Foundation assessment

Before starting work, the layer must check the base for compliance with basic requirements. If the preparedness of the foundation is in doubt, it is recommended to notify the Customer in writing of all identified deficiencies. In this case, it is necessary to indicate the possible consequences that will arise if the Customer decides to ignore these comments.

Complete data on the floor structure, such as the type of screed, types of mortars used, thickness of individual layers, insulation, sealing of building joints and sedimentary joints are determined during the design and planning of construction work. The relevant data must be contained in the list of floor installation works. The paver cannot begin work without the above information, since different types of preparatory work are carried out for different types of bases. Thus, in order to carry out high-quality finishing work, the craftsman needs to familiarize himself with the design documentation of the facility or obtain from the Customer accurate and reliable information about the floor structure. But what to do if there are no such documents, and the Customer cannot clarify the situation? You can only rely on your own professionalism, observation and hard work.

So, firstly, it is necessary to determine the material and design of the base. This needs to be done in order to assess what load it can withstand, as well as to select the right materials for preparing the base for laying ceramic tiles. Sometimes this is quite difficult to figure out, since the layer sees only the surface that needs to be covered and cannot be responsible for the design of the base as a whole. However, this does not mean that you need to distance yourself from these issues; on the contrary, your work should be guided by the rule “ more information about the object - a more predictable result .” Secondly, check the base for evenness, since an uneven base will lead to excess material consumption or affect the evenness of the coating. Thirdly, check the base for residual moisture, in order to avoid the formation of a “greenhouse effect” under the cladding. Fourthly, check the base for strength and absence of contamination. It is recommended to check using special measuring instruments.

Checking the flatness of the subfloor surface

Tools : two-meter control rod to check the evenness of the base in accordance with SNiP 3.04.01.87 ;
Methods: the deviation of the surface of the floor base from the horizontal plane over a length of 2 m should not exceed 10 mm - for a base under a screed, or 4 mm - directly under laying ceramic tiles (see SNiP 3.03.01-87).
To determine the deviation of the subfloor surface from the norm, it is recommended to use the 2-meter rule and a measuring wedge with a millimeter scale applied. If the base has an uneven profile, we lay the rule between the support points. Using a measuring wedge, we determine the largest deviation between the horizon (staff) and the maximum point of deviation. Measuring on protruding ends (if the edge of the rule is hanging in the air) is not allowed. Irregularities that do not meet the standard specified above must be leveled with appropriate compounds. Large irregularities or holes are filled with quick-drying (hardening) repair solutions before starting work with leveling compounds.

Checking the base for exceeding the maximum permissible humidity values

A wet screed requires a certain amount of drying time.
Drying time depends on many factors. Here it is necessary to take into account the type of base, its thickness, as well as the climatic conditions in the room. There is always some residual moisture remaining. This moisture, defined as “domestic” or “compensatory”, remains in the structure for a long time. The base is considered ready for installation when household humidity is reached. Excessive moisture can lead to severe damage during operation; this is especially pronounced when facing the surface with porcelain stoneware, due to its low water absorption and vapor permeability. Instruments: moisture meters, moisture analyzers, SM device (measurements based on the calcium carbide method).
Humidity measurement methods:
RH method (moisture analyzer test). Measuring humidity using a condensation protimeter.
To do this, a hole is drilled in the base and a special capsule is inserted. After reaching equilibrium air moisture in the capsule (usually 12-24 hours), the air humidity in the capsule is measured - we obtain the RH value in %. Measuring residual humidity using electronic moisture meters (hygrometer test) .
The operating principle of these devices is based on the correlation dependence of the dielectric constant of building materials on their moisture content. These devices do not measure the moisture content of the base, but the dielectric constant, and then the gravimetric moisture content is determined using available tables. This method also includes devices that measure the electrical resistance between electrodes immersed in the base at a certain distance from each other - the method of conductometric hygrometry. After measuring using the appropriate table, the residual moisture value is obtained. SM laboratory method: a sample of the material is taken using a hammer and chisel.
It is important that samples are taken from the bottom 1/3 of the base thickness. The sample is crushed and weighed (for concrete bases 20-50 g, for anhydrite bases - 100 g). Next, the weighed and crushed material is placed in a flask along with a carbide ampoule and steel balls. The flask is tightly closed and shaken several times. Moisture from the sample reacts with the carbide, forming a gas. The readings of a pressure gauge measuring gas pressure are determined from the table as the level of residual moisture in CM%. Method for determining the gravimetric moisture content of the base (laboratory) : a sample is taken from the lower 1/3 of the thickness of the base, crushed and carefully weighed (in grams to the fourth digit), heated to a temperature above 100 ^o C (102-105 ^o C) and maintained at 0.5-1 hours, then weighs himself again. The process is repeated until the weight of the sample becomes constant. The difference between the initial and final weights of the sample is the weight difference in grams, and the ratio of the weight of water to the initial weight of the sample is the percentage weight humidity. The process of determining the weight moisture for various building materials is regulated and is carried out mainly in certified construction laboratories. Currently, portable instruments for determining gravimetric humidity have appeared. They combine very precise scales and heating elements, have a microprocessor and immediately give the value of the weight moisture in the sample.

In a situation where there are no measuring instruments at hand, the readiness of the base can be judged based on the following:
Film method for assessing residual moisture. This method is good because any layer can do it by taking polyethylene and tape and clearing the area.
You need to take a piece of plastic film measuring 1x1 m. Secure it around the entire perimeter with adhesive tape. The base can be considered suitable for further work if after 24 hours the underside of the film is completely dry. The residual moisture value cannot be obtained using this method, but you will clearly see whether the base is dry or wet. There is also a calculation method for determining the drying time of cement and concrete bases . These are formulas obtained empirically at the French Institute of Concrete hcm*hcm*1.6=tdays (for a screed 8 cm thick we get 8*8*1.6=102 days, respectively). Naturally, the formula does not take into account the characteristics of the materials, the temperature and humidity conditions of drying, the design of the base, etc. But for a preliminary assessment of the residual moisture of cement bases (at a water-cement ratio of 0.4-0.5) for standard conditions (+20 ⁰ C and 60% relative air humidity), it gives a period faster than which the screed is unlikely to dry. Accordingly, you can determine whether it is worth using other methods of measuring humidity or not.

Table comparison of different methods for measuring residual moisture in the base:

Red color – excess moisture, laying coverings is not allowed.

Checking the foundation for strength

Tools: Kashkarov hammer, Ri-Ri device, etc.
Methods: The surface must be carefully inspected, the base must be strong and uniform in appearance. It is also recommended to tap the base. If, when tapping the screed or plaster, you find places with peeling, they need to be cut out and repaired. When placed on such a base, the tile will accelerate the peeling of the weak layer. Sometimes a dull sound is produced by a strong screed, at the base of which there is insulation - expanded clay, slag, etc. If large and strong layers are motionless in relation to each other, then such a screed can be used as a base for laying tiles. If the places of delamination are small and can be easily knocked out with a punch, they must be removed. After removal, the damaged area must be restored. Even if the base does not make a hollow sound when tapped, it may not be suitable for laying tiles. The fact is that the strength of such a layer may not withstand the load carried by the cladding. In this case, spontaneous tearing of the tile with adhesive along with the top layer of the base is possible. Experts determine the strength of concrete or cement screed using a special tool (Kashkarov hammer or analogues). They hit the surface, and then the strength of the mortar or concrete is determined by the size of the dent and the force of the impact using tables. In addition, to check the strength of the base, the method of applying notches to the surface of the base is used. You can use a special tool (Ri-Ri device, pre-set by spring tension to the level of strength required from the base). Using the device and template, 6 notches are made. Then the templates are turned at an angle and 6 more notches are made so that a diamond forms between the notches. The device should be pressed tightly against the template. If the sharp corners of the diamond are the same without chips, the lines are clear and sharp, then the screed corresponds to the specified value. Otherwise, the screed is unsuitable for the selected test stage.

Contaminated surfaces also pose a great danger . In addition to the fact that many types of subfloor contamination can lead to staining of the floor covering due to the migration of pigments to the surface, such stains can negatively affect the adhesion of primer, putty, adhesive, etc. All chemicals such as oils, wax, paint residues located on the subfloor must be completely removed. Laying coatings on a contaminated screed is prohibited .

Summarize . When assessing the substrate, special attention must be paid to the following defects (for a complete list see DIN 18365):

1. unevenness;
2. cracks in the base;
3. exceeding the maximum permissible moisture levels in the base;
4. insufficiently durable surface;
5. dirty surfaces, grease stains;
6. incorrect horizontal position;
7. unsuitable indoor climate conditions;

All these defects are fraught with serious problems in the future. And eliminating them at the preparatory stage is much cheaper and easier than dealing with the consequences, which sooner or later will make themselves felt. A good home begins with a strong foundation, and high-quality ceramic tile installation begins with a reliable, prepared foundation.

The article was based on the following materials:
“Instructions for laying floor coverings” by Tarkett;
"Residual moisture of the base" Rudchenko Yu.A. Uzin company.