Page 22 - New Trends in Green Construction

Circular Economy
Table 1 - Hardened-state properties results overview at 28 days
Mix and respective increase/decrease compared to the reference
Hardened-state properties
Compressive strength in cubes Splitting tensile strength Modulus of elasticity Ultrasonic pulse velocity Abrasion resistance
Shrinkage Creep
HPSCC (100%NA)
25% RA -1% -10% -5% -1% +6%
50% RA -3% -23% -12% -4% +13%
100% RA -8% -32% -26% -8% +39%
100% CRA -2% -19% -11% -2% +10%
100% FRA -5% -28% -21% -6% +36% +20% +84%
+3% +13% +23% +13% +20% +34% +97% +30%
The need of correction of the mixing water volume due to the high absorption by the RA means that, in particular CRA can retain (during the initial stage of mixing) large quantities of water. This water, which is neither used for workability nor for any hydration process in the initial stage, is available to contribute to the later reaction of FA with calcium oxide or calcium hydroxide (products from the cement hydration). The substitution of NA with RA causes a decrease of the hardened-state properties (namely compressive strength and splitting tensile strength). This is explained by the poor quality of RA, which has its origin in the adhered mortar and is responsible for increasing the porosity and cracking of the aggregates, making connections in the transition zone between the RA and the new binder weaker. After 28 days, decreases of 1% to 8% in the compression strength were recorded and from 10% to 32% in the splitting tensile strength.
According to the NP EN 206-1 (2007) standard classification, the 100% RA mix belongs to the C50/60 class resistance and the remaining mixes to the above class: C65/67. Thus, according to the mentioned standard, all concrete produced can be classified as high performance. A decrease in the modulus of elasticity is observed with RA increase, which is explained by the lower stiffness of RA (compared to NA), given the presence of old mortar adhered in RA and also to the lower deformability of the binder. At 28 days, there is a reduction of 5% to 26% in the modulus of elasticity. The lower stiffness of RA, compared to NA, decreases the overall stiffness of the concrete and, therefore, increases the shrinkage and creep behavior. There is also an increase in shrinkage and creep at 91 days larger than the one registered at 7 days due to the internal healing phenomenon triggered by RA. After 28 days, reductions of less than 23% of shrinkage occurs.
4. References
Silva P. M., de Brito J. and Costa, J. M. (2011) Viability of two new mixture design methodologies for Self-Consolidating Concrete SCC, ACI Materials Journal, American Concrete Institute Vol. 108, No. 6, pp. 579-588.
NP EN 206-1, Concrete, Part 1 (2007) Specification, performance, production and conformity, Lisbon, Portugal, IPQ.
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