ABSTRACT
One of the main problems of f r e s h concrete technology i s the d i f f i c u l t y i n r e t a i n i n g w o r k a b i l i t y , once the concrete has l e f t the mixer. As a r u l e , the time passing between mixing and p l a c i n g f r e s h concrete may be 50 or 60 minutes. During t h i s period the f r e s h concrete s t i f f e n s as a r e s u l t of physico-chemical processes. This changes i t s w o r k a b i l i t y , making i t d i f f i c u l t to place. This i s e s p e c i a l l y so i n the case of porous aggregates.
