What Is The Purpose Of Maintenance Of Concrete Laser leveling Machine?

    The determination of the initial setting time of concrete and the timing of the entry of the power trowel machine are the keys to ensuring that the floor is flat, solid and free of cracks. Below, I will elaborate in detail in two parts: one is how to determine the initial setting time of concrete, and the other is how to grasp the best time for the power trowel to enter the site for operation. Initial setting refers to a "critical point" at which concrete begins to transform from a plastic state to a solid state. At this point, the concrete loses its fluidity but has not yet acquired strength. According to the national standard "Standard Test Methods for Performance of Ordinary Concrete Mixtures", the initial setting time is scientifically determined by a penetration resistance meter. Method: Sift out the mortar from the concrete mixture and load it into a specific container. From the start of pouring, test with a penetration resistance meter at regular intervals (such as 30 minutes). The determination criterion: When the penetration resistance reaches 3.5 MPa, the corresponding time is the initial setting time. Applicable scenarios: Laboratories or major project sites, where precise data is required to guide construction.     In actual construction, workers rely on simple and effective empirical methods: Acupressure method (the most classic) : Operation: Press the concrete surface firmly with your fingers (or feet). Judgment Too early: If the fingers can easily press down very deep and cement slurry overflows, it indicates that it is far from beginning to set. Close to initial setting: When pressed hard, only a clear fingerprint of 3-5 millimeters is left on the surface, but no cement slurry overflows. This is a very important signal. Initial condensation: It requires a great deal of force to leave a very shallow mark. Footprint method Operation: Construction workers stand on the concrete surface and observe the sinking of the footprints. Judgment Too early: Deep footprints, obvious subsidence, and cement slurry being squeezed out around. Close to initial setting: The depth of the footprints is relatively shallow (about 5mm), and the edges of the footprints are clear, with no obvious precipitation of slurry water. At this point, people can walk on it, but there is a slight sense of sinking. Scratch method Operation: Use a nail, screwdriver or power trowel to make a mark on the concrete surface. JudgmentToo early: The scratch will close quickly and be filled with cement slurry. Near initial setting: The scratch can maintain its shape, with the edge slightly raised and no longer closed. This is a strong indication that the first power trowel operation can begin.   The entry and operation of the power trowel machine are not completed in one go, but are carried out in stages. Mistiming can lead to serious consequences: Entering the site too early: The concrete is too soft, and the machine will sink and stick to the slurry, damaging the concrete structure and causing the surface to peel and the strength to decrease. Entering the site too late: The concrete is already too hard, and the power trowel machine cannot effectively lift the slurry and smooth it out, which is time-consuming and laborious, and is prone to surface cracking. The best time and process for work The operation of a power trowel machine is usually divided into two key stages: Phase One: Initial smoothing (lifting the slurry and finishing the surface) Timing: The concrete is at or slightly above its initial setting point. Specific status When a person stands on it, the depth of the footprints is about 3 to 5 millimeters. By using the scratch method, the scratch can remain clear. When the power trowel blade is placed on it, it will not sink obviously. Assignment purpose: Press the aggregates (stones) slightly into the lower layer. Lift the surface cement slurry up. Carry out large-scale preliminary leveling. Key points of operation The operation is carried out using a disc (rubbing disc). The machine should not run too fast. It should move smoothly and slowly. Pay attention to overlapping areas that have been worked on to ensure no omissions. Phase Two: Fine grinding and polishing Timing: After the initial smoothing, when the moisture and luster on the concrete surface disappear and almost no footprints are left when stepped on (approximately 1-3 hours after the initial setting, depending on the environment). Specific status The color of the concrete surface begins to darken and become darker. There is a rough feeling when touched, but no hard or brittle sensation. At this point, the concrete has already possessed an initial load-bearing capacity. Assignment purpose: Eliminate the tiny scratches left by the disc operation. Further compact and smooth the surface to achieve the required flatness and smoothness. Seal the surface capillary pores to enhance wear resistance and impermeability. Key points of operation Remove the disc and use a blade (spatula) to carry out the operation. The rotational speed and moving speed of the machine can be appropriately increased. Cross-operation, multiple times of polishing, each time more meticulous than the previous one. Stage Description of concrete state Test method Operation of the power trowel machine Purpose Waiting period Soft, deep footprints, with bleeding Finger pressure/footprint method No entry allowed Let the concrete settle naturally and bleed water to evaporate Initial freezing point The footprints are 3 to 5mm deep, the scratches are clear, and there is no bleeding Finger pressure/scratch method Prepare to enter Determine the timing of the first operation First assignment Reach or slightly exceed the initial freezing point The same as above Install the disc and smooth it out at low speed Slurry extraction and initial leveling Fine grinding operation The surface water luster disappears, and the footprints are extremely shallow or non-existent Observe the color and footprints Replace the blade and polish at medium to high speed Compact, smooth surface, and seal Environmental factors influencing timing Temperature and wind speed: When the temperature is high and the wind is strong, water evaporation is fast, the initial setting time will be shortened, and all processes need to be advanced. Humidity: When the humidity is high, water evaporation is slow, and the initial setting time will be prolonged. Patience is required. The concrete mix ratio: The type of cement, water-cement ratio, and admixtures (such as retarders) all significantly affect the initial setting time. Final suggestion For important floor construction, "better late than early" is a safer principle. A little later, at most it will take more effort. However, entering the site too early will cause irreversible damage to the concrete structure. Skilled construction workers will dynamically adjust the entry and operation time of the power trowel machine through continuous observation and simple tests.   Note: The parameters provided in this document are for reference only and are not mandatory. Due to differences in technical characteristics between different brands and models of laser levelers, please consult the manufacturer for a suitable solution before actual operation. This reference document assumes no responsibility for any issues arising from failure to follow the manufacturer's instructions.    

    Technical objective: Through standardized technical management, precise process control, and scientific risk prediction, quality and safety hazards such as collapse, cracking, and leakage in concrete projects are eliminated from the technical level, ensuring the safety of personnel, equipment, and structures during the construction process, and guaranteeing that concrete projects comply with the "Code for Acceptance of Construction Quality of Concrete Structures" (GB50204) and relevant safety production regulations. ​ Scope of application: This measure is applicable to the entire life cycle of concrete engineering, including all links such as the selection of concrete raw materials, mix proportion design, mixing and transportation, pouring and vibration, curing and formwork removal, covering various concrete structure projects such as housing construction, municipal works, Bridges and tunnels. ​ Raw material selection and inspection: Strictly screen raw materials such as cement, sand and gravel, admixtures, and admixtures. Select cement products that meet the design strength grade (such as P.O42.5 and above), and the sand and gravel gradation should comply with the specification requirements (the particle size of the gravel is determined based on the structural dimensions, with the maximum particle size not exceeding 1/4 of the minimum cross-sectional size of the component). After each batch of raw materials arrives at the site, a third-party testing institution is entrusted to conduct performance tests (such as cement strength and stability, mud content and crushing value of sand and gravel, water reduction rate and setting time of admixtures). Only after passing the tests can they be used. ​ Technical requirements for raw material storage: Cement should be stored in a closed warehouse, stacked in zones according to the batches entering the site, and kept at least 30cm above the ground to prevent moisture and caking. The sand and gravel yard is hardened and treated, with rain shelters set up. Sand and gravel of different specifications are stored separately to avoid mixing. Admixtures are stored in dedicated tanks and labeled to prevent confusion with other materials. ​ Customized mix ratio: According to the type of engineering structure (such as beams, slabs, columns, foundations), strength grade (such as C30, C40), construction environment (temperature, humidity) and construction technology (pumping, self-compaction), entrust a qualified laboratory to design a special mix ratio. Clarify key parameters such as water-binder ratio, sand ratio, and admixture dosage (for example, the slump of pumped concrete should be controlled at 120-160mm, and the spread of self-compacting concrete should be ≥550mm). ​ Mix proportion optimization technology: By adding admixtures such as fly ash and mineral powder, the amount of cement is reduced, the heat of hydration is lowered, and temperature cracks in mass concrete are avoided. For winter construction, early-strength admixtures should be added to the mix proportion to ensure the early strength growth of concrete. During summer construction, adjust the dosage of retarder, extend the initial setting time, and prevent the initial setting of concrete during transportation. ​ Plan formulation and review: Prepare a special construction plan for concrete projects, clearly defining the construction process, technical parameters (such as the thickness of each pouring layer, vibration time, and curing period), and quality and safety control points. For special projects such as large-volume concrete (pouring volume ≥1000m³ or thickness ≥1m), ultra-high pumping concrete (pumping height ≥100m), and special-shaped structure concrete, experts are organized to conduct technical arguments on the plans, with a focus on reviewing temperature control, support systems, and distribution methods, etc., to ensure the scientific and feasible nature of the plans. ​ Technical briefing and Training: After the plan is approved, a comprehensive technical briefing will be conducted for technical personnel and construction teams, detailing the mix ratio requirements, pouring sequence, vibration key points, and emergency response measures. Organize technical training for operators, conduct practical exercises on key procedures such as the use of vibrators and the connection of pumps and pipes, and only allow them to take up their posts after passing the assessment. ​   Technical supervision of the mixing process: The mixing plant adopts a fully automatic metering system to ensure that the metering deviation of raw materials complies with the specification requirements (the metering deviation of cement and admixtures is ≤±1%, and that of sand and gravel is ≤±2%). The mixing time should be strictly controlled (for ordinary concrete, the mixing time should be ≥90 seconds; for concrete with admixtures or admixtures, it should be ≥120 seconds). During the mixing process, technicians should be arranged to inspect and observe the workability of the concrete (such as slump, cohesion, and water retention). If problems such as segregation and bleeding occur, the mix proportion should be adjusted immediately or the mixing should be stopped. ​ Technical support during transportation: Special tank trucks are used for concrete transportation. The inner walls of the tank trucks are thoroughly cleaned to prevent residual concrete from affecting the quality of fresh concrete. The transportation route should be planned in advance to avoid congested sections and ensure that the transportation time is ≤ the initial setting time of the concrete (≤2 hours at normal temperature and ≤1.5 hours at high temperature). During transportation, the tanker should maintain a low rotational speed (2-4r/min) to prevent concrete segregation. After the concrete arrives, technicians will test the slump on site. If the deviation exceeds ±20mm, the laboratory will issue an adjustment plan. It is strictly prohibited to add water at will. ​ Pouring sequence technical optimization: Follow the principle of "layered pouring, symmetrical advancement, and continuous operation". The concrete pouring of beams and slabs should be advanced from one end to the other, while the concrete pouring of columns and walls should be done layer by layer, with each layer thickness ≤500mm (when using insert-type vibrators), to avoid local accumulation causing overloading of formwork supports. The "inclined plane layering" pouring method is adopted for mass concrete, with a layering thickness of 300-500mm. The pouring speed is controlled (generally ≤2m/h) to reduce internal temperature stress. ​ Vibration Technical:  Specification The type of vibrator should be selected based on the slump of the concrete (high-frequency vibrators should be used for a smaller slump, and medium-frequency vibrators for a larger slump). When using an insertable vibrator for vibration, the spacing between the vibration rods should be no more than 400mm, and the insertion depth should be 50-100mm to the lower layer of concrete. The vibration time should be controlled at 15-30 seconds (until the concrete surface shows slurry and no air bubbles escape). Over-vibration (to prevent aggregate segregation) or missed vibration (to avoid honeycomb and pitted surfaces) is strictly prohibited. During the vibration process, avoid the vibration rod touching the steel bars, formwork and embedded parts to prevent structural deformation or displacement of embedded parts. ​ Temperature control for bulk concrete: The "internal reduction and external protection" technical measure is adopted. Temperature measurement tubes are pre-embedded inside (with one temperature measurement point set every 50-100 square meters) to monitor the internal and surface temperatures of the concrete in real time, with the temperature difference controlled at ≤25℃. Circulating cooling water can be introduced inside to lower the core temperature. The surface is covered with thermal insulation cotton and plastic film to reduce heat loss and prevent temperature cracks. When the temperature difference exceeds the limit, add insulation layers or adjust the flow rate of cooling water. ​ Conventional concrete moisture control: Within 12 hours after the concrete pouring is completed, cover it with moisture-retaining materials (such as gunny bags, geotextiles). During the hot summer, water it in time for maintenance to keep the surface moist. During winter construction, methods such as covering with electric blankets and steam curing should be adopted to ensure that the curing temperature is ≥5℃. The curing time should be carried out in accordance with the specifications (≥7 days for ordinary concrete, ≥14 days for concrete with retarding admixtures or with impermeability requirements) to prevent shrinkage cracks caused by rapid water loss in the concrete. ​   The determination of formwork removal time: The formwork removal time is determined based on the strength of the concrete test blocks under the same curing conditions. For bending members such as beams and slabs, formwork removal can only be carried out when the strength of the test blocks reaches 75% (for spans ≤8m) or 100% (for spans > 8m) of the designed strength. For vertical components such as columns and walls, the side formwork can be removed when the strength of the test block is ≥1.2MPa. Before formwork removal, technicians should issue a formwork removal application, which can be implemented after approval by the supervision unit. It is strictly prohibited to remove formwork in advance, which may cause structural cracking or collapse. ​ Technical Specifications for formwork removal operations: Formwork removal follows the principle of "install first and then remove, install later and then remove first, from top to bottom". Rough formwork removal is strictly prohibited. When removing large formwork, a crane should be used for hoisting, and a dedicated person should be assigned for command. No one is allowed to stand under the formwork. After formwork removal, promptly clean up the residual concrete on the surface of the formwork, check the flatness and deformation of the formwork, and repair the damaged parts before putting it back into use. ​ Structural entity inspection: 28 days after the concrete pouring is completed, a third-party inspection agency is entrusted to conduct structural entity inspection, including concrete strength rebound, steel bar cover thickness inspection, structural dimensional deviation inspection, etc. Conduct core drilling and sampling tests on large-volume concrete and important components (such as frame columns and main beams of Bridges) to ensure that the concrete strength meets the design requirements. For the parts that fail the inspection, a special treatment plan shall be formulated and implemented after being approved by the design unit (such as reinforcement by high-pressure grouting, external concrete coating, etc.). ​ Defect repair technology: For defects such as honeycomb, pitted surface and exposed bars on the concrete surface, the "surface treatment method" is adopted for repair: Clean the loose concrete at the defect area, rinse it clean with a high-pressure water gun, apply an interface agent, and then repair it with fine aggregate concrete or mortar in the same proportion as the original concrete. After repair, cover and maintain it. For crack defects, the repair method should be selected based on the crack width (surface sealing method for width ≤0.2mm, pressure grouting method for width > 0.2mm). During the repair process, technical records should be kept well to ensure the repair quality. ​ Establish technical archives for concrete engineering, collect and organize raw material inspection reports, mix proportion notices, construction logs, temperature measurement records, curing records, formwork removal applications, physical inspection reports and other materials to ensure that the materials are complete, accurate and traceable. Technical archives are filed and preserved in accordance with the prescribed requirements, serving as an important basis for project acceptance and later maintenance. ​ Regularly review and analyze the technical data of concrete engineering, summarize the technical problems during the construction process (such as the optimization effect of mix proportion and the accuracy of temperature control), form a technical summary report, provide technical references for subsequent similar projects, and continuously improve the safety production technology level of concrete engineering. ​   Emergency technology for concrete supply interruption: If the supply of concrete is interrupted due to a malfunction of the mixing plant or traffic congestion, immediately stop pouring, vibrate and compact the surface of the already poured concrete, and cover it with moisture-retaining materials. When the interval time exceeds the initial setting time of the concrete, handle it according to the requirements of the construction joint (set up a vertical construction joint, clean the surface floating slurry and loose aggregates, and apply an interface agent). After the concrete supply is restored, re-pour to ensure that the construction joint is tightly combined. ​ Emergency techniques for structural cracks: If early cracks are found on the concrete surface during the pouring process, stop pouring immediately, check the width and depth of the cracks. If they are surface dry shrinkage cracks, cover them with water in time and strengthen moisture retention and maintenance. If it is a temperature crack, add an insulation layer and adjust the temperature control measures. If the cracks continue to develop, immediately organize the evacuation of personnel, entrust the design unit to formulate a reinforcement plan, and adopt technical measures such as temporary supports and grouting sealing to prevent the cracks from expanding and causing structural safety accidents.   Instructions for Use: This plan is a general template. Before the specific implementation of each project, it should be refined and supplemented according to the characteristics of the project (such as super-high, super-heavy, large-span structures, etc.), especially the control measures for major hazard sources.    

    In the construction of super flat floors, the formwork system serves not only as lateral restraint but also as the baseline for elevation control. Since super flat floors typically utilize the "skip-pour method" or "long-strip paving," the stability and straightness of the formwork directly determine the flatness of the joints. The following are key points regarding the selection and installation of formwork systems for super flat floors: Traditional wooden formwork has been phased out in super-flat floor construction due to its susceptibility to deformation and low strength. The current mainstream solutions are as follows: Composition: Consists of two cold-drawn steel corner guards, load transfer bars, breakable bolts, and a lower steel plate. Advantages: No need to remove after construction; it remains in the floor as a permanent joint; effectively protects the concrete edges from impact damage by forklift wheels (prevents chipping). Applicable to: Longitudinal construction joints in very narrow aisle (VNA) areas. Requirements: Must possess sufficient rigidity, and the top surface must be machined and leveled. Advantages: When used with a laser leveling machine, it can serve as a physical reference for the leveling head. The accuracy of template installation directly affects the FL (Levelness) value, and the following steps must be followed: Precise elevation control: Use a high-precision leveling instrument (accuracy requirement within ± 0.5 mm ) for surveying. Denser measurement points: Set a fixed control point every 2 ~ 3 m along the template support trajectory to ensure straightness over long distances. Reinforcement design: The template must be firmly anchored to the base layer using steel reinforcement bars. In ultra-flat floor construction, the power trowel is heavy and vibrates strongly; even a 1 mm settlement of the template will result in unacceptable flatness in that area. Gap sealing: The gap between the bottom of the template and the base layer should be sealed with mortar to prevent leakage and the resulting "root rot" phenomenon, otherwise it will lead to honeycomb and pockmarked surfaces at the edges. Perpendicularity requirement: Dowel bars must be strictly parallel to the floor surface and perpendicular to the template surface. Displacement control: The use of dowel bar sleeves or supports is recommended. If the dowel bars tilt during pouring, it will cause irregular cracks in the concrete during shrinkage, rather than cracking at the predetermined joints. Key processes Technical Requirements Equipment coordination Top surface cleaning No old concrete or foreign objects shall remain on the top surface of the formwork. Manual inspection with a trowel is performed before pouring. Joint connection The connection between two formwork sections must have a "zero height difference" transition. A tongue-and-groove joint structure or reinforcing steel plates on the back are used. Demolding time The side formwork (non-armored joint) must be removed after the concrete strength reaches 2 to 3 MPa. Avoid using a sledgehammer to pry, to prevent damage to the edges of the newly poured concrete. When installing armored joints, it is important to pay attention to their three-dimensional adjustability. Adjustable height bolt supports are typically used, allowing for fine-tuning before concrete pouring to ensure the top surface of the angle steel is precisely at the design elevation. Misconception 1: Relying on laser Leveling without paying attention to formwork. Truth: When working near the edges of the formwork, laser Leveling are limited by the width of the Leveling blade and edge interference, often requiring the formwork as a physical reference. If the formwork is uneven, the flatness of the edges will inevitably be compromised. Misconception 2: Dowel bars are inserted directly through wooden formwork. Truth: This makes it very easy to dislodge the dowel bars during formwork removal, damaging the bond between the dowel bars and the initial setting concrete. It is recommended to use armored joints or steel formwork with professional sleeves.   Note: The parameters provided in this document are for reference only and are not mandatory. Due to differences in technical characteristics between different brands and models of laser levelers, please consult the manufacturer for a suitable solution before actual operation. This reference document assumes no responsibility for any issues arising from failure to follow the manufacturer's instructions.