As a child, you probably heard of the story The Three Little Pigs. This story taught us as a child that having a house made out of sticks or hay was not a suitable option for a home. However, it did teach that bricks made with concrete was a viable option to endure external forces such as the “huff and puff” from the big bad wolf.  Even though concrete is an appropriate building material, the variability of the strength in the concrete may not be proper for different forces such as earthquakes to strong winds equating to a tornado. To determine what is proper for different sizes and shapes of building, the strength of the concrete is essential.

          To determine the strength of the concrete there are many aspects of strengths such as compression, tensile, flexural, and shear strength.  According to Ken Tam, Manager of Technical Services at Clifton Associates, he explains, “the compression strength is usually measured in mega-pascals (MPa) at an age of 28 days. The most general used concrete has a compressive strength between 20 MPa and 35 MPa. High strength concrete will have compression strengths of at least 40 MPa. Some buildings that have a foundation of concrete will have compression strengths of 140 MPa” (K. Tam, personal communication, October 8, 2013).

Testing a 150x300-mm concrete cylinder in a compression machine

          Here is a video of a compression test on a concrete cylinder done by Koury Engineering, http://www.youtube.com/watch?v=AR_fjmV2Mpk.

          Normally, the tensile strength is approximately 8% to 12% of the compression strength. It is often estimated as 0.4 to 0.7 times the square root of the compressive strength. Similarly, 0.7 or 0.8 times the square root of the compressive strength defines the flexural strength. Moreover, the sheer strength can vary from 35% to 80% of  the compressive strength. Tam explains that there is a “correlation between compressive strength and flexural, tensile, and shear strength. It varies from the concrete ingredients.”

          As Tam described, ingredients are the most influential factor that gives variability to the strengths of the concrete. Concrete is essentially a mixture of two components called aggregates and paste. The paste is usually comprised of cement and water, which binds to the aggregate. Generally, the aggregate is sand and gravel or crushed stone. Once mixed, the paste will harden because of the chemical reaction that will occur from the cement and water.

          Aggregates are divided into two different groups called fine and coarse. The fine aggregate “consists of natural or manufactured sand.” Compared to the coarse aggregate, which are particles that are 1.25 mm to 56 mm. Tam further states that, “the most commonly used maximum aggregate size is 20 mm”

Ken Tam testing and recording the compression strength of the cylindrical concrete

          

          The strength for the concrete varies for the intended use. Some concrete with compression strengths of less than one MPa can be used for insulation of underground steam lines. Others such as roof fills are between 0.7 and 1.5 MPa. For a foundation of an average household is near 25 MPa. However, factors such as the environment could potentially change the compression strength acceptable for the building.

           If you have ever been in a tall building, there is a sense of motion that the building is swaying back and forth. This sway is due to the external forces acting upon the building. The external force is wind, which is measured in knots in the Beaufort Wind Scale. The higher the building, the more force is exerted on the top of the building because stronger winds exist at higher altitudes. Furthermore, as Tam explains, “the building becomes less stable the higher it becomes because it loses more of its center of gravity.” Because the building retains less of a center of gravity, the sway becomes increasingly prominent. Tam further describes the sways when he states, “the sway of the building can move upwards of three meters to each side. This is depending on how strong the wind is.”

          With a force of two on the Beaufort Wind Scale, which is four to six knots, the higher buildings will have no trouble withstanding these external forces. However, when the wind becomes stronger and has a force of nine classified as a strong gale, which is 41 to 47 knots, the building could potentially crumble to the earth. This is where the compression strengths of concrete play a major role in the building. With higher compression strengths for the foundation of the building, the more pressure it can tolerate from the external force.

          The main strengths that are essential for the concrete is the flexural, tensile, and compressive strength. The flexural and tensile strength is necessary for the swaying motion the building achieves from being pushed by the strong gale. If the building does not achieve the correct flexural and tensile strength, the concrete will slowly start to crack from the pressure and eventually lead to the building to tumble down. Moreover, the compression strength is highly important because of the stress the swaying motion causes to the overall building. Tam states, “As the wind acts upon the building, the weight shifts from side to side. The compression strength needs to be able to withstand these shifts in weight.”

A broken concrete cylinder with a compression strength of 22.4 MPa

          Since higher the building the more sway and unstable it is, therefore, the strength of the concrete need to dramatically increase. To make these stronger concretes “they must add less water to the mixture because with less water the stronger the concrete will become. They further will add chemical additives to enhance the strength.” With the proper aggregate and paste, the right strength will be achieved.

          Other external forces affect buildings and homes as well such as earthquakes. Earthquakes are measured on the Richter scale from 1-10. The Richter scale is calculated from the amplitude of the largest seismic wave recorded on the earth and is based on a logarithmic scale with a base of ten.

          A level five earthquake on the Richter scale can be enough to completely demolish a household. To avoid serious damage to a house, compression, tensile, and flexural strength is key. The tremors caused by the earthquake will violently shake the house causing the building to vibrate back and forth at a high frequency. This will cause the concrete to crack under pressure, but with a high enough flexural and compression strength, the building will be able to endure these movements because the concrete will fluctuate with the vibrations and the compressive strength will aid in the shifting weight. The tensile strength will prevent the concrete from cracking under the massive pressure exerted from the earthquake.

          A video was taken of a building swaying in the aftershocks of an earthquake, http://www.youtube.com/watch?v=uGYyZxKy4PI.

A seismograph and the Richter scale on the earthquake

          Overall, the strength of concrete is vital in buildings because it gives the foundation towards the building and prevents large buildings from breaking apart. However, even though the strength of concrete is important, against earthquakes the concrete will not be able withstand high levels on the Richter scale because of how powerful they are, but to wind it is crucial. Large buildings in particular need specific strengths of concrete for them to withstand the external forces that are exerted on them every day. Even households need certain strengths of concrete for the external forces that affect them on a daily basis. With the proper strength of the concrete, these external forces such as the “huff and puff” of the big bad wolf will keep your house standing. 

By Benson Tam and Tim Doolotbek