In Switzerland as well as in the rest of Europe the cement market is changing rapidly. These changes began in the middle of the 1990’s when in Switzerland the cement CEM II/A-LL (Portland-limestone cement) was launched. This cement has increasingly replaced the ordinary Portland cement CEM I. In the last few years a series of other cement types have been brought on the market. This trend runs parallel to the rest of Europe, with some country-specific differences.
Up to now it could be assumed, that with the use of the existing cements for new constructions the carbonation of concrete did not represent a direct and relevant risk for rebar corrosion any longer, since the actual standards (codes) for concrete constructions required a far higher concrete cover than earlier. With the expected new cement types this situation might change considerably. Thus, the innovations of the cement producers might imply certain risks for the durability of concrete constructions. It has to be considered in this context that critical chloride content for corrosion decreases with a decreasing pH of the pore water and, therefore, with the advancing carbonation of the concrete cover as well as.
With this project the influence of the concrete composition (cement type and content, w/c ratio) and the curing conditions on the carbonation resistance should be investigated. For this purpose the carbonation coefficient (mass for the carbonation resistance) of a variety of concrete compositions should be determined and correlated with physical and chemical parameters of the mixes.
The following goals should be achieved in the framework of this project:
- Definition of the requirements for the carbonation resistance of concrete mixes of the ASTRA (standard specifications of concrete mixes for civil and underground constructions) with new cement types and for the curing (performance-related design methods for concrete, possibly in dependence on the concrete cover and on the durability class acc. to SIA-Merkblatt 20029).
- Development of a proposal for a Swiss standard for a accelerated test methods of the carbonation resistance of concrete mixes (durability test, quality control).
Following steps are planned in the project:
- Production of concretes with different compositions in the laboratory
- Determination of different characteristic values for density (water conductivity, pore characteristics and gas permeability)
- Determination of the Na + -, K + - and Ca (OH) 2 content
- Determination of resistance to carbonation, depending on the preliminary storage and the CO2 content in the carbonation
- Correlation between carbonation with the sealing properties and the composition of the concrete (cement, cement content and w / c ratio and Na + - K + - and Ca (OH) 2 content).
The investigations led to the following findings concerning the carbonation resistance of concrete:
- As a measure of the carbonation, the coefficient of carbonation (constant of proportionality in the time function) can be used.
- Requirements for the concrete composition alone are not sufficient to ensure a certain carbonation resistance, since the last is influenced by several factors.
- The w/c respectively the w/ceq ratio, the type of cement and additives of type II according to SN EN 206-1 have a decisive influence on the carbonation resistance. Air entrained concretes have a lower carbonation resistance as similar concretes without artificially introduced air voids.
- The cement and binder content plays no or a very minor role at a given w/c ratio. This also applies to the w/ceq or w/b ratio if the ratio of cement to additive remains constant.
- Between the coefficient of carbonation and physical parameters (compressive strength, total porosity, water permeability and gas permeability) or chemical parame- 649 | Anforderungen an den Karbonatisierungswiderstand von Betonen 12 November 2012 ters (water- and acid-soluble Na2O equivalent and Ca(OH)2 content) no or only weak correlations exist. None of these parameters can be used for a reliable and quantitative assessment of the carbonation resistance of concrete.
- A prolongation of concrete curing from 1 to 7 days reduces the carbonation coefficient considerably. A prolongation from 7 to 28 days only brings a minor improvement. The type of cement has a significant impact on the results.
- Enhanced carbonation of edges (so-called edge effect) decreases with increasing carbonation depth and can be described by the interaction coefficient.
- In unweathered and weathered exterior conditions, concrete carbonates significantly slower than in laboratory conditions (increased air and concrete humidity, lower temperature). In unweathered conditions, the value of the carbonation coefficient reaches about 80% of the laboratory value. Additionally, instead of the simple √t-law (time exponent 0.5), a time exponent of 0.4 can be used.
Concerning the testing of carbonation resistance, the following conclusions could be developed:
- The acceleration of the carbonation does not fundamentally change the assessment of carbonation resistance of concrete. A quick test for the assessment of carbonation resistance is therefore possible.
- With an increasing CO2 concentr
- Findings about the influence of cement type and content, w / c ratio, pore characteristics, gas tightness and the chemical composition of the pore water in the carbonation.
- Builders / engineers: Recommendation for requirements (possibly depending on the coverage and exposure class or of the durability class according to SIA leaflet 2029).
- Testing laboratory: Test code for economic and expedient examination of carbonation resistance of concrete.
Special tools and infrastructure:
- Equipment for various quick carbonation tests
- Stevenson screen (specially designed container) for the unweathered outdoor storage of test specimens for carbonation
- Equipment for all required fresh and hardened concrete tests as well as for the chemical determination of Na and K content and the Ca (OH) 2 content in the concrete
The requirements for the carbonation to be included in the National Annex to SN EN 206-1. This standard forms for concrete structures is an important part of tenders and contracts for work.