Polypropylene Fiber Reinforced Expansive Concrete And Its Application In Pavement

This thesis deals with some issues of materials science and technology in Project "applied research of large dimension concrete pavement slab at Zhengzhou airforce airdrome", one of the Key Technological Research and Development Programs of the Tenth Five-Year Plan for Airforce. The paper focuses on two aspects: one is to design high performance paving concrete to enlarge concrete pavement slab at airports, another is to develop techniques for increasing resistance to cracking and enhancing durability of concrete.Cracking is one of the most important and hard-solving problems in concrete science and technology. Joint cracks are closely related with durability of concrete pavement slabs.At present, diminishing restriction and releasing most of deformation to decrease restriction stress are main measures to control cracking of concrete structures. A corresponding design principal of "combination of resisting and releasing, mainly by releasing" has been proposed. As a result, many expansion joints are set. However, setting of expansion joints may greatly lower construction efficiency, weaken structural functions and promote maintenance cost. It is believed that this measure may prevent concrete slabs to suffer large structural cracking problem and can not eliminate other harmful cracks in most cases. Cracking may cause structural leakage, which will cause durability problems such as erosion, corrosion as well as freezing and thawing.Normal concrete pavement slabs are designed as 3~5m wide and 4~6m long in order to diminish restriction of deformation produced by change of temperature and moisture. Low restriction will tend to lower contraction stress and prevent concrete slabs suffering from cracking. These small slabs will be accompanied by a great deal of joints, which may be origin of structural defects such as mixture of mud, false stage, arch, empty in the bottom of slab and crack. Joints may seriously affect life span of concrete, safety and easiness in flying (or driving). When concrete pavement slabs are attacked by long term of environmental and load, they may gradually lose surface functional characteristics and suffer structural deterioration, and will eventually be damaged. This deterioration may not be resumed by a simple repair.Based on analysis on techniques for making large dimension concrete pavement slabs and the corresponding material science problems, technical measures for enlarging dimension andimproving complex durability of concrete pavement slabs were determined.The requirements of large dimension cement-made concrete pavement slabs on materials performance may be classified into three aspects. The first is to prevent the concrete pavement slabs from rupturing and cracking. The second is to control opening length and its variation of joints. And the third is to satisfy demands on fatigue and deformation of the large dimension pavement slabs under load and temperature stress. In order to meet these three requirements, concrete should be modified by: 1. reducing elastic modulus and improving toughness to diminish brittleness and sensitivity to volume change; 2. decreasing deformation of dry shrinkage and temperature shrinkage to cut down shrinkage stress; 3. improving crack-resisting, mechanical and durability performance to increase capacity to resist deformation and stress.The technical route adopted in this paper is to incorporate both polypropylene fiber and expansive agent into concrete. Bulk volume stability of concrete may be improved by coupled action of restriction effect from fiber and compensation of shrinkage with expansion from expansive agent. Anti-cracking, mechanical and durability of concrete may be enhanced through modification of microstructures by virtue of expansion under filer restriction and fiber reinforcement under expansion.The thesis work was divided into three stages. The first was experimental carried out in the laboratory. The second was pavement of concrete in a testing segment. The third was application of fiber-reinforced expansive concrete in the airport.Orthogonal experiments were carried out to provide design principal for mixture of large dimension pavement slabs of polypropylene fiber-reinforced expansive concrete. Two polypropylene fibers and four expansive agents were used in four levels of amounts, respectively. Two water cement ratios were chosen. Properties tested included workability (slump or WB viscosity), strength, elastic modulus and deformation. Contrast analyses were carried out to systematically investigate the coupled effects of fibers and expansive agent.Two segments of pavements were constructed to examine the suggested technical measures. In the first segment, ordinary concrete, polypropylene fiber concrete, expansive concrete and polypropylene fiber-reinforced expansive concrete were adopted. Six kinds of concrete pavement slabs were cast. Size of the slabs were 5m X 10m, 5m X 12m, 5m X 15m, 6m X 10m, 6m X 12m and 6m X 15m, respectively. Thickness of the slabs was 30cm. Total areacast was 12220m2. In the second segment, polypropylene fiber-reinforced expansive concrete and polypropylene (PP) and polyvinyl alcohol (PVA) hybrid fiber-reinforced expansive concrete were used. The size of pavement slabs concrete were in 5m X 10m X 24cm. Area of slabs was 9600m2. For the two segments, practical effects of fibers, expansive agents and their mixtures were tested on crackings of pavement slabs, ruptures of cut slots, stretches of contract slots and their variations.In the application stage, the polypropylene fiber-reinforced expansive concrete and polypropylene fiber-reinforced concrete were used in the most important parts of runway, such as taking-off and landing areas. The size of concrete pavement slabs was in 5m X 8m X 30cm, and the area was 27000m2.Differential resistance strain meters were buried in pavement slabs with different concrete mixtures of refueling ground and runway to detect strain characteristics of different dimension pavement slabs.When fibers and expansive agents were simultaneously incorporated into concrete, expansion increment of concrete from 7d to 14d and fall of return-contract at 28d may be significantly reduced. Fiber may restrain expansion caused by expansive agent. This will promote ability of anti-cracking when shrinkage is compensated by expansion from expansive agents. Combined use of fibers and expansive agents may greatly improve resistance to plastic shrinkage crack, and are superior to use of fibers or expansive agents alone both in reducing amounts of plastic shrinkage cracks and in diminishing width of cracks.Compared with normal concrete, polypropylene fiber-reinforced concrete and expansive concrete, polypropylene fiber-reinforced expansive concrete show higher mechanical properties and modified durability. Its impact toughness (W) and impact resistance after first cracking (index of first cracking) AW are 4 times and 2.8 times that of normal concrete, respectively.Compared with normal concrete, loss of dynamic elastic module after 300 times of freezing and thawing cycles is decreased by 8.5% and 14.5% for the fiber concrete and fiber expansive concrete, respectively, and increased by 3.9% for expansive concrete. Height of water permeability is reduced by 55% and 58% for the fiber concrete and fiber expansive concrete separately, and increased by 5% for the expansion concrete. Coulomb passing the electrode of chloride ion permeability is lowered by 54%, 65% and 5% for the fiber concrete,fiber expansive concrete and expansive concrete, respectively. Wearing off per unit area is decreased by 45% and 47% for the fiber concrete and fiber expansion concrete separately, and increased by 20% for the expansive concrete, impedance spectrumFiber-reinforced expansive concrete has the lowest Mercury intrusion pore volumes less than 150nm and more than 150nm, being 72.48mm3/g and 9.10 mm3/g, respectively. Expansive agents tend to increase pore volume less than 150nm and decrease pore volume larger than 150nm as compared with normal and fiber concrete. Results of Mercury intrusion porosimeter and alternating current impedance spectroscopy reveals that fiber-reinforced expansive concrete occupies less harmful pore and more harmless pore. On the one hand, fiber may restrain the effect of pore increment and enlargement induced by the expansive agencies, and strengthen the effect of filling with pores and diminishing pore size. On the other hand, expansive agent weaken the effect of pores increment between fiber and matrix, and increase the effects of controlling deformation, reducing shrinkage and diminishing size of pores.When fiber and expansive agent are combined used, expansion resulted from the agent may strengthen adhesion between fiber and cement-based matrix, and improve interfacial microstructure between fiber and matrix. Randomly folded strengthening effect originated from interfaces between polypropylene fiber and aggregates may be the most important source for improving the microstructure of concrete by polypropylene fibers.It has been proved by the application in the testing segment and runway that polypropylene fiber can effectively prevent concrete pavement slabs from cracking in early construction period and improve deformation at later age. Expansive concrete can be used to partly compensate shrinkage of concrete pavement slabs, prevent breaking of slabs during construction, and diminish width of joints. The coupled use of fiber with expansive agent can effectively solve the problems of rupture and crack in large dimension concrete pavement slabs as well as the accretion of joints and its variation.Non-load strain, which is composed of temperature, humidity and self-volume deformation, is essential reason for causing rupture and crack in pavement slabs at early stage. The humidity and self-volume deformation in the slabs increase linearly at early stage and reach maximum at about the highest temperature in concrete.The humidity deformation and self-volume deformation in the pavement slabs of fiber-reinforced expansive concrete increase negatively and straightly before the maximumtemperature. After that, the negative strain diminishs gradually and turn into positive strain. The deformations become stable at 7d.Temperature deformation is dominant in the non-load strain. Therefore, foundation temperature of concrete is very important. It is the decisive factor in the non-load stress of concrete.Zhengzhou airforce airdrome has been put in use in October, 2003. Its slabs in parking apron was built from October to November, 2001, and the slabs in refueling ground from August to September, 2002, and the slabs in the runway from September to November, 2002. The characters of the concrete pavement slabs are well at present. Its long-term performance still needs to be proven.