| 1. What's the difference between cement and concrete? |
| Ans: Cement (hydraulic cement) is one of the ingredients of concrete. Concrete is basically a mixture of 3 ingredients namely cement, aggregates and water. Initially cement and water form a reactive paste which gives coating over the aggregates particles and binds the aggregates together. This bond hardens as a rocky mass and is strong because of the chemical reaction of cement and water. |
|
| 2. What makes concrete crack? |
| Ans: Concrete very often cracks due to High shrinkage.
Concrete "shrinks" slightly as it hardens. A normal shrinkage rate is approximately 3 to 4 mm per 30 liner meter. Shrinkage is caused by loss of excess water from the mix. More the water is used in the mix more will be the shrinkage and more cracks are likely to be observed. To reduce shrinkage rate and the resulting cracks, less water should be used and Control joints should be placed in the concrete at intervals of 2 to 3 times (in feet) the thickness of the concrete slab. For example, a slab 4" thick should have control joints every 8 to 12 feet. |
|
| 3. Why do concrete surfaces flake and spall? |
Ans: 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. |
|
| 4. Why should fresh concrete be properly cured? |
| Ans: These defects are generally a result of improper workmanship/finishing of the concrete. Surface dusting as well as scaling is the result of use of excess water in the mix and or excess compaction of the concrete. If excess water is added it comes out to the surface of the concrete during placement and finishing and the water/cement ratio on the surface may be drastically increased. This condition greatly reduces the strength of the concrete on the surface. Unfortunately, this is where the wear takes place. |
|
| 5. I've heard the term slump pertaining to concrete. What is slump? |
| Ans: The term slump simply refers to the consistency of concrete in a plastic state (prior to hardening). Slump is a measure of how wet or stiff the concrete is. Obviously, the more water used in producing the concrete, the wetter (or higher) the slump will be. Again, excessive water causes a dramatic loss of strength. Applications exist where higher slump (wetter) concrete is necessary because of difficulty in placement or specification requirements. These higher slumps can be attained through the use of water reducing admixtures (chemical additives). |
|
| 6. How long does concrete continue to gain strength after it is placed? |
| Ans: Concrete achieves almost 70% of its designed strength within the period of 7 days. The accepted time standard for measuring concrete designed strength is 28 days. However, concrete goes on gaining strength for even a century provided moisture is available to continue the process of hydration. |
|
| 7. What is fly ash? |
| Ans: Fly ash is a by-product from coal fired electric power generating plants. The inorganic or mineral constituents of the coal, such as clay, quartz and shale, fuse and chemically recombine during burning to produce various crystalline and glassy phases of fly ash. The fly ash is entrained in the flue gas and cools into spherical, usually hollow shaped particles. These particles are collected in electrostatic precipitators or bag houses and the gradation, or fineness, of the fly ash can be controlled by how and where the particles are collected. Fly ash reacts with the free lime generated by cement hydration to form cement-like compounds, which increase the strength and reduce the permeability of concrete. |
|
| 8. What is slag? |
| Ans: Slag is a by-product obtained in molten stage from the Iron making Blast furnace of steel plants. These are granulated by quenching with water/air till it achieves hydraulic property having glassy phases like that of cement making clinker. Granulated slag is used for making SLAG CEMENT. This cement has more resistance capacity than that of OPC marine and chemically aggressive environment. Slag cement is the best suited cement for aggressive environment amongst all other blended cement being used at present. |
|
| 9. What is the setting time of Cement Concrete? |
| Ans: The setting time of cement is the time required by concrete to harden and set itself according to the framework in which it has been placed. The time taken by concrete to set is normally 7 hours to 10 hours depending on the conditions under which the concrete is placed in the framework. But this does not mean that the framework may be removed after 7 hours. This initial setting of concrete does not impart enough strength for it to withstand its own self load so the framework should be removed only after 7 to 14 days of proper curing depending upon the size and span of the concrete. |
|
| 10. Does color have any bearing on its quality and strength of cement? |
| Ans: No the color of cement does not have any bearing on the quality and strength of cement in anyway. It just gives a look and finish to the concrete. The misconception that the darker in color the stronger cement is, is not true. On the contrary cement that is lighter in color not only has the same strength but also has a more pleasing finish and more capability of color rendering to the concrete than the cement that is dark in color. |
|
| 11. Addition of fly ash in concrete is detrimental to its strength? |
Ans: This is not true. In fact the advantages of addition of fly ash are many, a few of which are listed below.
It reduces permeability of concrete thus helping in making concrete structures water proof.
The addition of fly ash also helps in better bonding with the reinforcement.
It also increases the density of concrete by reducing the air voids.
It gives a pleasing appearance to finished concrete. |
|
| 12. What effect does the water/cement ratio have on the quality and strength of concrete? |
| Ans: The water cement ratio is very important with regards to the strength of concrete. The strength of concrete is inversely proportional to the water cement ratio i.e. with the increase in water content the compressive strength of concrete decreases and vice versa. Increased water cement ratio also increases shrinkage leads to possibility of getting cracks. Increase water cement ratio will increase permeability , hence will reduce durability ( life span ) of the concrete. |
|
| 13. Is concrete environmentally friendly? |
| Ans: Yes. Concrete is chemically inert and has no components made from petroleum products or other chemicals that might have an effect on our water supply due to rainwater runoff. |
|
| 14. How do you remove stains from concrete? |
Ans: Stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemical or water.
Common dry methods include sandblasting, flame cleaning and shotblasting, grinding, scabbing, planing 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 percent solution of vinegar to neutralize any remaining sodium peroxide. |
|
| 15. What are the decorative finishes that can be applied to concrete surfaces? |
| Ans: Color may be added to concrete by adding pigments-before or after concrete is place-and using white cement rather than conventional gray cement, by using chemical stains, or by exposing colorful aggregates at the surface. Textured finishes can vary from a smooth polish to the roughness of gravel. Geometric patterns can be scored, stamped, rolled, or inlaid into the concrete to resemble stone, brick or tile paving. Other interesting patterns are obtained by using divider strips (commonly redwood) to form panels of various sizes and shapes ¬ rectangular, square, circular or diamond. Special techniques are available to make concrete slip-resistant and sparkling. |
|
| 16. How do you protect a concrete surface from aggressive materials like acids ? |
| Ans: 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, 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. |
|
| |
| |
| |
