Thermal Analyser

This brochure outlines BCRC’s services in the area of thermal analysis of concrete and incorporates details of “Thermal Analyser”, a mathematical model for predicting the insitu temperature of concrete. It was developed by F.Papworth in conjunction with Curtin University.

Background

Hydration of cement is exothermic. This leads to a temperature rise in freshly placed concrete which can be of concern in thick structures (>600mm) for the following reasons:

• High temperatures at the centre of the pour may lead to
  • loss of strength (up to 30%)
  • microcracking
  • delayed ettringite formation (long term cracking)
• High temperature differentials between the centre of the pour and the surface may lead to differential movement and associated cracking
• Where the fresh concrete is restrained by existing concrete it can crack as it cools.

The control of mix design, placement temperature and placing methods can minimise temperature rise and temperature differentials and can reduce the strains that occur.
To determine if damage will result from the concrete’s insitu temperature cycle three things must be assessed in series i.e.

• the concrete’s adiabatic temperature rise (i.e. its heat generating fingerprint)
• the insitu temperature profile at any time for the specific concrete pour
• the strain and strain capacity of the element

Where the thermal strain combined with the early drying shrinkage strain exceeds the concrete’s strain capacity the reinforcement ration required to control cracking must be assessed.

Apart from providing an assessment of thermal effects on durability the temperature predictions are used to assess maturity.  Advice is provided on whether the internal accelerated curing can give construction advantage eg early cessation of curing and early stripping.

Adiabatic Temperature Rise Prediction

For adiabatic temperature rise a concrete sample is placed in an environment (oven or oil bath) maintained at the same temperature as the sample. This tricks the concrete into thinking it is at the centre of an infinite block as no heat is lost from the system. This is cumbersome, expensive and takes time.
As an alternative BCRC use a program called HYMOSTRUC to predict adiabatic temperature rise. The hydration process is simulated numerically as a function of:

• particle size distribution of the cement
  • the measured particle size distribution
  • the Rosin-Ramler coefficients b and n
  • the specific surface of the cement (Blaine Value)
• type of cement (C3S and C2S content and potential heat of hydration Qpot) . Models GP cement only
• water/cement ratio
• initial mix temperature
• (concrete) mix composition

The adiabatic hydration curves is used as input for “Thermal Analyser” where GP cements are used. “Thermal Analyser” has adiabatic temperature rise curves for a number of mixes built in. From these a first approximation of insitu temperatures can be obtained and this is often sufficient for construction purposes.

Insitu Temperature Rise Calculation

“Thermal Analyser” uses adiabatic temperature profiles and heat transfer principles for analysis of the temperature profile over time within an element. 

Figure 1 shows the data input and results summary screen. Only one case is shown. In the programme five cases are shown on one screen enabling a comparison of the effects of stripping time, insulation, mix and other variables.

Key input details are; time of shutter/insulation installation and removal, type of shutter, pour layers details and environment details. Daily temperature is input as a maximum and minimum from which the programme calculates the sinusoidal ambient temperature curve. The output provides the temperature differential and maximum temperature rise in graphical format so that the trends can be easily seen. Graphs of temperature profile through the concrete can also be viewed.

BCRC’s Construction Solutions

Where the maximum temperature in the bulk concrete is too high temperature can be reduced as follows:
Maximum temperature exceeded:

• replace cement with fly ash or slag to delay hydration giving heat time to escape. This is effective in pours 600-2500 mm thick. In thicker pours the efficiency decreases as the heat can not escape (figure 2).

• replace 3-4 parts of cement with 1 part silica fume to reduce the cementitious content & total heat evolved at all thicknesses.
• Minimise insulation
• Use a water pipes to cool the centre of the pour
• Use chilled water and/or cool aggregates to reduce concrete starting temperature

Temperature differentials to high

• The differential between the bulk concrete and the top and bottom surface needs to be considered. The ground (or blinding concrete) provides some natural insulation (figure 3) but this may not be adequate to prevent cracking.
• Consider an aggregate with lower thermal expansion coefficient
• Use a cement system with higher creep properties
• Use surface insulation to keep the surface of the concrete hotter. The analysis should indicate when insulation can be removed as if it is removed to early the thermal shock will create a worse condition than not insulating at all as the restraint conditions will be higher (figure 4).

Monitoring

BCRC are able to undertake insitu monitoring to measure the maximum temperature rise and differential. They also have 32 channel strain loggers to record thermal strains. However the intelli-rock temperature loggers are often the most efficient means for the contractor to log and report temperatures.

Recent Projects

• Thermal analysis of Jervoise Bay wharf decks to 3m thick
• Thermal analysis of various slab on grade projects
• Thermal analysis of Wellington Street station foundation
• Thermal analysis of Mt Henry bridge pile caps and piers
• Thermal analysis of Ravensthorpe mine foundations up to 5.5m thick
• Thermal analysis of Paraburdoo rail foundations
• Thermal analysis of Thornlie rail bridge pile cap