CRACKED CONCRETE CONSIDERATION IN DESIGN OF POST-INSTALLED ANCHORS

1. What is cracked concrete?

Joseph Monier, a 19th-century French gardener, was a pioneer in the development of reinforced concrete and later G. A. Wayss, a German civil engineer and a pioneer of the iron and steel concrete construction, made the first commercial use of reinforced concrete with contribution in advancement of Monier's system of reinforcing. He established it as a well-developed scientific technology; steel absorbs tension stresses, and the concrete only provides compression resistance.

Concrete naturally has high compressive strength but low tensile strength property. When it is subjected to tensile forces, it is prone to cracking (Fig. 1.1). Cracked concrete refers to concrete that has developed visible cracks due to internal or external stresses, i.e., typically at least 0.1 – 0.3 mm. These cracks can form from structural loads, thermal expansion, shrinkage, creep or environmental factors.

Due to the low tensile property of concrete, reinforcements generally take the tension stresses for cracked concrete. The compressive stress in the concrete section not affected by cracks remains relatively undisturbed and follows an approximately linear stress distribution pattern (Fig. 1.2). When a crack forms, the tensile stress at the crack tip becomes significantly high, while the stress within the cracked section drops nearly to zero. Between cracks, the concrete still experiences some tensile stress due to the bond between the concrete and reinforcement, which gradually decreases as the distance from the crack increases. State-of-the-art reinforced concrete design standards require reinforcement detailing to ensure that crack widths resulting at maximum permissible service static and quasi-static loads (i.e., dead loads plus a fraction of live load) do not exceed the value of 0.3 mm to 0.4 mm. Under accidental loads such as seismic actions crack widths may reach 0.5 mm to 0.8 mm.

2. Why cracked concrete is important consideration in anchor design?

2.1 Effect of crack on performance of anchor

Different loading conditions on concrete sections cause different tension zones and according to anchor placement the performance is influenced (Fig. 2.1).

In general when cracks form in a concrete member, it is highly likely that they will intersect the anchor location directly or tangentially (Fig. 2.2) and the radial stresses in the concrete are bisected by the crack Fig. 2.4 a)). This occurs because higher tensile stresses exist around the anchor as a result of the hoop stresses associated with the prestressing and loading of the anchor and the stress concentration caused by the presence of the anchor hole (notch effect) (Fig. 2.3 a)). Furthermore, the likelihood of seeing cracks forming at the anchor position is high, due to the weakening in the concrete matric caused by the fixing itself (Fig. 2.3 b)).

Fig. 2.4 b) shows a typical load-displacement behavior in cracked or uncracked concrete under tension loading. In uncracked concrete, the displacement is much less than in cracked concrete and load capacity is higher. An extensive analysis on the behavior of fasteners in cracked vs. uncracked concrete is documented by Eligehausen et.al. ([1]).

The qualitatively influence of crack in concrete on the pull-out resistance and displacements of post-installed anchors is shown in Table 2.1.

2.2 How performance of anchors in cracked concrete is assessed?

European and US frameworks define the tests to be performed for anchor’s performance for certain crack widths. A sample test program from EAD 330232 [2]and AC 193 [3] for post-installed mechanical anchors is shown in Table 2.2.

Crack widths used in qualification tests are based on solid research study on cracked width measured on structures at serviceability and ultimate limit states (SLS and ULS) as shown in Fig. 2.5 and Fig. 2.6.

Live load can significantly influence cracking in concrete at anchor locations. Crack widens and propagates with repeated loading cycles (Fig. 2.7). It can cause reduction in anchor’s resistance and gradual loosening or increased deformation and thus reducing reliability of the connection. Though there is a tendency of anchors slip out during crack opening and closing, the crack cycling tests (Fig. 2.8) allow anchor’s displacement of maximum 3 mm after 1000 crack cycles. This accounts for an expected design working life of 50 years. Modern assessment criteria require larger number of cycles, if the connection is designed for a longer working life (e.g., 100 years).

3. What do standards say about cracked concrete consideration in anchor design?

The design standards, both EN 1992-4 [7], cl. 4.5 and 4.7 and ACI 318 [8], cl. 17.10.5.4 recommend considering cracked concrete for design of post-installed anchors unless it is demonstrated that the concrete will remain uncracked during the entire working life (members in constant compression under permanent loads). EN 1992-4 [7]further provides some guidance on consideration of uncracked concrete for design of anchors in eq. (4.4).

EQUATIONS!!!!!

Additionally, as per EN 1992-4 cl. 9.2.2 and ACI 318 cl. R17.10.5.4 the design of anchors for seismic needs to be done for cracked concrete only. As per EN 1992-4 cl. D.1 anchors exposed to fire also should have an ETA for use in cracked concrete. To establish the base material as uncracked concrete under all circumstances is obviously quite cumbersome, hence it is recommended to consider always cracked concrete for design of post-installed anchors.

The equations given in EN 1992-4 [7] for calculation of resistance against different failure modes for tension loading clearly shows the difference in characteristic value for cracked and uncracked concrete (Table 3.1).

Additionally, as per EN 1992-4 cl. 9.2.2 and ACI 318 cl. R17.10.5.4 the design of anchors for seismic needs to be done for cracked concrete only. As per EN 1992-4 cl. D.1 anchors exposed to fire also should have an ETA for use in cracked concrete. To establish the base material as uncracked concrete under all circumstances is obviously quite cumbersome, hence it is recommended to consider always cracked concrete for design of post-installed anchors.

The equations given in EN 1992-4 [7] for calculation of resistance against different failure modes for tension loading clearly shows the difference in characteristic value for cracked and uncracked concrete (Table 3.1).

4. How to design anchors for cracked concrete?

The user-friendly, cloud-based structural engineering design software PROFIS Engineering by Hilti offers design options both for cracked and uncracked concrete. In the design input step, under “Base material” tab, the selection should be done for cracked concrete and accordingly, the suitable post-installed anchors for the desired load criteria, cracked concrete and other boundary conditions will appear. User can select from the options and further carry out the design and run analysis to see the result (Fig. 4.1). More details on step-by-step design methods in PROFIS Engineering is elaborated in Hilti Steel-to-Concrete Handbook [9].

5. Conclusion

In general, cracks in concrete are expected, and the probable location of the cracks can be easily predicted in the anchor position, implying a reduction of the load capacity or visible displacements. We recommend to always consider the concrete as cracked in the design, unless dealing with applications where it is clear that the concrete will never be tensioned, such as light fastening on pre-stressed concrete elements (to be proven, in any case). Otherwise, anchors qualified for use in tensioned concrete should be used to help ensure safety through a proper design, while solutions for which the performance has not been assessed in this condition cannot guarantee adequate reliability.

To start designing, visit https://profisengineering.hilti.com/

References

[1] R. Eligehausen, R. Mallee and J. Silva, Anchorage in Concrete Construction, Berlin: Ernst & Sohn GmbH & Co. KG., 2006. [2] EOTA EAD 330232-01-0601: Mechanical fasteners for use in concrete, Brussels: EOTA, 2021. [3] AC 193: ACCEPTANCE CRITERIA FOR MECHANCIAL ANCHORS IN CONCRETE ELEMENTS, ICC Evaluation Service, 2009. [4] K. Bergmeister, Stochastic in fixing technology based on realistic influenced parameters, PhD Thesis, Germany: University of Innsbruck, 1988. [5] P. Schiessl, Crack influence of the durability of reinforced and prestressed concrete components. Schriftenreihe des Deutschen Ausschuss für Stahlbeton, Berlin: Ernst & Sohn GmbH & Co. KG., 1986. [6] R. Eligehausen and A. Bozenhardt, Crack widths as measured in actual structures and conclusions for the testing of fastening elements., Germany: Univeristy of Stuttgart, Institute of Construction Materials, 1989. [7] EN 1992-4:2018: Eurocode 2 - Design of concrete structures - Part 4: Design of fastenings for use in concrete, Brussels: CEN, 2018. [8] ACI 318-19: Building Code Requirements for Structural Concrete, Farmington Hills: American Concrete Institute, 2019. [9] S2C Handbook: Steel to concrete connections using Post-installed systems, Schaan: Hilti Corporation, 2024.