Control and application of slump loss in concrete

Introduction:

Cement concrete construction has been widely used in all kinds of construction projects. The plasticity and fluidity of concrete will gradually thicken and harden with the passage of time, and gradually produce strength. The slump loss of concrete is a technical difficulty commonly encountered in the construction of concrete mixing stations and highway projects. Slump refers to the sinking amount of newly mixed cement concrete under its own weight, which is an important parameter to measure the workability of concrete and ensure the normal construction. Including the concrete stick strength, sand content, fluidity, cohesiveness and water retention. “Slump loss” of concrete refers to the difference between the slump of concrete mixed in the central mixing plant and the slump of concrete transported to the construction site. Excessive slump loss sometimes seriously interferes with the normal operation of construction, so controlling slump loss is an important task of construction.

1.Control and application of slump loss in concrete

The factors of cementitious materials, aggregate, admixture, water consumption per unit volume, heat of hydration and temperature are related to each other.

 1). cementitious materials

1.1 Cement

Among the main mineral components of cement, tricalcium aluminate C3A, quadalcium ferroaluminate C4AF, tricalcium silicate C3S and dicalcium silicate C2S, the adsorbability of water reducing agent is ranked as C3A > C4AF > C3S > C2S, respectively. When the content of C3A and C4AF in cement is large, a large number of water-reducing agent components are adsorbed by them, while the adsorption amount of C3S and C2S, which account for more cement components, is insufficient, resulting in the loss of concrete slump. Therefore, cement concrete with higher content of C3A and C4AF in cement has greater slump loss, and vice versa. This is the main reason for the collapse of concrete mixed with water-reducing agent.

The adsorption effect of different mineral composition on water reducing agent is different. The water reducing agent is mainly adsorbed on the surface of cement minerals. In cement with strength grades of P·O42.5 and above, the water consumption of the standard consistency of cement basically depends on the water consumption of the standard consistency of clinker. High C3A activity in clinker will cause rapid hydration reaction when it comes into contact with water. When the content of C3A is high, the water consumption of clinker standard consistency increases. It is suggested that the water consumption of clinker standard consistency can be effectively reduced when the content of C3A is controlled between 7.0% and 7.5%.

The water consumption of standard consistency of cement can be effectively reduced by appropriately increasing the content of ground semihydrous gypsum in cement.

1.2mineral additive

Proper addition of high quality fly ash or slag powder with small water demand is beneficial to improve the workability of concrete, and high quality fly ash can reduce the amount of admixtures. If the fly ash is coarse and the burning loss of fly ash is too large, the loss of concrete bleed and slump will be too large, and even affect the adaptability with cement, and may also cause the failure of high efficiency water reducer. Therefore, in the concrete mix ratio test, the first and second grade fly ash are preferred, and the mixed concrete will have better workability. The fly ash with small firing loss can effectively reduce the slump loss rate, and the mineral admixture can significantly improve the work, strength, volume stability and durability of concrete.

2).aggregate

Aggregate particle size, grading, mud content and sand rate will affect the performance of the mix. Good aggregate grading can effectively reduce slump loss, so as to prepare concrete with good flow and small slump loss. In particular, polycarboxylic acid superplasticizers have more stringent requirements for material quality.

3). Hydration heat cause

The hydration products produced by the hydration of the cement in the concrete mix act as adhesives and hinder the sliding of the particles. The temperature rise of the mix will accelerate the hydration of cement, and the reduction or volatilization of water will directly reduce the content of free water in the unit concrete, while the wind speed and humidity will also affect the evaporation rate of water in the mix, reduce the lubrication of the water film, and then cause slump loss. Therefore, adding a proper amount of retarder will have a great effect on the slump loss rate.

4).Temperature influence

When the weather is dry, the water on the surface of the mixture is easy to evaporate and cause slump loss. The higher the temperature of the concrete mix, the greater the proportion of free water to bound water and the faster the collapse loss. The lower the temperature of the mixture, the slower the loss of the slump of the mixture. Generally speaking, every time the temperature rises 10 ° C, the slump loss will increase by about 10% to 40%. Mass concrete should pay more attention to the addition of water-reducing agent to reduce the water consumption per unit volume, thus reducing the heat of hydration. The author has experience and lessons in a highway project: the slump of concrete mixed by mixing station is 220 ~ 225mm, and the concrete tank truck will be transported to the site after 20 minutes, and the slump damage will increase during pouring and the pipe will be blocked and the pouring will be interrupted. After finding out the reason, adding water-reducing agent with large water-reducing rate and appropriate amount of white sugar to the concrete mix will allow the construction to proceed normally. Before mixing concrete, reducing the temperature of sand, stone, cement, admixture and mixing water can effectively reduce the temperature of concrete mix, thus inhibiting slump loss; During the construction of mass concrete, the internal cooling water pipe can also reduce the temperature of the concrete. In addition, the outer surface of the concrete mixing truck is sprayed with water to cool down and wrapped with cloth to avoid direct sunlight; Measures such as sun protection, wind protection, and covering heat preservation after slurry collection can inhibit the slump loss during concrete pouring.

2.Measures to inhibit the loss of concrete slump

The excessive slump loss of cement concrete is caused by many factors, which are mainly controlled by the following methods.

1）After adding polycarboxylate superplasticizer

Sand, stone, cement, and water are mixed first and then added with water reducing agent, because when cement is hydrated, C3A and C4AF can quickly generate ettringite in an environment with gypsum. After C3A and C4AF are significantly reduced, water-reducing agent is added. At this time, the water-reducing agent added can be fully adsorbated by C2S and C3S, and the electrodynamic potential between mineral particles is significantly increased, and can remain relatively stable within a certain time range, so that the mixed material has obvious good workability and small slumps

2）Retarder method

Retarding agent can delay the hydration reaction of cement and prolong the setting time of concrete. Reasonable use of retarding agent can make the concrete mix less slump in a long time.

2.1 Organic retarder

Common citric acid, gluconic acid, salicylic acid and its salts, polyols and their derivatives, this kind of retarder is less affected by temperature. Sugars such as glucose, sucrose, molasses and their derivatives are widely used for their wide range of raw materials, low price and stable retarding effect. The experiment shows that the effect of adding the juice which is expired or discarded in the market after effective filtration is better than that of sodium gluconate.

2.2  Inorganic retarder

At present, phosphate and metaphosphate are widely used. Inorganic retarders often form a layer of insoluble film on the surface of cement, which acts as a barrier to the hydration of cement particles, resulting in the slowing down of cement hydration, prolongs the setting time of cement, and reduces the collapse rate.

2.3 Optimum PCE suitable for cement

In the process of adaptability adjustment, if the concrete mixture mixed with admixtures is found to be bleeding and grasping the ground after mixing (thin slurry is bleeding to the surface, sand and stone are sinking and close to the ground, it is difficult to shovel), it indicates that the adaptability of admixtures and cement is not good. The test should be readjusted. If the mixer is used to stir, the number of test mixing should not be less than 15L, and the same proportion of mortar should be used to moisten the inner wall of the mixer and the iron pan before the test mixing to prevent the small amount of test mixing and misalignment.

2.4 Do more slump loss tests to simulate the actual environment

When the concrete slump loss test is done, the author believes that most of the test personnel are only carried out at room temperature in the laboratory, and the construction site temperature, wind speed, light, humidity and indoor are different, but also should be targeted at outdoor or indoor imitation of high temperature, low temperature, wind, light and other environment slump loss test, in order to truly practical, real simulation of the site environment, targeted guidance for construction. Some admixture manufacturers in the pursuit of maximum profits, the quality of each batch of admixtures into the field is uneven, each batch of use should be sampled before mixing, meet the requirements to receive goods

END

In order to reduce slump loss of concrete, more control should be given to cement materials, aggregates, admixtures, water consumption per unit volume, hydration heat cause and temperature.

The quality fluctuations of the above factors are caused by a variety of reasons, from the source of various factors to analyze the causes of fluctuations and control, in order to reduce the material on the scene due to fluctuations and forced to do a variety of tests to reduce concrete collapse is the most important.