|M.Sc Student||Mandelboumm Liad|
|Subject||Influence of Old Paste on the Properties of Recycled|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Katz Amnon|
|Full Thesis text - in Hebrew|
This study investigated the effect of artificial fine aggregates made of 100% cement paste on the properties of concrete mixtures, in which this aggregate was used as a primary aggregate. Emphasis was subjected on the creation of a fine artificial aggregated by casting and by crushing. The purpose of using two alike materials was to find if there is a difference in the properties of the concrete prepared from this similar but different aggregates, as one was casted and the other crushed, and by that understand the effect of recycled aggregate production on the properties of the new concrete prepared with these aggregates.
The artificial cement paste aggregate describes a worst-case scenario of fine recycled concrete aggregate, which is a composite material of natural rock and old cement paste. In this research the effect of fine aggregate from paste on concretes physical properties were investigated.
The grinding process affects the strength of the cement paste as aggregates especially in the low strength range (25MPa). This insight is of important since recycled aggregates are partially composed of old cement paste. When the old cement paste as fine aggregate forms 100% of main aggregate and its strength is similar to that of the new concrete mix, the new concrete strength is expected to decrease by 20-25%, for water-cement ratios of 0.75-0.40 respectively. When the artificial aggregate is much stronger than the matrix the compressive strength is preserved but weak artificial aggregate in a strong matrix reduced the strength by 36%. In order to compensate for strength reduction, the water-cement ratio should be reduced accordingly.
Carbonation: For porous concrete, w/c=0.75, no difference in the carbonation rate was found between the different aggregates (natural, artificial made from high w/c or low). Dense concrete mixtures (w/c=0.40), with dense artificial aggregates or natural aggregates limestone, are very much resistant to carbonation. However, when porous artificial aggregates were use, CO2 penetrates through the porous aggregates that are in contact with the surface of the sample and passes deeper into the concrete.
Concrete with fine aggregates recycled from concrete or cement paste, shrinks approximately twice as much as natural aggregate concrete, depending on the total volume of the cement paste and the average water-cement ratio in mixture.
An approximately linear relationship was found between the overall shrinkage strain in lab conditions to the total volume ratio of cement paste times water-cement ratio. This relationship can roughly be used to estimate draying shrinkage.
Before demolition, it is recommended to classify the construction elements according concrete strength and during demolition - arrange the debris according to concrete strength.
In further studies, it is recommended to focus on concrete properties of a fine aggregate from recycled concrete aggregate passing sieve 2.36, i.e. very fine aggregate.