Green’s Dictionary of Slang

(N.Z./W.I.) any starchy, indigestible food, e.g. dumplings, fufu, usu. with peas (legumes) or beans mixed in.

1898		Star (Canterbury, NZ) 1 Oct. 7/8: In ‘Darkest England’ [...] baked bread and butter pudding [is called] ‘concrete’.
1943, 1958		cited in Cassidy & LePage Dict. Jam. Eng. (1980).