An Introduction to Polycarboxylates Concrete Admixtures

Concrete Admixture is one ingredient of concrete that is added to the mix before or during mixing to modify concrete properties. Concrete admixture are used to modify the properties of fresh and hardened concrete in order to achieve the desired objectives or purpose of the concrete mixture. It is also used for economic purpose when admixture allows reduction of concrete ingredients proportions or permits saving in construction practices. Concrete admixtures have their history for hundred of years. The latest generation is polycarboxylates or polymer based admixture.

Polycarboxylates or polymer based concrete admixtures are High Range Water Reducing admixture (HRWR) or we can also say "Ultra" High Range Water Reducer. Used at low dosage, polycarboxylates admixtures still can reduce water as much as high dosage of conventional admixtures.Used at higher dosage, polycarboxylates admixtures can produce Self Compacting Concrete SCC (concrete with very high flowability but still with good cohesion). Because of their strong water reduction capacity, they are particularly designed for production of High Strength Concrete.Polycarboxylates HRWR mode of action is based on steric hindrance, this means particle separation due to long polymer molecules.

Polycarboxylates or polymer basedadmixtures like any newest generation admixtures are expensive products compared to traditional admixtures (Melamine, Naphtalene or lignosulfonates based) but their performances (even at low dosage rate) can allow cement reduction which could make the mix more cost effective.For lower grades of concrete (lower than 40 MPa on cylinder samples), conventional admixtures are still generally the best solution (especially economically). Use of Polycarboxylate admixturesin RMC (ready-mix-concrete) is normally less required than in Precast concrete because of generally lower concrete grades. This admixture type is very suitable for underwater anti-washout concrete.

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An Overview of Shotcrete Or Sprayed Concrete

As a structural material, most people are familiar with concrete. However, not everyone is familiar with the form of concrete known as shotcrete. Here is some background on shotcrete, including how it is used in building projects today.

Essentially, shotcrete is projected concrete. Originally devised in the early 20th century, shotcrete was created as a means of using concrete to fill out molds. The concrete itself was a dry mix that was blown directly into the mold using compressed air.

As the concrete was released, the dry mix was moistened, allowing it to settle and set in the mold. The inventor of this method, Carl Akeley, received a patent in 1911 for both the concrete gun he developed, as well as the material that was produced, which he dubbed gunite.

Other applications for shotcrete were immediately obvious. Because the shotcrete could be applied to a horizontal or a vertical facing, the shot concrete could easily be applied to the sides of buildings, the shotcrete could be used as a patch and as a filler where cracks may have appeared in walls or in foundations. The method also hastened the construction of walkways in many front yards and backyard gardens as well, since shotcrete could be laid out in a fraction of the time it took to mix and apply concrete by hand.

The dry method of creating shotcrete remained in place until the middle of the 20th century, and continued to be refined. Still in use today, the dry method involves placing the dry mix into a hopper, where it is ran through a hose with a water attachment at the end of the hose. As the concrete is shot out of the hose and into the gun mechanism, the operator adjusts the amount of water that is added to the dry mix. The result is a concrete mixture that is easy to direct and will dry and cure in the same amount of time as any method using concrete.

By the middle of the 20th century, an alternate method to creating shotcrete was developed. Referred to as the wet method, this process involves using ready-mixed concrete. As with the dry method, compressed air is used to force the concrete mixture through a hose and out of a nozzle. With wet mixing, the operator does not have the ability to adjust the mix of water and dry concrete, since that process has already taken place. Fans of this method point to the fact there is no chance of introducing too much water into the mix, creating a poor concrete texture. Supporters of the original dry method state that the properly mixed concrete and water produce a finished product that is superior to the ready-mix used with the wet method.

With both applications, steel rods or webs of steel mesh are often used to help retain and reinforce the surface that is receiving the shotcrete treatment. By providing something extra for the shotcrete to cling to while it dries into place, the shotcrete patches or facades tend to last much longer.

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Accessing concrete Attributes of apparatus and materials, accomplish simpler with NDT

Currently there are various methods, techniques and equipments which are employed in different industries in order to evaluate materials and to produce efficient products that are better in quality and entail a lower manufacturing cost. NDT or Non destructive testing methods describe a popular type amongst all. Entailing an exclusive range of techniques employed for analyzing and assessing physical attributes of components in any system, these include many and not just one method.
The most popular forms of non destructive testing methods that find ample utility in various fields include
•    Ultrasonic Testing
•    Radiographic methods
•    Welding procedures
•    Leak testing
•    Eddy current method
•    Positive material identification
•    Ground penetrating Radar
•    Digital Imagining and many others
The popularity which NDT methods have earned recently has inadvertently made them emerge as an integral part of various fields. These include Electrical engineering, Forensic engineering, Mechanical engineering, Systems engineering, Civil engineering, Art, medicine and more. NDT methods have a wide area of application and are progressively more used in industries like automotive, construction, power plants, aviation, manufacturing, naval, military, railways and more.
Extremely beneficial for evaluating product and material quality, researching and troubleshooting, the biggest advantage NDT methods offer is, they do not affect the object that is being tested in even a slight manner. Many new methods and techniques are getting added in list of NDT methods to help all give their customers better quality and while enjoying lowered production costs.
Using reliable methods that can best cater to requirements of clients belonging to any field, there are many companies now emerging to help all. Like every other place, there has been a considerable rise in such firms in Canada too. The increasing number of companies and fast technological development has all led to the great hype in service providers. Canada, with its rapidly growing industrial sector now houses many such companies that offer such destructive and non destructive testing methods. One firm that enjoys ace popularity in this very scenario is what ABC Testing Inc. is all about.
ABC Testing is a leading firm that employs best brains and technologies to offer clients most recent and innovative in destructive and non destructive testing. With proven methods which are deciphered to screen flaws and weaknesses in any equipment, this firm can add to the productivity of any company easily.
So, with NDT specialists, who possess a certified license from NDT laboratories, get all the information you require to know about applications that can best suit your needs with ABC.

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Effects of Water Reducer and High Range Water Reducer on Fresh Concrete

1. How do water reducers work?
Water reducers (WR) have detergent like properties. They are called "surface active agents" and carry a negative charge. In cement pastes, there is tendency for cement particles to form large flocs (agglomerates). The water reducer will allow cement particle not to collect together, more cement surface area is available for reaction with water.
Modified lignosulphonates, naphthalene, melamine and polycarboxilates based high range water reducer (HRWR) are like WR surface active agents and perform in similar manner, but more powerful: more cement dispersion, more water reduction, higher strength achievable.

2. Effects of WR and HRWR on fresh concrete

2.1 Water reduction:
WR and HRWP can be used to decrease water, decrease water per cement ratio (W/C). It is resulted in increasing strength with same workability. If W/C is fixed, the workability is increased. Another affects from water reduction is to reduce cement content while workability keep unchanged.

2.2 Time of setting:
As per ASTM C494, set retarding admixture shall retard initial setting at least 1 hour later than control and retard final setting not more than 3.5 hours later than control. Set accelerating admixture shall accelerate initial setting at least 1 hour (maximum 3.5 hours) earlier than control and retard accelerate setting not more than 1.0 hours later than control.

2.3 Air entrainment
Air entraining properties of admixtures are generally controlled by formulation modifications unless specified in technical data sheet. WR and HRWR are not designed to act as air entrainers. If the project calls for air entrained concrete (typically with an air content of 4 to 6%) a special admixture shall be used.

2.4 Workability
Concrete using admixture shall show less segregation when vibrated and shall have better flowability. By reducing water content, coarse and fine aggregate content are increased and easier placing and good finishing can be obtained.

2.5 Bleeding
Generally use of WP or HRWR admixture has beneficial effect in terms of reducing the bleeding segregation of fresh concrete. But, when admixtures are used at high dosage for producing high slump concrete or flowing concrete, trials are still required to check actually bleeding.

2.6 Rate of slump loss
WR or HRWR with set retarding effect will reduce slump loss. But many factors can still affect the slump loss like dosage of admixture, cement characteristics, concrete temperature, outside temperature, initial slump... Trials are always required to fine tune mix design.

2.7 Heat of cement hydration
WR and HRWR can have only an indirect beneficial effect on heat generated by cement hydration. When the use of admixture allows water reduction and cement reduction (keep same W/C ratio than control mix). Thus, lower heat is generated than in control mix.

2.8 Finishing
Use of WP or HRWP admixtures gives general better concrete finishing quality. But for concrete with highly reduce water content; finishing can become more difficult due to decrease of bleeding. Trials are required.

2.9 Shrinkage and early cracking
In concrete with highly reduced water content, the decrease of bleeding can lead to possible risk of plastic shrinkage if surface is not cured properly (by fogging or use of curing compound). This is related to the fact that plastic shrinkage cracks result from a loss of water by evaporation higher than the rate of bleeding.
Overall, WP and HRWR admixtures still have generally a beneficial effect in term of fresh or wet concrete cracking reduction. This is due to the fact that cement paste shrinks but the aggregates build a rigid formwork.

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