Level, Thickness and Regularity Tolerances

CHAPTER THIRTEEN: PRACTICAL HINTS FOR SITE STAFF

C. PAVEMENT Subgrade

13. Level, Thickness and Regularity Tolerances

The Specification sets out level and thickness tolerances for all courses from sub-grade to top of wearing course. The contractor should be encouraged to keep to the lower -side of level tolerances.

Compliance can be checked by optical levelling or by "dipping" from a taut nylon line stretched across the pavement between level pins set out-side the pavement, so that they serve for all lay-ers without repeated replacement. Additional pins are required, however, adjacent to surfacing lanes for automatic level control devices of paving machines.

The surface regularity requirements (which apply only to surface courses) are also given,in the Specification and can be checked by rolling straight-edge, or hand-carried straight-edge and wedge.

D. CONCRETE

1. The Specification gives full details of a) the designation of Grades (Prescribed &

Designed),

b) Characteristic Strength,

c) Current Margins and the method of deter mination,

d) procedures for establishing mix design &

properties by trial,

e) compliance requirements (by test result) for incorporated concrete

It is important that all staff involved in supervis-ing and testsupervis-ing concrete works are fully conver-sant with all the Specification data.

2. It is important for the contractor and the supervisory staff to make an early start on the preparation of trial concrete mixes and their tests.

3. Sampling and testing regimes (including test frequencies etc.), for both constituent materials and finished concrete mixes (strength & worka-bility), are given in the Specification and/or

"Guidelines for Inspection and Testing of Roadworks" and should be carefully observed.

4. The moisture content of coarse and fine aggregates for concrete should be checked regu-larly in order to determine the quantities of added water. Frequent slump tests should also be taken on delivery of the mix to the structure in order to check water content and workability.

The specified mix properties established during the mix design trails give a workability range for each grade of concrete. The range may be too wide to provide a suitable mix for all the purpos-es for which the grade has been specified and it may be necessary to apply closer limits for par-ticular usages. For example high workability may be required for areas of congested rein-forcement or mould shapes which restrict the movement and compaction of fresh concrete.

Low water, slow setting mixes may be necessary for high strength deck slabs with large exposed areas. In such cases it may be desirable to incor-porate additives to produce these special effects..

It should be remembered however, that the addi-tive mix should be fully tested beforehand and the results compared with a basic additivefree control mix.

5. The requirements for fixing reinforcement,, constructing shuttering and placing, compacting and curing concrete are all fully covered in the

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Cawangan Jalan, Ibu Pejabat JKR, K.L

Specification but the following points are of par-ticular interest.

a) It is important to ensure that all faces of the placed concrete in the structure are kept alive with fresh material so that dead joints do not form. In emergencies it may be necessary to form specified construc tion joints.

b) Particular care should be taken with the curing arrangements for high strength bridge decks which are susceptible to shrinkage cracking. If high heat generat-ion and high ambient temperatures com bine to cause problems with setting and curing it may be necessary to take special steps to reduce concrete temperatures to the specified limits - e.g. by shading aggregate stockpiles or adding ice to the mixing water.

6. Non compliance of test cube results with specified strengths.

The first step is to ask the contractor for his pro-posals for dealing with the problem. In deciding whether to accept the contractor's proposals, or whether other remedial measures should be taken, the following points are relevant.

The action to be taken in respect of the concrete which is represented by the test cubes failing to meet the specified requirements, may range from qualified acceptance in less severe cases, to rejection and removal in the most severe cases.

In determining the action to be taken, the Engineer should have due regard to the technical consequences of the kind and degree of non-compliance and to the economic consequences of alternative remedial measures, either to replace the sub-standard concrete or to ensure the integrity of any structure in which the con-crete has been placed.

In estimating the quality of the sub-standard concrete and in determining the action to be taken, the Engineer should establish the follow-ing, whenever possible:-.

(i) the validity of the test result, and confir-mation that specimen sampling and test-ing has been carried out in accordance with the Specification,

(ii) the mix proportion actually used in the concrete under investigation.

(iii) the actual section of the structure repre-sented by the test cubes,

(iv) the possible influence of any reduction in concrete quality on the strength and dura-bility of the particular section of the stru-ture.

The Engineer may wish to carry out tests on the hardened concrete in the structure such as the taking of cored samples. However, the results of any such tests should not be admitted as taking precedence over the evidence of valid cube results.

7. Remedies for other defects

-If the contractor is permitted to repair defective concrete resulting from shutter movement or collapse, grout loss or poor compaction, rather than remove the whole structural element, care must be taken to ensure that the area affected is cut back to sound dense concrete before recast-ing fresh concrete. In areas of critical stress, it may be necessary to insist on repair with epoxy concrete or epoxy mortar.

If poor concreting and finish persist, it is advis-able to insist on the removal of all offending ele-ments and review the whole of the contractor's concreting and formwork techniques.

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