Report of the Postgraduate student of KhNUCEA, Director of UABIM Vladimir Kuchugura (Volodymyr Kuchuhura) at the 76th scientific and technical conference, timed to the day of science.

Ministry of Education and Science of Ukraine
Kharkiv National University of Civil Engineering and Architecture

Date: 13 May 2021
Location: Kharkiv National University of Civil Engineering and Architecture
The problem of installation and repair of high-strength floors of industrial and civil buildings is one of the priorities in research. During the preparation of this article, an analysis of organizational and technological solutions for the repair of high-strength floors of industrial and civil buildings, presented by the American Concrete Institute (ACI) and Portland Cement Association (PCA). These solutions will be briefly described in this article.
Crack repair. Before a proper repair can be prescribed, the cause(s) of cracking and impact on the structural capacity of the slab should be determined. The repair can be as simple as gravity feeding a semi-rigid, low-viscosity crack filler into the cleaned crack. Consider the structural integrity of the floor design before implementing a repair. For example, a slab thickness that relies on mechanical load transfer at joints and cracking occurs where mechanical load transfer is not provided, the cracks may not be sufficiently stable for a semi-rigid filler to restore long term load-carrying capacity. In this case, more substantial repairs such as epoxy injection or other stability-restoring devices may be required. Caution is advised to consider the potential for subsequent temperature expansion if a rigid structural epoxy repair is performed.
Fig. 1.1 – Drying-shrinkage cracks such as these are afrequent cause of complaint (Portland Cement Association 2001)
Fig. 1.2 – Plastic shrinkage cracks are caused by rapid loss of mixing water from the surface while the concrete is still plastic (Portland Cement Association 2001)
Fig. 1.3 – Crazing is a network of very fine superficialsurface cracks (Portland Cement Association 2001)
Restoring wear resistance-Before attempting to repair a surface prone to excessive wear, the depth of the low-strength paste should be determined. In some cases, application of a penetrating liquid surface hardener can bring a wearing slab surface to an acceptable level of wear resistance. In other cases, where there is a lack of calcium hydroxides present, diamond polishing may be required to remove a thin layer of weak material from the slab surface. In extreme cases, material with low wear resistance extends to a depth that makes surface hardeners and diamond polishing ineffective and cost-prohibitive. The low-strength material may have to be removed by shot blasting, milling, or both, and a bonded topping installed to restore wear resistance to an acceptable level.

The repair of a dusting slab surface is identical to that of the aforementioned low wear resistance. The depth of the weak material should be determined before attempting a repair method.
Fig. 3.1 – Dusting is evident when a fine, powdery mate­rial can be easily rubbed off the surface of a slab (Portland Cement Association 2001)
Repair of scaling and mortar flaking-If the scaling or mortar flaking was the result of finishing or curing the concrete with methods that decrease the effectiveness of the air-void system at the surface or impact strength development of the mortar, success has been observed by removing the non-air-entrained or weak surface. Removal should extend to a depth where an adequate air void system and mortar strength exist. Where scaled concrete is found to be dense (high compressive strength) with relatively low chloride penetration from deicing salts, as determined by laboratory studies, a proper penetrating sealer applied periodically to the repaired surface will prolong the life of the slab surface. If necessary, a bonded topping can be installed to maintain existing slab surface elevation. In conditions where large temperature swings occur, the topping material should have a coefficient of thermal expansion similar to that of the existing slab. If the entire slab was not adequately air entrained, a 2 in. (50 mm) thick, low-permeability, air-entrained topping should be used. Hard troweling is not recommended for air-entrained concrete.
Fig. 4.1 – Scaling is the loss of surface mortar, usually exposing the coarse aggregate (Portland Cement Associa­tion 2001)
Fig. 4.2 – Mortar flaking over coarse aggregate particles is another form of scaling that resembles a surface with popouts (Portland Cement Association 2001)
Measures that can be taken to alleviate the problem are:
a) Switching to a non-offending source of aggregate for floors and slabs, if possible
b) Using two-course construction with selected or imported aggregate without popout potential for the topping course
c) Using aggregates from which the offending particles have been removed by heavy-media separation, if available and economically feasible
d) Using wet-curing methods such as continuous fogging or covering with wet burlap immediately after final finishing. If reactive aggregates are used in the concrete, alkali-aggregate reactions near the slab surface can be problematic, as the internal restraint is minimal, resulting in little resistance to popouts. Wet-cure for a minimum of 7 days, as wet curing can greatly reduce or eliminate popouts caused by alkali-aggregate reactivity (Landgren and Hadley 2002). For alkali reactions to occur, an internal relative humidity (RH) of 80 percent is required. Often, the slab surface quickly dries to an RH below 80 percent. Impervious floor coverings or membranes, however, can result in an increase of moisture in this region, restarting the alkali-aggregate reaction, resulting in popout development, which is sometimes mistaken for blisters beneath flooring.
e) Using the lowest practical slump possible to prevent potential popout-causing particles from floating to the surface

In some areas and situations, these measures may not be practical. Specific local practices have been developed that have been helpful in minimizing popouts. For example, in some regions, concrete producers can supply popout-free concrete. Note, however, that while a popout-free aggregate will reduce the number of popouts that occur, it does not necessarily result in a slab surface without any popouts.
Fig. 5.1 – A popout is a small fragment of concrete broken away from the surface of a slab due to internal pressure, leaving a shallow, typically conical, depression (Portland Cement Association 2001)
To avoid blisters and delaminations, the following should be considered:
a) Avoid the use of purposely air-entrained concrete when the surface is finished using power equipment.
b) Avoid the use of concrete mixtures with a high water content, high mortar fraction, or both.
c) Avoid the use of concrete with excessively high slump.
d) Use appropriate cement contents (Table 8.4. lb).
e) Warm the base before placing concrete during cool weather. During hot, dry, windy weather, reduce evaporation over the slab by using an evaporation retardant (monomo-lecular film), a fog spray, or a slab cover (polyethylene film or wet burlap).
f) When placing a slab directly on a vapor retarder/barrier, consider the potential for a prolonged bleed period. Sometimes, the use of a minimum 4 in. (100 mm) thick layer of trimmable, compactable granular fill (not sand) to separate the vapor retarder/barrier from the concrete (6. 1 .5) alleviates finishing difficulties, but this detail is not recommended when either moisture-sensitive flooring or storage products are anticipated.
g) Avoid overworking the concrete, especially with vibrating screeds, jitterbugs, or bull floats. Overworking causes coarse aggregate to settle and bleed water and excess fines to rise to the surface. Properly vibrate concrete to release entrapped air.
h) Do not attempt to seal (finish) the surface too soon. Hand floating should be started when a worker standing on a slab makes a 1/4 in. (6 mm) footprint. For machine floating, the footprint should be only approximately 1/8 in. (3 mm) deep. If moisture is deficient, a magnesium float should be used.
i) Avoid early sealing during initial surface straightening and floating. Magnesium or aluminum tools should be used on air-entrained concrete. Slabs that incorporate a shake- on surface hardener are more prone to blister if the surface does not allow proper integration of the material (10.6. 1 and 1 0.6.2).
j) Use proper finishing techniques and proper timing during and between finishing operations (1 0.3). The formation of blisters is an immediate indication that the angle of the trowel is too great for the surface in that area at that particular time with the concrete and job conditions involved. The position of the trowel should be flattened and the blistered area retroweled immediately to eliminate and rebond the blisters. If frequent blistering occurs despite reasonable care in the timing and technique employed in the finished troweling, attention should be directed to the job and climatic conditions and to the concrete mixture as discussed as follows.

Most skilled finishers know when a concrete surface is ready for the increased blade angle and final troweling and closing of the surface, and how to accomplish this operation; however, circumstances are often beyond their control. For instance, if there are too few finishers for the climatic conditions, finishers may have to close some portions of a floor too early to get it troweled before it has set too much. Similarly, if supervisors insist that a floor be finished by a certain time, whether it is ready or not, blisters, trowel marks, and poor surfaces can result.
Fig. 6. 1 – Blisters (Courtesy of NRMCA)
The repair of joint spalling depends on the magnitude of the damage. In general, joints spalled less than 1/2 in. ( 1 3mm) wide can typically be routed out and refilled with a semi-rigid filler. If joint widening is essentially complete, a rapid-curing polyurea joint filler can be used. Joints spalled greater than 1/2 in. (1 3 mm) may require renosing by installing a vertical saw-cut along the spall perimeter, removing distressed concrete within the spalled area, installing a repair mortar, and reinstalling the joint and filler. In the most severe cases, when the spalling has reached a depth greater than half the slab thickness, full-depth repairs consisting of doweled mini-slabs may be necessary.
Fig. 7.1 – Spalled joint (courtesy of E. Tipping)
Fig. 7.1 – Spalled crack (courtesy of E. Tipping)
Thus, in the design of the production of repair work for high-strength floors of civil and industrial buildings, it is necessary to take into account as much as possible the data of the examination of the technical condition of existing structures, comply with the technological requirements of the processes of removing defective, damaged areas, preparation for repair, and use materials with similar indicators. This is the only way to ensure the organizational and technological reliability of the parameters of the technological process and the optimality of its technical and economic indicators.
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