- Compressive strength of material
- Calculation of the strength properties of masonry
- Determination of bearing capacity
- Practical advice
There is a lot of debate about which material is best for masonry walls. Today we are on the site Our site will explain what the strength properties of masonry depend on, what to look for when choosing materials and how to avoid common mistakes when erecting load-bearing structures.
Compressive strength of material
When constructing load-bearing walls, modern developers often prefer ceramic bricks produced in accordance with GOST 530–2012. The main argument is the strength grade, according to which this material is inferior only to monolithic concrete. However, you need to understand that the compressive strength of a masonry stone and the entire masonry in general are not identical things..
Confirmation of this is the design manual for SNiP II-22–81. In general terms, it is stated that masonry is a heterogeneous body and its destruction begins long before the application of loads, which are limiting for masonry elements. The reason for this is the transition from squeezing to bending and stretching, which the brick resists very poorly. Such phenomena are a consequence of the irregularity of the shape of the brick, uneven thickness of the joints, the presence of voids and inhomogeneities, laying the stone in an inclined position.
The standard defines stone quality and mortar strength as decisive indicators for determining the bearing properties of masonry. At the same time, due to a weak solution, the total compressive strength can be reduced to 10-15%, and with an irregular shape of stones – up to 5-8% of the strength of the masonry element with the lowest grade.
Some masonry materials, such as gas silicate blocks, completely exclude the effect of the properties of the binder solution on the strength of the entire masonry as a whole. Due to the small thickness of the seams or their complete absence, the transfer of compressive force from stone to stone is achieved. Due to this, the gas silicate wall perceives the load as a monolithic formation, bypassing accompanying deformations and reducing the effect of eccentric loads. However, the compressive strength of the autoclave gas silicate is only 3.5–5 MPa, but at the same time, the masonry strength grade almost completely corresponds to the actual block grade.
A similar effect can be achieved by increasing the thickness of the walls or reducing the number of binder layers. This can be seen on the example of cinder block buildings: due to the increased height of the masonry element, the number of horizontal joints decreases, while the stones themselves have an increased support area, which contributes to a more even distribution of loads.
Calculation of the strength properties of masonry
You can make a fairly simple initial conclusion: any masonry is a kind of sandwich. And the fewer layers there are, the more stable the supporting structure will become..
On the one hand, it is possible to approximately determine the resistance of brickwork to compression using the tables from section 3 of SNiP II-22–81. The main initial data are the compressive strength grades of concrete and stones. In this case, reduction factors should be applied to the tabular data, which are determined by the type of material, its voidness and the quality of the masonry. There are also multiplying factors that are valid, for example, for masonry that has been vibrated or aged for a year..
The data obtained will help determine the ability of the masonry to withstand its own weight and the weight of the superstructure. However, the calculations do not end there. In places where atypical bending, axial and bending tensile loads act, it is necessary to determine the design resistance of the masonry according to separate tables for brick and mortar brands. Examples of zones of action of atypical loads are vertical seams of prefabricated foundations, openings without reinforcing lintels, arches, support points of floor beams in the absence of an armored belt.
But that’s not all. Since the foundation is not an absolutely stable foundation, an allowable deformation threshold should be established, determined by the modulus of elasticity of the masonry. To do this, the calculated compressive resistance is multiplied by the elastic characteristic from the table, as well as by a factor of 2 for bricks and 2.25 for concrete blocks..
For reinforced masonry, the calculation procedure is different: the ultimate resistance is calculated using the formulas for longitudinal and mesh reinforcement, taking into account the percentage of reinforcement in the seams. The elastic characteristic for masonry with and without reinforcement is taken from one table.
Determination of bearing capacity
The bearing capacity of the walls is considered sufficient if, according to the method for determining the limit states of the first group, the total loads do not exceed the ultimate strength of the masonry, taking into account a number of factors. Section 4 of SNiP II-22–81 will help to make calculations, which describes the calculation method for centrally and eccentrically compressed masonry elements.
The centrally compressed masonry includes those in which the vector of application of the force from the acting loads is located on the longitudinal axis. An example of such a case is when a monolithic floor rests on the entire plane of the upper row of masonry. Off-center compression means that the load is applied eccentrically, for example, when the floor is not fully walled into the wall.
If the prefabricated floor rests on the wall pointwise in the places where the beams are bricked up, the calculation for local compression should be carried out. When supporting the rafter system on walls without a Mauerlat, it is necessary to calculate oblique bending loads. For all types of atypical influences, the standard presents calculation methods and diagrams of structural models.
In construction practice, a generally accepted scheme for the construction of brick envelopes has been formed. The bearing layer is represented by unreinforced rubble masonry, the tolerance for the curvature of which is determined by the capabilities of the internal plaster layer. From the outside, a finishing lining is carried out, which does not perform a load-bearing function.
This approach is fully justified: exemplary laying according to the textbook along the entire section of the wall requires time and spending money on the services of a bricklayer. And if the choice of material for such walls with the technology of their construction corresponds to the building standard, if at least a superficial design development was carried out, the choice in favor of such masonry can be considered successful.
However, this technology is not acceptable if the construction is carried out from large-format blocks, especially from autoclaved aerated concrete. Firstly, this material is quite expensive in comparison with rubble brick, its consumption must be carefully calculated in order to avoid overpayment. Secondly, non-compliance with the technology, for example, the use of a binder of dubious origin or poor quality material leads to the fact that the structure will not meet the design parameters..
In this regard, we can give several practical recommendations:
- When choosing a masonry material, it is not its compressive strength that matters, but the correctness and constancy of shape. For private construction, even the M100 brand of stones is excessive, it is much more correct to choose a less branded material, but of a higher grade.
- For masonry, do not prepare a mortar with an excessive cement content. It is imperative to look for a compromise between strength and deformability, because vibrations of the base are transmitted to the masonry, which means the elastic modulus must be high enough.
- Excessive cement in the mortar leads to increased shrinkage. Compression of the joint during curing leads to its flaking from the stone. Due to the appearance of micro gaps, the strength of the masonry is weakened, the wall becomes blown.
- The best binder for masonry is a lime mortar with a small addition of cement. Such seams are not only warmer, they have minimal shrinkage and provide additional deformability. An option for an even better solution – based on fluff and soaked hard-plastic clay.
- Masonry made of large-format materials with a high consistency of shape and size should be made thinly seamed or completely without seams. For example, for laying a cinder block with deviations of up to 3 mm per side, a joint thickness of 6–8 mm is allowed, while the cement content in the solution can be quite high due to the porosity of the material. Aerated concrete of normal quality does not make sense at all to be placed on a cement mortar, only an adhesive mixture, and for calibrated blocks – glue-foam.
- For high-quality masonry, an accompanying geodesy is required, which we have already discussed on the site in this article. It will not only help to avoid eccentricity, but also contribute to better alignment, which means less effort and money will be spent on finishing..
- It is very useful to use devices for standardizing the thickness of horizontal joints. All kinds of mortar stackers allow you to exclude from the calculation the reduction factors dictated by the inhomogeneous thickness of the binder.
- Seams must be filled to the full width and without voids. This also applies to vertical seams: contrary to popular belief, their unfilledness reduces the strength of the masonry and is very noticeable.
- Longitudinal reinforcement does not affect the strength of the seam, but it can increase deformability. Mesh reinforcement does not affect anything at all; it is used to bond multi-layer masonry.