This combination gives the highest bulk density and the lowest void content.
There can only be one optimum value for the fine aggregate content. An excess of coarse aggregate will increase the void content, and an excess of fine aggregate will lower the bulk density. The optimum ratio gives the smallest amount of gaps (voids) between the larger particles of aggregate, decreasing void content and maximising bulk density. Also the fine aggregate reduces the porosity of the concrete mix.
One of the major influential factors of bulk density is aggregate shape. The shapes of the aggregate influence the workability and performance. Rounded shape aggregates are good for workability while multi cubical shape provide the best bond for concrete. Similarly moisture content of the aggregate influences the composition of the concrete. The aggregate moisture content is required to determine the amount of water required for concrete mix.
The reason for finding this optimum value is to increase the compressive strength...
References: NRMCA, 2007, Aggregate grading, viewed 5 April 2010,
PCA, 2008, Cement & Concrete basics: Aggregate, 5 April 2010,
Obla, K. & Kim, H, 2008, On aggregate grading, 5 April 2010,
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