Frequently asked questions:
Here we provide you with a list of frequent questions about Cement and concrete.
Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and Portland cement. Concrete gets stronger as it gets older. Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver. Cement comprises from 10 to 15 percent of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. This hardening process continues for years meaning that concrete gets stronger as it gets older.
So, there is no such thing as a cement sidewalk, or a cement mixer; the proper terms are concrete sidewalk and concrete mixer.
As per cement international Standard, each type of cement must provide a minimum compressive strength at 28-Day age. So as European & Jordanian Standards, Cement strength classes are classified into 3 categories:
52.5 Strength Class: that should provide minimum of 52.5 MPa at 28-Day.
42.5 Strength Class: that should provide minimum of 42.5 MPa at 28-Day.
32.5 Strength Class: that should provide minimum of 32.5 MPa at 28-Day.
Those are the only strength classes permissible in the standard, which implies that no other strength class is allowed for construction purposes cements.
Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete's temperature stays within a suitable range. During the curing period-from five to seven days after placement for conventional concrete-the concrete surface needs to be kept moist to permit the hydration process. new concrete can be wet with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture.
Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is 0.4 cm in 3 meters. The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage.
Concrete is tested to ensure that the material that was specified and bought is the same material delivered to the job site. There are 12 different test methods for freshly mixed concrete and hardened concrete.
Slump, air content and unit weight and compressive strength tests are the most common tests.
- Slump is a measure of consistency & Workability, or relative ability of the concrete to flow. If the concrete can't flow because the consistency or slump is too low, there are potential problems with proper consolidation. If the concrete won't stop flowing because the slump is too high, there are potential problems with mortar loss through the formwork, excessive formwork pressures, finishing delays and segregation.
- Air content measures the total air content in a sample of fresh concrete, but does not indicate what the final in-place air content will be, because a certain amount of air is lost in transportation, consolidating, placement and finishing. Three field tests are widely specified: the pressure meter and volumetric method are BS EN/ ASTM standards.
- Unit weight measures the weight of a known volume of fresh concrete.
- Compressive strength is tested by pouring cubes of fresh concrete and measuring the force needed to break the concrete cube at proscribed intervals as they harden.
The real indicator is the yield, or the actual volume produced based on the actual batch quantities of cement, water and aggregates. The unit weight test can be used to determine the yield of a sample of the ready mixed concrete as delivered. It's a simple calculation that requires the unit weight of all materials batched. The total weight information may be shown on the delivery ticket or it can be provided by the producer. Some of concrete producers actually over yield by about 0.5% to make sure they aren't short-changing their customers. But other producers may not even realize that a mix designed for one cubic yard might only produce 98% of what they designed.
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.
Concrete surfaces can flake or spall for one or more of the following reasons:
- In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface.
- The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface.
- The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable.
It is concrete that is strong enough to carry a compressive stress of 25 MPa at 28 days. Concrete may be specified at other strengths as well. Conventional concrete has strengths of 50 MPa or less; concrete with strengths between 50 and 100 MPa is considered high-strength concrete.
The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the ratio of water to cement in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely affected by adding more water.
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 defense 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 sulfate-resistant cement to prevent sulfate attack), using suitable aggregates, and 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.
Each country has its own standard for Portland cement, so there is no universal international standard. Jordan Applies its own Standard JS 30-1:2007, which is based on the European Standard EN 197-1:2000. On the other hand, The United States uses the specification prepared by the American Society for Testing and Materials-ASTM C-150 Standard Specification for Portland cement. The European Cement Association located in Brussels, Belgium, publishes a book titled "Cement Standards of the World."
Alkali-silica reactivity is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolana, admixtures and mixing water. External sources of alkali from soil, deicers 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) use of blended cements, 3) use of proper Pozzolanic materials and 4) contaminant-free mixing water.
Though all Portland cement is basically the same, eight types of cement are manufactured to meet different physical and chemical requirements for specific applications:
- CEM I is a general purpose Portland cement, suitable for high performance concrete & quick setting requirements.
- CEM II is used for Medium to small range structures. It provides moderate sulfate resistance, or when heat build-up is a concern.
- CEM III is used in high performance concrete applications, when workability & heat build-ups are both crucial. It contains GGBS in various amounts.
- CEM IV combines two or more types of additives, to produce specific performance.
- Type V cement resists chemical attack by soil and water high in sulfates.
White Portland cement is made from raw materials containing little or no iron or manganese, the substances that give conventional cement its gray color.
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.