An assembly for the placement of concrete, said assembly including: - a large-profile screed guide/control joint suitable for use in the placement of concrete slabs of a depth superior to 15,24 cm; - a small-profile screed guide/control joint suitable for use in the placement of concrete slabs of a depth inferior or equal to 15,24 cm; each of the large-profile and small-profile screed guide/control joints (100, 100A) having a linear shape and comprising: a base member (102, 102A); and two walls (118, 118A; 120, 120A) extending upwardly from the base member and joined at an apex (122, 122A), forming a generally triangular-shaped longitudinal cavity (124, 124A), the base member (102, 102A) having a central arcuate member (104, 104A); each of the large-profile and small-profile screed guide/control joints also comprising an extension member (126, 126A), vertically extending upwards from the apex; upwardly and vertically being understood in an installed condition of the screed guide/control joint; and - a longitudinal cap member (130, 130A) able to be removably fitted onto the extension member of at least one of the large-profile and small-profile screed guide/control joints, so as to provide a removable height extension to said screed guide/control joint; the assembly being characterized in that: - the large-profile screed guide/control joint comprises unitary S-shaped side walls (106, 108) which extend outwardly from the central arcuate member and curve downwardly, each side wall terminating in an inwardly extending flange (110, 112), thereby defining two parallel longitudinal open channels (114, 116); - each side of the central arcuate member of the base member of the small-profile screed guide/control joint transitions to a closed flange having an upward slope (106A, 108A) which in turn transitions to one of the walls (118A, 120A). - a large-profile screed guide/control joint suitable for use in the placement of concrete slabs of a depth superior to 15,24 cm; - a small-profile screed guide/control joint suitable for use in the placement of concrete slabs of a depth inferior or equal to 15,24 cm; each of the large-profile and small-profile screed guide/control joints (100, 100A) having a linear shape and comprising: a base member (102, 102A); and two walls (118, 118A; 120, 120A) extending upwardly from the base member and joined at an apex (122, 122A), forming a generally triangular-shaped longitudinal cavity (124, 124A), the base member (102, 102A) having a central arcuate member (104, 104A); each of the large-profile and small-profile screed guide/control joints also comprising an extension member (126, 126A), vertically extending upwards from the apex; upwardly and vertically being understood in an installed condition of the screed guide/control joint; and - a longitudinal cap member (130, 130A) able to be removably fitted onto the extension member of at least one of the large-profile and small-profile screed guide/control joints, so as to provide a removable height extension to said screed guide/control joint; - the large-profile screed guide/control joint comprises unitary S-shaped side walls (106, 108) which extend outwardly from the central arcuate member and curve downwardly, each side wall terminating in an inwardly extending flange (110, 112), thereby defining two parallel longitudinal open channels (114, 116); - each side of the central arcuate member of the base member of the small-profile screed guide/control joint transitions to a closed flange having an upward slope (106A, 108A) which in turn transitions to one of the walls (118A, 120A).