Final answer:
Cold fudge and cold caramel are used in a concrete instead of hot sauces to prevent the custard from melting prematurely.
Step-by-step explanation:
Cold fudge and cold caramel are used instead of hot fudge and hot caramel when making a concrete because hot sauces can cause the custard to melt prematurely due to the increased temperature. By using cold sauces, the temperature of the custard remains lower, allowing it to stay solid for longer. This ensures that the concrete is more stable and doesn't melt too quickly.