Do you know the importance of kneading water?
Kneading water is the water used to mix and hydrate dry components during concrete production. It is responsible for joining the cement with the aggregates and additives in the mixture, forming a homogeneous mass with an adequate consistency for good workability in the application.
However, kneading water is much more important than just for mixing the mortar. The quality of the water used in the concrete will be fundamental to guarantee its performance in the future, and can profoundly influence the mechanical strength and durability of the concrete element after curing.
Thus, getting to know the kneading water used in the construction in depth is a fundamental step to ensure the quality of your construction. But what is the correct way to use water in concrete, and how do you know if the water used is suitable for this purpose?
NBR 15900:2009 normalizes and answers all these questions regarding water quality and the tests that can attest to its purity. In this article, we will learn more about kneading water and its importance in the construction. Check it out below!
What does NBR 15900:2009 say — kneading water?
NBR 15900, published in 2009 by the Brazilian Technical Standards Agency (ABNT), specifies the requirements for water to be considered suitable for preparing concrete and describes the sampling procedures, as well as the methods for its evaluation. Divided into 11 parts, this standard seeks to establish the requirements for water to be used to knead concrete properly and efficiently.
For this purpose, various types of water are classified and criteria are established to evaluate their chemical properties, as well as their possible contamination with excessive amounts of chlorides, sulfates, alkalis and others. Let's see below how it is possible to determine the quality of the kneading water, according to current Brazilian standards.
Kneading water quality
Briefly, the water you use to mix and cure should not have chemicals, suspended mud particles, or organic growth such as algae, weeds, etc. The general rule used on the construction site is that, if the water can be used for bathing, it can also be used for manufacturing concrete.
Thus, the quality of the water used in construction works is generally the same as that of drinking water, to ensure that the water is reasonably free of impurities such as suspended solids and dissolved salts, which can adversely affect the properties of the concrete.
However, to delimit the characteristics considered suitable for the use of water in concrete, we must follow certain criteria. In addition to the substances mentioned, water must be clean and must not contain sugar or its derivatives, nor sewage or oils, for example.
The use of contaminated kneading water can profoundly affect concrete formation processes, especially adhesion, hardening, strength, durability, and corrosion value.
Thus, the standard classifies kneading water according to its origin, and establishes certain requirements to assess its possible contamination. Let's see these criteria more clearly in the following items.
Classification of water types according to NBR 15900:2009
The NBR clarifies that it is possible to verify whether water is suitable or not for the preparation of concrete according to its origin. Thus, it stipulates 7 different types of water according to their source:
1. Public water supply
The water supplied by the public supply of cities has already been previously treated against impurities and is therefore considered suitable for use in concrete and does not need to be tested.
2. Water recovered from concrete preparation processes
Water recovered from concrete preparation processes water used to clean the inside of concrete mixers in mixing plants, concrete mixer trucks, mixers and pumps, or water from the process of recovering fresh concrete aggregates.
It can be used as water for kneading structural concrete, provided that it meets certain requirements, such as not exceeding established limits of additional mass of solid material in the concrete, and being distributed as evenly as possible when preparing the concrete.
3. Water from underground sources
Water from underground sources may be suitable for use on concrete but must be tested.
4. Natural surface water, rainwater and industrial wastewater
These types of water may be suitable for use on concrete but must be tested to prevent the presence of impurities. Examples of industrial wastewater are those recovered from concrete cooling, sandblasting, cutting, milling, and polishing processes.
5. Brackish water
This water can only be used for unreinforced concrete, but it must be tested. It is not suitable for preparing prestressed or reinforced concrete.
6. Sewage water and water from treated sewage
Under no circumstances is this water considered suitable for use in concrete, in accordance with the standard.
7. Reused water from a sewage treatment plant
Reuse water is a type of water treated by various processes, such as filtration and floating, in sewage treatment plants, starting from the affluent already treated for non-drinkable uses. However, the standard does not clarify whether or not it is suitable for use in concrete, saying that the use of this type of water is conditioned to specific applications in agreement between the water supplier and the person responsible for preparing the concrete, and all other requirements present in the standard must be met.
Purity tests
The standard also specifies tests to be performed to assess the presence of impurities in the kneading water, potentially harmful to the integrity of the concrete. For the use of potentially contaminated water, it is essential to use the NBR recommendations to assess whether it can be applied as kneading water or not.
Amount of Water
In addition to quality, the amount of kneading water to be used must be carefully observed. Often, despite using the best materials and tested water, concrete does not achieve the expected results.
In these cases, this occurs because the water-cement-sand ratio used was lower or higher than the recommended trace. Concrete hardening is a process: when water is mixed with mortar, it reacts with cement and forms a binder paste, helping to fill the spaces between sand grains and creating a cohesion of sand and cement particles.
Often, workers use more water than recommended when making the concrete mix, because this makes the mortar easier to work with. This is a negative practice because excess water weakens the strength of the cement paste and weakens the adhesive quality of the cement.
Another important point is to understand that once the mixture begins to harden, it cannot receive more water because this will reduce the resistance of the mortar.
curing
In addition to kneading water, water also plays a very important role after applying the mortar, in the curing process. The harmful effects of poor healing are not immediately visible, but they can completely compromise the strength and durability of the structure.
Irrigating the mortar for a certain period of time is necessary to control the rate and extent of moisture loss from the concrete, in addition to maintaining the optimum temperature of the concrete for hardening, which means fewer cases of uneven shrinkage and subsequent cracking.
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