What makes cement stronger

Cracks occur when tensile forces exceed the tensile strength of the concrete. Traditional concrete has a significantly lower tensile strength as compared to compressive strength. This means that concrete structures undergoing tensile stress must be reinforced with materials that have high tensile strength, such as steel.

It is difficult to directly test the tensile strength of concrete, so indirect methods are used. The most common indirect methods are flexural strength and the split tensile strength. The split tensile strength of concrete is determined using a split tensile test on concrete cylinders.

Flexural strength is used as another indirect measure of tensile strength. It is defined as a measure of an unreinforced concrete slab or beam to resist failure in bending.

In other words, it is the ability of the concrete to resist bending. Flexural strength is usually anywhere from 10 to 15 percent of the compressive strength, depending on the specific concrete mixture.

Results are expressed in a Modulus of Rupture MR in psi. Flexural tests are very sensitive to concrete preparation, handling, and curing. The test should be conducted when the specimen is wet. For these reasons, results from compressive strength tests are more typically used when describing the strength of concrete, as these numbers are more reliable. Other factors contributing to the strength of concrete include:. This refers to the ratio of water to cement in the concrete mixture.

A lower water-to-cement ratio makes for a stronger concrete, but it also makes the concrete more difficult to work with. The right balance must be struck to achieve the desired strength while maintaining workability.

Traditional concrete is made of water, cement, air, and an aggregate mixture of sand, gravel, and stone. The right proportion of these ingredients is key for achieving a higher concrete strength.

A concrete mixture with too much cement paste may be easy to pour—but it will crack easily and not withstand the test of time. Conversely, too little cement paste will yield a concrete that is rough and porous. Optimal mixing time is important for strength.

While strength does tend to increase with mixing time to a certain point, mixing for too long can actually cause excess water evaporation and the formation of fine particles within the mix. This ends up making the concrete harder to work with and less strong. There is no golden rule for optimal mixing time, as it depends on many factors, such as: the type of mixer being used, the speed of the mixer rotation, and the specific components and materials within a given batch of concrete.

The longer the concrete is kept moist, the stronger it will become. To protect the concrete, precautions must be taken when curing concrete in extremely cold or hot temperatures. A new concrete technology is available that has greater strength properties than traditional concrete across all strength ranges.

This innovative material is called Ultra-High Performance Concrete UHPC , and it is already being implemented in many state and federal government infrastructure projects given its exceptional strength and durability.

UHPC is very similar to traditional concrete in its composition. In fact, roughly 75 to 80 percent of the ingredients are the same. What makes UHPC unique are integrated fibers. These fibers are added to the concrete mix and account for 20 to 25 percent of the end product.

The fibers vary from polyester to fiberglass bars, basalt, steel, and stainless steel. Each of these integrated fibers create a progressively stronger end product, with steel and stainless steel delivering the greatest gains in strength. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement.

High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete, allowing it to be properly placed, consolidated, and cured.

A properly designed mixture possesses the desired workability for the fresh concrete and the required durability and strength for the hardened concrete. Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent. Almost any natural water that is drinkable and has no pronounced taste or odor may be used as mixing water for concrete.

Excessive impurities in mixing water not only may affect setting time and concrete strength, but can also cause efflorescence, staining, corrosion of reinforcement, volume instability, and reduced durability. Concrete mixture specifications usually set limits on chlorides, sulfates, alkalis, and solids in mixing water unless tests can be performed to determine the effect the impurity has on the final concrete.

Although most drinking water is suitable for mixing concrete, aggregates are chosen carefully. Aggregates comprise 60 to 75 percent of the total volume of concrete. The type and size of aggregate used depends on the thickness and purpose of the final concrete product. Relatively thin building sections call for small coarse aggregate, though aggregates up to six inches in diameter have been used in large dams. A continuous gradation of particle sizes is desirable for efficient use of the paste.

In addition, aggregates should be clean and free from any matter that might affect the quality of the concrete. Soon after the aggregates, water, and the cement are combined, the mixture starts to harden.

All portland cements are hydraulic cements that set and harden through a chemical reaction with water call hydration. However, the material does crack under too much pressure. Similarly, concrete can crack under the wrong kind of pressure.

Concrete is known to withstand compression, but it bends or cracks under tension weight. To make the concrete stronger and last longer under tension, companies add materials to the concrete during various stages of the process. Molecular materials are poured into the concrete to add strength while the concrete is being mixed.

Some of the nano-concrete mixtures are comprised of polyethylene or ethylene particles such as being developed by The Massachusetts Institute of Technology. Other nano-concrete, like that developed by the University of Florida, use the silicates from finely ground clay. According to MIT's "Technology Review Magazine," nano-molecules work their way into the microscopic holes found in the cement. As a concrete slab is poured, the nano-molecules inside the holes make it harder for salt and other contaminants to enter the holes and cause the concrete to break.

Another way to strengthen concrete is to incorporate steel into the mix.