Concrete Batching, Mixing, and Delivery
Core concepts
Understand batching by mass (weight) for cementitious materials, water, and aggregates, with admixtures measured by volume or mass per manufacturer guidance. Know plant versus site mixing, mixer types, and that mixing time must be sufficient for uniformity; too little mixing leaves dry pockets, too much can cause segregation or over-entrainment loss. Track delivery: transit time, drum revolutions, temperature limits, and slump loss. Know that retempering with water is generally not permitted beyond specified limits, and that adding water at the jobsite requires approval and must preserve w/cm.
Traps
Questions test the difference between mixing time and transit time, and between allowable drum revolutions and total time. A frequent trap is assuming you can always add water to restore slump; specification limits and w/cm caps usually forbid it. Another is confusing batch weights with yield-yield is the volume produced and must match the design. Watch for temperature limits (hot-weather limits are lower than cold) and for the rule that admixtures must be added separately unless compatibility is verified.
Worked example / rule of thumb
If design yield is 1.0 m³ and actual batch volume computes to 1.03 m³, the mix is over-yielding; check aggregate moisture and batching accuracy. Rule of thumb: verify batch weights against the approved mix design and correct aggregates for free moisture before mixing. Always confirm slump and air at discharge, not just at the plant.