The types used in Palestine mainly are Type G, Type D and Type F. In Palestine, either type G or type D is added at the mixing stage in the factory, and Type F is added in the site before pouring the concrete mix. The difference between type G and type D is that type G is high range water reducing and retarding admixture, and type D is water reducing and retarding admixture so choosing between them depends on the projects conditions.
As Eng. Haytham Hadarbeh claimed, material engineer from Al-Tarifi factory, type G or type D are used because the time needed for the transport of the concrete mix from the factory to the site is not less than 2 hours. Therefore, type G or type D are used in order to retard the initial set by at least 3 hours, therefore allowing longer time for interaction of concrete so that the concrete mix will be transported and poured on site without occurring any interaction.
In addition, Type F is used, which is high range water reducing and accelerating admixture, in order for the concrete mix to obtain flexibility only without interfering in the setting time. It is also used instead of adding water into the concrete mix, therefore obtaining higher strength.
The types of admixtures used do not differ depending on the structural element instead it differs depending on the specified strength of the concrete mix. One type of admixture is used when the strength of the concrete mix is specified until B300. Two types of admixture are used, when the strength of the concrete mix is specified from B300 until B450. In Palestine, B300 is the most common specified strength of the concrete mix. Special admixture (one type) is used, when the strength of the concrete mix is specified from B500 until B600.
Moreover, as Eng. Mohammad Sadi claimed, material engineer from Majdi Qandeel factory, that closed structural elements like columns and walls are handelled more carefully compared to other structural elements. When dealing with closed structural elements, higher quantity of admixtures are added in order for the concrete mix to obtain more flexibility. Thus preventing the honeycombing problem. Note that, honeycombing is a problem that occurs for the concrete mix after it hardens at which voids appear in it.
? Effect of temperature on admixtures
Based on Palestines moderate climate in winter and summer, the Palestinian factories need not change the type of admixture used. This is because the temperature in winter does not reach low temperatures (below zero) as the European countries. Therefore, it is possible to use the same type of admixture in winter and summer. However, the Palestinian factories commit to a few precautions during summer and winter through reducing the dose in winter and increasing the dose in summer.
As Eng. Haytham Hadarbeh stated that, in summer the percentage of the retarder will be higher in order to resist the high temperatures of the climate. Whereas, in winter (specifically from October), the factory requests from the super plasticizer supplier to reduce the quantity of retarder by about 10-15 %. In addition, as Eng. Mohammad Sadi said that accelerators can be used in cases that require quick hardening for the concrete.
Figure 1 represents the influence of temperature on initial setting time. It is noticed that the retarding effect is smaller at higher temperatures. Some retarders cease to be effective at extremely high ambient admixtures, about 60 °C (140 °F). (A.M.Neville, 1995). In addition, table 1 represents influence of air temperature on the retardation of the initial setting time of concrete by water-reducing and set-retarding admixtures. From the table, it is noticed that the retarding admixtures retard initial set approximately double the retardation caused by the water-reducing and retarding admixtures.
? Dosage of admixtures
The manufacture company of the admixture is the one that determines the quantity of dosage needed. The appropriate quantity of dosage is determined with respect to the quantity of cement (per 100kg of cement). As Eng. Mahmoud Sadi claimed that if higher dosage was used than required, this may result in delaying the concrete set, thus delaying the hardening of concrete, thus the probability of the occurrence of strength problems increases.
Site trials should be conducted to determine the best dosage for the purpose under site conditions. However, the dosage rate of addition used in the Palestinian factories ranges from 1- 1.5 % with respect to the cement mass. Generally, the dosage per 100 kg of cement is lower in mixes with a high cement content. (A.M.Neville, 1995). Figure 2 represents the influence of dosage of retarders on slump. It can be noticed that as the dosage increases of water-reducing admixture, the workability increases but the long term strength will not be effected.
? Advantages of Admixture
Regardless of the problems that may occur due to admixtures, admixtures have many advantages. At first, flowing concrete is obtained from concrete with a dry or plastic consistency without adding water. Note that, flowing concrete is a highly fluid (slump greater than 190mm) but workable concrete that can be placed with little compaction, and at the same time remaining free of excessive segregation. In addition, it is essential in water reduction and at the same time obtaining the same workability. Since it reduces the water percentage, thus it increases the strength of the concrete mix. The mechanical strength may increase to over 30%, when water is reduced at equal workability.
In addition, setting time and shrinkage remains practically unchanged. Moreover, admixtures help in the optimization of the cement content in order to get the desired mechanical strengths. The physic-chemical properties of admixtures allow the user to optimize the cement content, when a specified mechanical strength is requested.
When it was requested for Eng. Haytham Hadarbeh to tell about his experience in the construction market, he said that for every ready-mix concrete factory, there is a data sheet for the admixture that requires to be licensed and needs to be approved on the specifications and standards. However, he said that in Palestine the problem is that the internal supervision (the supervision within the factory itself) is not guaranteed. The factory itself may little bit reduce the admixture quantity in such a way that can not be noticed in order to save money. Also, the factory may bring the admixture in an illegal ways. Moreover, Eng. Mahmoud Shakarneh stated that locally there is a competition upon the cost of admixtures but not on its quality.
In addition, when it was requested for Eng. Mahmoud Sadi to answer how the occupation effects the use of admixture, he refused to answer. He considers that the level reached in Palestine considering the use of admixtures is higher than the level reached in Israel. Moreover, he said that the factory imports the raw material from the outside regions, and the Palestinian factories are capable of producing the appropriate admixtures.
Regardless of the advantages of the admixtures, however problems due to the misuse of admixtures may still occur. The first Case was that in a certain project, there was an increase from the required super plasticizer portion. This excessive portion was used after 6 to 7 months from the order of the portion. As a result, the admixture did not give its required effectiveness (it did not decrease water percentage so as to increase strength of concrete mix) instead it had a negative effect on the strength of the concrete. (Hadarbeh, 2019)
Obviously, the reason of the problem is due to expiry or the incorrect storage of the admixture. The quality of the admixture is guaranteed for 18 months from the manufacturing date, provided the product is kept sealed in its original packaging. The storage temperature should exceed 0 °C.
In addition the second case was that after 21 days from pouring concrete into a ground beam for a water tank base, the concrete did not harden. As a result, we had to remove the concrete mix with simple equipment that is water. (Shakarneh, 2019) This problem is due to excessive dosage of the super plasticizer. This case is one of the most common problems. Excessive dosage of the super plasticizer has vital effects on the mechanical properties of concrete mix. It increases workability and cause a decrease in abrasion resistance.
In conclusion, in order to avoid problems due to the misuse of admixtures, my proposed solution is that there must be strict regulations that forces every ready-mix concrete factory to examine their materials, by the continuous supervision of the designated authorities. Every ready-mix concrete factory must have a lab within it in order to test the materials, for example making tests for the density of the super plasticizer and for the initial setting time. In addition, the construction quality manager must be aware of any external change that occurs to the super plasticizer, for example if a foam appears or if during its use the effectiveness has reduced. If such things happen, the construction quality manager must tell the super plasticizer supplier directly, in order not to face any problems in the future.