The main focus of the foundation building process is on the formwork and casting requirements concrete mortar... However, the issues affecting the removal of the formwork are no less important. In particular, it is necessary to take into account the time interval during which a sufficient concrete strength is gained, as well as the technology of the dismantling of the structure itself.
The information gathered from laboratory research can also be used to develop a concrete repair mix design. As with other historic structures, concrete structures are cleaned for several reasons: to improve the appearance of concrete as a cyclical measure maintenance or in preparation for repair. The first question to be considered is whether the historical concrete structure should be removed at all. If cleaning is required, you should choose the softest system that will be effective.
Three main methods are used for cleaning concrete: water-based methods, abrasive surface treatment and chemical surface treatment. Low pressure water or steam purification can effectively remove surface contamination from sonic concrete; however, delicate or damaged surfaces require care. In addition, water vapor and steam methods are usually not effective for removing stains or heavy dirt. Powerful high pressure water washes are sometimes used to clean or remove coatings from sonic, high strength concrete, but high pressure washers are usually damaging and not suitable for concrete in historic buildings.
Standard time for formwork removal
There is an opinion that the optimal period for solidification of the foundation is 28 days... However, the answer to the question of when to remove the formwork after the concrete is poured in each case will be individual... The main regulatory parameter, from which one should build on when determining the time the structure is ready for dismantling, is tempering strength... This characteristic, in turn, depends on the following data:
Impact echo testing is performed on a concrete slab to help determine the depth of wear. In this method, a short pulse of energy is injected into the structure, and a transducer mounted on the impact surface of the structure receives reflected input waves or echoes. These waves are analyzed to help identify imperfections and deterioration in the concrete.
When used with proper control and at very low pressures, a micro-abrasive surface treatment using very fine particles such as dolomite limestone powder can sometimes be effectively cleaned. However, microabrasive cleaning can change the texture and reflectivity of the concrete surface. Some concrete can be damaged even by fine particles applied at very low pressures.
- Type and dimensions of the structure;
- Concrete grade used;
- Ambient temperature;
- Other hardening conditions;
- Loading size.
Chemical surface treatment can effectively clean, but can also alter the appearance of concrete by bleaching the concrete, removing the paste, etching the aggregate, or otherwise altering the surface. Detergent detergents or mild diluted acidic cleaners may be suitable for removing stains or stubborn dirt. Cleaning agents containing strong acids, such as hydrochloric acid or hydrofluoric acid, which can damage concrete and are harmful to people, animals, site and environment, should not be used.
The main criterion for determining the waiting time for concrete hardening is its appearance. In particular, the tempering strength for heavy concretes and mortars based on porous additives is 50%, and with autoclave hardening, the OP will reach 100%. When the structure reaches its maximum strength, the formwork can be dismantled. In other cases, the same time period is valid. 28 days... According to the standards, concrete for the foundation must acquire sufficient load strength no later than this period.
For any cleaning process, test samples must be performed prior to full implementation. The purpose of a cleaning program should not be to revert the structure to a new look. Concrete can grow gracefully, and as long as the contamination is not serious or harmful, many structures can still be assessed without thorough cleaning.
Maintenance and repair methods
The maintenance of historic concrete is often considered in terms of proper cleaning to remove unattractive dirt or contaminants. However, the implementation of the overall maintenance plan for the historical structure is the most effective way protection of historical concrete. For example, lack of maintenance on roofs and drainage systems can contribute to water damage associated with adjacent concrete structures. Reuse of antioxidant salts in winter climates can scour the surface of old concrete and can also promote decomposition in embedded steel reinforcements.
In addition, there are visual signs that signal that the formwork can be removed. For example, to determine when to remove the formwork from strip foundation, should pay attention to plywood boards... If they began to move away from their original position, this indicates that the concrete gives up water and the formwork can be dismantled. It usually takes 7 to 10 days for gaps to form between the plywood and the concrete base.
Influence of temperature on the timing of demoulding
Inadequate protection concrete wallsadjacent to access roads and parking areas may require repairs later. The preservation of historic concrete includes regular testing of the concrete to establish baselines and determine the necessary repairs. Inspection tasks include protection control systems including sealing joints, expansion joints and protective coatings; consideration of existing conditions for disaster development such as cracking and delamination; compliance with the terms of registration; and the development and implementation of a cyclic repair program.
To more accurately determine these terms, specialists use monitoring tools. In it, the calculations of the viscosity of concrete and its thickness are performed, taking into account the characteristics of the aggregates used. From the data obtained, the time for a more suitable formwork removal is calculated.
Stripping of monolithic unheated structures
Figure 17 The 63rd Street Beach House was built on the Chicago waterfront. High-carbon concrete on all sides of the beach house has been used for many buildings in Chicago parks as an alternative to more expensive stone construction. Photography: Leslie Schwartz Photography. Concrete failure included cracking, flaking and delamination caused by embedded rebar corrosion and concrete damage from freeze-thaw cycling. Various sizes and types of aggregates were compared to match the original concrete materials.
Monolithic foundations, which are installed without using heating technologies using a thermos, are demoulding in accordance with the regulatory document of the TP series. The following types of formwork are covered by this standard: side panel formwork on foundations, columns, walls and beams, load-bearing shield elements, structures with prestressing reinforcement, load-bearing formwork with spans about 6 m long, etc.
For comparison with the original concrete, models of the concrete repair mix were prepared. Considerations included cumulative type and size, cement color, proportions, cumulative exposure, and surface finish. The wizard created a surface to reproduce the original appearance in the layout on the structure. Here he used a nylon bristle brush to remove any loose paste and expose the aggregate, creating a variable surface to match the adjacent original concrete.
Sealants are an important part of maintaining historic concrete structures. Elastomeric sealants, which have replaced traditional oil-based fillers for many applications, are used to seal cracks and joints to avoid moisture and reduce air infiltration. Sealants are commonly used in the perimeters of windows and doors, at the joints between concrete and other materials, and in hinges to or across walls or roofs, such as lamps, signage or outdoor plumbing fixtures.
To determine the time when to remove the formwork after pouring the girder, column and other structures of this group, one should take into account the indicators of the actual load relative to the standard. Settable waiting time range - 6 to 72 hours... The main parameter for setting a specific time interval is the load, which can vary from 70 to 100% in relation to the design.
If used to repair cracks in historic facades, the finished sealant application needs to be considered as it can be visually intrusive. In some cases, sand can be transferred to the surface of the sealant to help hide the repair.
Urethane and polyurethane sealants are often used to seal joints and cracks in concrete structures, slabs and walkways; these sealants provide a service life of up to ten years. High performance silicone sealants are also often used with concrete as they provide a wide range of motion capabilities and a service life of 20 years or more. Some silicone sealants can stain adjacent materials, which can be a problem with more porous concrete and can also accumulate dust and dirt.
It is important not to forget that full loading of the structure is possible only from the moment when the concrete base will acquire full strength in accordance with the design index... In the future, all activities related to work on the site with reinforced concrete structure and its maintenance must be carried out on the condition that the strength of the concrete structure is at least 1.5 MPa... If intermediate spans are used, the ultimate strength for stripping can be reduced. In other words, the presence of supports increases the turnover of the concrete mass and shortens the construction time.
The effectiveness of joint and crack sealants depends on many factors, including proper surface preparation and application. Sealants should be considered as part of regular maintenance checks as these materials deteriorate faster than their bases and need to be replaced periodically as part of a cyclic service.
Repairs to historic concrete may be required to eliminate wear because the original design and construction do not provide long-term durability or facilitate a change in design use. Examples include increasing the concrete pavement to protect the reinforcing steel and reducing the penetration of water into the structure by repairing joints. Any such improvements must be carefully assessed for compatibility with the original design and appearance. Maintenance is required in all aspects of historic concrete renovation, including surface preparation; installation of formwork; development of the construction of concrete mixes; and placement, consolidation and curing of concrete.
Influence of temperature on the timing of demoulding
The dependence of the time of gaining strength on the temperature regime can be traced by the example of two concretes - from portland cement and slag Portland cement... The first, at a temperature of 10 ° C or more, gains for 5 days about 50% of the strength of the design indicator, and the mortar on slag Portland cement under similar conditions reaches the same state for 2 weeks... For comparison, in winter, with a negative temperature regime, these periods can increase to 3 months... And this is provided that the builders make antifreeze additives. Accordingly, in order to speed up the hardening process, when carrying out winter concreting, it is better to use electric heating systems.
Influence of air humidity
An appropriate repair program aims to address the existing disaster and reduce the rate of future deterioration, which in many cases is associated with moisture-related problems. The renovation program should include materials and methods that are sympathetic in nature and appearance to existing materials and that provide good long-term performance. In addition, the repair materials must withstand and withstand similar source materials... To achieve these goals, specific repair projects should be divided into three phases: development of test repair procedures, test repair and evaluation, and renovation work.
Proper maintenance also contributes to a quick set of strength. Concrete should be protected from direct contact with precipitation, sun and wind rays. In addition, a common question is when to remove the formwork after pouring the floor, provided that the optimal thermal and humidity conditions for hardening are created. If regular watering is carried out, and during the day the concreted base is covered with a film, then, provided the temperature is positive, the stripping can be performed on average after 10 days.
For any particular renovation project, the research process, laboratory analysis, prototypes, mock-ups and full-scale repairs allow continuous improvement of the repair work as well as the implementation of quality control measures. The repair trial process enables the owner, architect, engineer and contractor to evaluate the concrete mix design and installation and finishing methods for the renovation from a technical and aesthetic point of view. Final renovation materials and procedures must match the original concrete in appearance, subject to established criteria durability.
In device technology monolithic structures important stages of work are considered not only the competent installation of the formwork, pouring the concrete mixture and caring for it during the period of strength gain. No less important is the question, to which even specialists cannot give a sure answer. Among them long years there is a debate about when it will be more correct to remove the formwork from the foundation - before or after the generally accepted 28 days required for concrete to gain brand strength. It turns out that not everything is so unambiguous in the regulatory documents that indicate different terms and the conditions for starting the dismantling of the auxiliary structure. After how many days can the formwork be removed so that the concrete monolith does not collapse and what is the threat of its premature removal?
The information gathered through trial repairs and layouts is invaluable in clarifying construction documents before starting a general repair project. When preparing a surface for a historic concrete renovation, care must be taken to limit the removal of existing material while maintaining a proper repair substrate. This is especially important when it comes to ornamentation and small details. Preparing for a local repair usually starts with removing concrete concreteto determine the overall extent of the repair, and then saw around the perimeter of the repair area.
What is formwork
The device of a monolithic strip foundation begins with the installation of a formwork, which is a box without a top cover or only vertical walls located at a distance from each other. Internal dimensions and formwork configuration correspond to the design dimensions and outlines underground structure... With the help of removable or non-removable shields, monolithic parts of a building of a straight and complex shape are erected, which is one of the advantages of the technology. Other advantages include:- lack of seams and size restrictions;
- profitability;
- strength;
- the possibility of self-filling;
- carrying out zero work without involving lifting mechanisms.
The repair area should extend beyond the concrete failure area sufficiently to provide a sound underlay. When repairing concrete with exposed aggregates or other special surface textures, the edge of the saw blade may be too visual. To hide the edge of the repair, techniques such as lightly hand cleaning the edge of the patch can be used to hide the connection between the original concrete and the new repair material. The depth to which the concrete needs to be removed can be difficult to determine without invasive probing in the repair area.
Formwork is installed around the perimeter monolithic foundation... The panels are fixed using universal fasteners, spacers and supports. For the device of a removable structure, materials are selected that have a low degree of adhesion to concrete. The most commonly used wood planks, plywood, metal sheets or plastic.
This procedure greatly simplifies the dismantling of the formwork a few days after the foundation is poured. In this case, you do not have to remove the wooden boards or boards using impact forces. As a result, the integrity of the monolith is not compromised.
When can the formwork be removed?
It is generally accepted that concrete gains brand strength only 28 days after the foundation is poured. But this only happens under certain temperature conditions. Approximate data can be found in the table.
In addition, air humidity affects the process of setting and curing the monolith. It is not for nothing that the cement surface is abundantly moistened with water for the first few days and covered with a film, preventing moisture from evaporating too quickly. Otherwise, the cement will not hydrate properly and the concrete will not be able to gain the required strength. Cracks will appear in it, the sealing of which will not lead to the desired result. Eventually, load bearing capacity the foundation, even after 30 days, will not be sufficient to withstand the loads from the aboveground part of the house.
But is it necessary to leave the formwork for such a long time? Indeed, for a large house, in order to save money, it has to be rearranged from site to site, pouring the foundation in stages. How long, in this case, will it take to carry out zero work? This is where disagreements begin.
The SNiP 3.03.01-87 standards state that the minimum strength of a concrete structure, when it becomes safe to remove the formwork from the foundation, should be indicated in an individually executed or typical PPR. But private developers do not always have such a document on hand. In this situation, you can be guided by the following principles for the timing of dismantling the box or side walls from the concrete monolith:
- for horizontal and inclined elements - after gaining 70 percent strength with a structure length of up to six meters and 80 percent - with a length of more than six meters;
- for vertical elements - when the strength reaches 0.2 ... 0.3 MPa;
- to allow people to move around concrete surface or, if necessary, pouring of the structures located above - after the monolith has acquired a strength of more than 1.5 MPa.
Based on the data in the table, it can be determined that at a 20-degree average daily ambient temperature, the formwork from a monolithic foundation can be removed no earlier than six to eight days, and with a five-degree temperature, it is better to wait 28 days. But this is in the event that it is observed proper care behind concrete during its setting.
The dismantling of the formwork on the unloaded sections of the foundation is allowed to be performed earlier - after the concrete has been set at 50% strength. In this case, it is necessary to adhere to the rule - the structure must have a lower support over the entire area. For grillages towering above the ground, such a formwork condition is not acceptable.
You can find out the degree of hardening of concrete by means of control elements made in the process of pouring the foundation.
The data indicated in the above table are conditional, since a lot of factors affect the solidification process of the monolith:
- dimensions of the foundation;
- temperature and humidity conditions;
- use of additives in solution;
- quality of concrete care, etc.
It would be wiser to leave the structure alone until it reaches its design strength. But if it is necessary to remove the formwork after one or two weeks, then it is imperative to examine the control element. You should not bet on the numbers provided in the table alone.
The optimal season for the foundation is considered to be early autumn. During this period, there is no particular heat, and frosty nights do not come yet. There is nothing wrong with the fact that the future foundation of the house will have to spend the winter. This is even beneficial - in the spring there will be possible subsidence of the soil under load, and the quality of the monolith will make itself felt after frost and thaw. But it is not recommended to leave the structure in the formwork for a long time. In difficult weather conditions, wood will begin to swell and freeze, which can lead to a violation of the integrity of the structure of the concrete stone, and the metal will certainly interest those who like to profit at someone else's expense.
In fact, there is no need to rush to remove the formwork, especially in the case of self-development. Haste will not lead to anything good, except for deformations of the monolith, chips and cracks. It is almost impossible to determine at home after how many days the concrete will harden so that dismantling the box does not entail a loss of structural strength. And why do everything in a hurry? A solid house should be built thoroughly!
Dismantling the box should be started from the corners and protruding parts of the foundation. The fact is that hardening occurs much faster in these areas. In addition, until the turn has come to remove the formwork from the main massif, the monolith will continue to gain strength, being in a rigid form.
It is necessary to remove the shields carefully so that the integrity of the structure is not violated. This means that it is not necessary to tear off and beat off the formwork with the help of available tools. Otherwise, there will be a high probability that the edges will chip or pieces of concrete will fall off.
The formwork is dismantled from top to bottom. In this case, the following sequence must be observed:
- remove the upper metal ties after loosening the bolts;
- release the lower ties (they will remain in the body of the concrete monolith);
- separate the shields from the racks;
- remove the props;
- carefully remove the formwork.
Depending on the design of the box and the material of its manufacture, during the dismantling process, any work may or may not be present. But the very essence of removing the formwork remains. It is allowed to load the finished structure only after the concrete has set the brand strength. Under certain conditions, this can take more than 28 days. The foundation is not easy supporting structure, but the basis of the whole house, therefore, when building it, it is necessary to take into account all possible nuances.
