On the one hand, the introduction of pores in lightweight concrete endows ordinary concrete with such properties as light weight, sound insulation, heat preservation and high fluidity, which are not available in ordinary concrete.As the porosity of lightweight concrete is generally up to 50%-70%, and the pore diameter is more than 10 microns, the strength of lightweight concrete is much lower than that of ordinary concrete.As with normal concrete, the strength of the foam concrete is not a fixed value, different types of gelled material, dosage of cement, concrete mixture ratio, dosage of cement water, foam (i.e., the volume of different quality), blowing agent, curing system and other concrete additives used or not can affect the strength of the foam concrete.
1.The influence of mixing ratio
Lightweight concrete production is mainly cement, foaming agent as the main material, sometimes may add some mixed materials (such as silica fume, slag, fly ash, etc.), and may add fine aggregate (sand), add different materials on the impact of foam concrete is not the same!(1) in the first case, the main materials of cement and foaming agent are used only, and other auxiliary materials are not added.This situation is relatively simple, the higher the strength level of the cement used, the more dosage, the stronger the lightweight concrete prepared.Therefore, high strength grade cement should be selected when higher strength foamed soil is expected to be prepared.(2) in the second case, mixed materials are added: the addition of mixed materials will significantly reduce the early strength of lightweight concrete, while the influence on the later strength is not significant.The reduction of early strength can be reduced if appropriate strength exciter is added.If the use of ultrafine mixed materials, such as silica fume, ultrafine grinding slag powder, the strength decreases little, sometimes even increased.(3) the third case is mixing sand as fine aggregate, the strength of foam concrete in principle will be reduced to different degrees, but the sand mixing will be very beneficial to improve the volume stability of hardened foam concrete, reduce shrinkage.In the actual construction, mixed materials and sand are often mixed at the same time. Therefore, there is an appropriate range of mixing ratio of lightweight concrete, which needs to be determined according to the test.
2. The influence of water-cement ratio
If only from the analysis of the internal structure changes of hardened lightweight concrete, the increase of water-cement ratio will inevitably lead to the decrease of lightweight concrete strength.However, a large number of experiments have proved that when the water-cement ratio increases in a certain range, the strength of lightweight concrete does not decrease, but shows an increasing trend.
The preparation of lightweight concrete is different from ordinary concrete.In order to introduce the foam evenly into the cement slurry and distribute it evenly in the lightweight concrete system, the cement slurry is required to have good fluidity.The higher water-cement ratio is the necessary condition to ensure the good fluidity of cement mortar.On the contrary, if appropriate technical measures are taken to ensure the good fluidity of slurry under the condition of low water-cement ratio, high strength lightweight concrete can also be prepared.In fact, when the volume density of cement content of lightweight concrete is not changed, the content of sand and foam will inevitably decrease, leading to the decrease of porosity of hardened lightweight concrete, which is also one of the important reasons for the improvement of lightweight concrete strength when the water-cement ratio increases.
3. Influence of volume density
The smaller the volume density of lightweight concrete is, the lower the strength is.The strength of lightweight concrete with a certain density may vary in a large range when the mixing ratio, water-cement ratio and other process conditions change.
4. Impact of conservation system
The early curing system is very important for the performance and final strength of lightweight concrete. Lightweight concrete molding water cement is relatively large, so, to strengthen the early maintenance and water preservation, prevent the premature loss of water.This is of great significance not only to the strength of lightweight concrete but also to the prevention of cracking of hardened concrete.
5. The influence of concrete additives
The admixtures used in lightweight concrete mainly include cement strength initiator water reducer and foaming agent.Cement strength activator is mainly used in cement mixture material, which can reduce the degree of early strength reduction of lightweight concrete, but the use of activator will often reduce the final strength of lightweight concrete.
Concrete superplasticizer enables lightweight concrete to successfully mix slurry and foam even at a lower water-cement ratio to produce lightweight concrete with even foam distribution.Therefore, adding proper amount of efficient superplasticizer is one of the important means to prepare high strength lightweight concrete.However, because the price of water reducer is relatively expensive and some water reducer and foaming agent in some aspects of the opposite effect.So the type and amount of superplasticizer must be determined by the test.
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