This video talks about the importance of reinforcement on concrete for construction. Normally concretes can take high stress from compression but can barely take tension force, from the experiment in the video, we can tell the beam breaks from the bottom suddenly. in this case, adding reinforcement such as rebar during the casting of concrete can increase its tension, in the second experiment, shows not only this new beam can hold higher stress but it breaks very slowly. The video also talks about active reinforcement that gives the steel stress(stretching the rebar) before the beam is cured, this can activate the tension resistance of the reinforcement.

This video talks about how sinkholes are formed. Basically, water flow is the main factor for erosion, a process of taking soil and rock from one place to another place. One huge factor for why water is the biggest eroder is because it can dissolve many elements. For those sinkholes to happen, water will dissolve rocks and clay underground, creating space for soiling on the surface to flow away and this flow can also change the internal structure of that area. This process will continue until there is no more soil to flow away, and a sinkhole will appear at that time. With this principle, we can understand how sinkholes are formed in recent years. It is because the disconnection fo pipes under the ground give water space to create internal space and erosion.