July_1967 — Page 37

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an external waterproofing mem- brane to exclude the water. A third approach, which relies on an internal membrane to keep the basement dry, is not widely accepted.

The first method, which uses the structural concrete of the basement as a water-excluding barrier, requires careful mix design. Water bars at construction joints and integral water- proofing additives are often used. The usual areas of leakage are at the construction joints and where there is local segregation of the concrete mix.

To succeed, an exceptional degree of site quality control is required, but complete impermeability is seldom ac- hieved. and defects often do not come to light until sometime after- wards. In the event of failure to ex- clude water, a common remedy is to drill and pressure grout the defective parts of the structure.

The second method, known as tanking, relies principally on an im- pervious membrane, usually of as- phalt, around the basement to exclude water. The main shortcoming of this method is that, should there be a de- fect in the asphalt, the tracing of its location is almost impossible as this rarely coincides with the point of en- try of water into the structure. The making good of any defects in the membrane is therefore both difficult and expensive.

When leakage occurs, there is end- less argument as to whether the con- crete or the membrane is defective, adequate repairs often go by default, and the last resort of pumping from a sump has to be adopted. The depen- dence on such a membrane also tends to induce an undesirable sense of complacency in carrying out the con- crete work.

As neither of the above waterproof- ing methods appeared to be infallible, it was decided, in the case of the MSA Headquarters Building, to use a combination of both methods.

Structural Concrete

To achieve impermeability in the structural concrete, the following pre- cautions were taken:

(a) Splaying the concrete to avoid abrupt changes in its section. (b) The use of a carefully designed mix which incorporated the fol- lowing:

i) Granite coarse aggregate, gap graded in 3⁄44 in. and 11⁄2 in. sizes.

ii) Water

cement ratio not

greater than 0.50. iii) A plasticizer in the concrete,

Sealoplaz.

iv) Cube strength of 3,750 p.s.i. in 28 days, with under one per cent probability of fai- lure.

(c) Site control of the quality and quantities of the material used in the mix, involving weigh batching, frequent grading, silt and moisture content tests, and the making up of grading de- ficiencies as required on site. (d) Close supervision of the batch-

Far East Architect & Builder July, 1967

(e)

(f)

ing. mixing, transporting, plac- ing, compacting and finishing of the concrete.

Pattern vibration supplemented by rodding.

Careful removal of laitence and the use of an enriched mix in- corporating an integral water- proofer. Double Strength Pre- mix, and p.v.c. water bars at construction joints.

(g) Construction in bays not great- er than 30 ft. in each direction. with a minimum time interval of seven days between the con- creting of adjacent bays. A che- quer-board

(h)

(i)

following specification; Compressive strength

at 28 days Tensile strength at 28 days Coefficient of

permeability Non-corrosion of

reinforcement Resistance to acid and

alkali attack at

3,000 p.s.i.

300 p.s.i.

10-7

2<pH<12

Numerous tests on specimens from trial mixes were carried out and the following mix, which met the above requirements, was eventually adopt-

(a)

(b)

ed for the work. concreting pattern was used at first, but an im- proved technique using double joints 12 in. to 24 in. apart was used in the later stages of the contract.

The design of the timber form- work adequate laps at panel edges and its saturation with water prior to concreting to en- sure negligible loss of grout. Strict curing of the concrete for ten days.

The concrete used in the substruc- ture of the building had an aggregate cement ratio of 5.2 to 1, and a water cement ratio of 0.50. The slump varied between 11⁄2 in. and 21⁄2 in. The average cube strengths were 4,200 p.s.i. at seven days, and 5.740 p.s.i. at 28 days, with a standard de- viation of 870 p.s.i. at the latter age. The average density of the concrete was 151 lb. per cu. ft.

Grout Membrane

It was considered that, despite the stringent measures taken, the pos- sibility of local water penetration through the concrete could not be completely ruled out, and an ex- ternal membrane would therefore have to be provided in addition.

A grout membrane was adopted, in preference to asphalt. The former was considered to be more foolproof as it would be fully integral with the concrete work, and being injected under pressure would automatically make good any defective areas in the concrete, and so virtually eliminate any possibility of water penetration.

It was also less expensive than asphalt. The relative lack of plas- ticity of such a membrane was con- sidered to be of secondary importance, if it had adequate tensile strength. and if the concrete structure was not subject to large movements.

sur-

Basically the method comprised the provision of a porous layer be- tween the structure and the rounding soil, into which a cement based chemical grout was injected under pressure, via holes preformed through the concrete.

The use of cement based chemical grout in civil engineering strucures is not new, but it is believed that this is the first time it has been used to form a membrane around a base- ment, other than as a remedial mea-

sure.

The grout had to comply with the

112 lb.

Work-

+ 2 oz.

ability aids

Cement Sodium Sulphosuc- cinate base (c) Acrylic Polymer

antiprecipitant 4 oz.

(d) DSP (Di-sodium Phosphate)

(e)

AM-9 (Acrylamide & Methylenebis-

2 lb. Buffer

Water- proof

acrylamide)

211⁄2 lb. gel

with

(f)

AP (Ammonium Persulphate)

excess

±1⁄4 lb.

water

(g) KFe (Potassium

Ferricyanide)

(h) Water

→2 grms. Retarder

+44 gallons

The cement, with DSP (a weak alkali) as a buffer and controller, is the activator for the reaction to pro- duce the "gel". The AP is the initia- tor which triggers the reaction.

Grouting Operation

The structure lent itself to division into three sections for grouting: a basement section at each end, and the sub-basement in between. The three sections were grouted in separ- ate operations.

The following is a description of the grouting of the floors:

(a)

Preliminary. A sandwich layer of polythene sheet between bitu- menised felt was laid on the ground concrete to minimise loss of grout. (b) Porous Layer. A 11⁄2 in. thick

(c)

layer of hand screened and washed, crushed granite in. down to 3% in. was provided over the sandwich layer. "Chicken" wire mesh was used to hold the aggregate on sloped areas. The aggregate was floated over with a stiff 1-3 cement sand mix, 34 in. thick, which successfully withstood all traffic and fixing of heavy reinforcing steel.

Formers. Holes 11⁄2 in. diameter for fixing grout pipes through the structural concrete into the porous layer were formed by steel pipes, greased and wrapped in polythene: these were provid- ed at all lines of change of slope and at approximately 6 ft. cen- tres each way. Removal of the polythene proved difficult and a better method would be to leave

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