I am a concrete producer in the Upper Midwest and always must adjust my product for very hot weather conditions in the summer and cold temperatures in the winter. What advice do you have for producers like me who must figure how cool to make our aggregate in extreme weather conditions?

To answer this question, we turned to Brian Walker LDP engineer at Pizzagalli Construction Co., and formerly a construction management senior at Southern Illinois University Edwardsville, and Luke Snell, professor of construction management and director of the Concrete Construction Resource Unit at Southern Illinois University Edwardsville.

Many times concrete must be placed in less than ideal weather conditions. Specifications typically require that fresh concrete temperatures be maintained between 40 F and 90 F. Thus, the batch plant must develop procedures on how to produce a concrete mixture within these temperature ranges.

In cold weather, the batch plant operators have a couple of options of how to maintain the concrete temperatures above 40 F. They can heat the aggregate as some batch plants so that the aggregate will not freeze and restrict the flow through the bins/gates. Or they can use heated water.

In hot weather, the concrete temperature can be normally kept below 90 F by cooling the aggregate, usually by putting a sprinkler on the aggregate pile and allowing the evaporation process to cool the aggregates. Or you can use by chilled water or ice.

Although there are several ways concrete temperatures can be controlled, the method used at most batch plants is to control the temperature of the water. The American Concrete Institute (ACI) has developed formulas in ACI Committee Reports 305 and 306 to predict the temperature of fresh concrete.

These formulas use the proportions and the related specific heat of each material to determine the overall concrete temperature. Specific heat of both aggregates and cementitious materials is about 0.22. Water has a specific heat of 1.0. This is why the formula has the summation of the weight of each material multiplied by the temperature and then multiplied by the specific heat of the material. The formulas used to determine concrete temperatures are shown below:

Cold Weather

T = [0.22(TsWs + TaWa + TcWc) + TwWw]
[0.22 (Ws + Wa +Wc) + Ww]

T = final temperature of concrete mixture (deg F)
Tc = temperature of cement (deg F)
Ts = temperature of fine aggregate (deg F)
Ta = temperature of coarse aggregate (deg F)
Tw = temperature of added mixing water (deg F)
Wc = weight of cement (lb)
Ws = weight of fine aggregate (lb)
Wa = weight of coarse aggregate (lb)
Ww = weight of mixing water (lb)

Note: The equation was modified. Temperatures and the amount of free moisture on the aggregates were not used. The coarse and fine aggregates are entered separately.

Hot Weather

T = [0.22(TsWs + TaWa + TcWc) + TwWw ¡V 112Wi]
[0.22 (Ws + Wa + Wc) + Ww + Wi]

T = final temperature of concrete mixture (deg F)
Tc = temperature of cement (deg F)
Ts = temperature of fine aggregate (deg F)
Ta = temperature of coarse aggregate (deg F)
Tw = temperature of added mixing water (deg F)
Wc = weight of cement (lb)
Ws = weight of fine aggregate (lb)
Wa = weight of coarse aggregate (lb)
Ww = weight of mixing water (lb)
Wi = weight of ice (lb)

Note: The equation was modified. Temperatures and amount of free moisture on the aggregates were not used. The coarse and fine aggregates are entered separately.

Transportation Temperature Factors
For revolving drum mixers
T = 0.25 (tr-ta)
For covered dump body
T = 0.10 (tr ¡V ta)
For open-dump body
T = 0.20 (tr ¡V ta)

T = temperature drop to be expected during a 1-hour delivery time, deg F. (This value must be added to determine the required temperature of concrete at the plant).
tr = concrete temperature required at the job (deg F)
ta = ambient air temperature (deg F).

As you can see, these formulas are cumbersome; they use complicated mathematical formulas, so many people do not use them.

A better way
The Concrete Construction Resource Unit (CCRU) at Southern Illinois University Edwardsville (SIUE) has developed a program that uses Microsoft Excel to predict concrete temperatures with the above formulas. The user simply enters the information of the mix design and temperatures for each of the materials. The program will provide the temperature of the fresh concrete.

The user is then asked if this temperature is acceptable. If the temperature is not acceptable (out of specifications), the user enters the required fresh concrete temperature that must be achieved. The program then calculates the temperature of the water that is needed to achieve the required fresh concrete temperature.

The program allows the user to account for the temperature change due to transporting the concrete to the jobsite. The user enters the air temperature and is then given the new adjusted water temperature required for the type of equipment used to deliver the fresh concrete. The hauling options are revolving drum and open or covered dump trucks.

Example 1: Cold Weather Application

The concrete mixture is comprised of the following ingredients at these temperatures:
Course aggregate is 1674 pounds at 40 F
Fine aggregate is 1236 pounds at 32 F
Cementitious material is 643 pounds at 80 F
Mixing water of 270 pounds at 45 F.

After selecting cold weather concreting (because the temperatures of the materials were close to 40 F) and entering the data, the program calculates the fresh concrete temperature to be 45 F (Figure 1).

The specifications for the project require a concrete temperature to be at least 55 F; when this is entered; the program calculates the required water temperature to achieve this concrete temperature to be 86 F (Figure 2).

The program now asks you the ambient (air) temperature. If your air temperature is 30 F, the program calculates that the temperature of the water (for revolving drum mixers) must be 110 F (Figure 3).

Example 2: Hot Weather Application

The following is an example of hot weather concreting. The concrete mixture is comprised of the following ingredients at these temperatures:

Coarse aggregate is 1674 pounds at 100 F
Fine aggregate is 1236 pounds at 80 F
Cementitious material is 643 pounds at 140 F
Mixing water is 270 pounds at 75 F

After selecting hot weather concrete (because the temperatures of the materials were near 90 F) and entering the data, the program calculates the fresh concrete temperature to be 94 F (Figure 4).

The specifications for the project require a concrete temperature to be below 90 F. The program calculates the required water temperature to achieve this concrete temperature to be 60 F (Figure 5).

The program now asks you the ambient (air) temperature. If the air temperature is 100 F, the program calculates that the temperature of the water (for an open dump truck) to be 54 F (See Figure 6).

This program can be downloaded from www.siue.edu/CCRU/research.htm. Select the options entitled