The Effect of Bottom Reinforcement Spacers on Reinforced Concrete Structures Loaded by Explosion

1. Introduction

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After a long period of relative security, Europe is facing a deteriorating security situation. The security situation has deteriorated in all respects, including the military aspect. For this reason, the protection of critical infrastructure is a key task for the preservation of the functioning of states. Concrete is one of the key materials used for the design and construction of both the critical infrastructure facilities themselves and the facilities used to protect critical infrastructure elements. The results of experimental investigations of blast-pressure wave effects on reinforced concrete structures provide important input for refining the mathematical material model and for the process of designing reinforced concrete structures with the assumption of extraordinary loading by rapid dynamic action [ 1 3 ].

The correct function of the reinforced concrete structure is ensured by the appropriate position of the reinforcement in the cross-section of the concrete element [ 4 ]. The reinforcement shall be designed in the direction of tensile stress and as close as possible to the tensile edge. To ensure the durability of the entire structure, it is always necessary to ensure the distance of the reinforcement from the surface, the so-called cover layer of steel reinforcement.

2, this value gradually decreases from the surface of the structure to its core. This process is very slow, and placing reinforcement bars, e.g., 20 mm below the surface, will provide decades of protection for the reinforcement.

This layer prevents the steel members from being exposed to the surrounding environment. Exposure of the steel members to the open environment causes corrosion and consequent reduction of the cross-section of the member. Corrosion of concrete reinforcement occurs when it occurs in concrete with a pH value of less than 9. Newly made Portland concrete has a pH value between 12 and 13 [ 5 ]. These concretes thus provide sufficient protection against corrosion. However, due to the influence of airborne CO, this value gradually decreases from the surface of the structure to its core. This process is very slow, and placing reinforcement bars, e.g., 20 mm below the surface, will provide decades of protection for the reinforcement.

Another adverse effect of a conventional reinforced concrete structure is fire. The high temperatures that arise during a fire penetrate the surrounding structures, which become significantly hotter even if they do not burn. Exposing a reinforced concrete structure to high temperatures is particularly dangerous for the steel reinforcement, which loses its mechanical properties when it reaches 400 °C [ 6 ]. For the reinforced concrete structure to be safe even during a fire, it is necessary that the concrete again protects the steel reinforcement sufficiently. For example, for load-bearing columns, a cover layer thickness of between 40 and 70 mm may be required due to fire.

As can be seen from the previous two examples, the position of the reinforcement must be accurate for the proper functioning of the reinforced concrete structure. To fix the position during concreting, so-called spacers are used, which, by their presence, make it impossible to approach the formwork. Spacers are most often made of fine-grained concrete or plastic. The advantage of using concrete elements is that there is no significant weakening of the structure at the location of the spacer after the structure has been concreted. Concrete spacers are much more expensive than plastic elements, which is why plastic elements are most used.

Plastic spacers create a linear and systemic weakening of the structure and are designed so that the concrete will flow underneath them during the actual pouring of the concrete, and once the concrete has set, the strips themselves are no longer needed. This hypothesis is only correct for normal use of the structure; however, this is not the case under extreme loads (fire or explosion). In extreme loads [ 7 ], the structure tests the structure itself to the limit of its load-bearing capacity, or if locally, even exceeds this limit and thus reveals all its weak points, which are the spacers. An example of an experiment carried out where this problem was captured is the subject of this paper.

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In the design of reinforced concrete structures, the influence of spacers is not included in the calculation, although their influence on the quality of structures has been published in [ 8 11 ].

In the article [ 8 ], the effects of spacers on mass transport and the microstructure of concrete were studied. In total, 140 samples using plastic, cementitious, and steel spacers were investigated. These samples of various cover depths, aggregate particle sizes, and under different curing ages and conditioning regimes were tested. The paper concludes that plastic spacers exhibited the most elevated porosity at the interface between the spacer and concrete. Across all measured properties, samples with plastic spacers demonstrated the poorest performance. This is likely attributed to a fragile bond between the plastic material and concrete. Additionally, there is a more pronounced disparity in material properties concerning drying shrinkage and thermal behavior [ 8 ]. Considering the size of the tested samples, plastic A-shaped wheel and tower spacers were preferred to linear or bar spacers. Based on the above, it can be assumed that the test results for the linear spacers used in our RC slabs would show even worse results.

The paper [ 9 ] also studied the spacer–concrete interface using 216 cylindrical test samples containing plastic A-shaped spacers and cementitious spacers. Specifically, it investigated how the addition of supplementary cementitious materials, such as silica fume or fly ash, which are used in modern concrete mixes, could decrease the negative impact of the spacers. It is concluded that in some cases, the use of supplementary cementitious materials may mitigate the negative impact of spacers, but not in the samples where plastic spacers are used. Moreover, samples featuring plastic spacers displayed pronounced gradients in porosity and the presence of cracks, indicating debonding at the interface between the spacer and concrete [ 9 ]. According to Table 1 , the concrete mix design for our RC slabs does not contain any supplementary cementitious materials, so it is assumed that in the case of their use, there would be no significant improvement in the spacer–concrete interface in the slabs.

The research presented in [ 10 ] investigated the spacer–concrete interface in reinforced concrete columns with plastic or cementitious spacers under a 50 mm cover. The specimens obtained from the columns underwent testing with respect to several variables, including sample position, compaction frequency and duration, spacer type, and the conditioning regime. The article states that plastic, characterized by a smooth, non-porous surface, does not promote adhesion to concrete. Additionally, the thermal expansion coefficient for polyvinyl chloride, used for plastic spacers, is approximately 10 to 15 times higher than that of concrete [ 10 ]. A crucial finding for our tests is that, despite perforation of the plastic spacers by more than 25% of their gross plane area to alleviate these adverse effects, microcracking still occurred.

The article [ 11 ] investigated the corrosion of three pre-cracked concrete beams that were part of a larger test after 25 years of exposure to atmospheric, tidal, and seawater conditions. The plastic spacers, with 25 mm of cover, were used to keep the reinforcement in place during the casting of the beams. The research focused on the overall state of the corrosion of the beams, chloride and moisture content, and resistivity measurements. During the investigation, where some plastic spacers were exposed to saline water, high corrosion was observed, and it was attributed to the inhomogeneous and porous microstructure of the plastic spacers. In conclusion [ 11 ], it was hypothesized that corrosion at the weakest link, represented here by the highly exposed reinforcement at the plastic spacers, protects the steel in other areas, including cracks.

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These are relatively new articles from 2016 onwards. These articles confirm their conclusions that spacers have a negative effect on the homogeneity of concrete elements, which weakens the concrete elements. This weakening is caused by insufficient wrapping of the contact between the reinforcement and the spacers and by the porous interface between the spacer and the cementitious mixture; for more weakening, it goes through the entire cover layer of concrete. The effect of this weakening is to reduce the chemical [ 8 11 ] and mechanical durability of those elements. As described in the articles [ 8 11 ], the negative effect of the spacers on the mass distribution was due to the preferential flow through the pores and microcracks formed at the interface between the spacers and the concrete [ 9 ]. In the long term, this results in increasing the risk of premature corrosion or degradation of the surrounding concrete. This effect is not currently recognized by most researchers or practitioners.

Discontinuities in the material at the interface of the spacer, steel reinforcement, and concrete also have a major influence on the mechanical properties of concrete structures. When the structure is loaded with fast dynamic actions, these are the exact places where cracks will develop.

In addition, products intended as spacers for the placement of reinforcement do not belong to the specific families of construction products covered by the harmonized standard Regulation No 305/2011 of the European Parliament and of the Council [ 12 ]. For these reasons, it is not required to demonstrate any characteristics of these products. This fact allows the use of arbitrary shapes and materials. As shown by the experiment carried out, the use of a commonly used plastic spacer will result in a weakening of the cross-section due to the plastic spacer.

The research on the effect of spacers on the mechanical properties of blast-loaded reinforced concrete slabs is unique in the scientific community. Research of sources has found only a limited number of articles that discuss this negative effect.