Concrete is the most widely used building material in the world, and unlike steel or brick, it is made on site from a handful of raw ingredients. It is a composite material composed of fine and coarse aggregate bonded together with a fluid cement paste that hardens, or cures, over time. When aggregate is mixed with dry cement and water, the mixture forms a fluid slurry that is easily poured and molded into shape. The same material that forms a structural column can be finished into decorative surfaces, which is why products from colorful concrete tiles to polished industrial floors all start with the same basic mix.
Concrete is a site-made construction material unlike other materials of construction, and as such it can vary to a very great extent in its quality, properties, and performance. This article covers what concrete is, the role of each ingredient, how to mix it, and why supervision during mixing decides whether the finished product meets its design strength.
What Is Concrete?
Concrete is defined as the uniform mixture of cement, fine aggregate, coarse aggregate, and water in the appropriate proportion, which sets, hardens, and acquires strength after a particular period. To make concrete of good quality, you need a working knowledge of the different ingredients, because each ingredient contributes different properties to the finished mass. The quality of the paste determines the characteristics of the concrete, and the paste is only as good as the proportioning and mixing that produced it.
The Paste, the Aggregate, and the Voids
A concrete mixture which does not have enough paste to fill all the voids between the aggregates will not be possible to place and compact the concrete properly, leaving honeycomb and air pockets. A mixture with an excess of cement paste will be easy to place and will produce a smooth surface, but it costs more and can shrink more as it cures. The aim is a paste volume that coats every particle and fills every void, with nothing left over. The challenge of getting paste into every gap is why contractors use vibration and other methods to consolidate concrete in congested reinforced concrete members, where rebar leaves little room for the mix to flow.
The Three Components of Concrete
There are mainly three basic components which are used to make the concrete mix: cement, aggregate, and water. Each one has a distinct job, and the proportions between them control everything from workability to final strength.
Cement
Cement is one of the most common and widely used materials for making concrete, and it acts as the binder. Cement is initially in powdered form, and it acts as a binder after adding water: the chemical reaction, called hydration, hardens the paste, sets the mix, and adheres to the other materials. The cement paste coats each aggregate particle and bonds them into a solid mass as it cures.
Aggregate
Aggregate is the construction material which is used for the manufacturing of concrete and provides the concrete with its body and strength. Aggregates acquire 70 to 80 percent of the concrete volume, give body to the concrete, and reduce the shrinkage in concrete while acting as a filler to give a homogeneous mass along with the cement paste. The surface texture of the aggregate may be smooth or rough. Rounded aggregate with a smooth surface will require less cement paste and will increase the yield per bag, but rough-textured aggregate is generally recommended because it bonds more strongly with the paste.
Water
Water starts the hydration reaction and makes the mix workable. The strength of the concrete depends on the ratio of water to cement: the water-cement ratio is the weight of the water divided by the weight of the cement. High-quality concrete is produced by lowering the water-cement ratio as much as possible without reducing the workability of fresh concrete, so it can still be placed, compacted, and cured properly. Excess water makes the concrete easy to pour but leaves voids as it evaporates, and those voids become weak points. The importance of the water-cement ratio is why standard testing uses precisely made concrete cube samples to verify that a mix meets its specified strength.
| Component | Share of volume | Job in the mix |
|---|---|---|
| Cement | 10 to 15 percent | Binder that hardens and adheres |
| Aggregate | 70 to 80 percent | Body, strength, and shrinkage control |
| Water | 15 to 20 percent | Hydration and workability |
How to Mix Concrete
To achieve a strong and durable concrete, the careful proportioning and mixing of the ingredients are required. The mixing process determines whether the materials work as one mass or remain separate pockets of paste and stone. A practical mixing sequence for site work looks like this:
The Mixing Sequence
- Measure the cement, fine aggregate, and coarse aggregate to the mix ratio specified for the job.
- Mix the dry materials first until the color is uniform.
- Add water gradually while mixing, stopping at the target water-cement ratio.
- Mix until the paste coats every aggregate particle and the batch has a uniform consistency.
- Place the concrete within the working time of the mix, usually 60 to 90 minutes.
- Compact the concrete to remove trapped air, then finish and cure the surface.
Mixing by Hand vs. Machine
Small jobs can be mixed by hand in a wheelbarrow or mortar pan, but machine mixing produces a more uniform paste in less time. Drum mixers tumble the materials so the paste reaches every particle, and the drum keeps rotating during placement to prevent segregation. For large pours, ready-mix trucks batch the concrete at a plant and keep it agitated on the way to the site, which is why delivery distance and travel time are written into the specifications.
Consistency between batches matters as much as the mix itself. If the second batch is wetter or drier than the first, the wall or slab will cure unevenly and may show different strengths in different zones. The same care applies when concrete is placed against an older surface: the preparation steps for pouring new concrete over old concrete surfaces focus on bonding the new layer to the old, which requires a clean, roughened, and damp substrate.
Why Supervision Matters During Concrete Mixing
Concrete is a site-made construction material, which means the quality control happens at the mixer, not in a factory. Supervision during mixing catches the small mistakes that ruin large pours: an extra bucket of water, aggregate that never got mixed through, or a batch that sat too long before placement. A qualified person should watch every batch and reject any mix that does not match the approved proportions.
What the Supervisor Checks
- Water-cement ratio: the single biggest control on strength and durability.
- Consistency: the batch should look uniform, with no dry pockets or excess water sheen.
- Slump: the standard workability test, checked on fresh concrete at the point of delivery.
- Mix time: enough revolutions in the drum to blend the paste completely.
- Records: batch tickets and test samples logged for every pour.
Testing the Hardened Result
Supervision does not end when the concrete is placed. Cube samples are cured and tested at 7 and 28 days to confirm the mix reached its design strength, and the full sequence of checks from fresh properties to hardened performance is documented in post-concrete inspection and testing procedures. A pour that fails its tests triggers investigation of the mix records, which is why the supervision log matters as much as the concrete itself.
Mix Ratios and Concrete Grades
Concrete is specified by grade, and the grade defines the mix ratio and the strength the concrete must reach. Grades are expressed by their characteristic compressive strength: for example, M20 concrete is designed for a characteristic strength of 20 megapascals at 28 days. The standard grades of concrete and their mix ratios are published references that every site engineer works from, and choosing the right grade for the member is a structural decision, not a preference.
Common Grades at a Glance
| Grade | Typical mix ratio (cement:sand:coarse aggregate) | Characteristic strength | Typical use |
|---|---|---|---|
| M10 | 1:3:6 | 10 MPa | Mass concrete, leveling |
| M15 | 1:2:4 | 15 MPa | Footings, general work |
| M20 | 1:1.5:3 | 20 MPa | Slabs, beams, columns |
| M25 | 1:1:2 | 25 MPa | Reinforced structural members |
Choosing the Right Grade
Low grades such as M10 and M15 suit mass concrete and lightly loaded members, where the concrete mainly fills space and resists compression. M20 is the workhorse grade for slabs, beams, and columns in residential and light commercial construction. Heavily loaded or exposed members move up to M25 and beyond, and special applications such as prestressed members use high-strength mixes with low water-cement ratios. The engineer selects the grade, and the site team is responsible for producing it batch after batch.
From Mix to Finished Structure
The quality of the paste determines the characteristics of the concrete, and that quality follows the mix from the drum to the finished member. A well-proportioned mix placed and cured correctly gains strength for months, while the same materials mishandled on site can produce a member that looks fine and fails early. Curing is part of the process: fresh concrete must be kept moist and at a reasonable temperature so hydration continues, typically for at least 7 days on structural work.
How Concrete Becomes Part of the Structure
Concrete rarely works alone. It is cast around reinforcement to carry tension, combined with other concrete members into frames, or stressed to improve performance, and the choice of structural system changes how the material is used. The trade-offs between prestressed concrete, reinforced concrete, and arch are a matter of span, load, and economy rather than material quality alone. Whatever the system, the mix that goes into it has to be right, because nothing added later can fix a batch that was poorly proportioned at the mixer.
Not every concrete mix on a site is structural. A thin layer of lean concrete is often placed under footings to provide a clean, level working surface, and the differences between lean concrete and normal concrete are worth knowing before you order material for a foundation job. Lean mixes use less cement, cost less, and are not designed to carry structural loads, but they protect the soil and the reinforcement from contamination.