Concrete Curing and Strength

Curing

In all but the least critical applications, care needs to be taken to properly cure concrete, and achieve best strength and hardness. This happens after the concrete has been placed.

The cement paste hardens over time, initially setting and becoming rigid though very weak, and gaining in strength in the days and weeks following.

It does not set by drying out, but by the cementitious material chemically reacting with the water - hydrating. Properly curing concrete leads to increased strength and lower permeability, and avoids cracking where the surface dries out prematurely.

Care must also be taken to avoid freezing, or overheating due to the exothermic setting of cement. Improper curing can cause scaling, reduced strength and abrasion resistance and cracking.

Strength

Concrete has relatively high compressive strength, but significantly lower tensile strength (about 10% of the compressive strength). As a result, without compensating, concrete would almost always fail from tensile stresses - even when loaded in compression. The practical implication of this is that concrete elements subjected to tensile stresses must be reinforced with materials that are strong in tension. Concrete is most often constructed with the addition of steel or fiber reinforcement. The reinforcement can be by bars (rebar), mesh, or fibres, which provide the required tensile strength to concrete producing reinforced concrete. Concrete can also be prestressed (reducing tensile stress) using internal steel cables (tendons), allowing for beams or slabs with a longer span than is practical with reinforced concrete alone. Inspection of concrete structures can be non-destructive, carried out with equipment such as a Schmidt hammer is used to estimate concrete strength.

The ultimate strength of concrete is influenced by the water-cement ratio (w/c) [water-cementitious materials ratio (w/cm)], the design constituents, and the mixing, placement and curing methods employed. All things being equal, concrete with a lower water-cement (cementitious) ratio makes a stronger concrete than that with a higher ratio. The total quantity of cementitious materials (Portland cement, slag cement, pozzolans) can affect strength, water demand, shrinkage, abrasion resistance and density. All concrete will crack independent of whether or not it has sufficient compressive strength (In fact, high portland cement content mixtures actually crack earlier due to increased hydration rate) . As concrete transforms from its plastic state, hydrating to a solid, the material undergoes shrinkage. Plastic shrinkage cracks can occur soon after placement; but if the evaporation rate is high, they often can actually occur during finishing operations (for example in hot weather or a breezy day). In very high strength concrete mixtures (greater than 10,000 psi), the crushing strength of the aggregate can be a limiting factor to the ultimate compressive strength. In lean concretes (with a high water-cement ratio) the crushing strength of the aggregates is not so significant.

Experimentation with various mix designs begins by specifying desired "workability" as defined by a given slump, "durability" requirements taking into consideration the weather exposure conditions (freeze-thaw) to which the concrete will be exposed in service, and finally the required "28 day compressive strength", as determined by properly molded standard-cured cylinder samples. The characteristics of the cementitious content, coarse and fine aggregates, and chemical admixtures determine the water demand of the mix in order to achieve the desired workability. The 28 day compressive strength is obtained by determination of the correct amount of cementitious (and often chemical admixtures) to achieve the target water-cementitious ratio.

Our range of precast products include: Prestressed Concrete Panels; Retaining Walls and Concrete Barriers.