Cement can only be shipped wet, and it must be constantly moving to prevent the cement from hardening in transit. The cement can—and does—slosh inside the barrel of the cement truck, leading to an unstable load and high risk of rollover.
What do you do when concrete hardens in the truck? First of all, you do everything you can to prevent that from happening. If it happens anyway, it is necessary to get inside the mixer drum with an air hammer and break it out.
Cement trucks deal directly with this issue. The barrel, also known as the drum, prevents the concrete from drying as blades and the turning of the drum continuously mix the concrete throughout transportation. This process is called charging.
As a general rule of thumb, thirty to sixty minutes of transportation are acceptable on small jobs. At a central or portable ready-mix plant, concrete should be discharged from a truck mixer or agitator truck within two hours. If non-agitating transporting equipment is used, this time is reduced to one hour.
When the constituents of concrete (cement, aggregate and water) are brought together, the cement and water chemically react and eventually set to form a hard mass which adheres to and binds together the aggregates to form concrete.
Can I still use my cement if some has gone hard or there are lumps in the bag? No, we would not recommend that any cement containing lumps is used. Lumpy cement will not create a homogeneous mix. There is also a good chance it may not gain as much strength as you would expect.
RUB TEST: In this test, one must take a little cement between their fingers and rub it. If the texture feels smooth, it's considered good quality cement. If the texture is rough, it is likely adulterated with sand.
The ASTM C 94, Specification for Ready Mixed Concrete, indicates that the concrete shall be discharged on the job site within 90 minutes and before 300 revolutions after water was added to the cement. The purchaser may waive this requirement, when conditions permit.
When the latter takes place, those trucks transport material to the job site with help from one or more blades inside the rotating truck drum. The blades agitate the material through a process known as “charging” in order to prevent the concrete from hardening while en route.
One of the simpler methods is to simply dump the remaining concrete and let it harden; then crush it and sell it for base material. More complex reclamation methods, which also incorporate disposal of truck wash water, are based on screw classifiers or drag tank systems to separate out and wash the aggregates.
Portland cement reacts with water to form an amorphous paste through a chemical reaction called hydration. In concrete the formation of pastes causes the mix to harden and gain strength to form a rock-like mass.
Many people mistaken concrete trucks for cement trucks. The revolving cylindrical container or “barrel” that you see attached to trucks actually contains concrete, rather than cement. You often see these concrete trucks on site with workers so they can lay the concrete to build a parking garage or a highway.
However, concrete never stops curing and just gets stronger over the years.
Concrete driveways are typically poured four inches thick, giving the concrete driveway enough strength to withstand loads of regular vehicles (up to approximately 8,000lbs). A fully loaded concrete truck can weigh over 70,000lbs, almost nine times heavier than your driveway was meant to withstand.
How long does it take to empty a concrete mixer truck. Generally, it takes about six minutes to unload a yard of concrete.
The cement inside the truck hardens in around 90 minutes. If the driver does not make it to their destination on time, it can mean that the barrel inside has to have the hardened cement removed by using chisels, hammers, or even a jackhammer. That can slow down the driver's ability to make another run.
This happens because the dried cement fills the inner section of the drum. As a result, you won't be able to fill the drum as full as usual. Consequently, this dry cement also makes the truck heavier which in turn requires more fuel and isn't very cost-effective.
What Causes Slow Drying? Concrete releases moisture from the slab's surface as it dries. But if there is too much moisture in the air—also known as high humidity—the moisture from the concrete won't evaporate quickly. And instead, too much moisture in the air will slow down the drying process of concrete.
What is the ideal cement mix ratio? As per the experts, the safest cement mix ratio for all kinds of concrete used in different construction purposes is 4:2:1. It means four parts of crushed stones mixed with two parts of sand and one part of cement will create the ideal cement mix to prepare concrete for all purposes.
The 10-20-30-40 Rule. Let's start by discussing the essential ingredients of concrete. A typical concrete batch, by volume, contains roughly 10% cement, 20% water and air, 30% sand, and 40% gravel. The exact proportions vary up or down a little, but 10-20-30-40 is a good rule of thumb.
Concrete is usually dry enough after 24 to 48 hours to walk on. For concrete to dry and reach its full strength, it typically takes about 28 days per inch of slab thickness. Once conditions are conducive for the concrete to cure at 85-90% relative humidity.
Generally, for large infrastructures, the lifespan of modern concrete is about 100 years if properly maintained.
In short, the difference between cement vs concrete is easy to clear once we know all the basics of their compositions. Cement is a binding agent made from limestone and clay; whereas concrete is a building material created through the proper mixing of cement, water, sand, and rock.
We strongly recommend that you do not use any cement that has exceeded its use by date.