Concrete hardens and gains strength as it hydrates. The hydration process continues over a long period of time. It happens rapidly at first and slows down as time goes by. To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected by specification writing authorities as the age that all concrete should be tested. At this age, a substantial percentage of the hydration has taken place. He 28-day strength is commonly measured on concrete cubes (100mm or 150mm side length) that are removed from the moulds and cured in water at 23°C until testing.

Curing concrete is the term used for stopping freshly poured concrete from drying out too quickly. This is done because concrete, if left to dry out of its own accord, will not develop the full bond between all of its ingredients. It will be weaker and tend to crack more. The surface will not be as hard as it could be.
The longer you cure the better. Concrete will continue to hydrate and gain strength almost indefinitely, as long as moisture is present and a suitable temperature is maintained. If concrete is cured in cool temperatures, strength gain will be slow, but the concrete will eventually reach a high strength as long as moisture is continuously present. If the concrete dries out too early in the hydration process (within the first three days), long-term strength will be compromised even if moist conditions return. If your intention is to produce the highest quality concrete, keep freshly poured concrete moist for seven days.
SANS 2001-CC1:2012 Table 8 gives minimum recommended curing times for concrete placed in low temperatures of ≤ 5°C and different recommendations for concrete ≥ 15°C. This table also takes the type of cement into account.

The significance of curing of concrete is to improve its properties such as water- tightness, wear resistance, strength, volume stability and durability.

The most crucial time for strength gain of concrete is immediately following placement. In field conditions, heat and wind can dry out the moisture from the placed mixture, so care should also be taken during placement. Loss of water causes the concrete to shrink, which leads to tensile stresses within the concrete; as a result, surface cracking may occur, especially if the stresses develop before the concrete attains adequate tensile strength.

The timing of removal of formwork will depend on the rate of strength gain of the concrete:

Vertical surfaces can be struck when the concrete is strong enough to resist the adhesion on the concrete/form face surface.

Soffits cannot be struck until the concrete has gained sufficient strength for the element to support its self-weight and construction loads for all or part of the structural span. It is possible to speed up the removal of much soffit formwork by removing a small part of the formwork and then installing props to this area. Another area of formwork can then be removed and props installed. This sequence can be repeated until all the formwork has been removed and the soffit is fully propped.

When removing formwork it is important to avoid damage to the surface of the concrete and particularly to edges and corners. Following removal of the formwork, all exposed surfaces must be covered with polythene or wet hessian to prevent moisture loss for about 5 days, so that the concrete is sufficiently cured.

A MegaPascal (MPa is an SI unit for stresses and pressure) is a measure of the compressive strength of concrete at a required age. This is a calculation of force over area.

The 7-day is used as an early warning for the 28-day concrete strength. If the correlation between 7 and 28 days are known, the 7day is usually used to predict the 28-day results. Concrete mixes can be adjusted up or down based on this prediction. If low strengths are indicated, the site and engineer can be notified not to remove props, etc.

All concrete will gain strength as it hydrates. But the entire hydration process takes several years to be completed. Concrete needs to be tested to see how strong it is, but it would be pointless to wait several years to test the concrete for a job site. Concrete gains most of its strength by 28 days and therefore 28 days is a period of time long enough to let a substantial amount of the hydration to take place, yet not take so long that it would be impractical to test. For that reason it has become common to test concrete for its strengths after 28 days. 28 days is a practical number as it is 4 weeks from the day of casting, thus if you cast your concrete on a Monday you will test it on a Monday again.

Different strengths are produced by designing concrete at different cement to water ratios.

“Standard” concrete has a density of roughly 2400 kg/m3.
Lower densities of concrete may be required to save on weight.
Higher densities may be required for radiation shielding or as a counterweight

Normal strength concrete (15-30 MPa ) used in house construction can be produced with almost any type of sand and stone, as long as it is reasonably consistent. Cement, admixture and additional cementitious materials (Fly Ash, Slag, Silica fume) does not make such a big difference except for the economics of the mix.

There are different classifications for high-strength concrete. In the 1990’s and earlier, concretes of 40-60 MPa were considered to be high-strength. With the 2000’s concrete over 60 MPa being considered to be high-strength concrete. Today, concrete over 80 MPa is considered high-strength concrete in some countries. Concretes of over 100 MPa are considered as ultra high-strength concrete.

Be that as it may, when looking at high strength concrete the following should be considered:

• Aggregate crushing value (ACV) of aggregate – as the aggregate now becomes the weakest part of the concrete matrix.
• Aggregate size – higher strength concretes usually use smaller size aggregates to increase the surface area for bonding to take place.
• Surface texture of the aggregates – if it is smooth there would be higher potential for slippage
• Concrete grading – all material needs to be selected to give a continuous grading so that there is better load distribution and a denser matrix.
• Cement – cement with a high ultimate strength, and not necessarily early age strength, is preferred. The slower the strength gain of cement paste, the higher ultimate strength the crystals will reach.
• Complimentary cementitious material – the use of silica fume and/or ultra fine fly ash is advisable as the interfacial zone between the aggregate and the cement past is improved. Silica fume and ultra-fine fly ash are smaller than a cement particle so you will then, similar to aggregates grading, have a continuous cementitious grading.
• Admixtures needs to be used to reduce the water demand of the concrete and to improve the workability in the concrete.
• Water content cannot be too low, as water is needed to break down the stickiness of the concrete mix and for hydration of the cement. With this mixes having a high cementitious content there need to be enough water for initial and long term hydration to take place.

These are the most critical to consider.

Portland cement is hydraulic cement which means that it sets and hardens due to a chemical reaction with water. Consequently, it will harden under water.

Concrete like all other materials, will slightly change in volume when it dries out. Typically concrete shrinks 4 mm in 3 metres. Contractors put joints in concrete pavements and floors to permit the concrete to crack in uniform, straight lines at the joint when the concrete shrinks and changes its volume.

Many materials have no effect on concrete. However, there are some aggressive materials, such as most acids, that can have a deteriorating effect on concrete. The first line of defence against chemical attack is to use quality concrete with maximum chemical resistance, followed by the application of protective treatments to keep corrosive substances from contacting the concrete. Principles and practices that improve the chemical resistance of concrete include using a low water-cement ratio, selecting a suitable cement type (such as sulphate-resistant cement to prevent sulphate attack), using suitable aggregates, water and air entrainment. A large number of chemical formulations are available as sealers and coatings to protect concrete from a variety of environments; detailed recommendations should be requested from manufacturers, formulators or material suppliers.

Alkali-silica reactivity (ASR) is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures and mixing water. External sources of alkali from soil, de-icers and industrial processes can also contribute to reactivity. The reaction forms an alkali-silica gel that swells as it draws water from the surrounding cement paste, thereby inducing pressure, expansion and cracking of the aggregate and surrounding paste. This often results in map-pattern cracks, sometimes referred to as alligator pattern cracking. ASR can be avoided through (1) proper aggregate selection, (2) minimisation of cement content, (3) use of blended cements, (4) use of proper pozzolanic materials and (5) contaminant-free mixing water.

Polypropylene fibres are usually used in concrete to control cracking due to plastic shrinkage. They do not eliminate cracks but distribute the cracks into many small cracks and fewer larger visual cracks.

Concrete is chemically inert and has no components made from petroleum products or other chemicals that might have an effect on water supply due to rainwater run-off. So, yes concrete is environmentally-friendly.

Significant efforts have been made to reduce carbon dioxide (CO2) emissions associated with the manufacture of cement, primarily by making the process more energy-efficient and increasing the use of alternative fuels. Further reductions in CO2 can be achieved by lowering the clinker component of the cement because the pyro processing used to manufacture clinker produces approximately 1 tonne of CO2 for every ton of clinker. Traditionally reductions in the clinker content of cement have been achieved by producing blended cement consisting of portland cement combined with an extender like fly ash, slag, limestone and silica fume.
So use CEM II – CEMV class cements which have lower clinker contents compared to CEM I cements, or by adding additional Fly Ash or Slag on site, to reduce cement consumption.

No correlation between compressive strength and durability have been found in concrete.

Durability of Concrete depends upon the following factors:
Cement content – Mix must be designed to ensure cohesion and prevent segregation and bleeding. If water is added to improve workability, water / cement ratio increases, resulting in a highly permeable matrix.
Cement composition – cement blended with fly ash or slag produce more dense concrete due to the secondary crystal formation from these materials, making for a more dense matrix
Compaction – Concrete containing voids can be caused by inadequate compaction. Usually it is being governed by the compaction equipment used, type of formworks, and density of the steelwork.
Curing – It is very important to permit proper strength development to aid moisture retention and to ensure hydration process occurs completely
Cover – Thickness of concrete cover must follow the limits set in national codes
Permeability – This is considered the most important factor for durability. It can be noticed that higher permeability is usually caused by higher porosity .Therefore, a proper curing, sufficient cement, proper compaction and suitable concrete cover could provide a low permeability concrete. What cannot get in, cannot hurt the concrete.

No-fines concrete (NFC) consists of large, coarse aggregate (typically 19 mm stone) and cement paste. Aggregate particles are each covered with a thin layer of cement paste and are in point-to-point contact with each other. At each point of contact the paste forms a small fillet; in the hardened state these fillets hold the particles together and give strength to the concrete.

Typical application s for NFC include: drainage layers; backfill between walls and excavated ground; porous dams; roof screeds (below the waterproofing system); paving (foot traffic for quick drainage), etc.

Stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemicals or water.

Common dry methods include sandblasting, flame cleaning and shot-blasting, grinding, scabbing and scouring. Steel-wire brushes should be used with care because they can leave metal particles on the surface that later may rust and stain the concrete.

Wet methods involve the application of water or specific chemicals according to the nature of the stain. The chemical treatment either dissolves the staining substance so it can be blotted up from the surface of the concrete or bleaches the staining substance so it will not show.

To remove blood stains, for example, wet the stains with water and cover them with a layer of sodium peroxide powder; let stand for a few minutes, rinse with water and scrub vigorously. Follow with the application of a 5% solution of vinegar to neutralize any remaining sodium peroxide.