If concrete is placed when the weather is such that the temperature of the concrete would be less than 50°F, as determined by the engineer, the contractor shall employ effective means such as heating of aggregate and mixing water as necessary to maintain the temperature of the concrete as it is placed above 50°F. All surface with which concrete is to come in contact with must be free from frost or ice. The temperature of the concrete must be maintained above 50°F for a period of five days.