Concrete lasts a long time, even in heavy (weather) conditions. It is therefore suitable e.g. to make pit and manhole covers. Wooden copies become fast green, dangerously slippery and rot. Concrete also has the advantage that you can give it any desired shape and strength.
The mold you make or cover with smooth material, so that the concrete block can be removed easily after curing. The mold can be made of wood, a formwork. Or you can save it in a container with moist sand. Or dug into the ground. Or delimit with stones. Concrete is heavy, so the mold must be very firm. Before you pour concrete coat the inside with 2 layers of paper or foil. That you should stick or clamp so they are not sliding along when the concrete is filled. If you use smooth boards or planks (and possibly the setting can open) this is not necessary.
The lid will not be (or stay) tight if you do not reinforce it with steel bars. This can vary from chicken wire to rebar. Bend ends of iron in hook shape, so that rods not slide in the concrete. Make intersecting relationships in 2, depending on the shape and thickness even 3 dimensions, and bind them with iron wire to each other. You need at least 2.5 cm of concrete around the iron to avoid rusting. The iron itself may be rusted, that will give better adhesion of the concrete. Because it is isolated from the air it will not rust further.
Put some stones firmly under the reinforcement so that they are not at the bottom, but get stuck somewhere in the middle. Parts that are not cast will rust, and the expanding rust (up to 2 x steel volume!) will press the concrete apart and burst.
You can also first pour some concrete, put the reinforcement on it, and then continue molding.
Mold the concrete without trapping air bubbles. They weaken the concrete and give a chance to rust. You can float air from the wet concrete through vibrating. You can also knock against this form, or hold a sander against it.
Given concrete lids are very heavy you can make large specimens manageable by molding two halfs instead of pouring one complete.
Allow concrete to dry slowly. Cover it 48 hours with plastic or keep it moist. It is dry after a day or 4, and hard after 28 days.
Concrete lets, especially if it is sufficiently thick and dense, little water. Cracks (or joints) however do. Prevent cracks by uniform drying (unilateral or bilateral everywhere, even thickness of the parts, even temperature and humidity).
If you make a cover, it is useful to provide rings so that you can lift the lid. Concerned the rust problem that can best be galvanized, as well as the wire or rod that you anchoring the ring on. Preferably, you fasten that anchoring on the reinforcement.
If you make at the top of the concrete surface a hole that is large and deep enough, the ring can lie in it, and nothing protrudes above the surface.
You can also make boxes, poles, food- and water bowls etc. it this way. You can as in pottery, make sewage pipes and chimney. Keep for fitting work an eye on the shrinkage.
Much larger the object, much thicker the wall should be. You then better cast the work on the spot. Remember that a big thing will be too heavy. And will be hard or impossible to move. If the tank should stand outside then provide for holes through which the water can escape. Or, for a trough, fish tank,… foresee a tap so that you can let the water run out before winter. Otherwise the bucket freezes and breaks.
There are thus even built concrete boats. Water weighs 1 kg/dm3, concrete 2.5kg. The specific weight of concrete varies from not compacted concrete: 2.3 kg, over compacted to reinforced concrete: 2.5 kg. But by the empty space inside the displaced volume of water is greater than the weight of the boat (Archimedes). So it is perfectly possible. (As wel ass iron boats.) Even Hitler gave orders to build huge concrete tankers. Often concrete vessels were used as containers towed without its own drive. The walls were only 102 to 127 mm thick. Especially in wartime, with a shortage of wood and steel, this building technique lived on.
You can even make garden ornaments: elephant, sphinx, eagle... in concrete. Start with a skeleton of thin and pliable rebar. Modelling surrounded by chicken wire. Fill the inside with hay or old newspapers, that you can take out later. And finish the exterior with fine concrete or mortar.
To give fresh concrete to old concrete or stone better adhesion you can make the existing surface rough by cutting out pieces or grinding grooves or hammering steel nails in, or drilling a few holes. There are also commercially plastic emulsions to paint a kind of a bonding layer on the older rocks. Of this you can also mix in some new concrete.
Roman concrete
In the mid- 20th century concrete structures were designed to last 50 years. Now we design buildings that defy 100 to 120 years. Roman port facilities have survived 2,000 years of chemical attack and waves underwater. And without reinforcement. It is extremely resistant to fragmentation and nearly immune to corrosion by salt water.
The Romans made concrete by mixing limestone and volcanic rock. The difference is not in the fillers (sand and (little)stones), but in the binder cement and /or clinker.
For underwater structures, limestone and volcanic ash were mixed in order to form mortar, and were together with volcanic tuff packed in wooden forms lowered in the sea. The seawater had a reaction to this. The lime was hydrated and responded with the ash to cement all together.
Vitruvius, an engineer at Emperor Augustus, and later, Pliny the Elder wrote that the best marine concrete was made with ash from volcanic areas of the Gulf of Naples, near Pozzuoli. Ash with similar mineral properties, called pozzolana, is found in many parts of the world.
The research team of Monteiro found that Roman concrete differs essentially from the modern species. It has more aluminum and less silicon added.
The Roman recipe contains less than 10 percent of chalk (weight), and is burned to two-thirds or less (900°) of the temperature which is required for Portland cement (1450°). This makes the production more environmentally friendly.
Pozzolan is already by nature (volcanic) burnt earth that contains a lot of aluminum.
In reaction with lime it makes Al - tobermorite, that is giving high strength and durability.
Both the materials and the way the Romans used them are more durable than ours. Pozzolan can replace 40 percent of the world's demand for Portland cement.
And we use 19 billion tonnes of concrete per year!
The exact methods and compositions are, unfortunately, lost after the fall of the Roman Empire.