News

Strengths and weaknesses

Collecting the next bucket of stone

| Paul Gillis |

At the Stevens Brook Culvert Replacement project, the old concrete culvert has been removed and work installing the base for the new culvert is ongoing. Delivery trucks haul in 3/4″ stone, which is removed by the excavator and placed where the precast concrete culvert will go. The excavator operator has only 2″ to 3″ between the truck sides and the 2 cubic yard bucket in order to remove the stone. Operator skills and experience make such work possible. The excavator operator then levels the stone before the vibratory plate compactor is used to compact the stone. Vibrating at up to 4,000 times per minute, the compactor moves small particles into the areas between the larger stones and moves all the particles into a denser base.

Concrete and concrete reinforcement have come a long way in the last 94 years. Long ago, reinforcement bars were smooth, as can be seen in these photos. Later ridges were added to make the concrete adhere better to the reinforcement bar (rebar) and prevent the rebar from sliding in the concrete.  Epoxy coatings can now be applied to the rebar, particularly in bridges, to help prevent the rebar from rusting.

Why bother with rebar at all? Concrete is very strong when compressed (squeezed together), typically 3,000 to 5,000 pounds per square inch (PSI). It is much weaker when in tension (pulled apart) or flexed, perhaps only 10% to 15% of its compressive strength, say 300 to 500 pounds PSI. The steel reinforcing bars are used to provide concrete with the tension strength necessary, making reinforced concrete a great composite material.

Unfortunately, when water seeps into cracks in the concrete, the seasonal freeze/thaw cycles cause it to spall, and fall away. This, in turn, causes the structure to weaken. Such was the condition of the former Stevens Brook Culvert.

Quiz Time:

Question 1:
— If 3/4″ stone weighs 100 pounds per cubic foot, what weight of stone can a 2 cubic yard excavator bucket hold?
A. 2,700 pounds
B. 5,400 pounds
C. 8,100 pounds

Question 2:
— If a wheelbarrow holds 2 cubic feet, how many wheelbarrow loads would it take to equal one 2 cubic yard excavator bucket?
A. 14 loads
B. 27 loads
C. 54 loads

Smoothing the stone | Photos: Paul Gillis