Retempering of concrete; shape index

Retempering of concrete

Retempering is the addition of water and remixing of concrete that has started to stiffen (partially set, with a loss of slump) so as to restore its workability to a level that allows placing.

• It is allowed only if the concrete has not reached its initial set; concrete must be placed and compacted before initial setting begins, and good practice is to place it within about 30 minutes of mixing.
• Only a limited amount of water may be added (for example, water held back from the original batch), so that the specified maximum w/c ratio is not exceeded; mixing must then be continued. If more water is needed, cement must be added with it to keep the w/c ratio unchanged.
• Effects of adding extra water: lower strength (about 5-10 per cent), more shrinkage and higher permeability. Concrete that has set should never be retempered. Re-dosing with a superplasticiser is the better way to restore workability.

Shape index of aggregate

Shape index is a quantitative measure of how far the shape of the aggregate deviates from a cube. In IS practice, shape is expressed by the flakiness index (percentage by weight of particles whose thickness is below 0.6 times the mean size), the elongation index (length above 1.8 times the mean size) and the angularity number (IS 2386 Part 1). In BS EN 933-4, the shape index is the percentage by mass of particles whose length-to-thickness ratio exceeds 3. For a single particle, it can be expressed in terms of its length : width : thickness, or by its sphericity:

$$\text{Shape index} = \frac{\text{length}}{\text{thickness}} \quad(\text{or } \frac{L + W}{2T})$$

A value close to 1 means a cubical particle (good shape). Higher values show flaky or elongated aggregate, which gives poor workability and strength.