100 Questions about Ready-Mix Concrete (81-100)

Approximately how long is the setting time of 81 premixed concrete?

Answer: The initial setting time is generally 8-12 hours (4-6 hours in high temperatures). The final setting time is 12-16 hours (5-7 hours in high temperatures). In autumn and winter or during the transition between winter and spring, the final setting time may be between 18 and 24 hours. If it exceeds this limit, it is considered slow-setting concrete, and the mix ratio should be adjusted immediately or the admixture plant should be notified to reduce the slow-setting component.

What is the initial setting and final setting of concrete?

Answer: The initial setting is when the mortar in the concrete mixture begins to lose its plasticity, which is difficult to determine precisely. The final setting is when the mortar in the concrete mixture completely loses its plasticity, that is, when pressing the surface of the mortar with a finger leaves no mark.

What is slow setting of concrete? What are the reasons?

Answer: When the concrete mixture does not reach final setting after 24 hours of pouring, it is considered slow setting. Slow setting is caused by an excessive amount of slow-setting components added to the pumping agent. It can also occur after a sudden drop in temperature. A mix ratio suitable for normal temperatures may result in prolonged setting time and slow setting if the temperature suddenly drops by 5-10℃.

What is mass concrete?

Answer: Concrete is considered mass concrete when the minimum dimension of the concrete structure is ≥1m, or when it is expected that the heat of hydration of cement will cause excessive temperature difference between the inside and outside of the concrete (greater than 25℃), leading to cracks.

What issues should be considered for mass concrete?

Answer: (1) Minimize cement usage while ensuring strength, and increase the proportion of high-quality fly ash or mineral powder.

(2) Use a slow-setting pumping agent to extend the final setting time of the concrete and reduce the temperature peak. Control the temperature difference between the center and surface of the concrete component within 25℃. If it exceeds this limit, insulate the surface with insulation materials. Side formwork should not be removed too early; it must be removed only after the temperature difference between the inside and outside surfaces of the component is less than 20℃. When the length of mass concrete exceeds 50m, and no expansion agent is added, and the construction period is during a daily average temperature of 20℃ or higher, the bottom of the cushion layer must be flat; if the cushion layer is thick and there are many rocks embedded in the cushion layer, a flexible sliding layer should be laid on the cushion layer to allow for slight displacement during the hardening shrinkage of the mass concrete, preventing cracks in the middle. Use the 60d or 90d compressive strength as the basis for evaluating whether the concrete is qualified.

Why should water be pumped first, followed by cement mortar, to prime the pump before pumping concrete?

Answer: Because the pipe walls are dry, they easily absorb water from the mortar, causing the mortar to dry out and clog the pipe. If concrete is pumped directly after priming with water, the cement slurry in the concrete mixture will be bonded to the pipe walls, causing the mixture in the front section of the pipe to become sand and gravel, increasing the resistance in the pipe and leading to clogging.

Why should the pipe outlet not be inserted into the formwork before pumping mortar?

Answer: Because the mortar pumped first when priming the pump is mixed with water, making it too thin and with very low strength, it should not be pumped into the formwork. Sometimes, if the sand is not completely drained when cleaning the pump pipe, when pumping mortar next time, the clear sand and water mixture will be pumped out first (this kind of accident has occurred in some mixing plants, pumping clear sand and water into beams and walls). Therefore, the pipe outlet should only be moved into the formwork when good quality mortar is seen being pumped out from the pipe outlet.

Why should the priming mortar be distributed evenly?

Answer: Because the strength of the priming mortar is low, if it is concentrated in one place, it is easy to cause the concrete strength in that part of the component to not meet the design requirements.

What is the best way to prevent cracks in cast-in-place concrete slabs in spring?

Answer: The best method is: After pouring a section of the slab, consolidate the concrete with a vibrator, use a wooden trowel to smooth the slab surface, and then cover it with plastic film from front to back as it is smoothed, pressing the film against the concrete at the joints or ends with a trowel to ensure firm adhesion and prevent it from being blown away by the wind. If it is necessary to lay out lines when the concrete is finally setting, use a utility knife to cut away the parts obstructing the lines along the marker, do not lift the entire film.

Why should heavy objects not be hung on the concrete immediately after final setting?

Answer: Because the concrete has very low strength immediately after final setting. If steel bars or other heavy objects are hung at this time, the impact force at the moment of placing them on the slab, combined with the weight, will cause large deformation and cracks in the slab. Heavy objects should only be hung 24 hours after final setting or when the concrete strength reaches 1.2 MPa or more. If the construction schedule is tight, the stiffness of the formwork support can be increased to reduce the deformation of the formwork under load. When hanging heavy objects, try to place them on smaller spans, and along the walls in areas with shear walls.

What is winter construction?

Answer: Winter construction begins when the average daily temperature is below 5℃ for five consecutive days, and the lowest temperature is below 0℃.

What measures should be taken during winter construction?

Answer: The following measures should be taken: (Cold regions) Water tanks, pools, and admixture tanks should be insulated and covered; steel sand and gravel bins should be enclosed and insulated; when the lowest temperature is below -5℃, the remaining sand in the sand bin should be removed before the end of production to prevent freezing. Before loading each day, frozen sand lumps should be shoveled to one side, and unfrozen sand should be used.

Anti-freezing pumping admixtures should be used. The temperature of the concrete mixture leaving the factory should not be lower than 6℃. The transport truck tanks can be wrapped with cotton felt.

What is a post-pouring joint?

Answer: When the length of a project exceeds 45m, a 1m wide section of concrete is left unpoured in the middle of the slab to prevent structural cracks, and this reserved section is poured after the main structure is completed.

When should the post-pouring joint be poured?

Answer: It should be carried out when the main body is completed or nearing completion, and the average ambient temperature is below 10℃. Because after the main body is completed, the settlement and shrinkage that should occur are basically completed, and the temperature is below 10℃ from approximately November to March of the following year. During this period, the concrete structure is in a state of shrinkage. At this time, pouring the post-pouring belt is not easy to crack. Otherwise, the result is the opposite.

95. What is rebound testing?

Answer: Use a rebound hammer to strike the surface of the concrete structure. After the impact, the rebound hammer inside the instrument drives the indicator slider to move. Read the rebound value according to the scale on the indicator slider. Each location (test area) is struck 16 times. Remove the 3 largest and 3 smallest values, and take the average of the remaining 10 values as the rebound value of the test area. Then measure the carbonation depth value of the structure; use the rebound value and carbonation depth value of the test area to check the "Concrete Strength Conversion Table for Test Areas" to determine the strength value of the concrete in the test area.

96. What is carbonation depth?

Answer: The process in which calcium hydroxide in the surface layer of concrete reacts with carbon dioxide in the air to form calcium carbonate is called carbonation. The carbonation depth is to drill a small hole on the surface of the concrete, spray alcohol phenolphthalein solution, the uncarbonated part turns red, and the carbonated part remains colorless. The depth of carbonation is measured with a carbonation depth meter.

97. What is core drilling testing?

Answer: Use a concrete core drill (the drill bit is hollow, with an inner diameter of 100 mm, 89 mm, 75 mm) to drill core samples on the component, cut and grind the compressed surface, and press out the destructive load on the pressure testing machine to calculate the concrete strength.

98. Under what circumstances is core drilling testing used?

Answer: When the rebound method is not suitable for testing, the core drilling method can be used for testing.

99. What are the reasons why cracks easily appear in cast-in-place slabs on beams or wall tops?

Answer: There are three main reasons:

(1) The surface of the slab on the beam or wall is subjected to tensile stress, and the amount of negative reinforcement (eight-character reinforcement) designed is insufficient.

(2) The reinforcement is sufficient, but during construction, the personnel pouring concrete stepped on the negative reinforcement to the lower middle part of the slab, which cannot exert the effect of resisting the tensile force on the slab.

(3) Some individual houses have cast-in-place slabs with a thickness less than the design code requirements or with too low concrete strength.

100. What are the reasons why pipes are easily blocked when pumping concrete into deep foundation pits with a pump truck? What measures should be taken?

Answer: When the depth of the deep foundation pit exceeds 5m, the concrete mixture in the inclined pipe section from the ground to the bottom of the pit is subjected to a large downward gravitational force, causing the mixture to move downward, and the gas moves upward to the bend to form an air blockage. A 180° bend pipe can be set a few meters below the lower end of the inclined pipe section to solve the problem of pipe blockage; an exhaust valve can also be set at the upper end of the lower bend pipe to solve the problem of pipe blockage.