Hot Weather Concreting
|Publication Date:||1 January 1999|
Hot weather may create problems in mixing, placing, and curing hydraulic cement concrete. These problems can adversely affect the properties and serviceability of the concrete. Most of these problems relate to the increased rate of cement hydration at higher temperature and increased evaporation rate of moisture from the freshly mixed concrete. The rate of cement hydration is dependent on concrete temperature, cement composition and fineness, and admixtures used.
This report will identify problems created by hot weather concreting and describe practices that will alleviate these potential adverse effects. These practices include suggested preparations and procedures for use in general types of hot weather construction, such as pavements, bridges, and buildings. Temperature, volume changes, and cracking problems associated with mass concrete are treated more thoroughly in ACI 207.1R and ACI 224R.
A maximum "as placed" concrete temperature is often used in an effort to control strength, durability, plasticshrinkage cracking, thermal cracking, and drying shrinkage. The placement of concrete in hot weather, however, is too complex to be dealt with by setting a maximum "as placed" or "as delivered" concrete temperature. Concrete durability is a general term that is difficult to quantify, but it is perceived to mean resistance of the concrete to weathering (ACI 201.2R). Generally, if concrete strengths are satisfactory and curing practices are sufficient to avoid undesirable drying of surfaces, durability of hot weather concrete will not differ greatly from similar concrete placed at normal temperatures. The presence of a desirable air-void system is needed if the concrete is going to be exposed to freezing cycles.
If an acceptable record of field tests is not available, concrete proportions may be determined by trial batches (ACI 301 and ACI 211.1). Trial batches should be made at temperatures anticipated in the work and mixed following one of the procedures described in Section 2.9, Proportioning. The concrete supplier and contractor are generally responsible for determining concrete proportions to produce the required quality of concrete unless specified otherwise.
According to ASTM C 31/C 31M, concrete test specimens made in the field that are used for checking adequacy of laboratory mixture proportions for strength or as a basis for acceptance or quality control should be cured initially at 60 to 80 F (16 to 27 C). If the initial 24 h curing is at 100 F (38 C), the 28-day compressive strength of the test specimens may be 10 to 15% lower than if cured at the required ASTM C 31/C 31M curing temperature (Gaynor et al 1985). If the cylinders are allowed to dry at early ages, strengths will be reduced even further (Cebeci 1987). Therefore, proper fabrication, curing, and testing of the test specimens during hot weather is critical, and steps should be taken to ensure that the specified procedures are followed.