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When using an internal vibrator for consolidating concrete in a compressive strength test specimen, the recommended practice is to use two insertion points for each layer. This approach ensures that the concrete is effectively consolidated, reducing the risk of air pockets and optimizing the density of the specimen.

The use of two insertion points allows for better distribution of vibrational energy throughout the concrete layer, leading to more uniform consolidation. This is particularly important in ensuring that the compressive strength test results are accurate and representative of the mix being tested.

If only one insertion point were used, there might be areas within the layer that remain inadequately consolidated, leading to unreliable test results. Similarly, using three or four insertion points could lead to over-consolidation or excessive vibration, which may cause segregation of the mix or alter the characteristics of the concrete negatively. Therefore, two insertion points strike a balance that fosters proper consolidation while minimizing the risk of negative impacts on the test specimen.