What is a common cause of early stiffening in transit concrete?

• A. Low cement content only.
• B. Temperature drop during transit.
• C. Long idle time at site after loading.
• D. Time elapsed and temperature rise during transport leading to accelerated set.

Answer: (D)

Concrete stiffening happens as the cement hydrates, and the rate of that hydration is driven by both time and temperature. During transit, the mix can heat up from the ongoing hydration and from hot surroundings. When enough time passes and the temperature is elevated, the hydration reactions accelerate, causing the concrete to stiffen sooner than expected. That combination—time elapsing while the temperature rises during transport—leads to accelerated set, which is why this option is the best answer.

Lower cement content tends to reduce the heat released and slow the stiffening process, not speed it up. A temperature drop during transit slows hydration, delaying set rather than causing early stiffening. Idle time at the site after loading can contribute to some set, but the defining factor for early stiffening in transit is the time coupled with a temperature rise during transport.