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The laboratory director is one of the difficult positions to hold in concrete companies.

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Currently, the ready mixed concrete industry in Hubei Province is standing at a critical juncture of green transformation.

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With the intensive introduction and implementation of a series of policies and regulations, the certification of concrete enterprises has changed from an “optional” to a “mandatory” option.

Certification is not a matter of whether or not to do it, but rather if we don’t take action, it will be too late.

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Traditional concrete material research has long relied on “trial and error” experimental mix proportions and empirical formula calculations.

Researchers gradually approach the optimal performance solution through limited cementitious material combinations, water cement ratios, aggregate gradation, and curing system schemes, supplemented by mechanical testing and microscopic characterization.

However, as a typical multiphase heterogeneous composite, the hydration process of concrete involves multi-step chemical reaction of cement water admixture, complex microstructure evolution of interface transition zone (ITZ), and multi-scale coupling effect from nanoscale C-S-H gel to macro aggregate skeleton.

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In addition, the introduction of industrial solid waste (such as fly ash, slag, silica fume, etc.

) and recycled aggregates has led to a rapid expansion of the spatial dimension of mix proportion parameters – traditional methods are like searching for a needle in a haystack, which is not only time-consuming and laborious, but also difficult to capture the collaborative optimization potential hidden in nonlinear interaction relationships.

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Many people think that the quality of concrete only depends on cement, but in fact, sand and gravel aggregates account for more than 70% of the volume of concrete.

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The quality of aggregates directly determines the strength, compactness, and durability of concrete, and is also the core cause of cracking, sanding, and insufficient strength.

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Recently, the Ministry of Housing and Urban Rural Development released the draft of the “Concrete Quality Control Standards (Revised for Soliciting Opinions)”, which directly addresses the pain points of the industry: steel slag should not be used as coarse aggregate, nor should it be used as fine aggregate.

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In just two short sentences, it completely ended the industry operation path of “steel slag mixed with concrete and solid waste disposal and reuse” in China for many years, and blocked all channels for steel slag to be used as building concrete aggregates from the source.

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1、 This chapter is applicable to the testing method for the uniformity test of concrete mixtures for hydraulic concrete.

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From the perspective of the author’s contact, some technical personnel believe that reducing costs and increasing efficiency of concrete is a technical route that can be implemented under the advancement of material technology; Another group of frontline practitioners directly classify it as a pseudo proposition.

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The fundamental reason for the divergence is the confusion between cost reduction through compliant technology and speculative cost reduction through sacrificing quality.

In fact, most experienced personnel know that concrete can achieve cost reduction and efficiency improvement through technology, proportioning, management, and supply chain optimization while meeting specifications, strength, durability, and construction performance.

This is the beneficial way to reduce costs and increase efficiency.

Simply relying on reducing adhesive materials, using inferior materials, and minimizing necessary testing and after-sales service, only pursuing a decrease in numbers on paper, without considering the comprehensive performance of concrete workability, strength, durability, etc.

, is a false cost reduction, which may lead to greater losses such as cracking, insufficient strength, and even rework in the later stage.

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(1) The interface bending tensile, diagonal shear, tensile and splitting tensile tests are the main testing methods for evaluating the interface bonding performance.

Among them, the interface bonding strength measured by oblique shear test is the highest, reaching about 27 MPa; The strength obtained from the pull-out test is relatively low, usually not exceeding 2.

5 MPa.

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The interface failure mode is closely related to the testing method and substrate surface treatment method: samples without surface treatment mainly suffer from interface failure; After surface treatment, the damage location shifts more from the interface to the repair material or the interior of the matrix concrete.

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Concrete is a brittle material with a tensile strength of only about 1/10 of its compressive strength.

The cross-sectional size of large volume concrete is relatively large, and the hydration heat of cement will cause a sharp increase in the internal temperature of the concrete; And in the future cooling process, significant tensile stress will be generated under certain constraints.

In large volume concrete structures, only a small amount of steel reinforcement is usually placed on the surface, or no steel reinforcement is provided.

Therefore, tensile stress should be borne by the concrete itself.

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