MultiMechanics Blog

Why so Hard? Tips for the Analysis of Mass Concrete

Posted by MultiMechanics on Mar 10, 2015 1:58:36 PM

What’s the most used composite in the world?

While advanced composites (like carbon and glass fiber reinforced plastics) are the most popular “composites” out there, they are certainly not the most used. That title belongs, resoundingly, to concrete.

Concrete is used more than any other man-made material in the world and is the 2nd most consumed substance on earth behind water.  As of 2012, the concrete industry was worth around $40 billion (compared to $14.7 Bil for reinforced plastics) and it employed more than 2 million employees in the United States . About 10 billion tons of concrete are produced every year.

Barrage_de_Roselend_Dam

Similar to Fiber Reinforced Plastics, concrete is subject to the same complex behavior and failure mechanisms.

 

Concrete Failure

concrete_cracks

Failure of mass concrete applications results from distresses in the material, such as permanent deformation and fatigue cracking. These distresses may result from many factors, such as improper choice of materials, excessive loads, environmental effects, and aging.  In massive structures or the ones where safety requirements are extremely stringent, such as nuclear reactors and dams, the risk of manufacturing-induced cracking must be accurately evaluated.

In many cases the cracking of the concrete at early ages is often the result of volume variations due to temperature changes during the hydration process.

The dimensions of mass concrete structures cause large temperature variations and differentials between the interior and the outer surface of the structure. These volume changes result in strains and stresses, which can cause an unacceptable cracking state and compromise of the performance or appearance of the structure.

In other words, the cracks within the structure are forming before the concrete is even dry. Therefore, the central problem of the mass concrete is to quantify the generation of heat, the volume changes and the evolution of the properties due to hydration. 

Because of MultiMech's Chemo-Thermo-Mechanical coupling, we are able to simulate this hydration process for mass concrete structures. 

 

Manufacturing Variables

stack_concrete

When performing concrete analysis, the potential for cracking is directly affected by five main variables: 

  1. Maximum temperature - the higher it is, the greater the likelihood that cracks will occur.
  2. Ambient temperature - helps define the temperature gradient
  3. Cooling rate - the  higher the cooling rate, the greater the cracking potential
  4. Height of the layers of concrete
  5. Time between the pouring of layers

Using MultiMech, these variables can be accurately captured.

Example Use-Case in MultiMech

Consider a concrete cube with 0.4m sides. After preparation, concrete is poured in wood mold. The lateral mold is removed at 8.6h. Concrete initial temperature is 26C and room temperature is 20C. Temperature is monitored using both infrared thermography as well as embedded sensors, as shown in Figure below. Thermography as a technique for monitoring early age temperatures of hardening concrete. Monitoring data is provided in M. Azenha, R. Faria, H. Figueiras (2011) 

Heat generation rate is measured experimentally for this specific concrete mixture using an isothermal
calorimeter. The data is curve fitted into our thermally activated model – meaning that the heat
generation rate is affected by temperature. MultiMech's fitted model is compared below along with
experimental measurements for different temperatures. The run time for this simulation was 1 minute on a standard desktop computer. 

 

cube_concrete

 

conc_res2

 

 

 

 

 

 

 

 

 

 

This type of modeling is just the tip of what MultiMech is capable of. If you are modeling a heterogeneous material, odds are we can help.  Reach out to learn more.

 

  MultiMechOpener

Topics: Composite Analysis, Concrete