Question 3 of 7: Concrete Pavements — Materials, Joints and Curing

Question 3: Concrete Pavements — Materials, Joints and Curing (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Ten short definitions at two marks each; each answer below gives the definition first and then the reason it matters in pavement practice, which is what the second mark is normally awarded for. Canadian practice is cited throughout, since these are Engineers Canada national examinations.

(a) Why is air-entrained concrete used? Air entrainment deliberately disperses several percent by volume of microscopic, stable air bubbles (typically 4–7 % of the mix, with bubble spacing factors below about 0.2 mm) throughout the mortar fraction by means of a surfactant admixture. Its dominant purpose in a Canadian pavement is freeze-thaw durability: when pore water in the paste freezes it expands by roughly 9 %, and the entrained bubbles give that expanding water a nearby empty space to be driven into, relieving the hydraulic pressure that would otherwise crack the paste and cause progressive surface scaling. Entrained air also markedly improves resistance to de-icing-salt scaling and, as a secondary benefit, improves workability and reduces bleeding and segregation. The cost is strength: each additional percent of air reduces compressive strength by roughly 5 %, which is allowed for in the mix design rather than avoided. CSA A23.1 sets the required air content by exposure class and aggregate size for all Canadian pavement concrete.

(b) What is the modulus of subgrade reaction? The modulus of subgrade reaction, k, is the constant of proportionality between the vertical pressure applied to a soil surface and the vertical deflection it produces, k = p/Δ, expressed in MPa/m (or in pounds per cubic inch in the AASHTO literature). It is measured with a plate-bearing test on a 760 mm diameter plate, conventionally at a deflection of 1.25 mm, and it idealises the foundation as a bed of independent springs — the Winkler foundation — which is the model underlying Westergaard’s slab analysis and the AASHTO rigid-pavement equation. It is a system property, not a soil property: it depends on plate size, on the subbase placed over the subgrade, and on season, which is why designers work with a composite or effective k adjusted for the subbase type and thickness and for loss of support.

(c) What are transverse contraction joints, and what is their purpose? These are the closely spaced joints running across the traffic lanes, usually formed by sawing a groove into the fresh-but-hardened slab to a depth of one quarter to one third of the slab thickness within hours of placing. The groove creates a plane of weakness so that the crack which drying shrinkage and thermal contraction will inevitably form occurs at a chosen, straight, sealed location rather than at a random ragged one. They are spaced so that the panel stays roughly square and generally no longer than about 4.5 to 5 m in plain concrete, and they are normally dowelled so that load transfers across the crack without a step forming.

(d) What are transverse construction joints, and what is their purpose? A transverse construction joint is a full-depth joint formed at the end of a day’s paving, at a planned interruption, or at an equipment breakdown, where fresh concrete is later placed against hardened concrete. Its purpose is simply to provide a sound, square, vertical bulkheaded face at the stopping point rather than a feather-edged or contaminated one. Because it is a genuine discontinuity through the full thickness, it is always dowelled, and it is planned to fall at the location of a regular contraction joint so the panel layout is not disturbed.

(e) What are transverse expansion joints, and what is their purpose? An expansion joint is a full-depth transverse gap, typically 19–25 mm wide, filled with a compressible non-extruding filler board and sealed at the top, with smooth dowels that are bonded on one side only and fitted with an expansion cap on the free end. Its purpose is to give the slab room to grow when it heats up, relieving the very large longitudinal compressive forces that would otherwise cause blow-ups at joints or damage to abutting fixed structures. Modern Canadian practice uses them sparingly — only at bridges, structures, intersections with existing pavement, and sharp changes in alignment — because a wide sealed gap is a maintenance liability and because well-spaced contraction joints, if kept sealed and incompressible-free, already provide the necessary movement.

(f) What are longitudinal joints, and what is their purpose? Longitudinal joints run parallel to the centre line, at lane lines and at the pavement edge, and are formed either by sawing a weakened-plane groove in a wide paved strip or as a construction joint between adjacent paving passes. Their purpose is to control the longitudinal cracking that would otherwise develop from transverse warping and curling of a wide slab, and to define the lane geometry. They are held together with deformed tie bars rather than smooth dowels, because the intent is to prevent the lanes from separating and to keep aggregate interlock working, not to allow movement.

(g) Explain pumping of joints. Pumping is the ejection of a slurry of water and fine-grained subgrade or subbase material through a joint, crack or pavement edge under the action of repeated heavy axle loads. It requires three ingredients simultaneously: free water trapped beneath the slab, a fine-grained erodible support layer, and frequent heavy loads. As a wheel approaches the joint the approach slab deflects and displaces the trapped water at high velocity beneath the leave slab; the jet erodes the support layer and carries the fines out through the joint, where they appear as fan-shaped stains on the shoulder. The consequence is a void under the slab corner, which produces faulting, loss of load transfer, corner breaks and eventually slab cracking. The remedies attack the three ingredients directly: seal the joints, provide positive drainage and a permeable drainage layer, and use a non-erodible stabilised subbase — which is why the AASHTO drainage coefficient C_d appears explicitly in the rigid-pavement design equation used in Question 7.

(h) Why is curing of concrete necessary? Curing is the maintenance of adequate moisture and temperature in the concrete for long enough for cement hydration to proceed. Hydration is a chemical reaction that consumes water, and it effectively stops when the internal relative humidity falls below about 80 %; concrete that dries out early therefore never develops the strength, and especially never develops the dense low-permeability surface skin, that the mix was designed for. In a pavement this matters disproportionately because the exposed surface area per unit volume is enormous and the surface is precisely the part that must resist abrasion, scaling and chloride ingress. Poor curing shows up as low flexural strength, a dusting or scaling surface, and early plastic-shrinkage cracking; in cold Canadian conditions curing also means protecting the concrete from freezing before it reaches roughly 7 MPa.

(i) What are the various methods of curing of concrete pavements? The methods divide into those that supply water and those that retain it. Water-supplying methods include ponding or immersion, continuous or intermittent sprinkling and fogging, and wet coverings such as burlap, cotton mats or damp earth and sand kept continuously moist. Water-retaining methods include liquid membrane-forming curing compounds sprayed on the finished surface, which is by far the commonest choice on highway paving because a single machine pass covers the full width, and impervious sheeting of polyethylene film or waterproof paper lapped and weighted down. Accelerated methods — steam curing, heated enclosures, insulating blankets — are used in precast work and in cold-weather paving where the object is to protect against freezing as much as to cure. On white concrete pavement a pigmented, light-reflecting compound is usually specified so that coverage can be inspected visually and solar gain is reduced.

(j) Define fineness modulus of fine aggregate. The fineness modulus is an empirical index of the overall coarseness of a sand, computed by adding the cumulative percentages by mass retained on each sieve of the standard series — 4.75, 2.36, 1.18, 0.600, 0.300 and 0.150 mm — and dividing the sum by 100. It is a single number that increases as the sand becomes coarser, and CSA A23.1 and ASTM C33 require a natural concrete sand to fall between about 2.3 and 3.1 and, once a source is accepted, to remain within ±0.20 of the value used in the mix design. It is a grading average and not a grading curve: two very different sands can share a fineness modulus, so it is used to detect drift in a known source rather than to qualify an unknown one.