Search

Is compressive strength test required for standard mixes of concrete?

In some countries like Britain, specification for concrete does not normally require cube tests for standard mixes of concrete. The quality control of standard mixes in Britain is achieved by checking if the appropriate mix proportions are adopted during the mixing of concrete. However, in Hong Kong the requirement of testing for compressive strength is still required for standard mixes in the specification because it is impractical to inspect and check all constituent materials (e.g. cement, aggregates etc.) for concrete for compliance. As there is high variability in mixing materials owing to variance in the origin of production of constituent materials in Hong Kong, there is a risk that the end-product concrete does not comply with the design requirements even though the mix proportions of standard mixes are followed closely by engineers.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Is Schmidt hammer test a standard test for testing concrete strength?

The Schmidt hammer test involves hitting the in-situ concrete with a spring-driven pin at a defined energy, and then the rebound is measured.

The rebound depends on the surface hardness of the concrete and is measured by test equipment. By referring to the some conversion tables, the rebound result of the test can be used to determine the compressive strength of the concrete. Although past investigations showed that there is a general relationship between compressive strength of concrete and the rebound number, there is a wide range of disagreement among various research workers regarding the accuracy of estimation of strength from Schmidt hammer. In fact, there is about a variation of 15-20% in concrete strength measured by the method.

Schmidt hammer is in not a standard test for acceptance testing of concrete strength. It is only a test used for estimating the strength of concrete in structure and it can hardly be considered as a substitute for compressive strength test.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What are the major problems in using pumping for concreting works?

In pumping operation, the force exerted by pumps must overcome the friction between concrete and the pumping pipes, the weight of concrete and the pressure head when placing concrete above the pumps. In fact, as only water is pumpable, it is the water in the concrete that transfers the pressure.

The main problems associated with pumping are the effect of segregation and bleeding. To rectify these adverse effects, the proportion of cement is increased to enhance the cohesion in order to reduce segregation and bleeding. On the other hand, a proper selection of aggregate grading helps to improve the pumpability of concrete.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What are the potential problems for retardation of fresh concrete?

Retardation of fresh concrete has several advantages as follows:

(i) A rapid hydration process results in loss in concrete strength because the concrete will have a poorer structure with a higher gel/space ratio compared with the concrete with a lower hydration rate.

(ii) During the hydration process, a substantial heat of hydration will be generated. If the hydration process is carried out too swiftly, it will cause a rapid rise in temperature and results in considerable early thermal movement in concrete.

(iii) In hot weather concreting, the loss of workability is substantial. In order to ensure sufficient compaction of fresh concrete, it is necessary to extend the time for fresh concrete to remain plastic.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Where are the desirable locations of construction joints?

Construction joints are normally required in construction works because there is limited supply of fresh concrete in concrete batching plants in a single day and the size of concrete pour may be too large to be concreted in one go.

The number of construction joints in concrete structures should be minimized. If construction joints are necessary to facilitate construction, it is normally aligned perpendicular to the direction of the member. For beams and slabs, construction joints are preferably located at about one-third of the span length. The choice of this location is based on the consideration of low bending moment anticipated with relatively low shear force. However, location of one-third span is not applicable to simply supported beams and slabs because this location is expected to have considerable shear forces and bending moment when subjected to design loads. Sometimes, engineers may tend to select the end supports as locations for construction joints just to simplify construction.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Search

anglo.com