[Key words] Concrete; Water content; Slump; Control
Chinese Library Classification Number: TU522.3+1 Document Identification Code: A
At present, concrete has been widely used in various construction industries. In concrete pouring construction, how to strictly and effectively monitor the quality of concrete has become the focus of construction. In my opinion, the common method is to test the slump of concrete mixing on site to determine the fluidity, viscidity and workability of concrete.
I. Brief Introduction of Engineering
The Laizhou Port Area of Yantai Port is located in the North Sea area of Haibeizui Village, Jincheng Town, Laizhou City, Shandong Province. The east side of Sanshan Island is about 8 km away from the West Port Pool of Laizhou Port Area. There are 2 general berths (breakwaters) of 35,000 tons, 1 berth for working vessels, breakwaters, diversion dikes, cofferdams and land reclamation, harbour basins and waterways, etc.
(1) Wharf and breakwater
The wharf and breakwater are arranged at a depth of about - 7.5m, and the direction is E-W. The berth length of the wharf is 490m (including working vessel berth 50m), the width is 100m, the top elevation of the front is 4.0m, and the bottom elevation of the front design is - 14.1m. The breakwater extends westward from the end of the wharf, with a length of 150 m. Mooring waters are arranged at the front of the wharf with a width of 61m and a design bottom elevation of - 12.5m. Swing waters are arranged in front of the wharf with a diameter of 380m and a design bottom elevation of - 12.0m.
(2) Dike
The diversion levee root is located in the northeast corner of the dredger filling land area, and its strike is N-W to 13.5. It is 621.371 m long and 18 m wide.
(3) Cofferdam and Sand Retaining Dike
A cofferdam is built at the base of the approach dike, and dredged soil from the harbour basin and some waterways is filled into the cofferdam to form a land area. The total length of the cofferdam is 1642m, of which the East-West section is 1000m and the North-South section is 321m. The cofferdam is connected with Laizhou Power Plant. The sand-retaining submerged dike is arranged on both sides of the seawater intake of the power plant in the north section of the western cofferdam, and the length of each section of the submerged dike is 80m.
(4) Land area behind the wharf
The land area behind the wharf is formed by dredged soil filling in the harbour basin, and the width of the wharf surface is 100m. In front of the wharf is the loading and unloading operation area, which is 30m wide. Each berth is equipped with three 25t-40m portal cranes, followed by the front substations, dispatching and waiting buildings.
(5) Land area of cofferdam reclamation area
The land area of cofferdam reclamation area is formed by reclamation. In the middle of the site, an 18m wide dredging Harbour Road is arranged, which is connected with the approach road of the wharf in the north, the dredging road outside the harbour in the south, and the seawater intake channel of the power plant is arranged on the west side of the cofferdam.
Basic concepts
Slump refers to the workability of concrete, including water retention, fluidity and cohesion of concrete. Convenience refers to whether concrete is easy to operate and uniformly compacted, which is a very comprehensive performance, including fluidity, cohesion and water retention. The main factors affecting the workability are water consumption, water cement ratio, sand ratio, cement varieties, aggregate conditions, time and temperature, additives and so on.
3. Factors Affecting Slump of Concrete
(1) Impact of raw materials
The aggregates stacked in Prefabrication Yard are mostly open storage. Because the storage time of sand is different, the moisture content of sand is unstable, especially after rain, the moisture content of sand varies greatly. New moisture content in aggregate should be determined by experiment. Because of the different water absorption of aggregate, the slump of concrete must be deviated in different degrees.
(2) The influence of mixing time
The long time of concrete mixing will result in the increase of aggregate water absorption, the decrease of free moisture in concrete clinker, and the loss of concrete slump.
(3) Impact of Transportation Machinery
The longer the transportation distance and time of concrete mixing tank truck, the less free water content of concrete clinker due to water evaporation, aggregate water absorption and other reasons, resulting in concrete slump loss.
(4) Influences of pouring time
In the morning and evening, the temperature is low, the evaporation of water is slow, and the influence is small; in the afternoon and noon, the temperature is high, the evaporation of water is fast, and the impact is great. The faster the loss of water is, the greater the slump loss of concrete is, the worse the fluidity and cohesion of concrete are, and the harder the quality of concrete is to be guaranteed.
4. Examples
In the prefabrication construction of the twist-king block of the 2 x 35,000-ton wharf and waterway project in Laizhou Port Area of Yantai Port, the amount of 1 m3 concrete mix material is as follows: (kg)
(1) Before raining, the humidity of sand in Prefabrication Yard is stable, the moisture content of sand is 3.0% by sampling, the concrete mix ratio of 1 m3 in sampling and mixing station is gravel: sand: fly ash: cement: water: admixture = 1007:782:75:354:174:6.435, and the actual water consumption W = 174+782*3.0% = 197.5 kg, which is more than the designed water use W'= 197.5-175 = 22.5 kg.
The slump of concrete measured is 145 mm and the water-binder ratio is 197.5/(354+75)=0.46, which is less than the maximum allowable value of water-binder ratio in the Standard for Quality Control of Concrete for Waterway Engineering [0.55].
(2) After raining, the moisture content of sand in the Prefabrication Yard is relatively high. The moisture content of sand measured by sampling is 5.0%. The concrete mix ratio of 1 m3 in the sampling and mixing station is gravel: sand: fly ash: cement: water: admixture = 1007:799:75:354:188:6.435. The actual water consumption is W=188+799*5.0%=228 kg, which is more water W'=228-175=53kg than the designed mix ratio.
The slump of concrete measured is 155 mm, and the water-binder ratio is 228/(354+75)=0.53, which is less than the maximum allowable value of water-binder ratio in the Standard for Quality Control of Concrete for Waterway Engineering [0.55].
In actual construction, the water consumption of concrete in mixing station is on the high side. With the hot weather in summer, the temperature rises, the water evaporation is too fast, and the evaporation is too large. It is permissible to increase the appropriate amount of concrete mixing water, but it can not exceed the maximum allowable value of water-cement ratio. Under a certain mixing ratio, the slump of concrete is proportional to the water consumption. Because of the inaccurate detection of sand moisture content or the inaccurate water addition of mixing station, the actual water addition of concrete will be directly affected, resulting in the deviation of concrete slump. In the construction of prefabricated concrete with twisted king block, I think that the slump should not be too large. The slump will directly produce the following consequences:
1. Loose roof: The top of the twist King block is too thick, there is obvious mortar layer, gravel sinks, resulting in the top concrete strength can not reach the design strength.
2. Sand patches and sand lines: The vertical sand of twisted King block has not been fully cemented by cement slurry, resulting in sand pattern defects, which affect the appearance quality.
3. Input modulus of concrete is insufficient: the slump is too large, the amount of concrete into the formwork is full when pouring, but with the evaporation and sinking of water, the height of prefabricated twisted King block is lower than the design size.
4. The quality of concrete vibration is poor: it is not conducive to the operation of vibration, the number of bubbles is large and the volume is large, which seriously affects the external quality of the block.
In concrete construction, the stability of slump must be guaranteed so that it has good strength, fluidity, viscidity and workability. Mainly from the following aspects of control:
1. Strictly inspect the raw materials in accordance with the specifications, and do a good job in measuring the moisture content of aggregate (especially the moisture content of sand).
2. Insist on the test of concrete slump of each bench in the prefabricated site, especially when the water consumption changes greatly, the number of slump tests should be increased, and the water consumption should be adjusted in time according to the test results.
3. Water consumption of concrete should be increased or decreased in a moderate and small range, and should not exceed the maximum allowable value of water-cement ratio.