The relation between compressive strength of concrete cube and cylinder is complex. The various factors including inherent variations in concrete quality and comparison between cube and cylinder strength affects the strength ratio.

Factors affecting Concrete Cube and Cylinder Strength Ratios are:

1. Concrete cube and cylinder casting, curing and testing procedures

2. Geometry of the specimen

3. Level of strength

4. Direction of loading and machine characteristics

5. Grading of aggregates

1. Effects of concrete cube / cylinder casting, curing and testing procedures:

The method of casting and capping of cube and cylinder affects the strength ratios of both. The use of rigid and non-rigid moulds affects their strength. Also the method of capping these moulds affects the strength as out of plane surface also influences their strength ratio.

Proper curing and testing procedure is necessary to associate a proper relation between concrete cube and cylinder compressive strength ratios, otherwise the ratio obtained will be misleading.

2. Effects of geometry of specimen

Geometric factors such as volume of concrete, shape of concrete and h/d ratio (height to lateral dimension) of specimen affects the concrete cube and cylinder strength ratio. The following figure shows the effect of height/diameter ratio to concrete strength ratio.

concrete-cube-cylinder-strength-comparison

concrete-cube-cylinder-strength-ratio-comparison

3. Effect of concrete strength level

Nominal strength of concrete has been shown to affect the concrete cube and cylinder strength ratio. Research by Evans indicates that this ratio decreases with increasing concrete strength. Cylinder to cube strength ratio ranges from 0.77 to 0.96 depending on concrete strength level.

4. Direction of loading and machine characteristics

Concrete cubes may be loaded in the direction perpendicular to casting while cylinders are always loaded in the direction of casting. Since these concrete cubes and cylinders are casted in multiple layers, their strength will differ based on direction of loading.

concrete-cube-cylinder-strength-based-on-loading-direction

5. Grading of aggregates

Grading of aggregates in concrete affects the strength of any structure or specimen. The effect of compression test on concrete specimens are large due to relative size of aggregate particles to specimen dimensions. Most standards sets limits for the ratio of diameter or size of specimen to maximum nominal size of aggregates. Typically this allowable minimum is around 3 to 4.