BS 7542:1992 download.Method of test for Curing compounds for concrete.
1 Scope
BS 7542 describes a method for the laboratory determination of the water retention efficiency of organic solvent based, membrane-forming curing compounds for concrete.
NOTE The titles of the publications referred to in this standard are listed on the inside back cover.
2 Definitions
For the purposes of BS 7542 the following definitions apply.
curing
maintenance of a sufficient moisture content in freshly placed concrete to ensure hydration of the binder and hence development of strength
2.2
curing compound
liquid applied as a coating which dries to form a curing membrane
2.3
curing membrane
film formed on the surface of freshly placed concrete to assist curing
3 Principle
The curing compound to be tested is applied as a uniform coating, usually by spraying, to the levelled surface of freshly mixed mortar in a metal mould. The coated specimen and an uncoated control specimen are then transferred to a warm curing cabinet in which dry air is circulated. The specimens are kept in the cabinet for 3 days and then weighed. The efficiency of the curing compound in forming a membrane which reduces moisture loss is calculated from the measured loss in mass of the specimens, after correction for the loss of solvent from the coated specimen.
4 Materials
4.1 Portland cement, complying with BS 12 that has been specially selected for the purpose of testing admixtures. It shall be identified as “CAA Reference Cement” and shall be stored in an airtight container.
NOTE BCA Specification Cement, meeting the requirements of the Cement Admixtures Association Reference Cement (CAA Reference Cement), and its specification can be obtained from Wexham Developments Limited, Wexham Springs, Slough
SL3 6PL.
4.2 Natural silica sand, oven dry with a rounded particle shape complying with BS 882, except that the grading shall be as given in Table i’.
Table 1 — Grading of sand
Sieve size
Proportion retained
4.3 Mould release agent, comprising petroleum jelly, mineral oil or a similar commercial mould release agent.
5 Apparatus
5.1 Moulds. that are of corrosion resistant metal, do not distort, are watertight and of the following size (see Figure 1).
a) Top, internal: 150 mm x 300 mm.
b) Depth: (50 ± 2) mm.
c) Top flange: width at least 12 mm.
5.2 Balance, capable of weighing the filled moulds (each weighing about 6.5 kg) and readable to the nearest 0.1 g.
5.3 Cabinet, complying with BS 2648, for storing specimens at a temperature of (38 ± 1) °C and at a relative humidity of (35 ± 5) %. It shall have three perforated or mesh shelves each capable of supporting two specimens during testing so as to ensure a clear space of at least 40 mm on all sides of the individual specimen and between specimens and the side walls of the cabinet2. The cabinet shall be equipped to circulate air over the specimens at an approximate rate of 0.5 mIs.
NOTE A relative humidity of (35 ± 5) % can be produced by placing a large open vessel containing a saturated aqueous solution of magnesium chloride in the bottom of the cabinet during the test.
5.4 Spray equipment. designed to permit the curing compound to be aspirated and applied evenly to the surface of the test specimen2.
5.5 Mixer, as described in 8.3 of BS 455 1:1980, electrically driven and of nominal capacity 12 kg.
5.6 Compacting equipment, either a vibrating table or a vibrating hammer with a foot 40 mm square, or a compacting bar made from a non-absorbent material, approximately 200 mm long and with a foot 40 mm square.
5.7 Metal screed, (148 ± 1) mm long, of L-shaped cross section approximately 50 mm x 25 mm, the shorter side having a sharpened leading edge. The screed shall be supported across the top of the mould by a 200 mm long rigid member that can slide on the flanges while holding the screed horizontal. The height of the screed shall be adjustable to give a uniformly flat surface finish to the mortar (7 ± 1) mm from the top of the mould (see Figure 2).
5.8 Metal tray, with sides at least 3 mm high and having an area approximately equal to that of the surface of the test specimen (see Figure 3).
5.9 Hydrometer, complying with BS 718.
5.10 Pain brush, 50 mm wide, medium soft.
6 Preparation of mortar
6.1 Number of specimens
Prepare at least three pairs, each comprising a test specimen and a control specimen, for each curing compound to be tested.
6.2 Batching and mixing
The mortar shall contain one part by mass of cement (4.1), three parts by mass of sand (4.2) and 0.44 parts by mass of water.
The mortar batch size shall be just sufficient to prepare two specimens, i.e. one test specimen and its corresponding control specimen.
Bring all the materials to a temperature of (20 ± 5) °C before starting the mixing and carry out the mixing in a room at this temperature.
Place the sand and cement in the mixing bowl and mix dry for I mm. Add the water and continue mixing for a further 2 mm. Stop the mixer and scrape any unmixed material from the bottom and sides of the bowl. Restart. the mixer and mix for a further 2 mm.
6.3 Moulding specimens
Prepare each pair of specimens within 20 mm after the completion of mixing.
Thoroughly clean two moulds (5.1), lightly coat them with mould release agent (4.3), weigh them to the nearest 0.1 g and record the masses as m1 and m9. Then place a layer of mortar approximately 25 mm deep in each of the moulds. Compact this layer and a second layer as described in a) or b).
a) Tamp the mortar 50 times with the compacting bar (5.6). Place a second layer of mortar, sufficient to overfill the moulds slightly and compact it in a similar manner. Fill indentations formed by tamping and approximately level the surface using a vigorous compactive effort (slapping action) with the palms of the hands.
NOTE Rubber gloves should be worn to prevent contact of
the hands with the wet mortar, which is alkaline and can cause skin irritation.
b) Compact the layer using the vibrating table or the vibrating hammer (5.6), place a second layer to overfill the moulds slightly and compact it by the same means.
Level the mortar (7 ± 1) mm from the top of the mould by a shaving action using the metal screed (5.7). Make the minimum number of passes of the screed, working along the length of the mould in both directions, to produce a uniform surface, free from undulations and surface defects.