For small projects involving the use of concrete, such as repairs to a sidewalk or driveway, bags of premixed concrete can be economical alternatives to purchasing large quantities of premixed concrete. In many areas this product, available in bags of dry material already mixed, can be purchased in specialized shops for building renovations and building materials in general.
Steps
Step 1. Calculate how much pre-mixed concrete you will need for your project
Multiply the length by the width by the depth of the space you need to fill. This will give you the cubature - or volume - of the concrete you will need. Then divide the volume (in cubic meters) by the yield of each bag of the material you are going to use. Premixed cement is usually sold in bags of 5, 15, 25 and 50 kg, with the 25 kg bags having an average yield of 0.015 per cubic meter.
Step 2. Prepare the shapes you will need to keep the concrete in shape, then level and compact the ground or material you will be casting on
Place any iron reinforcements, and get ready for your concrete.
Step 3. Purchase the pre-mixed product you have chosen to use
Here are some examples of different commonly available blends:
- Blend of gravel, sand and Portland cement with a compressive strength of 3000 PSI (pounds per square inch). It is a basic and inexpensive type of concrete, suitable for most repairs and for the laying of poles.
- The 4000 PSI mixture is used for structural construction concrete or for related repairs, for example for sidewalks or paths, in which cases the greater resistance increases the durability of the finished surface over time.
- The 5000 PSI ready-to-set concrete mix is very resistant thanks to the higher ratio of Portland cement to fine and grainy aggregates, typically used in cases where quick setting is required and greater strength is desirable.
- Sand cement does not contain gravel or crushed stone (granular aggregates) and is used for grouting or coatings, where a smoother surface is preferable.
- Other mixes consist of premixed mortars, non-shrinking non-metallic-based fillers and high-strength concrete. These are special mixtures intended for specific uses not covered in this article.
Step 4. Collect all the materials you need to complete your project
For a complete list see the list below called "Things You Will Need", considering that dry premixed concrete, clean water, a shovel and a mixing container are essential.
Step 5. Open a bag of your premixed cement and pour it into the container
Wheelbarrows (as shown in the illustrations) are ideal for mixing small amounts of concrete. Avoid spilling dry material onto treated surfaces or garden grass, and as much as possible try to stay upwind to avoid inhaling product dust.
Step 6. On the dry material, in the center of the container in which you prepare the mixture, make a small depression or a hole using the shovel or a trowel
It will function as a basin to accommodate the water you are going to add. In this hole pour 3 liters of water for every 25 kg of dry mix. Don't worry about any spills or splashes, as all the contents of the container must be thoroughly mixed before you can use the concrete.
Step 7. Otherwise, if mixing concrete in a wheelbarrow, first pour the water and then add the dry premix
Portland cement, the essential ingredient in concrete, is poured over still water to initiate hydration, and not the other way around. Mixing is easier and easier if done with a shovel. Just the action of pouring the mixture into the water starts the hydration process without having to move the blade. The secret lies in the right amount of water that must be used for each bag.
Step 8. The water / concrete ratio is determined by the volume of Portland cement contained in the bag, not by the gross weight of the bag
This ratio is usually 3 liters per 25 kg bag. However, one fifth of a 15 liter cauldron is difficult to mix by hand with the shovel in a wheelbarrow with 25 kg of Portland cement, various aggregates and mixtures. In concrete mixers it is much simpler. With the shovel you would end up on the carpet mixing a resistant and semi-rigid mixture to the point of not being able to move. A better technique is to start by pouring 6 liters of water into the wheelbarrow, then dipping the first bag, and mixing it all together to obtain a fluid mortar, then adding the second bag as well, assuming you are physically strong enough to be able to mix the everything. Otherwise, pour half a 25 kg bag into 3 liters of water, mix thoroughly, and then add the rest, always mixing carefully.
Step 9. When mixing with the shovel, as strange and stupid as it may seem, the shovel goes like a paddle into the mixture, just as if you were going in a canoe
You stick the shovel into the mixture left over the water in the rounded front part of the wheelbarrow, and row backwards picking up the concrete and bringing it back to touch the water and dipping it in the front. All shovel movements must be done in such a way as to bring the cement mixture into contact with the water, since the chemical interaction between water and cement will do the rest. Repeat these movements continuously with the shovel, for as many times as necessary, until each grain of the mixture has come into contact with the water (usually 2-3 minutes), thus no longer finding any hidden grain of dry mixture. on the bottom, on the edges and nowhere else. You understand that you have mixed enough when you can grab a punch that won't hold its shape if you squeeze it, but which still isn't too dense either. If a ball forms, the mixture is too dry. If everything is leaking, it means that too much water has been added. The right mix for the strongest concrete lies in the middle between dry and liquid, and is only found with experience. The strongest concrete is the one made with 0.45 parts of water for each part of Portland cement.
Step 10. Mix the water and the material, using a shovel or trowel, so that all the material is wet
Add more water until the concrete takes on the malleable consistency you need to make your project. You must avoid making concrete that is too watery, or dense, since excess water weakens the resulting concrete, and would also allow the aggregates to separate from the rest of the mix.
Step 11. Continue mixing for a couple of minutes to make sure you completely even out the cement mix in the water
The concrete pulls through the hydration process, so continuing to mix the material ensures that the reaction kicks in completely.
Step 12. Cast the concrete in the form you have prepared, smoothing the surface with a shovel or other tool, so that any additional concrete quantity can be easily estimated and calculated
Step 13. After casting, spread the concrete and level it with a straight plank or a bar to level the concrete
To better compact the material, it may be necessary to tap the concrete with the tool used to level it, removing any air bubbles that may have formed when the casting was made.
Step 14. Finish the concrete according to your needs or those of the project
Step 15. Fence the area around the concrete to prevent passersby from walking on it (ruining your project), and allowing the concrete to harden and dry
Clean and tidy up tools, tidy up the area you've worked in, and throw away the empty bags when you're done.
Step 16. The right technique to adequately dry the concrete cast horizontally, so that it hardens slowly allowing maximum reliability and resistance, is to prevent the water added and mixed with the cement in the wheelbarrow from evaporating from the casting. just made and smoothed with a trowel
Step 17. A possible alternative to a shovel and wheelbarrow is to mix the mixture in a regular 20 liter bucket using a mixer drill with a "whisk" tip
This system is best suited for mixing mortar, but it also works with concrete. Fill the bucket a little less than 1/3 full with water and add a whole 30 kg bag of premixed cement, and mix everything.
Advice
- Think ahead about how you can get rid of any cement that may be left over.
- Before you begin, make sure you have a good source of water ready when needed. This will allow you to mix all the materials, clean the tools and remove any concrete splashes that may occur during the work.
- If you have an assistant (physically robust), the concrete can also be mixed and thrown easily with a large waxed sheet: pour the dry mixture on the sheet, then the water in the depression (as described above), and then with your helper lift holding the cloth at the four corners, shake the mixture and roll it until it is well mixed (this will take about 90 seconds). This system requires the strength to be able to keep a certain weight off the ground for a small amount of time, but many people find it very quick and simple.
- Buy easy-to-handle sized pre-mixed cement bags. 50kg bags, which need to be lifted several times, transported long distances or handled excessively, may be too heavy for you, so consider purchasing the product in smaller bags.
Warnings
- Cement can burn the skin in case of contact. So always cover up wearing suitable long-sleeved clothing, trousers and gloves.
- When handling concrete, wear a mask or respirator, safety goggles, and chemical resistant gloves.
- Cement mixes can harden much faster than you might expect, so be prepared and get help if needed.
